Nikki Tuning - Wideband Style
12-30-19, 02:14 PM
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Nikki Tuning - Wideband Style
Ok, this thread is coming from me never having tuned using a wideband in my life, and I have to say that seeing what's going on via the gauge in different scenarios is very intriguing. That said, I've done some research, but the only thread that I've found to be of much help so far is Tuning a Nikki Carb Using a wideband AFR Gauge (air to fuel ratio). That gave me an idea of the target afr for various conditions, but not so much on the actual tuning philosophy. So then, in no particular order I've got some questions and/or assorted info for interpretation.
First off, where do those of you who have done this before usually start. I understand the starting at idle and working from there bit (as that's essentially what I've done without the gauge thus far), but what's the whole deal with air bleed tuning, tip-in, afr spikes (like when letting off the gas or slamming the pedal down - should those even be occurring in high levels?), etc.
I should preface this next paragraph quickly by saying that I am not 'too' concerned with tuning my stock carb, and so as of now these are just things that I've noticed in the past 24 hours and would appreciate any insight on. The current setup is a stock 1982 carb with smog equipment disabled or removed (aside from the choke and alt comp), and then all the rest of what's in my sig.
Right now my stock carb - which I have done zilch to tune other than messing with idle adjustments - is running way lean on part throttle cruise (~14.2-15.8), ~13.5-14.2 under wot, and decides at random to idle between 12.4 and 13.5 (the idle never fluctuates that much when sitting, but between two stops it can easily vary that much - it also greatly affects my idle speed which I have been noticing ever since I reinstalled my stocker; now I have a clue why). When I let off the gas I can get spikes to 17 and usually this doesn't level back out until the idle has steadied itself - sometimes this lean spike is enough to kill the car, but usually only when cold. I've also noticed that my misfires show the afr going a little lean, which is interesting...
My modded carb which is in the process of being revamped and should go back on in a couple months is as follows:
- 24mm primaries (for now - probably to stay that way)
- to be 30mm secondaries (unless someone convinces me otherwise)
- trimmed boosters with the 'airfoil mod'
- fully stripped carb
- retimed mech secondaries
- ap mod for ^^^
- 124 primary fuel jets
- 60 primary no.2 air bleeds
- 70 primary main air bleeds
- solder filled secondary long slows
- to be 80 secondary main air bleeds
- stock secondary no.2 bleeds
- ?? for the secondary fuel (I think a 140 would be a safe starting place?)
When I had my modded carb last installed, it would not idle cold below 2k but as of now I have no data to back up that fact. It was actually a smoother carb to drive, probably because it wasn't so terribly lean on the cruising afr (I'm just guessing here). Additionally, I've always had some sort of issue with the car wanting to die when cold and returning to idle, as well as a consistent tendency to buck anywhere below 3k when decelerating in gear.
So at this point I should probably actually ask a couple questions. When going about tuning carbs, how normal are the extreme spikes when letting off the throttle, ect., or perhaps I should say that if they are normal about how extreme are they typically? I'm also wondering if anyone's target afr's have changed since that post back in 2015 (although I don't really expect this to be so, I'm just curious).
Any other tuning tips or tricks (doesn't have to be wideband specific) would be greatly appreciated.
Thanks in advance, and a happy new year to all!
Edit: One big thing I forgot to mention is that this is NA tuning - although perhaps that was assumed. Boosted (blow-thru) application knowledge is still appreciated, although as of now I'm running NA.
First off, where do those of you who have done this before usually start. I understand the starting at idle and working from there bit (as that's essentially what I've done without the gauge thus far), but what's the whole deal with air bleed tuning, tip-in, afr spikes (like when letting off the gas or slamming the pedal down - should those even be occurring in high levels?), etc.
I should preface this next paragraph quickly by saying that I am not 'too' concerned with tuning my stock carb, and so as of now these are just things that I've noticed in the past 24 hours and would appreciate any insight on. The current setup is a stock 1982 carb with smog equipment disabled or removed (aside from the choke and alt comp), and then all the rest of what's in my sig.
