zenith volume adjustment screw

[dgaley]

Can someone give me the right way to adjust the volume screw on my zenith carb. I think it is running rich. thanks in advance

[mechman]

This is probably far more info than you were looking for, but the problem you are describing is often not just an adjustment.

First, you want to make sure the screw is in good shape. With the engine shut off (I'd pull the carb and do this work on the bench), remove the volume screw and check the taper. It should be smooth and clean, with no indentations or corrosion. If it's OK, screw it back in GENTLY until it JUST bottoms out (DO NOT FORCE IT!! You'll damage the seat in the carb body). Then back it out 2 to 2-1/2 turns. Reinstall it and try it out. turn it in to lean it out, or back out to make it richer, 1/4 to 1/2 turn at a time, making sure it accelerates smoothly without stumbling off idle after every adjustment.

If the volume screw in your Zenith has a ring shaped indentation on the taper from being screwed in too far, you'll need to polish this out. Use a sheet of extra fine sandpaper on a piece of thick safety glass, and spin the screw (in a lathe, drill or what-have-you) and hold the glassed sandpaper against it gently until it's smooth again. Take care not to change the pitch of the taper...

If you have a rebuild kit, DO NOT just screw in the new one. Zenith carbs have a much finer thread on the volume screw than the later Solex-Zeniths (what the kits are made for). Putting a later screw in an earlier carb body will ruin the carb.

I have found that rich running in a Zenith is often a problem in the accelerator pump, that vertical polished aluminum piston under the top of the carb. First, this needs to move freely in the bore - polish both the piston and bore gently with a piece of crocus cloth until they are smooth. Second, in the bottom of the pump bore is a ball check valve under a spring wire clip. This needs to be clean and free to rattle around a little bit. If a piece of crud hangs it up, the carb will dump fuel at idle as if it were accelerating.

It's not hard to fix these carbs, but they have to be clean, and all of the components need to work smoothly.

[gudjeon]

Incorrect float setting (fills up the bowl too high) will cause it to run rich as well.:thumb-up: