Playing with big Nikkis
10-23-16, 02:05 PM
#1
Playing with big Nikkis
This post was directed at wankel=awesome but ended up containing far more Nikki specific info than intended, so I decided to give its own thread so I don't clutter up his MSD ignition problem thread seen here: https://www.rx7club.com/1st-generati...stall-1106669/
------------------------------------
As for your Nikki, it is 24.5mm primary and 29mm secondary, right? Tuned perfectly on stock cap and rotor ignition? Now runs too rich on the MSD?
Install DLIDFIS.
Set the trailing back to stock as well, but move the wires to the leading side of the cap for one less gap to jump. Engine will run better and probably too lean.
I was right about the Nikki mods, wasn't I? Do you think I could also be right about ignition as well?
If you're still not convinced at this point, I'll tell you about the Nikki mods I just tested a couple days ago. It is the biggest hogged out Nikki anyone has ever done to the best of my knowledge. 26.55mm primaries and 30.8mm secondaries. I'm at the physical size limits of the primary venturis to the point where the metal around the little vacuum secondary hole is about as thin as aluminum foil. This carb ran 100% perfect on primaries and secondaries. Zero sec delay. Remember how that used to plague me? Not anymore. Zero flooding issues too. Drove like EFI. It easily broke them loose in gear on primaries only, even with a passenger who weighs 245 pounds; I didn't even feel his body weight. And then the secondaries feel like another rotor kicks in. And remember this car used to have an NA streetported 20B in it, but it was not powerful like this Nikki+DLIDFIS and stock S5 turbo combination on a basic R5 4 port 13B. I gave the same friend a ride when it had the 20B in it, but that thing was small potatoes compared to now. He says I'm in V8 killer territory now.
Granted the MegaSquirt wasn't tuned, but I'm glad I got away from all that EFI stuff and decided to specialize in the Nikki. I even considered Holleys for a while, like we all do. No shame in that. And even got a pretty ragged ugly boost prepped double pumper but decided to not mess with it and instead see if I could unlock the Nikki's secrets, which has been a roller coaster of discoveries as you similarly experienced, so you know what I'm talking about. You helped me a lot in the early days.
I'm not bragging about the V8 territory. It's what he said. He drives a 1990 Corvette which is quick and powerful in its own right, was dynoed at 400 torque and 375HP the other day after a full tune job, but there is just something about the rotary he likes. I don't know what my 13B is making powerwise but it can only get more powerful from this point forward when I add an intercooler and crank up the boost a little more from the current 9psi or so. I just needed to get the carb figured out first before I take the next steps.
I assume you are still wanting to add boost as well? But just wanted to finish your widebody and ignition first?
As far as your carb, I think it's fine other than needing an FB top for the properly shrouded air bleed casting this style has.
A stock ported 12A doesn't need much more than 24.5mm primaries but if you wanted to try a bigger size experimentally, I'd say don't go much bigger than 25.4 to 25.5mm and pay attention to how well it runs etc. That's kind of a given. I plan to build a nice boosted 74 ported Y 12A over the winter that is getting a Nikki within the size ranges of 24.9mm up to maybe 25.45ish. It largely depends on how well either carb runs on it, ultimately ending up with whichever runs best. I just don't have enough experience doing big carbs on 12As to know for sure yet. I also have a set at 24.5 and 24.6 venturis to play with if needed.
The secondaries can stay stock or you can try up to 30mm or so, but you don't need to make them very big with boost. I'd say leave yours at 29mm alone. I only did big secondaries experimentally to correct some that were improperly cut by that SVT guy, to see if they worked after I corrected them, and yes they do, but I won't be doing any other big secondaries. However, you do need large primaries on every Nikki because at low RPM when you're running in vacuum, the large primaries make more low end torque and provide better drivability than smaller primaries.
Sterling was dead-wrong about this aspect of Nikkis as there is a very strong reversion pulse that Carl's flow bench simply could not mimic for testing. Peejay first brought this to our attention so I'm mentioning it now; with stock 20mm venturis or Sterling's recommended but still too-small 22mm, by the time the flow gets going in the right direction, up comes a reversion pulse which stalls out the incoming air and requires it to start all over again. This reduces low end torque and leads to painfully weak low RPM power and is the main reason why everyone and their dog associates rotaries with having no low end torque.