Right now my stock carb - which I have done zilch to tune other than messing with idle adjustments - is running way lean on part throttle cruise (~14.2-15.8), ~13.5-14.2 under wot, and decides at random to idle between 12.4 and 13.5 (the idle never fluctuates that much when sitting, but between two stops it can easily vary that much - it also greatly affects my idle speed which I have been noticing ever since I reinstalled my stocker; now I have a clue why). When I let off the gas I can get spikes to 17 and usually this doesn't level back out until the idle has steadied itself - sometimes this lean spike is enough to kill the car, but usually only when cold. I've also noticed that my misfires show the afr going a little lean, which is interesting...
My modded carb which is in the process of being revamped and should go back on in a couple months is as follows:
- 24mm primaries (for now - probably to stay that way)
- to be 30mm secondaries (unless someone convinces me otherwise)
- trimmed boosters with the 'airfoil mod'
- fully stripped carb
- retimed mech secondaries
- ap mod for ^^^
- 124 primary fuel jets
- 60 primary no.2 air bleeds
- 70 primary main air bleeds
- solder filled secondary long slows
- to be 80 secondary main air bleeds
- stock secondary no.2 bleeds
- ?? for the secondary fuel (I think a 140 would be a safe starting place?)
When I had my modded carb last installed, it would not idle cold below 2k but as of now I have no data to back up that fact. It was actually a smoother carb to drive, probably because it wasn't so terribly lean on the cruising afr (I'm just guessing here). Additionally, I've always had some sort of issue with the car wanting to die when cold and returning to idle, as well as a consistent tendency to buck anywhere below 3k when decelerating in gear.
So at this point I should probably actually ask a couple questions. When going about tuning carbs, how normal are the extreme spikes when letting off the throttle, ect., or perhaps I should say that if they are normal about how extreme are they typically? I'm also wondering if anyone's target afr's have changed since that post back in 2015 (although I don't really expect this to be so, I'm just curious).
Any other tuning tips or tricks (doesn't have to be wideband specific) would be greatly appreciated.
Thanks in advance, and a happy new year to all!
Edit: One big thing I forgot to mention is that this is NA tuning - although perhaps that was assumed. Boosted (blow-thru) application knowledge is still appreciated, although as of now I'm running NA.
Last edited by Benjamin4456; 12-30-19 at 02:17 PM.
12-31-19, 10:30 AM
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the nice thing about the wideband is that it gives your seat of the pants dyno, an objective marker. for instance, you notice that it idles better at 12.5 than 13.5, it then helps you to get it closer to its happy place faster and more consistently.
the lean spikes with throttle movement are normal, air is lighter than fuel, so it moves faster when you move the throttle suddenly. this is why it needs an accelerator pump. as long as it doesn't stumble/hesitate its fine.
it should be lean at part throttle cruise too, basically as lean as you can go and have it still run ok, (without cats, with cat it needs to be 14.7), any more than that is wasting fuel.
13.5-14.2 at WOT is probably too lean for best power. if you take a step back, in theory every air molecule combines with every gasoline molecule at 14.7:1. however in real life, things are not perfect, so to make sure you use every molecule of air, we add a little extra fuel, this ensures that all the air is used, and the extra fuel helps keep things cooler. 13.5 is about as lean as i would go in a street car, if it sees track time i like it in the high 12's, it gives you a little extra margin for a cold day, or a clogged filter or something like that
the lean spikes with throttle movement are normal, air is lighter than fuel, so it moves faster when you move the throttle suddenly. this is why it needs an accelerator pump. as long as it doesn't stumble/hesitate its fine.
it should be lean at part throttle cruise too, basically as lean as you can go and have it still run ok, (without cats, with cat it needs to be 14.7), any more than that is wasting fuel.
13.5-14.2 at WOT is probably too lean for best power. if you take a step back, in theory every air molecule combines with every gasoline molecule at 14.7:1. however in real life, things are not perfect, so to make sure you use every molecule of air, we add a little extra fuel, this ensures that all the air is used, and the extra fuel helps keep things cooler. 13.5 is about as lean as i would go in a street car, if it sees track time i like it in the high 12's, it gives you a little extra margin for a cold day, or a clogged filter or something like that
12-31-19, 04:45 PM
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01-01-20, 12:59 PM
#5
The fuel jets really are the last thing you tune. They are how you custom tailor the fuel using a wideband to get the AFR numbers you want. Follow j9fd3s' advice here. You never want to tune on a hot day because as soon as it gets cold (including like when the sun goes down), the way carbs work is they will go lean with colder air because it is more dense (has more oxygen for a set amount of fuel) and your engine will not enjoy that. You can fudge it slightly with NA but you don't want to mess around if boost is in the picture.
