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I have a stripped (emissions and oil metering pump as well as vacuum secondary disconnection) nikki carb on my 83. The carb is from 85 and has been rebuilt. Recently I took apart the accelerator pump due to a choking/bogging condition when I would first get into the pedal to accelerate. I found nothing notable and just sprayed the ports out as best as I could. Seemed to really help and the problem was eliminated...but only for about a half an hour of driving. Then it came back.
Now it isnt as bad...it seems to happen when fully in gear and accelerating slowly at lower rpms (up to 1500 or so). The car will buck in this rpm range and also in neutral it is a little rough when you get into the pedal...but not like before.
Warm engine,shut off. Remove air cleaner,operate throttle and look for accelerator pump discharge. Should be a steady stream into each primary venturi,if not,revisit your work on the pump. Does fuel level look to be 1/2 way up in each window on float bowls? If cleaning out accel pump passages again improves performance as it did 1st time and then degrades as before,possible carb needs cleaning/rebuilding. Have you looked at/changed fuel filter recently?
Warm engine,shut off. Remove air cleaner,operate throttle and look for accelerator pump discharge. Should be a steady stream into each primary venturi,if not,revisit your work on the pump. Does fuel level look to be 1/2 way up in each window on float bowls? If cleaning out accel pump passages again improves performance as it did 1st time and then degrades as before,possible carb needs cleaning/rebuilding. Have you looked at/changed fuel filter recently?
Performed this 'test'. Both venturis flow very smoothly and fully when the throttle is operated manually with a warm/shut off engine. Also, I narrowed in more on the 'symptoms.' When in neutral (or driving) the engine will be smooth and responsive up UNTIL about 1500 rpm at which point it stops accelerating...starts to become rough/starved...and if you stay on the pedal you may break free of this condition and end up accelerating. What I have had to do is shift at higher RPMs to completely avoid this spot in the rpm band (~1500-2000rpms).
If you accelerate slowly from about 1000rpm (in gear) you will get to the ~1500rpm mark and the car will start to buck at which point its actually best to take it out of gear and bring engine RPMs up higher than about 2000rpms and fully engage the clutch there.
In other words, it seems as if it may not be at the first drop of the pedal.
Both float bowls are at about 40% when idling and under load. They could use adjustment, however would that still be the cause potentially?
2 fuel filters, brand new and clear (see through) in line on the fuel supply.
At times the mismatch between the main body venturi positions and the machined bores where the butterflies sit in the baseplate can be off by quite a bit. This mismatch will produce a flat spot around the RPM you are seeing. My findings on the large hogged out venturis I do is anywhere from 1700 to 2300 RPM or so. It is the 2000 RPM stumble most Nikkis suffer from and is mentioned on the forum here and there, mostly among wankel=awesome's posts and mine.
The fix is to do a very slight funnel shape in the baseplate above the butterflies with a grinding stone in a dremel or a die grinder. This has reduced the flat spot to something that is drivable and no longer requires fancy foot work (accel pump squirts and such that you've needed to do).
This is pretty advanced stuff but it does make sense, doesn't it? Think about it. You start out with a nasty lip that can impede flow. Combine this with the rotary engine's tendency to have a really strong reversion pulse as the ports are closed by the edge of the rotor face as it passes the opening. This reversion pulse is even worse on larger ports. The key to reducing its unwanted effects is to create an anti-reversion lip in the baseplate by introducing a slight funnel shape. Don't believe me? Even Holley does this, though it is directly cast into their zinc main body above their cast aluminum baseplate. Of course they did this out of necessity on a mass produced carb to get past the mild mismatch they have. Only an enthusiast for those carbs would take the extra time to trim off any mismatches no matter how slight. As for our Nikki carbs, the mismatch can sometimes be brutally apparent and it can make some carbs run worse than others. But all the carbs I've done this funnel shape on the baseplate have seen a marked improvement in this lean zone area.
Two more points to consider. 1) The bottom edge of stock venturis have a nasty lip that is part of the casting. This lip slightly deflects the reversion pulse but can hurt forward flow. If you have access to a Nikki main body that has been separated from its baseplate, you can feel and even see this lip for yourself and you can carefully trim it on an otherwise assembled carb just by separating them (but you do need to funnel the baseplate at this point as well so there are no more lips seen from above). 2) The intake manifold employs an anti-reversion lip itself. Just look at the seemingly mismatched short runners of the manifold compared with the tall runners in the intermediate plate (middle iron). Also measure the outer end ports verses the end plates (irons) and you will see the manifold is smaller than the engine in port area here as well. Why is that? Anti-reversion. It's important. That is why you must never port match your manifold to your engine. If you do, the Nikki gods will smite you.
At times the mismatch between the main body venturi positions and the machined bores where the butterflies sit in the baseplate can be off by quite a bit. This mismatch will produce a flat spot around the RPM you are seeing. My findings on the large hogged out venturis I do is anywhere from 1700 to 2300 RPM or so. It is the 2000 RPM stumble most Nikkis suffer from and is mentioned on the forum here and there, mostly among wankel=awesome's posts and mine.
