half bridge 12A, with a twist
03-14-11, 01:13 PM
#76
I had a chance to make some progress on the manifold. One of the copper crush washers for the brake booster fitting wasn't perfect so I heated it up so it will be soft and pliable when it goes on. Also made a blockoff plate and found a bolt to block another hole. Lastly I found a working(?) PCV valve. If the PCV valve turns out to be bad, I can swap in an FV266 and a heater core fitting from an old school rear plate (like from an R5 or 3B, ask glazedham42). The next step is to throw it on the engine in between rain storms. heh
lol
03-18-11, 01:07 PM
#80
I tested the old school carb. It works great! It could fire up on a cold engine and idle at 800rpm without choke (it's adjusted a bit rich at the moment). The accel pump is weak and turned out to be leaking a little so I'll deal with it.
I tested it at 1psi fuel pressure (fuel level too low so it stumbled big time), then 2psi (gettin' better), then 3psi (in the middle of the window) so I set it down to 2.5psi which is typical for a Nikki. Now it could free rev once it got past the weak accel pump shot.
I reved it up and manually forced the secondaries open (they won't open while free reving normally, so you have to grab the linkage and turn it etc). Man this thing flows! It had a nice amount of output out of all four venturis.
This will be an excellent carb for the half BP!
I tested it at 1psi fuel pressure (fuel level too low so it stumbled big time), then 2psi (gettin' better), then 3psi (in the middle of the window) so I set it down to 2.5psi which is typical for a Nikki. Now it could free rev once it got past the weak accel pump shot.
I reved it up and manually forced the secondaries open (they won't open while free reving normally, so you have to grab the linkage and turn it etc). Man this thing flows! It had a nice amount of output out of all four venturis.
This will be an excellent carb for the half BP!
03-20-11, 02:45 AM
#81
I've got some info you Nikki lovers will appreciate.
The specs for this old school carb are:
primary fuel jets: 94
primary air bleeds: 80
secondary fuel jets: 150 (that's not a typo)
secondary air bleeds: 160
typical Nikki numbers are more like this:
primary fuel jets: 92 (91 and 93 less common)
primary air bleeds: 70, 80 or 90 (60 less common)
secondary fuel jets: 160 (always)
secondary air bleeds: 140
So I left the primary fuel jets alone as 94 is actually pretty cool. I chose to leave the primary air bleeds at 80 because it's in the middle of the range. Their heads are the size of secondaries so no swapping to smaller numbers is possible, only larger heh. The number and spacing of the holes in the emulsion tubes are the same as 70 primary air bleeds from a modern Nikki. I'm happy with 80s. The secondary fuel jets at 150 were too small for my liking so I swapped to some 170s. I also swapped the too large 160 secondary air bleeds to some rare 120s.
So the new numbers are:
primary fuel jets: 94 same as before
primary air bleeds: 80 same as before
secondary fuel jets: 170 (up from stock 150 and still bigger than a typical 160)
secondary air bleeds: 120 (smaller than a typical 140 and much smaller than stock 160)
It makes sense to leave the primary circuit alone as the primary ports are going to be left alone. Only the secondary circuit required some lovin. I'm just glad I could offer up such a dramatic change of +20 for fuel and -40 for air. This means it will run richer up to a much higher RPM than stock. Or from the carb's perspective, it will maintain fuel up to what it thinks is a high RPM but in reality is less RPM but much more CFM through the bridgeports. Does that make sense?
So if a stock carb is calibrated to run out of breath near the top of the engine's (stock) RPM range, what if that engine breathes like a larger engine but at a lower RPM, but only while the secondaries are open, and if so, what can be done to the secondary circuit to, in a sense, trick the carb into providing more fuel not only when the secondaries first open, but also up to a higher rev limit than stock if necessary? The solution I came up with is to increase secondary fuel jets and decrease secondary air bleeds.
Now if you're just joining and you don't really know what air bleeds do, they simply make it so carbs don't go pig rich at high RPM. The smaller the air bleed, the more fuel is available as RPMs rise, or in other words the higher working RPM range of the carb. Likewise a larger air bleed can reduce fuel consumption and performance. You have to tune to find the right balance for you.