I've of course transcended that entire incorrect notion. The issue is all but eliminated by going bigger than Sterling's 22mm. He cut several venturis of different sizes and styles and sent them all to Carl for flow bend testing. He said 22mm was the perfect compromise between flow and vacuum. He misguidedly thought high vacuum was the way to go. Nope. It's good to keep in mind but not the whole picture. What he simply could not account for was the reversion pulse. This reversion problem is probably even worse on turbo engines due to the exhaust gas dilution caused by the giant cork in the exhaust system (turbo) and larger than stock exhaust ports which a lot of us like to run. I try not to go bigger than T2 spec port timing. High overlap is not a good thing with boost, but that is another subject for another time.
As far as opening up the venturis, they say if some is good, more is better right? Up to a point. But when you do go bigger than 22mm, you have the added benefit of less intake restriction at higher RPM in boost, so you end up making more midrange power too. It's win-win. Just don't go bigger than your engine's vacuum can handle if it leads to the air stalling out which fails to pull enough fuel in, and the stock transition circuit can only account for so much (the 1700RPM glitch that you've experienced), thus it's good to not gamble too much because it leads to unusable venturis, or just sell them to someone who has a 13B. I've found my engine's limits, even though I still have a leaky break booster which lets in unwanted air in the secondary runners, so at low RPM it effectively makes the Nikkis seem bigger than they actually are. You now, it's possible my engine thinks it's got a Nikki at 26.7 or 26.8mm... maybe.
I actually decided to carefully cut two more sets of venturis bigger than anyone should; at 26.65 and 26.75 respectively. Why the .05mm at the end of my numbers? Because that is where they end up after finishing work. I aim for a basic number with the die grinder, which leaves high spots, that are then smoothed down by about .025mm on each side, that add up to a .05mm increase in ID. The vac sec hole metal is like aluminum foil but still barely intact. I guess it would be zinc foil at this point. All I'm saying is my leaky brake booster makes the carbs seem like they're bigger than they actually are, which proves that a 74 ported 13B can handle bigger than what I've tested at 26.55mm so far, so I decided to see what else I could do. Please don't ever go this big.
------------------------------------
Hey, here's a disturbing thought. Anyone want to take the primary venturis out of a Nikki and see if it still runs? The resulting hole is 28mm. I know some weber guys remove their venturis to get more flow with only a slight reduction in drivability. Think the Nikki can handle it?
------------------------------------
As for your Nikki, it is 24.5mm primary and 29mm secondary, right? Tuned perfectly on stock cap and rotor ignition? Now runs too rich on the MSD?
Install DLIDFIS.
Set the trailing back to stock as well, but move the wires to the leading side of the cap for one less gap to jump. Engine will run better and probably too lean.
I was right about the Nikki mods, wasn't I? Do you think I could also be right about ignition as well?If you're still not convinced at this point, I'll tell you about the Nikki mods I just tested a couple days ago. It is the biggest hogged out Nikki anyone has ever done to the best of my knowledge. 26.55mm primaries and 30.8mm secondaries. I'm at the physical size limits of the primary venturis to the point where the metal around the little vacuum secondary hole is about as thin as aluminum foil. This carb ran 100% perfect on primaries and secondaries. Zero sec delay. Remember how that used to plague me? Not anymore. Zero flooding issues too. Drove like EFI. It easily broke them loose in gear on primaries only, even with a passenger who weighs 245 pounds; I didn't even feel his body weight.
And then the secondaries feel like another rotor kicks in. And remember this car used to have an NA streetported 20B in it, but it was not powerful like this Nikki+DLIDFIS and stock S5 turbo combination on a basic R5 4 port 13B. I gave the same friend a ride when it had the 20B in it, but that thing was small potatoes compared to now. He says I'm in V8 killer territory now. Granted the MegaSquirt wasn't tuned, but I'm glad I got away from all that EFI stuff and decided to specialize in the Nikki. I even considered Holleys for a while, like we all do. No shame in that.
And even got a pretty ragged ugly boost prepped double pumper but decided to not mess with it and instead see if I could unlock the Nikki's secrets, which has been a roller coaster of discoveries as you similarly experienced, so you know what I'm talking about. You helped me a lot in the early days. I'm not bragging about the V8 territory. It's what he said. He drives a 1990 Corvette which is quick and powerful in its own right, was dynoed at 400 torque and 375HP the other day after a full tune job, but there is just something about the rotary he likes. I don't know what my 13B is making powerwise but it can only get more powerful from this point forward when I add an intercooler and crank up the boost a little more from the current 9psi or so. I just needed to get the carb figured out first before I take the next steps.
I assume you are still wanting to add boost as well? But just wanted to finish your widebody and ignition first?
As far as your carb, I think it's fine other than needing an FB top for the properly shrouded air bleed casting this style has.
A stock ported 12A doesn't need much more than 24.5mm primaries but if you wanted to try a bigger size experimentally, I'd say don't go much bigger than 25.4 to 25.5mm and pay attention to how well it runs etc. That's kind of a given.
