holley 1920 carb question

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bbrroowwnn

bbrroowwnn

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I have a holley 1920 on a slant six and it runs beautiful, starts beautiful hot and cold, good acceleration etc...but the damn thing won't idle worth a crap.....So I purchased a factory rebuilt 1920 and put it on so I could rebuild the original and still drive the car....and the new onw idles great and accelerates great, runs good, starts good cold, but hot it has to crank about 20 revolutions or so until it starts and then it stumbles for awhile into it clears out..... I did the rerouting of the fuel line over the valve cover to help with heat soak but it still won't start hot worth a damn....So, I figured well when I get my old one rebuilt I will just put that one on and be all set....Well , I got it totally rebuilt and it does the same thing as it was originally doing....runs beautiful, starts beautiful hot and cold, good acceleration etc...but the damn thing won't idle worth a crap....When idling bad, if I hold my hand over the throat the idle picks up and smooths out.....If I turn the idle up with the screw it also idles clean and clear, then when I put my hand over the carb it wants to die out......So It is something in the carb.....because the other carb idles great....I tried backing out the idle air mixture screw and that helped a little but then same thing....I took screw out and blew air back in the idle mixture screw and was OK for awhile then started acing up again.....Is there any other reason why it wouldn't idle? The brass seat of the screw was marked up a little bit so I have another one on order in case that is the problem....suggestions? thanks
 
It sounds like a vacuum leak to me. Try spraying carb cleaner in short bursts around suspected areas (gaskets, etc). The carb cleaner will bubble and get sucked into the leak. That bubbling is the sign of a vacuum leak that is causing your idle problems. Your idle might pick up a little bit as carb cleaner is very combustible.
 
Yes. My first suggestion is convert to a late-model(77-79) Holley 1945. It's a better design with easily acquired parts and kits. You do need the matching electric choke unit for a 1945. There are those who love either but not both carbs, but in my experience the 1945 eliminates the headaches the 1920 causes.

Regarding the pair of 1920 carbs you are trying to make work:

The one that won't idle is probably dead. Those use a metering block and small passages in the carb that can corrode internally and according to most, you have to have the powers of Harry Potter to ever get them cleaned out again. I would move on from that one and forget it existed.

The second one that will actually idle sounds to me like it is not "venting" after it sits and heat soaks. In other words, the gas vapors that accumulate in the float bowl because of fuel perc have nowhere to go, so they hang around and get sucked into the intake when you try to start it, causing an overrich mixture until the engine pulls enough fresh air in.

If you are referring the '72 in your sig, there should be a metal or plastic nipple on the top front of the carb where the hose to the charcoal canister connects. Adjacent to that is a lever that acts on a small valve that permits fuel vapor to flow to the charcoal canister at closed throttle but not otherwise.

You can't plug that hose up. That'll cause the situation I just described. If you still have the canister, make sure it's connected properly. If no canister, then just run a length of line from the carb to a spot where the excess vapor and possible miniscule traces of fuel can safely exit.

The reason I went into this long sermon is that way back when, I had a friend with a '71 Demon slant car. In that year, cars with Federal emissions routed the carb bowl vent line into a port on the fuel pump because they didn't have charcoal canisters yet on those cars. It worked terrible and his car spun and spun like you describe on each and every hot start.

It also goes without saying that if the bowl vent isn't your issue then check your float level and also make certain the choke plate is totally open.

Lastly, I haven't had a Mopar that started quickly when hot since the gas turned into the ethanol junk. It just vaporizes too easily and causes problems.
 
Well like I said, it is definitely a carb issue and not a vacuum leak such as an intake gasket leak or carb gasket leak as both the problems get solved by swapping out the carbs.....I am going to try a new air mixture idle screw in the one that won't idle and see what happens.....As this is the one that starts great , even when hot.....It doesn't even turn one revolution when hot or cold and it fires right up strong....As far as the one that idles great but won't start hot, I do have the charcoal canister on it and the bowl vent line is connected to it.... Your quote as follows

"The second one that will actually idle sounds to me like it is not "venting" after it sits and heat soaks. In other words, the gas vapors that accumulate in the float bowl because of fuel perc have nowhere to go, so they hang around and get sucked into the intake when you try to start it, causing an overrich mixture until the engine pulls enough fresh air in.