01-01-20, 02:13 PM
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Awesome, thanks both of you. So Jeff if I understand what you're saying, it's best to be tuning just about everything else in the carb before the fuel jets - unless perhaps something is really off and the fuel jets aren't at a useable starting place. And then of course not tuning on a hot day because you'll go lean when it cools down. Out of curiosity, do any of you bother with a summer versus winter tune, or do you just keep it safe for winter by running a little rich in the hotter days?
So I guess my question is now, how do you go about tuning other aspects of the carb? Up until now I've just gotten it 'close enough' by adjusting mixture, the ap, and the primary fuel jets but obviously there is more than just that. What is the whole deal with tuning the air bleeds; how they affect 'tip in' (what exactly is that?), etc. Perhaps someone wouldn't mind outlining a basic 'map' of the process you go through when tuning a carb. I know I'm sounding like a noob here, but in all honesty I've never fully understood how to properly tune a carb (really just enough to get things running decently). I understand the ideas behind air bleeds and all in what I might call a basic sense, although an explanation from those of you who know the task would be greatly appreciated.
As a side note, the hogged carb probably won't be going on for a couple months yet, but I'm starting the secondaries today so we'll see. Whenever that happens I'm sure I'll revive this thread for tuning it.
Thanks again for the advice thus far.
So I guess my question is now, how do you go about tuning other aspects of the carb? Up until now I've just gotten it 'close enough' by adjusting mixture, the ap, and the primary fuel jets but obviously there is more than just that. What is the whole deal with tuning the air bleeds; how they affect 'tip in' (what exactly is that?), etc. Perhaps someone wouldn't mind outlining a basic 'map' of the process you go through when tuning a carb. I know I'm sounding like a noob here, but in all honesty I've never fully understood how to properly tune a carb (really just enough to get things running decently). I understand the ideas behind air bleeds and all in what I might call a basic sense, although an explanation from those of you who know the task would be greatly appreciated.
As a side note, the hogged carb probably won't be going on for a couple months yet, but I'm starting the secondaries today so we'll see. Whenever that happens I'm sure I'll revive this thread for tuning it.
Thanks again for the advice thus far.
01-02-20, 10:20 AM
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when i was tuning my Weber i ran across a Keith Francks white paper on DCOE tuning, and i'll summarize, it works for every carb
so the first part is the setup, so step one is to get all the mechanical things working properly. the throttle linkage, accel pump, etc. the most important thing is here are the float levels. every carburetor sucks fuel from the bottom and combines it with air to form an Emulsion before it goes to the actual jets. if there is magic in a carb it is here, as an Emulsion can either flow like a liquid or a gas. anyways, the carb has a tube, Weber calls it an Emulsion tube where the fuel jet is at the bottom, and the air is at the top, and its mixed in the center. the Nikki has difference parts, but uses the same principals. the float level dictates how high up in this tube the fuel is. the higher up the less force is needed to pull fuel into the engine, ie it will be richer. the Weber is nice because there is one big float chamber, the Nikki has two small ones. its kind of why we like to leave the float levels alone. you should measure it like the FSM, and then check it with the carb installed in the car and the fuel pump running. with the Weber this is easy, the Nikki, no so much....
