The Stovebolt Site Forums The Shop Area Engine Shop Lean out air fuel mixture

Hi Everyone,
From the looks of my spark plugs I think I need to lean out the fuel air mix. I did a search here and consulted my Motors, and factory manuals and didn't get what I wanted. So could somebody give me the proper procedure for this, and how do I know if I have the optimum air fuel setting? Thanks.

What engine? Are you fouling out plugs, producing black smoke, engine running rough? If none of the above, I don't think I'd be too concerned about a rich fuel mixture. Leaning the mixture out too much will make your engine run hotter, and a TOO lean of mixture will end up wearing on your valves.

Pakrat48,
Sorry I am doing six things at once this morning. It is a 235 motor. I am getting some black smoke out of the tailpipe, the plugs have a dry black soot on them. The motor runs pretty smooth, but I can smell what I think is unburned fuel comming ouut of the tailpipe.

Jimmie,

If you have the correct carburetor on your engine I would suspect you need to spend some time tuning and tweaking as opposed to actually re-jetting something.

The carburetor float setting must be correct and the power valve should be free and not sticking. Your choke should be full 'open' when the dash knob is pushed all the way in.

The ignition system can contribute to your problem too. It must be in top shape with properly gapped points and good cap, wires and rotor.

Your stock carburetor was made to work well with the engine and shouldn't need to be modified.

This is how I see it.

Stuart

Jimmie, I would check the things that Stuart mentioned. A partially closed choke will cause the symptoms you describe. Check all ignition components as well. If all is well, and your air/fuel mixture does need adjustment, I've always backed out my mixture screw (with engine running) until you hear the engine start to "fall off" or run poorly. From this point start turning the screw in, counting the number of turns until it starts running poorly again. Then back the screw out 1/2 the number of turns that you turned it in. That should get you in the ballpark. Make sure points gap, and timing are adjusted properly also.

If it has the model B roch. carb. there is a vacuum passage in the bottom of the carb. It must have a passagway in the gasket to the intake manifold. If all these things the guys have mentioned are correct you might try putting about 2 sizes smaller main jet in it. These carbs are so simple it;s easy to experement with them.

Thanks for all your replys. I am fairly certain that I just need to tweek things. In the last 18 months I have installed new plugs, plug wires, points, condenser, rotor, coil, a remanufactured Carter Yf carb made for the chevy 235, new wiring harness, new battery, new ground strap, and last but not least a new ignition switch. And now that it has warmed up some here in Western Colorado I think I will go work on the old girl. Thanks again, Jimmie.