If you are referring the '72 in your sig, there should be a metal or plastic nipple on the top front of the carb where the hose to the charcoal canister connects. Adjacent to that is a lever that acts on a small valve that permits fuel vapor to flow to the charcoal canister at closed throttle but not otherwise.

You can't plug that hose up. That'll cause the situation I just described. If you still have the canister, make sure it's connected properly. If no canister, then just run a length of line from the carb to a spot where the excess vapor and possible miniscule traces of fuel can safely exit."

Yes, it is a 72 dart...has factory charcoal canister....bowl vent hooked up and bowl vent valve is working properly.....I hooked it up exactly as the carburetor that starts great when it is hot does.....That is why I know it is something in the carb itself....The routing of the hoses is exactly the same....I am stumped
 
Best guess is that the regular idle mixture feed hole is full of dirt and needs serious cleaning. Putting your hand over the carb increas the vacuum signal and that sucks more fuel mixture out. Opening the idle speed screw is likley moving the idle slightly into the idle progression slot and fixing it that way.

So I would pull it and work on the idle mixture hole under the throttle plate, work also on cleaning out the idle air bleed at the top of the throat, and when done, do a wet float check.

Check here for a good idle circuit diagram: http://www.slantsix.org/forum/viewtopic.php?t=24736
 
nm9stheham said:
Best guess is that the regular idle mixture feed hole is full of dirt and needs serious cleaning. Putting your hand over the carb increas the vacuum signal and that sucks more fuel mixture out. Opening the idle speed screw is likley moving the idle slightly into the idle progression slot and fixing it that way.

So I would pull it and work on the idle mixture hole under the throttle plate, work also on cleaning out the idle air bleed at the top of the throat, and when done, do a wet float check.

Check here for a good idle circuit diagram: http://www.slantsix.org/forum/viewtopic.php?t=24736
Click to expand...


good info...thanks....
 
Hi, related to this, from diagrams of holley carbs, there is an air emusifier tube in the jet well that is usualy vented to the venturi throat or up to top. If I look into my 1920 there is the accell pump squirt hole and right next to it a dimple that might have been (?) a tiny air inlet hole. ?clogged over? Probing these passages , jet block side with block off, with wire, ones finds the squirter but the other one takes a 90 to where? Turns sort of upward. I think it is the air bleed inlet? If the air bleed for idle is blocked , you get no emusifying so badly screwed up idle? where does idle air intake for emulsifier come out on the main body ? Also related, comparing two 1920, the throttle plate on one shows a bit of vacuum advance port ,that will advance ignition. Not supposed to be open at idle, fit of plate etc is all factory, looks good but not covering advance port--on one. . Its supposed to be covered--- right-- at idle? if not ----advance moving at idle could cause erratic rpm.

On discussion of bowl vent , it is vented anyway direct to air into venturi top , (that pressed in metal pipe cut at 45 goes straight right into it !) , into the chamber where the poppet valve opens at idle , and the larger line from cannister. So not having charcoal thing connected does nothing to anything ? It looks like idea of all that is when engine is running a slight vacuum exists in there sucking vapor into venturi, from bowl and from cannister . Or runs other way air and vapor into cannister?---- Note that there is second real vacuum line to cannister at bottom of carb, , maybe that sucks out both vapors, and gas tank too. stored in cannister from bowl (poppet is closed driving along) but end of day, the upper large pipe/ hose is just going to air cleaner air in throat ,all the time . With poppet closed , engine off, hot soak vapors would end up in carb, gets pulled in when you start , or back to cannister, once vacuum...in smaller line at bottom

One more thing, one of my 1920 "rebuilt" had power valve gasket on top of power valve, I looked into that , that is wrong (see sheets) if you do that power valve is set too deep into bowl, it is holding open the enrichment balls. Also 58 jet is like 100% better.

Sorry for bold, not sure why
 
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