I'm making an educated guess on my choices above based on what was already there to work with, how the engine will be ported, and my previous experience working with these carbs.
The specs for this old school carb are:
primary fuel jets: 94
primary air bleeds: 80
secondary fuel jets: 150 (that's not a typo)
secondary air bleeds: 160
typical Nikki numbers are more like this:
primary fuel jets: 92 (91 and 93 less common)
primary air bleeds: 70, 80 or 90 (60 less common)
secondary fuel jets: 160 (always)
secondary air bleeds: 140
So I left the primary fuel jets alone as 94 is actually pretty cool. I chose to leave the primary air bleeds at 80 because it's in the middle of the range. Their heads are the size of secondaries so no swapping to smaller numbers is possible, only larger heh. The number and spacing of the holes in the emulsion tubes are the same as 70 primary air bleeds from a modern Nikki. I'm happy with 80s. The secondary fuel jets at 150 were too small for my liking so I swapped to some 170s. I also swapped the too large 160 secondary air bleeds to some rare 120s.
So the new numbers are:
primary fuel jets: 94 same as before
primary air bleeds: 80 same as before
secondary fuel jets: 170 (up from stock 150 and still bigger than a typical 160)
secondary air bleeds: 120 (smaller than a typical 140 and much smaller than stock 160)
It makes sense to leave the primary circuit alone as the primary ports are going to be left alone. Only the secondary circuit required some lovin. I'm just glad I could offer up such a dramatic change of +20 for fuel and -40 for air. This means it will run richer up to a much higher RPM than stock. Or from the carb's perspective, it will maintain fuel up to what it thinks is a high RPM but in reality is less RPM but much more CFM through the bridgeports. Does that make sense?
So if a stock carb is calibrated to run out of breath near the top of the engine's (stock) RPM range, what if that engine breathes like a larger engine but at a lower RPM, but only while the secondaries are open, and if so, what can be done to the secondary circuit to, in a sense, trick the carb into providing more fuel not only when the secondaries first open, but also up to a higher rev limit than stock if necessary? The solution I came up with is to increase secondary fuel jets and decrease secondary air bleeds.
Now if you're just joining and you don't really know what air bleeds do, they simply make it so carbs don't go pig rich at high RPM. The smaller the air bleed, the more fuel is available as RPMs rise, or in other words the higher working RPM range of the carb. Likewise a larger air bleed can reduce fuel consumption and performance. You have to tune to find the right balance for you.
I'm making an educated guess on my choices above based on what was already there to work with, how the engine will be ported, and my previous experience working with these carbs.
03-20-11, 04:21 AM
#82
I also did the accel pump mod while I was in there changing out the old leaking accel pump diaphragm.
Something about the old school carbs, they don't have as long of a pump shot as the newer ones, but as such they squeeze out more fuel in a shorter space. Well, you'd think they would. The banjo bolt actually has the tiny jet at the bottom just like a modern Nikki. The only Nikkis that don't seem to have the tiny jet are the 79-80 carbs. Go fish.
I did the mechanical secondary mod to this carb just in case the half BP causes a change in the time it takes to open the vacuum secondaries. Figured I'd make 'em mechanical now to avoid future problems. So something that goes along with mech secs is the accel pump mod to avoid a bog when you punch it and force the secondaries to open before they normally would.
I happened to have two new accel pump diaphragms, one old style and one newer style, and two types of accel cover pieces. I found the old style diaphragm has more travel or is more supple than the new style. Then I discovered the modern AP cover has a taller cap allowing more travel. Oh and the modern ap lever is slightly longer than the old school, by a few mm.
I decided to go with the old style diaphragm for more travel/volume. I went with the modern cover cap for more travel/volume.
I went with the old school AP lever because it fits this carb better than the newer style, and I went with a 1" extension to more than make up for it, and to increase duration. Testing so far has revealed it still is compressing the AP diaphragm as the secondaries open. Success!