Part two is the actual tuning the fuel delivery.
if you have a wideband, and you're tuning from scratch, like you would on a Weber, you shoot for an AFR that is consistent, and a little rich, just pick a number from 12.5-13. we start by tuning the idle, and then work our way up through the circuits of the carb until we hit WOT. the nice thing about the Nikki is that its already close. on the Weber we actually pull the main jet stack out, which leaves you only running on the idle jet, on the nikki you can disable the secondaries, etc.
once the fuel is right where we want it, we can touch up the transition with the air jets
when you start physically pulling jets out, it really shows you what part of the carb does what, and half of this tuning process is just training yourself what part of the carb does what. once you know that it makes it quite easy to make changes in a specific spot. a lot of things work like this too. when we play with suspension at the track, we basically try all the adjustments, and see what it does. once you know what the adjustments do its easy to know what change to make
so the first part is the setup, so step one is to get all the mechanical things working properly. the throttle linkage, accel pump, etc. the most important thing is here are the float levels. every carburetor sucks fuel from the bottom and combines it with air to form an Emulsion before it goes to the actual jets. if there is magic in a carb it is here, as an Emulsion can either flow like a liquid or a gas. anyways, the carb has a tube, Weber calls it an Emulsion tube where the fuel jet is at the bottom, and the air is at the top, and its mixed in the center. the Nikki has difference parts, but uses the same principals. the float level dictates how high up in this tube the fuel is. the higher up the less force is needed to pull fuel into the engine, ie it will be richer. the Weber is nice because there is one big float chamber, the Nikki has two small ones. its kind of why we like to leave the float levels alone. you should measure it like the FSM, and then check it with the carb installed in the car and the fuel pump running. with the Weber this is easy, the Nikki, no so much....
Part two is the actual tuning the fuel delivery.
if you have a wideband, and you're tuning from scratch, like you would on a Weber, you shoot for an AFR that is consistent, and a little rich, just pick a number from 12.5-13. we start by tuning the idle, and then work our way up through the circuits of the carb until we hit WOT. the nice thing about the Nikki is that its already close. on the Weber we actually pull the main jet stack out, which leaves you only running on the idle jet, on the nikki you can disable the secondaries, etc.
once the fuel is right where we want it, we can touch up the transition with the air jets
when you start physically pulling jets out, it really shows you what part of the carb does what, and half of this tuning process is just training yourself what part of the carb does what. once you know that it makes it quite easy to make changes in a specific spot. a lot of things work like this too. when we play with suspension at the track, we basically try all the adjustments, and see what it does. once you know what the adjustments do its easy to know what change to make
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01-02-20, 02:32 PM
#8
You asked about air bleeds. The old thinking was that air bleeds only trim the upper RPM range. It turned out this was incomplete. In fact air bleeds influence the entire RPM range but can affect the upper part because... well duh, the upper range is part of the whole range, isn't it? My personal findings regarding Nikki air bleeds is that it's a 'one size fits all' approach that works. It works every time regardless of NA or boost usage, and I will quote them below.
The only part that can be "tuned" for boost would be the short slow secondary bleeds that come stock as nickel plated 60s. These are perfectly fine for NA use but if boosting you will want to do some work here. I've found that .35mm to .37 is pretty good but it depends on the drill bits you use and what you can get away with while drilling through solder. I would not go any larger than .40 but no one sells pre-drilled any smaller than 50. No one sells brass blanks but I happened to have a couple of old dead Hitachi carbs with usable blanks that I painstakingly drilled out at .20 or .25 by hand a couple of years ago just to try them out. I do not recommend this size as it was too small and lead to some floody issues. I later corrected them by redrilling out to .40 and are now used on NA carbs intended for really huge secondary venturis. Not that it matters all that much for NA, but why not. And I've already got a couple complete sets of solder filled ones in the .35 to .37 range which I'll much rather use with boost anyway.
Whatever you do, DO NOT use fully solder filled bleeds here because the carb uses the tiny hole as an anti-siphon feature. Talk about floody! Yes I've tested this and yes it floods quite spectacularly!
As for the reason we go from stock nickel plated 60s to .35 to .37 is to further speed up when the fuel begins to flow through them secondary emulsion tubes and on out and through the boosters into the engine. I had several spare primary brass 170s and was willing to try filling them with solder experimentally. Also, my .20 and .25 will break in solder due to how grabby it is compared to brass. And it turns out that .60mm is just too big for a fast response while boosting (the short slow circuit is connected with the transition slots down in the baseplate - take a look and follow the path visually next time you have a Nikki apart). You do not want a lean condition when you open the secondaries in boost because it can pop your engine, and the accel pump mod can only cover so much of the transition... It gets worse with bigger secondary venturi sizes so I like to keep it around 30mm for safety purposes. Any bigger and the delay gets longer, but any smaller and the transcendent power just isn't quite there... even with boost.
