Time to remember how to tune a 1920 1 bbl carburetor

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timk225

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I've got the /6 in my 1973 Duster running. I took my time with it. I got rid of the CDI ignition, and today I went over the vacuum hoses and found a few leaks or just plain unplugged hoses. I replaced the distributor vacuum advance hose, and the hose going to the fuel vapor canister purge port. The PCV hose and the carb vent to vapor canister hose looked okay. I also disconnected the vacuum hose going to the power brake booster and tried to suck through it, it seems to not be leaking.

I also bought a timing light for the first time in 20+ years and checked the timing. It's got about 4 degrees advance with the distributor vacuum advance hose unplugged, and around 16-20 with it plugged in. I was cheap and got the #3551 timing light at Autozone, they didn't have the #3555 with the dial back knob option.

So the engine runs, but once it warms up, getting it to idle right is a problem. It will only idle around 1500 rpms or so, no less. I tried the needle valve from 1-1/2 to 3 turns, didn't seem to matter. I've got the idle adjustment screw turned in about 5-1/2 turns or so, making sure it is off the fast idle cam. But once it is idling fast, if I turn the screw in only 1/4 of a turn, it stalls out.

And it is not fun to have to do the old move of pull the throttle open by hand to put fuel in the intake, reach inside the car to crank it, and as soon as it starts, get your hand back to the carburetor FAST, to pump the throttle more or it will die.

Any ideas where the problem may be, that it will not idle any slower once warmed up?

I was able to drive the car around the storage lot briefly, but it takes some fast moves to keep pumping the gas and put the shifter in D or R during the brief second where engine rpm slows down, then keep pumping the gas to keep it running.

This Holley 1920 is the #7031 version. I noticed during the carburetor rebuild I did that the main fuel tube leading into the main bore was shooting carb cleaner out as I sprayed through it, but from only 1 or 2 holes, I wonder if one or more is still clogged. And how to clean it?

I used 90 octane ethanol free gas from Sheetz, what we used to refer to as "GAS".

I paid attention to which way I put the carb to intake gasket on, as it is not the same both ways or flipped upside down. But the gasket was close to blocking the idle adjustment screw, I wonder if I might need to trim it a bit so the idle screw can function better. Should have done that the first time.
 
Spit balling here. What vacuum do you have? Haven't had a slant that year. Does it have a stuck EGR valve or bad PCV valve? Also you can put your key on and jump the starter relay with a screwdriver to start it at the engine compartment.
 
Vacuum is around 16 to 18 when it was idling, before it warms up. I haven't checked the PCV or EGR yet, but they are old and sitting for years, so it wouldn't surprise me if they were not in perfect condition. I remember the old screwdriver trick, just hadn't done it yet.
 
timk225 said:
I've got the /6 in my 1973 Duster running. I took my time with it. I got rid of the CDI ignition, and today I went over the vacuum hoses and found a few leaks or just plain unplugged hoses. I replaced the distributor vacuum advance hose, and the hose going to the fuel vapor canister purge port. The PCV hose and the carb vent to vapor canister hose looked okay. I also disconnected the vacuum hose going to the power brake booster and tried to suck through it, it seems to not be leaking.

I also bought a timing light for the first time in 20+ years and checked the timing. It's got about 4 degrees advance with the distributor vacuum advance hose unplugged, and around 16-20 with it plugged in. I was cheap and got the #3551 timing light at Autozone, they didn't have the #3555 with the dial back knob option.

So the engine runs, but once it warms up, getting it to idle right is a problem. It will only idle around 1500 rpms or so, no less. I tried the needle valve from 1-1/2 to 3 turns, didn't seem to matter. I've got the idle adjustment screw turned in about 5-1/2 turns or so, making sure it is off the fast idle cam. But once it is idling fast, if I turn the screw in only 1/4 of a turn, it stalls out.

And it is not fun to have to do the old move of pull the throttle open by hand to put fuel in the intake, reach inside the car to crank it, and as soon as it starts, get your hand back to the carburetor FAST, to pump the throttle more or it will die.

Any ideas where the problem may be, that it will not idle any slower once warmed up?

I was able to drive the car around the storage lot briefly, but it takes some fast moves to keep pumping the gas and put the shifter in D or R during the brief second where engine rpm slows down, then keep pumping the gas to keep it running.

This Holley 1920 is the #7031 version. I noticed during the carburetor rebuild I did that the main fuel tube leading into the main bore was shooting carb cleaner out as I sprayed through it, but from only 1 or 2 holes, I wonder if one or more is still clogged. And how to clean it?