The fix is to do a very slight funnel shape in the baseplate above the butterflies with a grinding stone in a dremel or a die grinder. This has reduced the flat spot to something that is drivable and no longer requires fancy foot work (accel pump squirts and such that you've needed to do).
This is pretty advanced stuff but it does make sense, doesn't it? Think about it. You start out with a nasty lip that can impede flow. Combine this with the rotary engine's tendency to have a really strong reversion pulse as the ports are closed by the edge of the rotor face as it passes the opening. This reversion pulse is even worse on larger ports. The key to reducing its unwanted effects is to create an anti-reversion lip in the baseplate by introducing a slight funnel shape. Don't believe me? Even Holley does this, though it is directly cast into their zinc main body above their cast aluminum baseplate. Of course they did this out of necessity on a mass produced carb to get past the mild mismatch they have. Only an enthusiast for those carbs would take the extra time to trim off any mismatches no matter how slight. As for our Nikki carbs, the mismatch can sometimes be brutally apparent and it can make some carbs run worse than others. But all the carbs I've done this funnel shape on the baseplate have seen a marked improvement in this lean zone area.
Two more points to consider. 1) The bottom edge of stock venturis have a nasty lip that is part of the casting. This lip slightly deflects the reversion pulse but can hurt forward flow. If you have access to a Nikki main body that has been separated from its baseplate, you can feel and even see this lip for yourself and you can carefully trim it on an otherwise assembled carb just by separating them (but you do need to funnel the baseplate at this point as well so there are no more lips seen from above). 2) The intake manifold employs an anti-reversion lip itself. Just look at the seemingly mismatched short runners of the manifold compared with the tall runners in the intermediate plate (middle iron). Also measure the outer end ports verses the end plates (irons) and you will see the manifold is smaller than the engine in port area here as well. Why is that? Anti-reversion. It's important. That is why you must never port match your manifold to your engine. If you do, the Nikki gods will smite you.
Much appreciated. I will keep this in mind as a next step after more diagnosing/tuning. Again, thank you
Upon further investigation...it appears that a relatively reasonable amount of fuel is flowing into the primary port closest to the firewall between ~800 and ~1500 rpm (once the accelerator is depressed or the arm is manually turned on the carburetor). It isn't until, say about ~1700-1900 rpm that "equal" amounts of fuel flows into the primary port nearest the nose of the car.
Last edited by dankekong; 07-21-17 at 08:53 PM.
Reason: Info omitted
At times the mismatch between the main body venturi positions and the machined bores where the butterflies sit in the baseplate can be off by quite a bit. This mismatch will produce a flat spot around the RPM you are seeing. My findings on the large hogged out venturis I do is anywhere from 1700 to 2300 RPM or so. It is the 2000 RPM stumble most Nikkis suffer from and is mentioned on the forum here and there, mostly among wankel=awesome's posts and mine.
The fix is to do a very slight funnel shape in the baseplate above the butterflies with a grinding stone in a dremel or a die grinder. This has reduced the flat spot to something that is drivable and no longer requires fancy foot work (accel pump squirts and such that you've needed to do).
This is pretty advanced stuff but it does make sense, doesn't it? Think about it. You start out with a nasty lip that can impede flow. Combine this with the rotary engine's tendency to have a really strong reversion pulse as the ports are closed by the edge of the rotor face as it passes the opening. This reversion pulse is even worse on larger ports. The key to reducing its unwanted effects is to create an anti-reversion lip in the baseplate by introducing a slight funnel shape. Don't believe me? Even Holley does this, though it is directly cast into their zinc main body above their cast aluminum baseplate. Of course they did this out of necessity on a mass produced carb to get past the mild mismatch they have. Only an enthusiast for those carbs would take the extra time to trim off any mismatches no matter how slight. As for our Nikki carbs, the mismatch can sometimes be brutally apparent and it can make some carbs run worse than others. But all the carbs I've done this funnel shape on the baseplate have seen a marked improvement in this lean zone area.
Two more points to consider. 1) The bottom edge of stock venturis have a nasty lip that is part of the casting. This lip slightly deflects the reversion pulse but can hurt forward flow. If you have access to a Nikki main body that has been separated from its baseplate, you can feel and even see this lip for yourself and you can carefully trim it on an otherwise assembled carb just by separating them (but you do need to funnel the baseplate at this point as well so there are no more lips seen from above). 2) The intake manifold employs an anti-reversion lip itself. Just look at the seemingly mismatched short runners of the manifold compared with the tall runners in the intermediate plate (middle iron). Also measure the outer end ports verses the end plates (irons) and you will see the manifold is smaller than the engine in port area here as well. Why is that? Anti-reversion. It's important. That is why you must never port match your manifold to your engine. If you do, the Nikki gods will smite you.
Which part of the carb does this refer to? I think of baseplate as the plate which mounts under the carb and has all the vacuum ports. Thanks!
07-21-17, 07:38 AM