And finally I drilled out the tiny jet at the bottom of the AP banjo bolt. Sterling recommended you drill through it with a .040" drill bit. Then drill the two upper holes with .030" and then drill the nozzle/squirters with .025". What I did was to drill the tiny jet at the bottom totally out which takes longer, but looks more like a 79-80 so it makes me feel more satisfied. Then I drilled the upper holes to .040" (smallest bit I have) and tested this on another old school carb this afternoon. I did not drill the nozzle/squirter thing at all. The result was plenty of fuel and the transition to secondaries was great. Also low RPM quick taps of the pedal sound like the accel pump shot is just right.
Something about the old school carbs, they don't have as long of a pump shot as the newer ones, but as such they squeeze out more fuel in a shorter space. Well, you'd think they would. The banjo bolt actually has the tiny jet at the bottom just like a modern Nikki. The only Nikkis that don't seem to have the tiny jet are the 79-80 carbs. Go fish.
I did the mechanical secondary mod to this carb just in case the half BP causes a change in the time it takes to open the vacuum secondaries. Figured I'd make 'em mechanical now to avoid future problems. So something that goes along with mech secs is the accel pump mod to avoid a bog when you punch it and force the secondaries to open before they normally would.
I happened to have two new accel pump diaphragms, one old style and one newer style, and two types of accel cover pieces. I found the old style diaphragm has more travel or is more supple than the new style. Then I discovered the modern AP cover has a taller cap allowing more travel. Oh and the modern ap lever is slightly longer than the old school, by a few mm.
I decided to go with the old style diaphragm for more travel/volume. I went with the modern cover cap for more travel/volume.
I went with the old school AP lever because it fits this carb better than the newer style, and I went with a 1" extension to more than make up for it, and to increase duration. Testing so far has revealed it still is compressing the AP diaphragm as the secondaries open. Success!And finally I drilled out the tiny jet at the bottom of the AP banjo bolt. Sterling recommended you drill through it with a .040" drill bit. Then drill the two upper holes with .030" and then drill the nozzle/squirters with .025". What I did was to drill the tiny jet at the bottom totally out which takes longer, but looks more like a 79-80 so it makes me feel more satisfied. Then I drilled the upper holes to .040" (smallest bit I have) and tested this on another old school carb this afternoon. I did not drill the nozzle/squirter thing at all. The result was plenty of fuel and the transition to secondaries was great. Also low RPM quick taps of the pedal sound like the accel pump shot is just right.
03-20-11, 01:29 PM
#85
I'm also boost prepping another old school Nikki. It has a much longer accel lever extension to continue to flow AP shot while the secondaries open. bad 83 suggests you leave the primaries and sec air bleeds alone but up the sec jet size to a 200 Mikuni harley jet (he says they're measured differently than Nikki jets). Huh? He also doesn't mod the accel pump. Why not?
So what I chose to do is half boost prep this other carb now by leaving the choke and secondary vacuum housing alone and will remove them later when the guy is ready to boost his engine. I'll also swap the secondary fuel jets from their stock 160 to a set of drilled 190s (or 200, whatever).
I'm thinking of swapping the stock 92 primary fuel jets to a set of new 95s I've got sitting here right now. Might as well, and maybe change the primary air bleeds to a set of 70s for better (richer) high RPM fuel ratios. This other old school carb isn't as old school as the old school carb featured in this thread. It's kind of a middle school carb being older than an SA carb but not nearly as old as the old school carb. That was unpleasant to type.
So what I chose to do is half boost prep this other carb now by leaving the choke and secondary vacuum housing alone and will remove them later when the guy is ready to boost his engine. I'll also swap the secondary fuel jets from their stock 160 to a set of drilled 190s (or 200, whatever).
I'm thinking of swapping the stock 92 primary fuel jets to a set of new 95s I've got sitting here right now. Might as well, and maybe change the primary air bleeds to a set of 70s for better (richer) high RPM fuel ratios. This other old school carb isn't as old school as the old school carb featured in this thread. It's kind of a middle school carb being older than an SA carb but not nearly as old as the old school carb. That was unpleasant to type.
03-20-11, 01:35 PM
#87
Here is the thread where he talks about boost prepping a Nikki. He says to leave the primary jets alone.
https://www.rx7club.com/showthread.p...1#post10321601
Edit: Oh I see you've seen it already. Nevermind. This is for the noobs.
https://www.rx7club.com/showthread.p...1#post10321601
Edit: Oh I see you've seen it already. Nevermind.
