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Discussion Starter · #1 · (Edited)
I was reading the manual on how to adjust the fuel mixture screws on an Autolite 4100. The manual says to "turn the screws in (leaning it out) until the engine starts to run rough. Then back them out (making the mix richer) until the engine starts to "ROLL". Then turn them back in until the engine smooths out. So, my question is, "What do they mean by engine starts to ROLL?
 

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What manual is that?
 

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If you have a vacuum gauge, hook it up to a vacuum source on the intake manifold. Adjust each mixture screw one at a time until you achieve the highest vacuum reading. You may have to adjust each screw multiple times to fine tune.
 

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Woodchuck's 4100 idle mixture adjustment 101.....

1. Turn each mixture adjustment screw in until VERY LIGHTLY seated.
2. Turn each mixture adjustment screw out exactly 1-1/2 turns.
3. Connect vacuum gauge to intake manifold source (preferably a tee on the PCV hose). DO NOT use a vacuum source tapped into an individual intake runner! If no vacuum gauge is available, connect a tachometer as directed by the instrument's manufacturer.
4. Turn each screw, alternately and no more than 1/4 turn at a time, in whichever direction results in an immediate increase in manifold vacuum and/or rpm.
5. Continue turning in that direction, while maintaining curb idle speed as close to specification as possible, until vacuum and/or idle speed begins to decrease.
6. Return to the last "good" setting.
7. If you have to have more than 1/4 turn difference between the individual mixture screws you have other issues that need to be corrected.

PS: Don't forget about the Secondary Idle Adjustment, which must be made with the carburetor off as it faces up from the base of the carburetor.
 

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On the above, be careful with tightening the idle screws too hard so they don't strip the threads in the carb body. @Woodchuck is one of the VMF's best and it shows.
 

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I'm guessing when they say "roll" they mean the rpm will start to drop. Maybe that means rollback from the highest rpm. The problem with what they said about turning the screws in until it runs rough does not take into account if the screws were all turned out an equal amount. We will discuss that.

All non-feedback carburetors (computer controlled) idle air fuel ratios are adjusted exactly same. Except that you can have one, two or four A/F mixture screws. Some four barrels only have two A/F mixture screws, some can have four, Gently bottom the screws in and mark the top of the screw with a dot of whiteout so you can accurately count the turns. Then back them out 1 turn, 1 1/2 turns doesn't really matter, just so it idles.You just want to make sure you are keeping them all turned exactly the same amount.

Propane is your friend. Crack open a propane bottle just a little and pass it over the carb. If the idle goes up the engine is lean if the idle goes down the mixture is too rich. If the idle doesn't change your close to where you need to be. If its lean turn all the screws out an additional 1/4 turn and test it again with the propane. You will reach a point where the propane no longer raises the idle. Your not done yet. I will discuss it further below.

I'll give you a little propane tune-up history. Back in the day we were suppose to adjust every single carburetor during a CA State Smog Test. The State mandated that you have a special propane enrichment tool that screwed onto a propane bottle. You don't need that stupid tool just a propane bottle but once a year a State Inspector would come by the shop and ask to see if we had our propane enrichment tool. It was part of the tool requirements for having a CA Smog Station License.

I can tell you that the State wanted you to adjust the A/F mixture using propane until you reached the highest idle rpm but then they wanted you to turn the mixture screws in until the rpm dropped 50 rpm. Testing it with propane should raise the rpm 50 to verify it was properly adjusted to the lean side. That was to reduce emissions at idle.. The problem was the engine was not happy adjusted that way. You would get a complaint from the customer if you adjusted it that lean, the way the State wanted you to do. It might run rough adjusted like that, it might even stall coming to a stop and it would absolutely hate it if you turned on the AC compressor. That's because if you adjust the mixture with propane on a hot day its not the same as adjusting it on a cold cold day. You need to adjust the mixture slightly rich to compensate for temperature and barometric change with the weather. That was not the State's problem but it was yours when the customer came-back wanting to know what you did to their car?

Getting back o adjusting the A/F mixture. You definitely want it adjusted slightly to the rich side to deal with those environmental issues that can effect the A/F ratio. You will have less problems if it tuned slightly towards rich than slightly lean or even right in the middle. I would adjust the screws just until the propane makes no difference in the rpm but not past that point. Then give the screws an additional 1/8 to 1/4 of a turn to richen it just slightly and you should be right in the ballpark. There is no better way than using propane to adjust your idle A/F mixture.

I looked and there is not one YouTube video showing how to adjust a carb with propane. Only videos on looking for a vacuum leak using propane. When I was helping write the questions for the State Smog Test, 15-year's ago, the sub jest came up about should we have any carburetor test questions. They asked the ten of us, when was last time any of you even worked carburetor? Then he said, no more carburetor questions. One of the guy's on the team, who also taught the two-year smog test certification course at a college said he had students in the class that had never even ridden in a automobile with a carburetor.

All of sudden everybody wants you to think a car can't drive with points or a carburetor. I can tell you I have seen just as many come in on the tow truck with fuel injection and electronic ignition as I did with a carburetor and points. When a carb or points start to have a problem they will usually still get you home. With fuel injection and electronic ignition there is usually no grey area. It either goes or it doesn't. I would be a fool to say that fuel injection and distributorless ignitions are not leaps and bounds superior. But you can drive across country without any problems with a carburetor and points in your distributor (if they are maintained properly).
 

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I found a propane enrichment tool for sale. For $136.00 you get an additional piece of hose and a momentary button to release the propane. You could save your $136.00 and just open the propane bottle a little and hold it over the carburetor but that is not as cool as getting to push the button.

OTC Propane Enrichment Kit - 7148
 
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