Holley carburetor has the flu and won't stop vomiting fuel

I have a Holley 4150 that won’t stop regurgitating fuel out of the overflow on the front of the carb. Number stamped on the carb is C9AF-9510-N.

I have taken out the float needles, cleaned and reinstalled but it still does the same thing.

It will run fine for a few seconds until the carb fills with fuel, overflows and drowns itself almost to the point of dying but I cut it off before it did.

What else should I do to fix?

What carburetor kit do I need to rebuild it? I’ve tried to find a troubleshooting guide but no luck.

What else could be causing these issues besides the float needles?

The rubber o ring on the needle assembly is bad. Happens all the time with alcohol fuel

Best resource for a quality replacement?

Only buy the genuine Holley version. It can be bought direct from Holley, but I generally use Jeg’s or Summit Racing. They make a version suitable for use with ethanol - laced fuel.

Got a part number? I’ll run by summit and pick them up. Also what carburetor kit for a C9AF-9510-N if needed? See several different ones listed so I’m not sure

Several of them are correct, some are more complete than others. Honestly Google is your friend.

Google is what has me confused as to what to get. I believe the needles themselves are fine and was hoping to just buy the o-rings. There’s the Holley 34-7 Needle and seat hardware kit, There’s the 26-37 fuel transfer tube o-ring, there’s the 6-504 Needle and seat, there’s the STD-542B rebuild kit…

You need to be sure and buy only HOLLEY brand parts. The P/N STD-542B is a Standard Parts Co part number. Chinese garbage that will make the carburetor non - functional.

Honestly you need to replace both needle and seats with real Holley parts. Replacing just the O - rings is not going to end well. You need the needle, seat assemblies and the paper gaskets for same.

Yep this fixes your problem if you follow the instructions for float adjustment. You also need bowl gaskets and metering plate gaskets.


All Holley carbs I worked on in the past I basically adjusted on the car to where the fuel barely trickles out the site window on the side without taking apart the carburetor. If that didn’t work then I would take the carburetor off and dive into it. I would set the float level by turning the bowl upside down and adjusting the float level until it was perfectly horizontal with the float bowl without being at any type of angle. Better way?

Yes it is just that simple. Carry on.

By the way, be absolutely SURE you lubricate the Oring or you may cut them and you’ll have another leak.

With brake fluid, vaseline, wd-40… or is something else better?

Vaseline works fine.

That’s handy! Bookmarked that one for possible future needs.

Here are some carb kit bookmarks I had saved that may be useful.


AllCarbs has the full rebuild kits available, but they have the blue gaskets. At the very least, this should give you an idea of what you are looking for on the Summit website.

Yes. non-stick blue gaskets. You’d be better off getting the entire kit. I bought the .090 viton needle and seats for my Shelby to help control the pump pressure. 55.00 for just needle and seats. I stepped down from the .110 widow size because my vehicle is a factory dual quad car.

Replaced the needles, lubed with vaseline and still vomiting fuel. Doesn’t seem to matter what I do, it throws up within seconds of cranking.

How are you setting the float level?

I took the current float needles out and reinstalled new ones. The front one was 6.25 turns counter clockwise to take out and the rear was 4.25 turns.

That’s not the way to do it. You just put them in where they were.