I plan to build a nice boosted 74 ported Y 12A over the winter that is getting a Nikki within the size ranges of 24.9mm up to maybe 25.45ish. It largely depends on how well either carb runs on it, ultimately ending up with whichever runs best. I just don't have enough experience doing big carbs on 12As to know for sure yet. I also have a set at 24.5 and 24.6 venturis to play with if needed.The secondaries can stay stock or you can try up to 30mm or so, but you don't need to make them very big with boost. I'd say leave yours at 29mm alone. I only did big secondaries experimentally to correct some that were improperly cut by that SVT guy, to see if they worked after I corrected them, and yes they do, but I won't be doing any other big secondaries. However, you do need large primaries on every Nikki because at low RPM when you're running in vacuum, the large primaries make more low end torque and provide better drivability than smaller primaries.
Sterling was dead-wrong about this aspect of Nikkis as there is a very strong reversion pulse that Carl's flow bench simply could not mimic for testing. Peejay first brought this to our attention so I'm mentioning it now; with stock 20mm venturis or Sterling's recommended but still too-small 22mm, by the time the flow gets going in the right direction, up comes a reversion pulse which stalls out the incoming air and requires it to start all over again. This reduces low end torque and leads to painfully weak low RPM power and is the main reason why everyone and their dog associates rotaries with having no low end torque.
I've of course transcended that entire incorrect notion.
The issue is all but eliminated by going bigger than Sterling's 22mm. He cut several venturis of different sizes and styles and sent them all to Carl for flow bend testing. He said 22mm was the perfect compromise between flow and vacuum. He misguidedly thought high vacuum was the way to go. Nope. It's good to keep in mind but not the whole picture. What he simply could not account for was the reversion pulse. This reversion problem is probably even worse on turbo engines due to the exhaust gas dilution caused by the giant cork in the exhaust system (turbo) and larger than stock exhaust ports which a lot of us like to run. I try not to go bigger than T2 spec port timing. High overlap is not a good thing with boost, but that is another subject for another time.As far as opening up the venturis, they say if some is good, more is better right? Up to a point. But when you do go bigger than 22mm, you have the added benefit of less intake restriction at higher RPM in boost, so you end up making more midrange power too.
It's win-win. Just don't go bigger than your engine's vacuum can handle if it leads to the air stalling out which fails to pull enough fuel in, and the stock transition circuit can only account for so much (the 1700RPM glitch that you've experienced), thus it's good to not gamble too much because it leads to unusable venturis, or just sell them to someone who has a 13B. I've found my engine's limits, even though I still have a leaky break booster which lets in unwanted air in the secondary runners, so at low RPM it effectively makes the Nikkis seem bigger than they actually are. You now, it's possible my engine thinks it's got a Nikki at 26.7 or 26.8mm... maybe. I actually decided to carefully cut two more sets of venturis bigger than anyone should; at 26.65 and 26.75 respectively. Why the .05mm at the end of my numbers? Because that is where they end up after finishing work. I aim for a basic number with the die grinder, which leaves high spots, that are then smoothed down by about .025mm on each side, that add up to a .05mm increase in ID. The vac sec hole metal is like aluminum foil but still barely intact. I guess it would be zinc foil at this point.
All I'm saying is my leaky brake booster makes the carbs seem like they're bigger than they actually are, which proves that a 74 ported 13B can handle bigger than what I've tested at 26.55mm so far, so I decided to see what else I could do. Please don't ever go this big. ------------------------------------
Hey, here's a disturbing thought. Anyone want to take the primary venturis out of a Nikki and see if it still runs? The resulting hole is 28mm. I know some weber guys remove their venturis to get more flow with only a slight reduction in drivability. Think the Nikki can handle it?
10-23-16, 08:49 PM
#2
but how can a carb work without a venturi shape?
Wanna really play take the carb to a machinist and get him to cut the bores bigger and drop in the secondary venturies into the primaries. Then take some even bigger secondaries and drop them in. Now that would be epic.
Wanna really play take the carb to a machinist and get him to cut the bores bigger and drop in the secondary venturies into the primaries. Then take some even bigger secondaries and drop them in. Now that would be epic.
10-24-16, 10:56 AM
#4
Qingdao, it's what some weber guys do. I once tried a Nikki without secondary venturis and it didn't feel any different from stock 28mm so wasn't worth the possible risks of introducing a secondary delay.