I used 90 octane ethanol free gas from Sheetz, what we used to refer to as "GAS".

I paid attention to which way I put the carb to intake gasket on, as it is not the same both ways or flipped upside down. But the gasket was close to blocking the idle adjustment screw, I wonder if I might need to trim it a bit so the idle screw can function better. Should have done that the first time.
Click to expand...

As long as you don’t have any vacuum leaks, try this.

Screw your idle mixture screws in lightly until they stop and count the turns.
Take em out and shoot some carb cleaner, WD40 or whatever into the open holes and blast air into them both.
The liquid will help carry junk out of the idle circuit.
Put the idle screws back in and and bottom them out again.
Start with both 1.5 turns out go from there.

I have one car with that carb that has a screwy idle that I need to do the same with.
 
I messed around with it some more today, made minor progress at best. I took the carburetor off and trimmed out the gasket around the one hole I thought might be an issue. Turns out it is the hole leading to the fuel vapor canister purge port. I screwed the one idle screw all the way in, felt it on the inside of the barrel, then turned it back out 2 turns.

After getting the engine running, I spent some time checking for vacuum leaks. I blocked off the tiny EGR vacuum port, which didn't change anything, then used Vise-Grips to pinch the vacuum reservoir line shut. No difference there either.

I did get it to idle a little lower, maybe 1200-1300 rpms or so, vacuum gauge showed around 15, but still the same issue, turn the throttle stop screw down another 1/4 turn and it dies. I tried changing the idle screw from 2 to 2-1/2 turns out, didn't make a noticeable difference there either.

That flapper valve in the exhaust manifold seems to work, it spun freely, but the spring was broken, so it did not snap to one direction or the other. I tried with it turned all the way one way then the other, no difference.

The one modification I had planned for the car was a Super Six Carter BBD setup, maybe I should just switch it over if I can find all the parts in one spot. Aluminum intake, throttle cable bracket, kickdown linkage, a Carter BBD carburetor and air cleaner, etc.
 
Carburetor operation and repair manuals and links to training movies and carb repair/modification threads (with a lot of 1920 content) are posted here for free download.
 
I read the articles and looked at the diagrams and learned some things. In the carburetor main body, I didn't do anything to shoot cleaning fluid down the high speed air bleed or the idle air bleed, I'll try blasting some carb cleaner back and forth through that. And even though I cleaned out the metering block pretty well, I'll hit it some more. Maybe I'll even get some acetone and let the whole thing soak in that for a day or two. I know someone who has an ultrasonic cleaner, maybe we will try that as well.

I know the accelerator pump nozzle works, I had fuel coming out of it pretty well when I was running the engine and when I was emptying out the carburetor.

For the main discharge nozzle out in the throttle body, is there only one point at which fuel comes out of the pipe? Judging by the shape of the extra pipes in there, it looks like it could have multiple points of exit, and if some are clogged up, that could be a problem also.

Holley should have designed this thing better, made it easy to take apart and clean every little piece. Surely they knew that in 50+ years, some fanatics like us would be trying to rebuild them.......
 
Update. I went out to the carburetor parts and got out the can of carburetor cleaner spray. I blew through all the holes in the carburetor body, and they seem to be free and unclogged. The idle air bleed hole had no problem blasting a solid stream of cleaner spray out the transfer slot and the idle adjustment screw hole. The high speed air bleed hole is very tiny, but it looked like liquid was getting through it also. The main discharge nozzle also had a solid stream of cleaner spraying through it.

So if anything is clogged, I'm guessing it is in the metering block. But when I was spraying through the block I thought it was okay as well. Who knows. I cut the top off a plastic bottle and put the metering block in it to soak in vinegar. The choice of vinegar over acetone was because the vinegar was in the kitchen, whereas the acetone was a mile away at the nearest auto parts store. I'll let it soak and see if that helps anything. Then maybe hit with acetone as well.
 
Update. After soaking in vinegar for almost 2 days, I took the metering block out, cleaned it, and blasted it every way possible with carb cleaner through all the holes. While drying it off, I was getting a gray colored residue, similar to what anti-seize compound looks like. I guess the vinegar was strong enough to affect the metering block casting a little.

Anyway, I put the carburetor back together, and went and put it back on the engine. Once I got fuel back into it, it was running better and idling a lot more normally. I drove it around the storage lot for about a half mile, just off idle. The carb needs a little more tuning, but it is doing much better than before.
 
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