Ok, I need the experienced experts only here, no wild guessing please! I am trying to help a friend tune his brand new Holley 4 bbl (model 4175) on his relatively fresh Chevy 350. I can get the car to run fairly smooth in park, but there is an anoying steady shake in gear unless I crank up the rpm's which is hardly a proper fix. The reason I know the carb is the problem is because I get the smoothest response with the left idle needle 1/6 turn open, and the right needle fully closed. That's got to be wrong, but it runs best that way right now. There is also a secondary idle screw which I have not dealt with before and the instructions are the most vague and useless instructions I have ever seen. Even with the secondary screw backed off fully, the engine is getting gas from somewhere and judging by the shake, it is not properly atomized. My experience tells me there is an internal leak. Power valve maybe? Its all brand new and supposedly bench tested at the factory. I have already fixed both float valves after some metal shavings in the carb got caught in the needle valve and flooded bowls. Are there any good starting points before I tear the thing apart?
what's the list number for the carb? 4175 holleys are vacuum secondary so there should be no need for secondary idle screws since the secondaries have nothing to do with idle. what kind of vacuum is the engine pulling? the fact that you said there were metal filings in the float needles also worries me that some may have gotten in the idle passages. i'm sure you know, but the idle screws should be equal for both sides. i'm sure you have already, but have you checked the timing?
The model number for this carb is 0-80555S. There is a screw that opens the secondary throttle slightly for some reason, but the instructions only show where it is, not what to do with it. I've been looking up info on the Holley website and it sounds like the power valve is opening on idle. I don't have a vacuum gauge, so I'm not sure what it is pulling, but the power valve is supposed to be a 6.5 inch valve. I don't think the vacuum is nearly that low on idle. There is a test for a blown power valve that tells you to screw the idle needles all the way in, and if the engine dies, the valve is good. I assume that means that if the engine still runs, the power valve is bad. But it is brand new! I don't think timing is an issue since the engine is getting enough gas to run, with the needles closed. Would timing affect the amount of vacuum that much? I'll check it tomorrow anyway.
you mentioned metal shavings in the first post, while the engine is running, inside the choke horn there are little holes, these are air bleed passages and if any metal has contaminated them, clogged one or more of them it will run rough with very poor idle characteristics. You can cover them one at a time with you finger and see if the idel changes. A normal running holley with a good idle will run bad when these are covered with your finger, if you get no change in idle then tear the carb down and remove the metering block, soak it in a good cleaner to loosen the clog and blow out with compressed air. you will have to blow out backwards in the metering block and the body. Probably have to remove the choke butterfly. hope this helps
Problem solved! Thanks everybody for your input. I called the Holley tech line this morning and the guy told me to check the float levels first. They were supposed to be correctly set at the factory, but they were obviously not level with the top of the bowls when I took a look inside. After I adjusted them, it ran bitchin'! I still don't understand why it seemed to have too much gas when it was actually starving, but the timing and vacuum checked out ok and the floats were the only adjustment I made, so that must have been it. I also removed and re-tightened the power valve, but I don't think it was the problem. I'm gonna go veg-out now, I deserve it.
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