My carbs had a delay back then that's since been solved, so I was curious how a set of properly hogged secondaries would do; my mentioned 30.8mm, and they did just fine with no delay and seemed to make plenty of power so I might venture into cutting secondaries some day as I think the improved shape over the dull stock shape actually helps despite the larger ID.
TG, if you study the aspect ratio or "venturi profile" of the secondaries and compare that with stock primaries, you can see how the Nikki company was going for a larger ID than what the bean counters at Mazda wanted. Studying them, it looks like Nikki wanted about 23mm, which maintains the same or similar profile shape to the secondaries. However we know the stock profile is just a casting with a lathed hole cut for 20.00 and 28.00, which could be mass produced easily. But I have a feeling some enterprising individuals at the Nikki plant probably took it open themselves to make various custom profiles installed in several test carbs and probably shipped them to Mazda for dyno testing. I mean it makes sense, right? And only after extensive testing, the CEO probably decided on going smaller than ideal: 20mm for fuel mileage concerns and probably emissions as well, even though the larger carbs do actually run more efficiently and can actually save fuel with tighter fuel jet and air bleed tuning than mass produced for general consumption carbs all the new 1st gens ended up with. Our stock jet sizes ranged from 91 to 92 on FB carbs and 93 to 94 on SA carbs, with 60 to 70 air bleeds on FBs and 80 to 90 air bleeds on SAs. And then our idle air bleeds range from 150 to 190 which only works with a functional choke flap to keep the carb from going so rich it braps the engine due to richness beyond what the Mazda engineers decided a general consumption carb could get away with. When you remove the choke flap for performance, you have to make the idle air bleeds smaller. I found 118 to 124 to work best.
Simple rules to follow: Make your air bleeds the smallest you can get away with, without brapping. Then tailor your jet size to the AFR you want. I like to see:
12.2 idle
13.0 part throttle cruise
11.5 in boost
If NA, you can aim for 12.5 at full throttle, but follow j9fd3s' advice and don't follow what I just said or any piece of paper with a jet tuning guide. Instead, you should always give it what it wants and ignore everything else. If it runs happier at another number, go with that. Once you find your the engine's sweet spot, go one step richer for safety. Done.
TG, I don't know how big your venturis are, but I can take an educated guess. I think they are 23mm. I say that because it looks right. If you were using the secondaries as a guide, 23mm gives the same profile shape or aspect ratio. However, 23mm is too small on a stock port. I've determined this only after a lot of testing and what others have told me. It turns out a stockport 12A with a decent exhaust like the RB long primary can handle up to 24.5mm just fine. But can we go bigger? Not sure. I'd like to test a 24.9mm and a 25.45mm on a 74 ported Y 12A with a 79 manifold and an S5 turbo.
My carbs had a delay back then that's since been solved, so I was curious how a set of properly hogged secondaries would do; my mentioned 30.8mm, and they did just fine with no delay and seemed to make plenty of power so I might venture into cutting secondaries some day as I think the improved shape over the dull stock shape actually helps despite the larger ID.
TG, if you study the aspect ratio or "venturi profile" of the secondaries and compare that with stock primaries, you can see how the Nikki company was going for a larger ID than what the bean counters at Mazda wanted. Studying them, it looks like Nikki wanted about 23mm, which maintains the same or similar profile shape to the secondaries. However we know the stock profile is just a casting with a lathed hole cut for 20.00 and 28.00, which could be mass produced easily. But I have a feeling some enterprising individuals at the Nikki plant probably took it open themselves to make various custom profiles installed in several test carbs and probably shipped them to Mazda for dyno testing. I mean it makes sense, right? And only after extensive testing, the CEO probably decided on going smaller than ideal: 20mm for fuel mileage concerns and probably emissions as well, even though the larger carbs do actually run more efficiently and can actually save fuel with tighter fuel jet and air bleed tuning than mass produced for general consumption carbs all the new 1st gens ended up with. Our stock jet sizes ranged from 91 to 92 on FB carbs and 93 to 94 on SA carbs, with 60 to 70 air bleeds on FBs and 80 to 90 air bleeds on SAs. And then our idle air bleeds range from 150 to 190 which only works with a functional choke flap to keep the carb from going so rich it braps the engine due to richness beyond what the Mazda engineers decided a general consumption carb could get away with. When you remove the choke flap for performance, you have to make the idle air bleeds smaller. I found 118 to 124 to work best.
Simple rules to follow: Make your air bleeds the smallest you can get away with, without brapping. Then tailor your jet size to the AFR you want. I like to see:
12.2 idle
13.0 part throttle cruise
11.5 in boost
If NA, you can aim for 12.5 at full throttle, but follow j9fd3s' advice and don't follow what I just said or any piece of paper with a jet tuning guide. Instead, you should always give it what it wants and ignore everything else. If it runs happier at another number, go with that. Once you find your the engine's sweet spot, go one step richer for safety. Done.
TG, I don't know how big your venturis are, but I can take an educated guess. I think they are 23mm. I say that because it looks right. If you were using the secondaries as a guide, 23mm gives the same profile shape or aspect ratio. However, 23mm is too small on a stock port. I've determined this only after a lot of testing and what others have told me. It turns out a stockport 12A with a decent exhaust like the RB long primary can handle up to 24.5mm just fine. But can we go bigger? Not sure. I'd like to test a 24.9mm and a 25.45mm on a 74 ported Y 12A with a 79 manifold and an S5 turbo.
10-27-16, 11:06 AM
#5
carb whisperer
I cut my venturi step angles at a specific 11* inlet, and 7* outlet. Apply that to whatever diameter you choose, and it should work great!
Mine fogs up the boosters at the slightest throttle angle, so I know the venturi shape is doing its job at pulling more air through the jet.
Btw. Keyword, MORE.
If you take away the Venturi *shape*, it will still function as a carburetor because air will still be diving through the boosters when you open the throttle. As long as it keeps going the right direction (it will), the air moving through and around the booster will force it to draw the fuel through itself as a physical reaction to the *Booster* effect.
Removing the Venturi shape will simply slow the air down (a lot) and make it seem as if your jet or fuel supply is lesser. It will have a linear effect at less fuel at any given level, because the air speed and low pressure has been reduced/eliminated.
It DOES work for an all out performance carb, but be prepared to jet up a LOT, and have a poor idle quality and much reduced part throttle efficiency. Remember, that Venturi shape was engineered in the heart of the fuel circuitry. The Nikki already has a tiny accelerator pump discharge to cover transition, and a fairly weak transition ciruit, The Venturi shape actually promotes both of them. The low pressure at the throttle blades, and increased speed at the booster forces every part of the system to draw fuel easier, including the acc. pump shot.
Dont believe me? Set your idle to 500 RPM and quickly open the throttle while looking down the throat of the carb. The pump shot will look like a short stream at relatively low pressure firing down the primary throats.
Now set your idle to 4k RPM and do the same test. The pump shot now exits the nozzle instantly, and at a seemingly much higher pressure, and appears to be instantly emulsified.
My secondaries are 28MM jeff, with the aforementioned angles. They work flawlessly.
Although, the gasket paper I made my spacers out of has proven to be garbage and has been slowly leaking down on to my engine. I removed them all this morning and replaced the entire stack with a new, stock acc pump with no spacers. Its only temporary until I can make some new spacers, but it surprisingly still runs pretty well.
Mine fogs up the boosters at the slightest throttle angle, so I know the venturi shape is doing its job at pulling more air through the jet.
Btw. Keyword, MORE.
If you take away the Venturi *shape*, it will still function as a carburetor because air will still be diving through the boosters when you open the throttle. As long as it keeps going the right direction (it will), the air moving through and around the booster will force it to draw the fuel through itself as a physical reaction to the *Booster* effect.
Removing the Venturi shape will simply slow the air down (a lot) and make it seem as if your jet or fuel supply is lesser. It will have a linear effect at less fuel at any given level, because the air speed and low pressure has been reduced/eliminated.
It DOES work for an all out performance carb, but be prepared to jet up a LOT, and have a poor idle quality and much reduced part throttle efficiency. Remember, that Venturi shape was engineered in the heart of the fuel circuitry. The Nikki already has a tiny accelerator pump discharge to cover transition, and a fairly weak transition ciruit, The Venturi shape actually promotes both of them. The low pressure at the throttle blades, and increased speed at the booster forces every part of the system to draw fuel easier, including the acc. pump shot.
Dont believe me? Set your idle to 500 RPM and quickly open the throttle while looking down the throat of the carb. The pump shot will look like a short stream at relatively low pressure firing down the primary throats.
Now set your idle to 4k RPM and do the same test. The pump shot now exits the nozzle instantly, and at a seemingly much higher pressure, and appears to be instantly emulsified.
My secondaries are 28MM jeff, with the aforementioned angles. They work flawlessly.
Although, the gasket paper I made my spacers out of has proven to be garbage and has been slowly leaking down on to my engine. I removed them all this morning and replaced the entire stack with a new, stock acc pump with no spacers. Its only temporary until I can make some new spacers, but it surprisingly still runs pretty well.
Last edited by wankel=awesome; 10-27-16 at 11:08 AM.
10-27-16, 11:11 AM
#6
carb whisperer
Also, the AFR here is exactly as I have experienced working with carbed rotaries for NA.
Idle quality is best at around 12.0:1-12.5:1
Cruise works well at anywhere from 11.0:1-14.0:1
WOT pulls the hardest from around 11.5:1-13.0:1
On the nikki, the reason for tuning the cruise on the richer side is simple: it helps transition into the power circuit. Its very common with 4BBL carburetors.
Idle quality is best at around 12.0:1-12.5:1
Cruise works well at anywhere from 11.0:1-14.0:1
WOT pulls the hardest from around 11.5:1-13.0:1
On the nikki, the reason for tuning the cruise on the richer side is simple: it helps transition into the power circuit. Its very common with 4BBL carburetors.
10-27-16, 03:26 PM
#7
Still 28mm, huh? Ok I must have remembered it wrong.
I'll check out the inlet and outlet angles of some of my venturis today.
Yes, I've completed a couple of carbs that exhibit very similar tiny flat spots at the bottom of the RPM range as you drag it down when you let out the clutch, however one carb wants really rich jets (as you explained) while the other wants really lean jets. That one threw me for a loop. Same size venturis too, with a margin of error of .05mm.
What I think is happening here is the one carb that wants richer jets just doesn't have enough venturi shape left after my extreme hogging job at 26.25, so it got a set of 131 to 132ish drilled jets and they helped a lot. But then I've always had trouble tuning this carb regardless if its size increases over the last 2.5 years from 24.7, to 25.25 then to 26.25 or 26.30ish-ish. Remember I cut these by hand so, I average it.
On the other hand, another carb that got some identically cut 26.25mm wanted 126 jets after I tried 131, 128 and lastly 126. It has a very mild flat spot and runs a little bit lean so I might jet it back up to 128 which should reduce or eliminate the flat spot. However I think my car's leaky brake booster is the real culprit here. But then it is running a little bit lean at 13.1 to 13.3 AFR at 126, so I think the 128s will get it down to 12.7 to 13.0 which will run better (especially in this cooler weather we're having now).
Lastly, I retuned a 26.55mm carb which has always ran great for me at it previous 25.35mm size, and continues to do so now at 26.5mm. This carb seems to be impervious to the leaky brake booster and actually drives better than most on it. It was first tested with the 131 jets (was too rich at 12.2 to 12.5) but has since gotten 129s and is now perfect at 12.7 to 13.0 AFR.
Even though this carb is bigger than the others, it doesn't have any of those flat spots or other problems at low RPM in the transition circuit. I don't think it's the venturis. I think it's the carb. Imagine that; some carbs always have issues while others don't. Could be a manufacturing flaw.
I've got another carb at 26.50mm which feels like I'm driving with my e-brake on. It just feels sluggish. 130 jets, and could handle the 129s from the 26.55 carb I'm sure. Thus the same amount of air is getting through the boosters and pulling the same amount of fuel at part throttle and low RPM, but this carb just doesn't have the grunt I want. What it does have are mild glitches and unsteadiness at part throttle cruise, but is otherwise stable enough to run on an engine with a light flywheel. It only has a mild flat spot at lor RPM when you drag it down.
I had a carb like this last year that ran about the same on my leaky brake booster. I thought about going to a heavier flywheel etc. However when I tested it on a similar 13B with a healthy brake booster, it ran perfectly. Perhaps the transition circuit of some carbs is far more sensitive to a leaky brake booster than others? That's my guess. I've tried a few different sets of long slow 46 air bleeds and it does make a difference in idle quality. The mixture screw does need to be cranked in or out a little. So yeah, maybe.
My dark grey gaskets for the accel pump don't drip and my blue baseplate gasket does absorb OMP oil over time but I'm not worried about it. They're from a set of gasket paper types from any auto parts store.
I'll check out the inlet and outlet angles of some of my venturis today.
Yes, I've completed a couple of carbs that exhibit very similar tiny flat spots at the bottom of the RPM range as you drag it down when you let out the clutch, however one carb wants really rich jets (as you explained) while the other wants really lean jets. That one threw me for a loop. Same size venturis too, with a margin of error of .05mm.
What I think is happening here is the one carb that wants richer jets just doesn't have enough venturi shape left after my extreme hogging job at 26.25, so it got a set of 131 to 132ish drilled jets and they helped a lot. But then I've always had trouble tuning this carb regardless if its size increases over the last 2.5 years from 24.7, to 25.25 then to 26.25 or 26.30ish-ish.
