My carburetor knowledge is extremely limited, and trying to help a good friend of mine here. He has an 830 Holley HP 80509-1 on his stroker small block. This carb has been on the engine for a good 4 yrs, and has always ran real well. He just started having an issue with it. Out of the clear blue, it all of a sudden doesn't want to idle.

Started to investigate, so he took the air cleaner off, and also took off the stud that goes in the center of the carb which holds the air cleaner on. Fired it up with these two items removed, and now the engine now idles fine, just like it used to before it started acting up. Put the air cleaner stud back in the hole, and the idle goes crappy...to where the engine dies.

Took the air cleaner stud back out, fired the engine up. Idles fine. Placed finger over the hole where the air cleaner stud goes, and there is strong vacuum there at idle and also at any engine rpm. Revving the engine to any rpm and placing your finger over the hole makes an immediate difference in how the engine runs.

What's going on with this carb, that there is vacuum where the air cleaner stud screws in, and that this is effecting how the engine runs and idles?

Thanks in advance!
TJ2

I don't know it all either, but I think there's an Idle adjustment screw in that hole just like the Demon carbs.

I don't claim to know it all especially with carbs but I'd look for a stuck float, debris in an air bleed or the metering block and main body passages. Also possibly a loose or bad power valve and gasket.

I know nothing as to why air is apparently being introduced through the valve cover stud area. You may have a problem there or auxiliary idle air is used on that model.

Did you look down in there as some newer carbs have an air adjustment screw in the stud hole as to let more air around the throttle blades instead of drilling holes in the throttle plates. It may have the screw in there and if so maybe it has backed out. Thats what I would look at first. Ron

That's a good one. I just checked Holly's instructions for the HP 80509-1 on-line. There is no mention of a center adjustment screw like the Demon has. The only pictures I was able to pull up show the threaded casting area sealed at the bottom. The likely culprit would be the throttle plate to throttle main body gasket but that doesn't explain the vacuum at the center hole.

http://static.summitracing.com/global/images/instructions/HLY-199R10016-2.pdf

My Ultra HP has an adjustment screw in the bottom of the air cleaner stud hole. Don't know what the factory/base setting is, but it's a handy adjustment to have.

My old HP carb, the air cleaner stud hole goes all the way through the body and baseplate so the carb has to be adjusted with the air cleaner stud installed or it creates a vacuum leak. If it ran good before and is now acting up, it may need to be cleaned, but check the float levels and spray out the air bleeds with carb cleaner first.

I'd shine a light down there and see what's there. I know Holley added that demon-like adjustment screw to their carbs, but it was in the past few years so im not sure if your friend's carb has it.

On my demon, once i adjust the screw, putting the air cleaner stud back in has no effect.

The air bleed screw under the air cleaner stud is a feature that wasn't introduced until the later aluminum "Ultra HP" 4150 carbs. Those are the 8080# series, such as the 80805 850 version.

Somehow or another your carb has gone rich. As suggested, check the air correctors to make sure something is not clogged. Alternative of the loose power valve also makes sense. Essentially, when you removed the stud you opened up the throttle introducing more air which it clearly needed.

Kind of doubt all the air correctors clogged at once, so maybe a quick check of the power valve is in order, or as I think someone suggested is the needle stuck open? Think you would see a stuck needle coming out the bowl vent though.

Quick way to tell if you have the air bypass is to look and see if there are 4 holes in the boss around the air cleaner stud. This comment is more for reference as I doubt you have that.

The old 950 HP I used to have the threaded hole went all the way through. When the stud was removed the idle sped up, which is what you would expect.

Good luck.

Best regards, Dave