120 (118 to 125 range) primary short slows
stock 46 primary long slows
stock 70 main primary air bleeds (some Nikki models came with 60 but can be drilled out to 70 if you have the bit for it)
------
modded secondary main air bleeds to 80 (machined, slotted, drilled and threaded to accept Hitachi pilot jets drilled to .80mm)
solder filled long slows upper section only -- leave lower section open
stock nickel plated 60 for NA or do as described above for a smaller size between .35 and .37 for boost
This all assumes larger than stock venturis and obviously mechanical secondaries with preferably re-timed secondary opening linkages.
The only part that can be "tuned" for boost would be the short slow secondary bleeds that come stock as nickel plated 60s. These are perfectly fine for NA use but if boosting you will want to do some work here. I've found that .35mm to .37 is pretty good but it depends on the drill bits you use and what you can get away with while drilling through solder. I would not go any larger than .40 but no one sells pre-drilled any smaller than 50. No one sells brass blanks but I happened to have a couple of old dead Hitachi carbs with usable blanks that I painstakingly drilled out at .20 or .25 by hand a couple of years ago just to try them out. I do not recommend this size as it was too small and lead to some floody issues. I later corrected them by redrilling out to .40 and are now used on NA carbs intended for really huge secondary venturis. Not that it matters all that much for NA, but why not. And I've already got a couple complete sets of solder filled ones in the .35 to .37 range which I'll much rather use with boost anyway.
Whatever you do, DO NOT use fully solder filled bleeds here because the carb uses the tiny hole as an anti-siphon feature. Talk about floody! Yes I've tested this and yes it floods quite spectacularly!
As for the reason we go from stock nickel plated 60s to .35 to .37 is to further speed up when the fuel begins to flow through them secondary emulsion tubes and on out and through the boosters into the engine. I had several spare primary brass 170s and was willing to try filling them with solder experimentally. Also, my .20 and .25 will break in solder due to how grabby it is compared to brass. And it turns out that .60mm is just too big for a fast response while boosting (the short slow circuit is connected with the transition slots down in the baseplate - take a look and follow the path visually next time you have a Nikki apart). You do not want a lean condition when you open the secondaries in boost because it can pop your engine, and the accel pump mod can only cover so much of the transition... It gets worse with bigger secondary venturi sizes so I like to keep it around 30mm for safety purposes. Any bigger and the delay gets longer, but any smaller and the transcendent power just isn't quite there... even with boost.
120 (118 to 125 range) primary short slows
stock 46 primary long slows
stock 70 main primary air bleeds (some Nikki models came with 60 but can be drilled out to 70 if you have the bit for it)
------
modded secondary main air bleeds to 80 (machined, slotted, drilled and threaded to accept Hitachi pilot jets drilled to .80mm)
solder filled long slows upper section only -- leave lower section open
stock nickel plated 60 for NA or do as described above for a smaller size between .35 and .37 for boost
This all assumes larger than stock venturis and obviously mechanical secondaries with preferably re-timed secondary opening linkages.
01-05-20, 10:49 PM
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Thanks for that great info, both of you. My apologies for not getting back sooner, I've had quite a few projects on my hands lately. I probably won't be able to put this info to the test for a little while yet, but hopes are to have the carb done within the next couple months. I'll probably try the .35 to .37's for the secondary no. 2's if only for attempting to reduce the secondary delay - my secondaries open very quickly as I made a new linkage set for them.
Any tips for tuning the AP nozzle size? I've seen all sorts of different opinions on the subject, but since it's a 'once and done' sort of mod, I'd appreciate some more current opinions. I realize it may vary greatly by the carb setup, but is there a general rule of thumb? I have a .6 as well as what I believe are a few .5 and .55's which I can of course drill out if it's warranted.
Any tips for tuning the AP nozzle size? I've seen all sorts of different opinions on the subject, but since it's a 'once and done' sort of mod, I'd appreciate some more current opinions. I realize it may vary greatly by the carb setup, but is there a general rule of thumb? I have a .6 as well as what I believe are a few .5 and .55's which I can of course drill out if it's warranted.
