Topic: Carburetor Woes

Trying to get this art car roadworthy. '64 Dart with 225 slant six fed by a 1-barrel Holley 1920. I got the carb from a friend, who had it reportedly running well on this same engine before we pulled the engine from its donor car a year and a half ago. I believe it was a store-bought reman unit.

The engine behaves like it has a big vacuum leak. When hot, it will only stay running with the choke engaged. It won't run at all below 1500ish RPM, and the need for choke drops off with higher RPM. I checked everywhere with a can of starting fluid and can't find a leak, including carb base, vacuum lines, and intake manifold to head connection. Blocking off the PCV line helps a little, but not enough. Either I'm missing a big source of unmetered air, or its just not getting enough fuel in the main circuit. I took the bowl off, and everything looked clean, no obvious gunk. The main jet was clear. I tried stepping up two jet sizes and it didn't make much difference, if any. There may be a clog somewhere deeper in the carb, but who knows where. I installed a new fuel filter before firing it up, and the gas in the tank isn't that old.

The kicker is that it was behaving fine in the garage yesterday (in neutral). Then when I was idling it today to check the trans fluid, it suddenly died and started exhibiting the above symptoms upon restart. The only thing I know that has changed is the exhaust manifold choke stove stopped working, but I don't recall the engine running with very much choke engagement before that. You don't just develop a huge vacuum leak that suddenly, and I can't find a leak anyway, so I'm thinking something in the main fuel circuit suddenly got partially clogged.

I was planning to take this thing on a road trip this weekend. Anything else I should check before tearing this carb apart, or wiring the choke plate partially closed and dealing with a very narrow band of usable throttle?

Re: Carburetor Woes

sounds go me its clogged up. Had simiar issue with mine. Till it it hit the track, then it fixed it self.
I had partial success running fuel system cleaner, it would ungunk the carb a bit.
Does the engine response to fuel mixture screw?
also, what the vacuum you pulling at "idle"?

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Re: Carburetor Woes

Just throwing this out there, fuel pump/pressure, or just gunked up. My 2c

Re: Carburetor Woes

Disconnect each vacuum line and plug it at the carburetor one by one. This will rule out the vacuum lines.

Try blocking off the carb vents.  This will draw extra suction on the fuel side and sometimes pull the dirt through.

On most carbs, the main jet does not flow any fuel at idle. There is a separate idle circuit. This circuit may be plugged. There is usually an idle mixture screw. May want to make sure it is set properly. Perhaps someone playing a joke and turned it down.

Re: Carburetor Woes

rlchv70 wrote:

Disconnect each vacuum line and plug it at the carburetor one by one. This will rule out the vacuum lines.

Try blocking off the carb vents.  This will draw extra suction on the fuel side and sometimes pull the dirt through.

On most carbs, the main jet does not flow any fuel at idle. There is a separate idle circuit. This circuit may be plugged. There is usually an idle mixture screw. May want to make sure it is set properly. Perhaps someone playing a joke and turned it down.


Look at this!  Most carbs have more then two circuits. There is the idle circuit and then there's another to cover the change over from the idle to power circuit. Many carbs use the idle circuit to add fuel well beyond idle speed. Some carb have a small slot in the venturi where the throttle plate sits. Make sure that it's clean as well.

Re: Carburetor Woes

I think most Holleys have an economizer valve that helps the transition between idle and the main circuit.  If you close the idle mixture screw all the way and the engine still runs, you have a blown valve. They're like twelve dollars at the auto parts store.
...and that concludes all I know about Holley carbs.

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Re: Carburetor Woes

Turns out the idle circuit was clogged from sitting. I pulled the idle mixture screw and blasted it with B-12 chemtool with the engine running. It runs fine now!

I didn't realize the idle circuit was still responsible for such a significant portion of the fuel flow at higher RPM.

Re: Carburetor Woes

Just to take advantage of a teachable moment...
The choke acts on the main circuit by dropping the pressure at the booster
Venturi, since the vacuum of the engine is pretty cut off by the throttle blade. That then activates the main circuit,
adding fuel for cold enrichment. Not all carbs use the idle circuit so deep into the load/rpm range, but 1-bbl carbs
on cars seem to be a likely candidate for this.

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