Hello, I have been fighting a stumble on my car for several months now, and i'm about fed up! I live at a high elevation (about a mile high), but it seems like no matter what i change i still have a stumble. When i hold the pedal down to where the throttle is about 1/8th of the way open, the engine will start to stumble unless i let up and pump the gas again. It used to be even worse until i added some pieces that reduce the size of the airbleeds. Watching the voltage on my oxygen sensors, the voltage dips down into the .1-.2volt range, indicating a severely lean condition. The reason i am confused is that with my high elevation i have a setup that should be running it on the very rich side. I have jets that are two steps down from stock (but that shouldn't matter because with the throttle only 1/8th of the way open they shouldn't even be used). A guy at the local performance shop suggested a larger power valve, like a 10.5, but that seems a little high to me? At idle in drive my engine pulls about 7.5inHg, so it is my understanding that i should have about a 5.5" power valve? My understandign is that the power valve is strictly to richen up the mixture under high-load conditions. So, i've done air-bleeds, power valve (changed from 5.5-8.5 with only a slight help), shooters (currently have a 31 in), jets, and also pump cam, and nothing seems to help! I'm wondering if i should do a compression check, possibly i have burned seats? This is a Demon carb, by the way...same as a holley...625 vac. secondary. Thanks!
According to Holley you are right to use a power valve below that of idle. Are you sure that you have only 7.5 inches of vacuum at idle? That is quite low unless you have a long duration cam. Sounds like there is a problem in the transition from the idle circuit to the intermediate circuit. Trying to get a power valve to fix this seems not the right fix but may seem to help. I would be looking to get the intermediate circuit to pick up the fuel a little sooner. With the high altitude, I would be inclined to make the circuits larger so the air pressure you have were you live will make it eaiser for the fuel to be drawn out of the bowl by the engine vacuum. At the vacuum you have, opening the throttle lowers it even more and draw suffers. When putting a very large carb on a small engine, the main jets have to be larger not smaller in order for the vacuum signal from the smaller engine to have a better chance to draw on the fuel. Just my thought on the matter.
If I were you I`d call The company who made the carb(Barry Grant makes the Demon line I think?) and let them tell you what you should do. After all they did design and build the carb right?
I have had problems like this years ago--could have sworn it was the accelerator pump---though I could be wrong it was a while ago.
I am at 6000 ft. and I had a similar problem 12 years ago with mine. I had to go back to the stock jets (they DO affect part throttle, don't know why), a 8.5 power valve, and finally, I dicsovered that the vacuum advance on my distrubutor wasn't working--apparently the diaphragm was blown. Replacing it cured the problem 100%. Your power valve is supposed to be 2 steps less than what your engine idles at (in Drive with your foot on the brake if it's an automatic). I was pulling 10.5 inches of vacuum with mine, so the 8.5 power valve should be correct. It really depends on the carb though--Just for kicks I installed a friend's carb on my car and took it around the block...it had a 2.5 power valve and ran smooth as silk with no hesitation whatsoever. If I were you, the FIRST thing I would check is the vacuum advance and hose on the distributor---they are dirt cheap to replace...
I re-checked my vacuum in drive and i'm pulling 9in Hg. So i should be using the 6.5 power valve that came stock in my carb...and it was still stumbling when i had that in there. I have one step down in jets right now...69 in the primaries and 76 in the secondaries (70/77 is stock). I've had the stockers in there before too and it was still stumbling...even tried going up a few steps! What about shooters? What size is good to start at? Are there any other adjustments i can really make at this point? I have a new vacuum advance and i've checked the whole system for leaks. Also the accelerator pump is new and doesn't leak, but your comment makes me think i might want to consider going up another size in shooters. I think i'll just call Barry Grant tomorrow....thanks for the suggestions guys!
Just a point to make and that is in a well set up carb, if the pump were diabled and you (slowly) run the engine RPM up, there should not be a stumble under a no load condition. Only a fast opening needs the pump shot to make the engine rich for acceleration. This is one way to tell what could be a problem.
Carb Stumble Just a thought on it, not being familiar with the Demon but having had this problem with a Holley, I would go with a 34 to 36 squirter.But first I would check the pump accuating lever. Holley recommends holding the pump lever down and then adjusting the accuating lever screw until .015 (haven't got exact numbers handy) clearance is obtained. I personnaly leave the pump and lever in up (idle) position, if there is no clearance between the screw and pump lever, adjust it until it has a bit of clearance ie. .010. This always seemed to help stumbles in my case but different carbs and engines are all not going to react the same so it's a trial and error thing.