01-06-20, 09:35 AM
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Thanks for that great info, both of you. My apologies for not getting back sooner, I've had quite a few projects on my hands lately. I probably won't be able to put this info to the test for a little while yet, but hopes are to have the carb done within the next couple months. I'll probably try the .35 to .37's for the secondary no. 2's if only for attempting to reduce the secondary delay - my secondaries open very quickly as I made a new linkage set for them.
Any tips for tuning the AP nozzle size? I've seen all sorts of different opinions on the subject, but since it's a 'once and done' sort of mod, I'd appreciate some more current opinions. I realize it may vary greatly by the carb setup, but is there a general rule of thumb? I have a .6 as well as what I believe are a few .5 and .55's which I can of course drill out if it's warranted.
Any tips for tuning the AP nozzle size? I've seen all sorts of different opinions on the subject, but since it's a 'once and done' sort of mod, I'd appreciate some more current opinions. I realize it may vary greatly by the carb setup, but is there a general rule of thumb? I have a .6 as well as what I believe are a few .5 and .55's which I can of course drill out if it's warranted.
for example, the engine has a lean limit, its the leanest you can go and it still runs. if you picked a fairly rich cruise mixture, the engine is very far away from the lean limit, and will not need much accelerator pump.
02-03-20, 04:30 PM
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Well I finally got the hogged Nikki back on the car this past weekend. It's interesting to see it with the wideband now - some of my prior guesses have been confirmed and I've learned a couple things as well. First off, it cruises far better than the stock carb, which makes sense since I never rejetted the stocker for the ported engine (it was supposed to be temporary). The low end torque boost was also immediately apparent, as was the power increases when my clutch would actually hook (transmission work is now planned for this coming weekend).
So now then, what I have suspected for a long time with the carb has finally been backed up by the wideband - my transition circuit just isn't doing it's job correctly. From 850rpm (on this carb I idle at ~550-700) to 2000 rpm the carb is far too lean (15-16 afr is very common). This gets a bit better by 2500 and by 3000 it's fine, in fact a little rich if anything (secondaries are definitely richer than need be for NA). So that begs the question, can the transition circuit be tuned on the Nikki? If I recall correctly, it's pretty well tied in with the main primary circuits - although I haven't had a chance to work on a Nikki for a bit now so the actual circuit pathways are eluding me. Could something, even after the many times the carb has been apart and cleaned, still be clogged somehow? If not, where would I start to try alleviate this issue? What keeps coming to mind is that little pressed-in jet in the t-body, which if I remember correctly is the supply for the transition circuit? I know it's not a great idea to drill it, so where else can the circuit (or another to compensate) be adjusted without messing up other things - slow air bleeds? I should mention that this lean scenario is most noticeable at part throttle cruise (which at such a low rpm is rather rare) but is also particularly apparent when holding the throttle under no load. Since when driving around I don't spend much time there, it's not usually an issue I notice on the road - besides bucking on decel, which starts just under 3k and at which point I usually just put in the clutch.
I'm betting this issue is closely related to, if not the cause of, my hard cold start issues. To start the car cold it must remain over 2k rpm for at least a few minutes or else it will die. Seriously, anything below ~2k and the engine will just drop, almost like the ignition was shut off. Sometimes it can be kept near idle (although not smoothly) if I continually 'bump' the accelerator to give it fuel using the AP. I also cannot keep the throttle slightly above idle (think 900-1000rpm) by foot without a misfire starting after the fuel from the AP has been consumed - even when warm, although it's not as severe. I've also noticed that this carb will only idle nicely (and still not as smooth as the stock carb) at 12.7 afr @ 45F, 200ft elevation - the stock carb liked 12.2, but could go as lean as 13.4 without major issues. The idle MAS is also really touchy on this carb, perhaps that's related as well?
Anyway, if anyone has some tips concerning the transition circuit, shoot. Perhap it really is just a clog that I've missed somehow each time this carb has been apart, who knows. Do Nikki's sometimes have an issue like this, or is a clog/my carb just being special more likely? Thanks for the help thus far.
