Holly 1920 carburetor adjustments?

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Ryansrt

Ryansrt

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I have a single barrel Holley 1920 carburetor, and everyone keeps saying 2 and a half turns out for the fuel/air mixture. Well I do that and it runs like crap, I have to do about 5 turns for it to run right. It never floods and starts every time so it's probably good?
 
There is no set number of turns for the mixture screw — that's not how this works. The correct way of adjusting the idle mixture is described below the carb operation and repair manuals and links to training movies and carb repair/modification threads in this thread.

That said, 5 turns out suggests there's a problem with your carb -- trash (dirt) in in the idle circuit, for example. Try the technique described here.
 
I went thru 4 Holley 1920 rebuild carbs from auto parts over decades before I got one that would idle smooth. I expect the metering block in all the others was clogged. They don't rebuild that and it isn't normally opened. Some guys here know how to blow them out w/ compressed air without taking the carb apart. You blow thru some of the holes on the underside. When clogged, the engine idles lean, which is why even all the way out you never find an optimum adjustment. To verify, close the choke slightly (richens) and if it runs better it was on the lean side. In general, too-lean makes it shake and too-rich makes it slow down but still run smooth, eventually pouring black smoke out the tailpipe and eventually die.

When lean, it will slow way down in going from N to D and stutter as you drive away from a stop sign. You will find that it wants much more spark advance at idle. Indeed, it gets into a positive feedback mode in P or N at the normal ~5 deg advance setting since as the engine speeds up it gets more advance, speeds up more, etc.

I never tried the later Holley 1945. I have a Carter BBS on my 1964 slant and it idles unbelievably smooth. When I first drove it home, I even actuated the starter once on the side of the highway, thinking the engine wasn't running (had died from "no charging", funky wiring). The small factory air cleaner helps keep it quiet. You can't even see the engine moving at idle, whereas my 1969 slant used to shake until I got the good 1920. The only smoother engine I have seen is a neighbor's 1950's Buick straight-8. You wouldn't believe his engine is idling if you didn't see the pulleys turning.
 
The 1920 has a plug you can remove on the metering block on the top that has a brass looking tube (emulsion tube?) That can be cleaned also. I did mine trying to chase a rough idle.

I have also never been able to get my mixture screw to kill the engine. I can turn it in to the point of it killing the engine. But never out far enough on the super rich end. And I have cleaned this carb very thorough. I just get it to the lean shake then set it 3/4 turn rich.

Turns out rough idle was the made in China rotor and cap. Now I have a supply of made in USA n.o.s. and some long tip echlin

Luckily now I have a few 1920 parts in my stash. Like the elusive float and spring.
 
cdnEHbody said:
I have also never been able to get my mixture screw to kill the engine. I can turn it in to the point of it killing the engine. But never out far enough on the super rich end.
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Not surprising. Idle circuits were redesigned starting in the late '60s to limit the amount of richness that could be dialled in via the idle mixture screw, no matter how far out it was turned.
 
It only smokes (black) a little when it's first started but when it's warmed up and I give it a good rev the exhaust smoke it barely noticeable. Sorry I'm 24 and not used to how these old cars are supposed to smell/act and the sounds they should and shouldn't make haha. Grew up in the wrong era sadly.
 
Ryansrt said:
It only smokes (black) a little when it's first started but when it's warmed up and I give it a good rev the exhaust smoke it barely noticeable. Sorry I'm 24 and not used to how these old cars are supposed to smell/act and the sounds they should and shouldn't make haha. Grew up in the wrong era sadly.
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I'm not sure what's in the links SSD posted, but click on them & read, then check the obvious like vacuum leaks at the carb-bad hoses-stuck PCV-manny gaskets-etc.
 
for other information about working on your car, you might also want to buy a copy of martin stockel's auto service and repair, as well as the factory service manual for your year of car. you should be able to find both of these on amazon or ebay, and you will find a wealth of helpful information in both. in fact, last weekend i had to pull BOTH out dealing with a question about the oil pump. the stockel's actually had some information and pics about my specific vehicle that the factory service manual didn't have.

if memory serves me, i would bet old slantsixdan would tell you the same. ;)
 
Ryansrt said:
... Sorry I'm 24 and not used to how these old cars are supposed to smell/act and the sounds they should and shouldn't make haha. ...
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The only real difference is in the controls. Some people retrofit these engines to modern computer control, even MPFI. They then run just like a modern engine. Mechanically, not much has changed. Indeed, the 3.8L V-6 in my 2002 T&C looks very familiar to an old Mopar guy. It has pushrods and stamped-steel rockers on a shaft that look just like my 1965 383. About the only difference internally is the roller lifters (after-market available for V-8's). Indeed, you can interchange the block of a Magnum 5.2L (until 2002) with a 1960's small-block V-8 (heads, transmission, and engine mounts bolt-up). In some ways, the mechanicals have become poorer (no forged cranks, ...). But, less options for a slant since it stopped production in the 1980's, so no MPFI manifolds and such available. The legendary AMC (Jeep) 4.0L straight-six is a better deal, having run from 1964 until 2006, so factory retrofits to fuel injection exist.

Perhaps a more important difference is that when the controls in a modern engine fail, the engine often doesn't run at all, which pushes you to repair it, as does the "check engine" lamp. Diagnostics are often easier too. When these old controls fail, it is more incremental, so you are tempted to just live with it. It is often hard to tell what is going on inside a carburetor. Adding an O2 sensor is a great diagnostic, especially a "wide-band" type.
 
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It runs fine, doesn't stall out and starts every time, I guess what I'm trying to get at is, are these old engines supposed to have black smoke come out of the exhaust when you rev them up when they are not fully warmed up?
 
Again: black smoke there should never be. And you should not rev up the engine right when you start it -- that's bad for it.
 
slantsixdan said:
... And you should not rev up the engine right when you start it -- that's bad for it.
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Definitely. In my wife's developing country in Asia, people start a cold engine, then rev it for 30 sec at 3000 rpm in N, then drive up and down the street real fast. Then it is ready to take the family out. Perhaps a mystical idea about waking the mechanical gods. I cringe whenever I see that. Any wonder why their cars only last 2 years. Many clueless gomers like that in the U.S. also.
 
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