Box Stock

Fuel pump?

D

Ducar 212

Member
I'm still having issues at WOT, motor is strong till I'm on it up to 6500 or so and I don't know if my fuel pump is the culprit, but my question is I've heard these pulse pumps work better from vacuum instead of off the breather. I don't know if I'm losing my fuel at high rpms. I thought I had this issue figured out, but I'm geared right on a bigger track now, the motor spits and pops like it's lean at the same rpms every lap. I've gone through new plugs and they all look great as well as spark. Coil gap is .060. Carb has turned out clean. I was goin up on jet sizes in the main. Got a #135 in it now. Should be too much fuel
 
Have you checked float level and or cleaned inlet needle? Pump has to be very shot to cause a problem in my experience because the fuel demand is so low.

Also check entire fuel line from pickup to carb looking for potential air leaks.
 
It's not even been used for a whole season. An air leak would be something to check.
I've cleaned and checked the jets and have found a very small amount of things.
I believe I will drain the tank and replace the lines, and running off of vacuum, is that better in any way? I'm running it off the breather cover
 
flattop1 said:
What carb?
What rpm ?
How long at full throttle before it happens .
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Nibbi 24mm slide carb. #38 pilot jet, #135 main jet (tried to go rich to experiment) coming out of the middle of the corner after letting off to the flagstand is where it starts missing and popping. It does it like clockwork. Wish I could send a video
 
RadialFin said:
Post to YouTube and paste a link.
Come to think of it. How’s the head gasket?
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Head gasket should be fine, it is a .009 shim gasket but I don't think its blown.... not real sure on that but I would think I would have bigger issues with it than that
 
Ducar 212 said:
I would think I would have bigger issues with it than that
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Not necessarily, you can have a small leak that pulls in air and causes it to run lean. You can even eyeball the fins where the head and cylinder meet.

Just something else for the list in case you run out of other things to check.
 
I would look at the fuel inlet and needle and seat well . Where the needle rides . It may not be passing enough fuel . Couple ways to change that . One is lowering the float level .
Seems counter productive .
Common thinking is raise it . Thus closing the needle.
 
Ducar 212 said:
Nibbi 24mm slide carb. #38 pilot jet, #135 main jet (tried to go rich to experiment) coming out of the middle of the corner after letting off to the flagstand is where it starts missing and popping. It does it like clockwork. Wish I could send a video
Click to expand...
The Popping is most probably a leaking exhaust gasket.
 
2Aron said:
My Nibbi pops and afterfires when its too rich on main jet. Its pretty consistent.
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What main jet do you run? It seemed to do good with a #120, yet I couldn't take it at the rpms I would have ran at the miller track, it was at a smaller track.
 
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