Remember I cut these by hand so, I average it.On the other hand, another carb that got some identically cut 26.25mm wanted 126 jets after I tried 131, 128 and lastly 126. It has a very mild flat spot and runs a little bit lean so I might jet it back up to 128 which should reduce or eliminate the flat spot. However I think my car's leaky brake booster is the real culprit here. But then it is running a little bit lean at 13.1 to 13.3 AFR at 126, so I think the 128s will get it down to 12.7 to 13.0 which will run better (especially in this cooler weather we're having now).
Lastly, I retuned a 26.55mm carb which has always ran great for me at it previous 25.35mm size, and continues to do so now at 26.5mm. This carb seems to be impervious to the leaky brake booster and actually drives better than most on it. It was first tested with the 131 jets (was too rich at 12.2 to 12.5) but has since gotten 129s and is now perfect at 12.7 to 13.0 AFR.
Even though this carb is bigger than the others, it doesn't have any of those flat spots or other problems at low RPM in the transition circuit. I don't think it's the venturis. I think it's the carb. Imagine that; some carbs always have issues while others don't. Could be a manufacturing flaw.
I've got another carb at 26.50mm which feels like I'm driving with my e-brake on. It just feels sluggish. 130 jets, and could handle the 129s from the 26.55 carb I'm sure. Thus the same amount of air is getting through the boosters and pulling the same amount of fuel at part throttle and low RPM, but this carb just doesn't have the grunt I want. What it does have are mild glitches and unsteadiness at part throttle cruise, but is otherwise stable enough to run on an engine with a light flywheel. It only has a mild flat spot at lor RPM when you drag it down.
I had a carb like this last year that ran about the same on my leaky brake booster. I thought about going to a heavier flywheel etc. However when I tested it on a similar 13B with a healthy brake booster, it ran perfectly. Perhaps the transition circuit of some carbs is far more sensitive to a leaky brake booster than others? That's my guess. I've tried a few different sets of long slow 46 air bleeds and it does make a difference in idle quality. The mixture screw does need to be cranked in or out a little. So yeah, maybe.
My dark grey gaskets for the accel pump don't drip and my blue baseplate gasket does absorb OMP oil over time but I'm not worried about it. They're from a set of gasket paper types from any auto parts store.
Trending Topics
10-30-16, 01:50 PM
#10
I tested two big Nikkis yesterday with swapped baseplates. Both carbs run FAR better now, but one went lean and the other went rich. It's all in the baseplates.
The first one I tested is the 26.50mm carb. It ran horrible before but now its part throttle cruise went rich at 12.1 to 12.6 and ran pretty well with a lot of grunt. It only had a minor flat spot between 1700 and 2100 that I'm going to blame on my leaky brake booster (I will retest this carb on its intended engine and retune it if necessary). Its secondary delay was .1 second or just a very minor cough that I'm attributing to the baseplate as it exhibited this exact behavior on a different carb at 26.65mm.
Yes, the baseplate itself can cause a secondary delay if you're boosting. Pretty strange, right?
I also tested the 26.65mm carb with a different baseplate and it now runs very lean for my setup at 13.1 to 13.4 but a lot better than before despite the new leanness. Lots more grunt though. This carb still has the same wide flat spot as before, which is encouraging because it means I can jet it up now and this will reduce or eliminate the flat spot entirely, without going pig rich, which I couldn't do before. Plus its secondaries are perfect now, instead of that minor .1 second delay it had with the other baseplate.
I'll say it again. The baseplate can have a secondary delay. There could be nothing visually wrong with it. It just runs a certain way.
One more thing. j9fd3s says it's best to find the perfect tune, then jet up one step for safety. I think the carb that went rich by swapping the baseplates is a good example because when it goes onto a different setup that doesn't have a leaky brake booster, the reduction of unwanted air will richen it up further which could act like jetting it up one step for safety, but I won't have to actually swap any jets into it. I will only need to richen up the other carb.
I'm saying this because going up by just one step would not be enough with my leaky brake booster, but should be enough on everything else. My leaky brake booster would probably want two steps.
In my case, each step = .02mm such as 126, 128, 130, 132 etc in the 26mm and up venturi range. The carb has 130 jets in it so if I go to 132 and eliminate the unwanted air, it should work like going up to 134 if I kept the unwanted air. Of course the unwanted air only affects idle and the lower RPM ranges in vacuum.
The first one I tested is the 26.50mm carb. It ran horrible before but now its part throttle cruise went rich at 12.1 to 12.6 and ran pretty well with a lot of grunt. It only had a minor flat spot between 1700 and 2100 that I'm going to blame on my leaky brake booster (I will retest this carb on its intended engine and retune it if necessary). Its secondary delay was .1 second or just a very minor cough that I'm attributing to the baseplate as it exhibited this exact behavior on a different carb at 26.65mm.
