Going to a show this weekend my 351w with a 4160 holly would not idle after getting off the highway. All primary checks at the show by sight showed nothing. Did see some drips of fuel in the carb when running. It ran great on the way home off idle. Today I had a little time and checked a few things. Hit the needle with a hammer, fuel level is good in the bowl. Vacuum is about 25 and is jumping around from the poor idle. Runs like a vacuum leak. Carb is only a year old with less then 2k miles. Kind of thinking I have a warped meter block area. But on such a new carp? Or do you think there is just some junk stuck in there? Think of removing the idle screws and blowing some cleaner through it.
I had one running methanol on my race car, every couple weeks it would idle bad but run good. I always had carb cleaner on the trailer to spray down the air holes in the metering plates. I guess with out a air cleaner they would get something in them, a quick shot and it was back to normal till they got dirty again.
Newer Holley come with PV blowout preventers . What "air holes" in metering plates ? Hitting a carb with a hammer doesn't help a thing , this isn't a 1930's fuel delivery system .
What kind of gas are you running? Clear gasoline, or some of the ethanol mix crap most states sell these days?
Fuel dripping from the boosters at idle is an indication of main well fuel initiation. At idle it should not be dripping. This is a common problem with a big cam, but with 25 in hg vacuum it sounds like you have a mild cam. Try closing down the throttle plates. That may require you increase the size of the idle air bleed restriction. I like to convert from pressed in bleeds to screw-in. I have also found the Holley main bodies to have metering plate surface flatness issues. I fly cut mine to insure a flat mating surface for the metering plate. The carb on the left did not clean up after the first pass. In the photo below also note the "spongy-ness" of the main body. It may have an internal porosity leakage problem.
Drove my 57 T-Bird for almost 30 years with a "modern" Holley carb. In fact, over the years I had 3 different Holley carbs on the car. My main problem was an inconsistent idle. It would idle fine then it wouldn't. Most of the time a bit of tweaking on the mixture screws would get a good idle back. I had the front metering block on and off so many times I finally had one bowl screw start to pull the threads out of the body on the second carb. What I think the cause of the problem was was fine rust in the fuel tank. I had checked the tank by pulling the sending unit and it looked pristine. However, after having the carb apart for the umpteenth time I took a mirror and looked at the top of the tank. It was covered in rust. The tank came out and was cleaned and sealed at the last old time radiator shop in Portland. That seemed to cure my problem.
Check that the air bleeds are not plugged up. Set the float level then double check it stays there . Needle and seats on these do mess up . I keep spares in my Vette. Watch the fuel pressure . Check the power valve. Spray something in all the lines , carb base , Check the diaphragm on the vacume advance , transmission modulator and such. If it ran good before and its a actual carb issue it should be fairly simple to sort out . If its dripping fuel out id be double checking needle and seat / float level first
Back in the day I'd say a blown power valve, but it's rare to see one now, they all have power valve protection....plugged air bleed would be my guess....if you really think it's a vacuum leak don't forget to check your brake booster if you've got power brakes. My daily driver f100 had a leak I couldn't find, finally sprayed some brake cleaner on the back of the booster, and voila, idled up a few hundred rpm. Booster had a crack on the seam.
What I ment to say in my first post above was the air bleeds in the main body, not the metering blocks. It's been 30 years since I had the Holley running on a car. Shoot some cleaner down all the bleed holes and see how it acts.
If it ran good and suddenly started doing that after a highway run, I would say its a small piece of something in the needle and seat.
OK, here's a dirty little secret about Holley needle and seats: 90% of the ones that get replaced are fine - they didn't leak and didn't need replacement! What is far more common is for the tiny O-ring that seals the needle in the bowl dries out, shrinks or cracks, and incoming fuel leaks past the needle assembly, causing an erratic fuel level. This is especially true for carbs that have been on the shelf for a long time or race car carbs that see lots of down time between outings. Before you replace the (rather pricey) needle/seat assemblies inspect the O-rings. Are they smashed flat instead of still round? Are they torn or cracked? I carry spare O-rings in my race car trailers in case a float level issue creeps up (pun intended). Good luck
There is also some weird things going on with gas with ethanol, Seem to be with Old gas tanks & Older carbs & some New Holleys 2000 - 2010 Seeing / resembles like real fine crush Glass , & some times past . Seeing also on brass needle and seat, and you can feel it in your fingers but it's hard to see, it's gritty. Seem ,I am thinking A chemical reaction to the coating in old fuel tanks and also the zinc of carbs , Not seeing this in billet carbs because Anodized .
David Freiburger does a fantastic job showing how to set up a Holley carb. Do yours in the exact order that he does. Do NOT skip steps.
Except to check accel pump lever clearance early in diagnostic steps, before changing jets, air bleeds, etc. Otherwise good basic info, definitely worth the watch.
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