The Stovebolt Site Forums The Shop Area Engine Shop Basic mixture setting ?

Hi all,
This is my first post. I have just purchased a 52 3/4 ton truck. The engine fires first time and i have rebuilt the carburettor. But i was wondering if someone could give me a basic setting for the mixture screw please as it seems to be running a little rich. The carburettor is a single barrel Rochester.
Any help would be most welcome.
Regards
Tim (UK)

A good start would be to seat the mixture screw all the way in (just 'til it stops) then back out 1 1/2 turns.

Be aware the mixture screw only affects idle, and to a slight extent, part-throttle mixture. Once the engine speed is up to normal cruise RPM, the main jet (non-adjustable) and the power valve affects the mixture. Adjusting the mixture screw only changes the smoothness of the idle.
Jerry

Welcome to the forums!

There are three major contributors to an engine running rich. These are, in order of the normal procedure in addressing:

(1) defective compression
(2) defective ignition
(3) defective carburetion

A major study was once done of vehicles diagnosed with "carburetor problems" by professional mechanics. The sample consisted of 1000 vehicles. The results:

(1) defective compression (2 percent)
(2) defective ignition (93 percent)
(3) 5 percent actually had carburetor problems.

If you are going to attempt the adjustment of the carburetor, Jerry's post above is worth rereading. The idle mixture control screw adjusts the mixture ONLY at idle. On the typical Rochester B, this setting should be from 1 turn to 1 3/4 turns from lightly seated.

Jon.

Thanks Jon, excellent information. I hope I remember it when I need it.

My method. Start with the screw about 2 turns out. With engine idleing slowly turn the mixture screw in till the engine starts to slow down. Then back the screw up till it speeds back up, usually about 1/2 turn.

What's good start for the Roch 2G? Seat both, back both out the same (1.5 to 2 turns), and then fine adjust each one in and out individually to optomize the idle?
I assume this is with a warm engine and choke wide open.
Cavebull

On your 2G they should both be set the same. Otherwise follow instructions above. If they need to be set differently it may indicate a mechanical problem.

Devin

Cavebull - the early 2G series with the short taper needles from 3/4 to 1 1/2 turns. The later 2G series (post-1968 smog emission) with the long taper needles from 1 1/2 to 3 1/2 turns.

Carter, Stromberg, and Rochester all went to a long taper screw with the implementation of smog emission. The longer taper gave a much finer adjustment; thus the difference in adjusment specifications. Since we don't do Holleys, I cannot comment on them.

Jon.

thanks Jon, just what I needed.
Cavebull

thanks for your help jon. Most appreciated. I have done the basic settings now on the carb, but have yet to run thru the ignition system.. this vehicle has just arrived in the UK from Minnesota. And is a stock truck. I have checked over the vehicle and absolutely everything i can find on it is totally original and untouched. Rebuilding the brakes right now, then we will get into the ignition and give her a clean up one piece at a time and see what we've got. I sure hope the old girl will run just fine as i wish to use this truck every day as a working vehicle.
Just for the record. I was amazed that even after so long standing on a farm for so many years, that all of the bolts came undone without too much trouble. I just wish the British had taken a tip from you guy's across the pond when we made old trucks. 9lol)
Regards
Tim

Thanks Jerry, much appreciate the information.
regards
Tim

Tim - the problem with the British trucks was that all the electrons leaked out of the Lucas ignition systems, and through electrolysis, glued the bolts in place ☺ ☺ ☺

Jon.

Jon, that's why the Brits drink lukewarm beer- - - - -they have Lucas refrigerators. Back in the days of the Jaguar XKE, it was possible to drown out a Jag by spitting on the sidewalk two blocks away!
Jerry

excellent reply jon !!!!!!!!!!!!!!!
The frightening thing is that you could be correct

Not often that a question begs for a double entendre as did this one.

Electrolysis can play an important part in corrosion. We often use an acetylene torch to disassemble items composed of dissimilar metals. The heat from the torch burns the oxygen from the corrosion, reducing the space required. This is a technique that should be practiced on junk prior to use on non-replaceable parts.

As to Lucas, they take their fair share of insult, all of which are probably deserved. But Lucas did invent a very important electric item..............the three position light switch: (1) off, (2) on, (3) flicker. ☺

Jon.