This is for the noobs.
03-20-11, 02:26 PM
#88
Old [Sch|F]ool
Porting makes no difference to a vacuum secondary. The "vacuum" used is venturi vacuum (airflow through the primaries) and not manifold vacuum.
Easiest way to convert to mechanical is to just remove the spring. Any airflow through the primary will open the secondaries as soon as the blocking linkage will allow it. I played the spring changing game on my '80 and found no driveability issues with running with no spring, so I ran with no spring.
Bear in mind, this is with the '80 carburetor. I don't think I ever had an FB Nikki that worked worth a damn, so when I got my FB, one of the first modifications I did was to install the SA carb.
I kinda miss the days of playing with 12As. I didn't have half the problems I have nowadays. I keep thinking, hmm, I have two spare GSL-SE center housings, if I could scare up enough 12A parts to go around it then I can make an EFI 12A that will bolt to my FC subframe, and use one of my many Nikki intakes lying around (including three 79-80 intakes), hull out a junk carb so that it's nothing more than a progressive throttle body, and stick the good ol' blue air cleaner on top.
Maybe I'll do that.
Easiest way to convert to mechanical is to just remove the spring. Any airflow through the primary will open the secondaries as soon as the blocking linkage will allow it. I played the spring changing game on my '80 and found no driveability issues with running with no spring, so I ran with no spring.
Bear in mind, this is with the '80 carburetor. I don't think I ever had an FB Nikki that worked worth a damn, so when I got my FB, one of the first modifications I did was to install the SA carb.
I kinda miss the days of playing with 12As. I didn't have half the problems I have nowadays. I keep thinking, hmm, I have two spare GSL-SE center housings, if I could scare up enough 12A parts to go around it then I can make an EFI 12A that will bolt to my FC subframe, and use one of my many Nikki intakes lying around (including three 79-80 intakes), hull out a junk carb so that it's nothing more than a progressive throttle body, and stick the good ol' blue air cleaner on top.
Maybe I'll do that.
03-20-11, 04:15 PM
#90
peejay, I bet you could do a set of half bridgeports and run a carb without all the problems you're having with EFI. Just gotta block all communication from primary to secondary runners!
Hmm, maybe do what the 6 port 12A aussies are forced to do and BP the primary ports. They can only use side motor mounts in whatever car they came in (HB Cosmo?) so 4 port intermediate Y plates are not useable. Those 6 port 12As have very small runners and very late opening ports. You with your GSL-SE plates will have NO port timing which opens at 12A spec (I believe), much sooner than a 6 port 12A, but still will allow a 5mm bridge the same as any 4 port 12A. Then you can delay closing timing as late as possible, being careful about the water jacket. Maybe get close to 4 port 12A closing timing. You're always saying how you've gotta bridge the primaries and all that. 
Worth an experiment I think.
Hmm, maybe do what the 6 port 12A aussies are forced to do and BP the primary ports. They can only use side motor mounts in whatever car they came in (HB Cosmo?) so 4 port intermediate Y plates are not useable. Those 6 port 12As have very small runners and very late opening ports. You with your GSL-SE plates will have NO port timing which opens at 12A spec (I believe), much sooner than a 6 port 12A, but still will allow a 5mm bridge the same as any 4 port 12A. Then you can delay closing timing as late as possible, being careful about the water jacket. Maybe get close to 4 port 12A closing timing.
You're always saying how you've gotta bridge the primaries and all that. Worth an experiment I think.
03-20-11, 04:53 PM
#91
Old [Sch|F]ool
EFI can be tuned for free and in moments, at any time, including while driving.
EFI also works the same no matter the forces acting on the car as long as the fuel pump inlet stays covered. Carbs don't.
And remember, I played the block-the-runners game, and the engine was much happier with them unblocked.
03-22-11, 12:32 PM
#93
Still experimenting with carbs over here. Something about the transition from idle to primary circuit is choppy so I've swapped the #1 and #2 primary slow air bleeds to a set from a 79-80 carb. I'll test on the engine today.
Yes I already made sure the accel pump is not the problem here.
