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Engine runs like ass after a carb rebuild


Veteran Member
Senior Member
This is my friends recently purchased 64 Galaxy with a 390. It’s a bit rough so he’s trying to get it running reliably. Last time we ran it there was fuel pouring out of vent holes on top of the carb so we thought there was some crap preventing the needle valve from seating.

Carb is an Autolite 4100. Rebuild was seemingly easy but it runs terrible afterwards. Doesn’t want to stay lit unless you keep giving it some gas. Sometimes won’t even start. Very rough, the whole car shakes and much more black smoke that I thought was there beforehand. It actually ran great before we took the carb off to rebuild but there was a lot of fine sediment in the carb so it had to be done.

After scratching our heads for a few minutes we pulled off the top cover of the carb and I was surprised to find the front bowl almost empty but rear one full. I thought maybe the needle was stuck closed but if you crank the engine with the cover off, fuel shoots out of the valve into the bowl (and elsewhere). We poured about a 1/2 inch of fuel into the bowl and watched at it slowly drained down. But where does the fuel go is the question?

The carb is this:
The front bowl has an accelerator pump. The floor of the front bowl has 2 jets and a power valve. Correct my if I’m wrong but the jets feed the primary venturi cluster so if the bowl is going to leak down through the jets wouldn’t it have to go uphill as the Venturi are higher than the floor of the fuel bowl. If the fuel were to leak past the power valve, where would it end up going?

The day was coming to an end so we ran out of time but I assume I could just prop the carb up on a bench, fill the fuel bowl and see where it leaks out the bottom but I thought this question might be child’s play to some gents more experienced than I

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Sounds like the front inlet needle and seat is stuck closed ? Buy a kit and install a new one.
It looks like it has a single inlet that feeds the front and rear. The rear must be working whereas the front is stuck closed.
I see you already rebuilt it. Maybe check the front inlet seat to make sure trash didn't lock it up ?
If that doesn't fix it, I would buy something new.

Slap on a Street demon. Don't let the name scare you. It's a very mild street carb.
Is the manifold a spreadbore : small primary, bigger secondary ? This carb is a spreadbore type.
Talk about inflation ! This carb was $310 last time I looked ! Now $440

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Yeah, we rebuilt it. There is only one fuel inlet (in the front of the carb). The needles and seats are brand new and we confirmed that fuel comes up through the the needle/seat by cranking the engine with the top off the carb.
It’s definitely leaking down pretty quickly in the front bowl while the rear bowl stays full. I think the accelerator pump is working as it should. My guess is that it’s leaking through the power valve. I found a few online threads that suggest this is not a rare problem. In some cases it can be caused by covers (such as the power valve cover or the accelerator pump cover) warp easily.
Check to see if the power valve is loose. I had my sons Holley 650 rebuilt and it ran like shit. Pissing too much gas out the overflow tube?
Couldn't figure it out. Brought it back to the guy that rebuilt it. He bought another rebuild kit and he still couldn't figure it out. My son`s buddy, who is a mechanic, found the power valve was loose, he tightened it and it runs great now.
The original guy, who has been rebuilding Holley carbs for at least 55 years couldn't figure it out? Sometime things slip by even the most experienced.
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