Edit: I went and checked my old misfire thread (this one: Won't idle cold, misfires, bucking, and more... (Nikki) and rediscovered a couple things. It sounds like the transition jets are not able to be tuned - so perhaps a clog is more likely? If those with Nikki wideband experience who have noted the behavior in the transition phase could chime in it would be greatly appreciated. All the info and issues in that thread are still relevant as nothing on the carb has changed since I went to the larger ~120 jets (starting post #5). With those jets (aka the current ones) the 3k rpm part throttle cruise is about perfect (~13.5) and with just the primaries wide open I see the low 13's. When I open up the secondaries it goes to the low 12's so... a bit low. Although I haven't touched them yet and they'll be getting swapped for 30mm venturis not so far off in the future, so I'll have to retune them then anyway.
Edit 2: I also just listened to the audio clip attached to the last post in the thread I linked above and it is indeed still accurate.
So now then, what I have suspected for a long time with the carb has finally been backed up by the wideband - my transition circuit just isn't doing it's job correctly. From 850rpm (on this carb I idle at ~550-700) to 2000 rpm the carb is far too lean (15-16 afr is very common). This gets a bit better by 2500 and by 3000 it's fine, in fact a little rich if anything (secondaries are definitely richer than need be for NA). So that begs the question, can the transition circuit be tuned on the Nikki? If I recall correctly, it's pretty well tied in with the main primary circuits - although I haven't had a chance to work on a Nikki for a bit now so the actual circuit pathways are eluding me. Could something, even after the many times the carb has been apart and cleaned, still be clogged somehow? If not, where would I start to try alleviate this issue? What keeps coming to mind is that little pressed-in jet in the t-body, which if I remember correctly is the supply for the transition circuit? I know it's not a great idea to drill it, so where else can the circuit (or another to compensate) be adjusted without messing up other things - slow air bleeds? I should mention that this lean scenario is most noticeable at part throttle cruise (which at such a low rpm is rather rare) but is also particularly apparent when holding the throttle under no load. Since when driving around I don't spend much time there, it's not usually an issue I notice on the road - besides bucking on decel, which starts just under 3k and at which point I usually just put in the clutch.
I'm betting this issue is closely related to, if not the cause of, my hard cold start issues. To start the car cold it must remain over 2k rpm for at least a few minutes or else it will die. Seriously, anything below ~2k and the engine will just drop, almost like the ignition was shut off. Sometimes it can be kept near idle (although not smoothly) if I continually 'bump' the accelerator to give it fuel using the AP. I also cannot keep the throttle slightly above idle (think 900-1000rpm) by foot without a misfire starting after the fuel from the AP has been consumed - even when warm, although it's not as severe. I've also noticed that this carb will only idle nicely (and still not as smooth as the stock carb) at 12.7 afr @ 45F, 200ft elevation - the stock carb liked 12.2, but could go as lean as 13.4 without major issues. The idle MAS is also really touchy on this carb, perhaps that's related as well?
Anyway, if anyone has some tips concerning the transition circuit, shoot. Perhap it really is just a clog that I've missed somehow each time this carb has been apart, who knows. Do Nikki's sometimes have an issue like this, or is a clog/my carb just being special more likely? Thanks for the help thus far.
Edit: I went and checked my old misfire thread (this one: Won't idle cold, misfires, bucking, and more... (Nikki) and rediscovered a couple things. It sounds like the transition jets are not able to be tuned - so perhaps a clog is more likely? If those with Nikki wideband experience who have noted the behavior in the transition phase could chime in it would be greatly appreciated. All the info and issues in that thread are still relevant as nothing on the carb has changed since I went to the larger ~120 jets (starting post #5). With those jets (aka the current ones) the 3k rpm part throttle cruise is about perfect (~13.5) and with just the primaries wide open I see the low 13's. When I open up the secondaries it goes to the low 12's so... a bit low. Although I haven't touched them yet and they'll be getting swapped for 30mm venturis not so far off in the future, so I'll have to retune them then anyway.
Edit 2: I also just listened to the audio clip attached to the last post in the thread I linked above and it is indeed still accurate.
Last edited by Benjamin4456; 02-03-20 at 05:00 PM.