Yes, the baseplate itself can cause a secondary delay if you're boosting. Pretty strange, right?
I also tested the 26.65mm carb with a different baseplate and it now runs very lean for my setup at 13.1 to 13.4 but a lot better than before despite the new leanness. Lots more grunt though.
This carb still has the same wide flat spot as before, which is encouraging because it means I can jet it up now and this will reduce or eliminate the flat spot entirely, without going pig rich, which I couldn't do before. Plus its secondaries are perfect now, instead of that minor .1 second delay it had with the other baseplate.I'll say it again. The baseplate can have a secondary delay. There could be nothing visually wrong with it. It just runs a certain way.
One more thing. j9fd3s says it's best to find the perfect tune, then jet up one step for safety. I think the carb that went rich by swapping the baseplates is a good example because when it goes onto a different setup that doesn't have a leaky brake booster, the reduction of unwanted air will richen it up further which could act like jetting it up one step for safety, but I won't have to actually swap any jets into it. I will only need to richen up the other carb.
I'm saying this because going up by just one step would not be enough with my leaky brake booster, but should be enough on everything else. My leaky brake booster would probably want two steps.
In my case, each step = .02mm such as 126, 128, 130, 132 etc in the 26mm and up venturi range. The carb has 130 jets in it so if I go to 132 and eliminate the unwanted air, it should work like going up to 134 if I kept the unwanted air. Of course the unwanted air only affects idle and the lower RPM ranges in vacuum.
Last edited by Jeff20B; 10-30-16 at 02:00 PM.
10-31-16, 09:46 PM
#11
I've got sets of 134, another 134, 135, 136, and 137 jets now. That should be enough to go one or two steps richer on several carbs.
Hundredths of a mm is a bit OCD, but if you've got it, flaunt it.
Hundredths of a mm is a bit OCD, but if you've got it, flaunt it.
Last edited by Jeff20B; 11-01-16 at 10:02 AM.
11-10-16, 01:07 PM
#12
I cut ten sets of secondaries in sizes ranging from 29.4mm up to 30.7mm, intended to go on several different engines. The pile of metal shavings was rather large and filled up my dust pan all the way. It had some actual weigh to it. I wonder how many venturis worth of metal shavings I had there. Crazy.
I mean, you could have made probably two or more full venturis just from the shavings alone.
It took three days, working for 6 hours each day, to get through 20 individual venturis like that. Oh and I hogged out and rough cut two sets of primaries too. One set is fully cut out to 25.85mm and the other set was just roughed into shape so far. I was pretty spent by the time I got to them, so I went for broke. Got a lot done on them but I'll revisit them some other time.
Yes, a lathe would be easier in many ways, but nothing beats the hands on approach you can only get with a skilled use of a die grinder and other finesse tools like a dremel for finishing work.
I mean, you could have made probably two or more full venturis just from the shavings alone.
It took three days, working for 6 hours each day, to get through 20 individual venturis like that. Oh and I hogged out and rough cut two sets of primaries too. One set is fully cut out to 25.85mm and the other set was just roughed into shape so far. I was pretty spent by the time I got to them, so I went for broke. Got a lot done on them but I'll revisit them some other time.
Yes, a lathe would be easier in many ways, but nothing beats the hands on approach you can only get with a skilled use of a die grinder and other finesse tools like a dremel for finishing work.
Last edited by Jeff20B; 11-10-16 at 01:28 PM.
11-10-16, 02:37 PM
#13
Took me a month to cut 4. I screwed up the first two. And the second two I took way too many breaks and cut them slowly.
This kind of work would be perfect for the HF lathe. At least getting rid of the majority could be done with a Chinese tool. The last bit would have to be done by hand.
This kind of work would be perfect for the HF lathe. At least getting rid of the majority could be done with a Chinese tool. The last bit would have to be done by hand.
11-12-16, 11:13 AM
#14
Agreed.
The funny thing is, the finishing work with a dremel is actually more difficult than the initial rough cutting/metal removal plus final shaping is with the die grinder. I can do that stuff for hours with only some fatigue. However, the finishing work always gets to me quickly. The finishing work on the 25.85mm primaries wore me out. I was already spent from the rough/final cutting and shaping of 22 venturis though.
The funny thing is, the finishing work with a dremel is actually more difficult than the initial rough cutting/metal removal plus final shaping is with the die grinder. I can do that stuff for hours with only some fatigue. However, the finishing work always gets to me quickly. The finishing work on the 25.85mm primaries wore me out. I was already spent from the rough/final cutting and shaping of 22 venturis though.