Yes I already made sure the accel pump is not the problem here.
03-22-11, 12:38 PM
#94
I'm thinking if I can't tune out the choppy problem, I'll just go with the other old school carb that I've been boost prepping (about half way complete) as it runs fantastically well on this old school intake manifold. I'll then need to boost prep another carb but it will be an FB carb because Sterling said they are able to refill with fuel faster than the old school, and work better as a starting point for boost prepping. Also something about the idle is more stable or tuneable on the 84-85 specifically. Well, so far I've gotten a robust idle on the half boost prepped carb but a weak choppy idle on the old school carb. I'll keep this thread updated.
03-22-11, 02:39 PM
#95
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how long are the runners in the manifolds?
03-22-11, 03:33 PM
#97
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lol, and how long is that?
03-23-11, 12:29 AM
#99
Still got a lumpy transition and weak rich idle so I switched focus on to the other old school carb that's about half way boost prepped. 
This carb has been tuned just now with the following:
95 primary jets
170 secondary jets
70 primary air bleeds
120 secondary air bleeds
Accel pump shot and duration mod. Also banjo bolt drilled like a 79-80. The flow is like a healthy prostate.
All vacuum nipples capped and zip tied. richer solenoid replaced with a dummy fitting. Don't ask where I got it because I found it in my pile. Looks like a truncated richer solenoid.
The idle on this carb is superb. The transition is flawless. Punch it and the secondaries roar to life. There is a cloud of activity in all four boosters when you free rev it (mech secs).
Other info. Primary #1 slow air bleeds are 170. #2 slow air bleeds are 50 (larger than 79-80 and probably FB too). The secondary slow air bleeds #1 100 and #2 can't remember but way different from modern Nikkis so not able to compare.
Carb still has its secondary diaphragm housing hanging on the side because I've been too lazy to block the little hole. Gotta do it right so boost won't leak out. I'll get around to it later. Carb also has a simplified choke which does actually function. As the carb won't be boosted right now, I'll leave the choke as well.
This is going to be an awesome daily driver carb when it goes on the half BP!
I just have one more task to perform before I can get started on the half BP. This current engine I'm testing on requires a working intake. I've gotta prep an FB manifold. That means cutting a channel where the shudder valve used to be (Nikki and Mani thread). Then I can test the old school carb and see if it's going to work better on a channeled manifold. That's the theory anyway, from what I've discovered testing a channeled carb on a sep-runner manifold. The result is a super rich weak idle (that hates low RPMs below 1k) but gets better when you swap to a channeled manifold.
This carb has been tuned just now with the following:
95 primary jets
170 secondary jets
70 primary air bleeds
120 secondary air bleeds
Accel pump shot and duration mod. Also banjo bolt drilled like a 79-80. The flow is like a healthy prostate.
All vacuum nipples capped and zip tied. richer solenoid replaced with a dummy fitting. Don't ask where I got it because I found it in my pile. Looks like a truncated richer solenoid.
The idle on this carb is superb. The transition is flawless. Punch it and the secondaries roar to life. There is a cloud of activity in all four boosters when you free rev it (mech secs).
Other info. Primary #1 slow air bleeds are 170. #2 slow air bleeds are 50 (larger than 79-80 and probably FB too). The secondary slow air bleeds #1 100 and #2 can't remember but way different from modern Nikkis so not able to compare.
Carb still has its secondary diaphragm housing hanging on the side because I've been too lazy to block the little hole. Gotta do it right so boost won't leak out. I'll get around to it later. Carb also has a simplified choke which does actually function. As the carb won't be boosted right now, I'll leave the choke as well.
This is going to be an awesome daily driver carb when it goes on the half BP!
I just have one more task to perform before I can get started on the half BP. This current engine I'm testing on requires a working intake. I've gotta prep an FB manifold. That means cutting a channel where the shudder valve used to be (Nikki and Mani thread). Then I can test the old school carb and see if it's going to work better on a channeled manifold. That's the theory anyway, from what I've discovered testing a channeled carb on a sep-runner manifold. The result is a super rich weak idle (that hates low RPMs below 1k) but gets better when you swap to a channeled manifold.