02-16-20, 03:30 PM
#12
3D Printed
Thread Starter
It's been a couple weeks now and with temps still around freezing I decided to pull off the modded Nikki and throw back on the stocker. Main reason is I can idle at 1.5k warming up rather than 2k+ which, for my current excuse of an exhaust, makes quite a difference. It does drive worse as it's not tuned for the ported engine, but oh well. I'll swap back on the other carb once it or my exhaust are in better standings.
So then, that brings me to a couple things I'm curious about. When those of you who have widebands on your Nikkis do a cold start, where about afr and rpm wise does it sit at fast idle? On both carbs I can't go any leaner than 15.2ish on a cold start until it stalls - for my modded Nikki that's at a little above 2k and on the stocker it's slightly below 1.5k. Those numbers are just at for the first minute or so though, as over time it gets progressively richer. I'm also wondering about y'all's afr at part throttle, low rpm operation under load or free rev. On the modded Nikki I see the 16's with relative frequency when down in the 1500 rpm territory, be it cruising on a level patch in my neighborhood or testing it in the driveway. To me that seems off, but perhaps that's just a tendency of the Nikki? My float levels are spot on, fuel pressure is solid, and as far as I know everything else is well adjusted. Idle with the modded carb is rougher than the stocker, but not terribly much and is far more consistent than the stocker (sometimes the stock carb will idle at 700 and sometimes at 1000, although that's in part due to the bizarre clutch issues I'm facing). Idle afr for both carbs is generally in the mid 12's.
Essential the couple running conditions I am most curious about are (and this under load or not, warm or cold - but please note such):
- afr just above idle, constant rpm
- afr 1500 to 2000 rpm, constant rpm
- cold start fast idle: rpm and afr
I should also note that this carb does not have a choke, in case that wasn't already mentioned.
If anyone would be willing to gather a little data in the above couple scenarios I'd be grateful and just plain out interested. For reference, current jetting is in the first post of the thread. I've also noticed that there is a slight hitch right around 1900 rpm where all of a sudden power seems to kick in with the modded Nikki. Haven't noticed what the afr is doing then, but it feels like the main circuit coming online. It's actually feels more like the carb is hesitant until it hits 1900 rpm in 2nd or 3rd (neighborhood driving) rather than a hitch, but it's noticeable regardless.
So then, that brings me to a couple things I'm curious about. When those of you who have widebands on your Nikkis do a cold start, where about afr and rpm wise does it sit at fast idle? On both carbs I can't go any leaner than 15.2ish on a cold start until it stalls - for my modded Nikki that's at a little above 2k and on the stocker it's slightly below 1.5k. Those numbers are just at for the first minute or so though, as over time it gets progressively richer. I'm also wondering about y'all's afr at part throttle, low rpm operation under load or free rev. On the modded Nikki I see the 16's with relative frequency when down in the 1500 rpm territory, be it cruising on a level patch in my neighborhood or testing it in the driveway. To me that seems off, but perhaps that's just a tendency of the Nikki? My float levels are spot on, fuel pressure is solid, and as far as I know everything else is well adjusted. Idle with the modded carb is rougher than the stocker, but not terribly much and is far more consistent than the stocker (sometimes the stock carb will idle at 700 and sometimes at 1000, although that's in part due to the bizarre clutch issues I'm facing). Idle afr for both carbs is generally in the mid 12's.
Essential the couple running conditions I am most curious about are (and this under load or not, warm or cold - but please note such):
- afr just above idle, constant rpm
- afr 1500 to 2000 rpm, constant rpm
- cold start fast idle: rpm and afr
I should also note that this carb does not have a choke, in case that wasn't already mentioned.
If anyone would be willing to gather a little data in the above couple scenarios I'd be grateful and just plain out interested. For reference, current jetting is in the first post of the thread. I've also noticed that there is a slight hitch right around 1900 rpm where all of a sudden power seems to kick in with the modded Nikki. Haven't noticed what the afr is doing then, but it feels like the main circuit coming online. It's actually feels more like the carb is hesitant until it hits 1900 rpm in 2nd or 3rd (neighborhood driving) rather than a hitch, but it's noticeable regardless.
Last edited by Benjamin4456; 02-16-20 at 03:33 PM.
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