by David Schwarze
First, check your plugs, wires, and points. Set everything to specs. Check for vacuum leaks. Make sure your timing isn't too far off. No point fiddling with the carbs till this is done - it will be time wasted.
Now turn two side screws in then 2.5 turns out as described. Start bus. On left carb, turn in the side idle adjustment screw until the RPM starts to drop. Stop and back it out 1/2 turn. Do the same on the other carb. If these screws are misadjusted (too far in) it will cause the engine to backfire during deceleration (my experience).
Now pull all four wires at the coil one at a time and make sure the RPM drops an approximately equal amount for the left and right banks of cylinders. If so, you have the idle mixture set fairly well, and your idle shutoff solenoids are working (ugh). If you have a dead cylinder at idle it's probably either the mixture screw overtightened or a bad shutoff solenoid.
Now fiddle with the small adjustment screw on the top of the left carb. On my bus, it doesn't seem to make much difference where it is set, so I just set it 2.5 turns out and leave it. The bus runs fine.
One other thing I have found with the dual carbs: it is VERY important that the linkage is set to open them at the same time. Tug slightly on the accelerator cable where it attaches to the crossbar (don't grab any part of the crossbar though!) and watch the stops on each carb. Make sure that the linkages are beginning to move at exactly the same time. Also check that the accelerator pumps are being actuated at the same time. Adjust the linkage if necessary. For me, this almost always makes a HUGE difference in driveability. The carbs being slightly off makes your part throttle acceleration suffer badly.
"Also one more problem, carb related. When cold, the engine idles
nice and fast and smoothly, but after its warmed up, and I kick down
the choke, it idles too slowly and backfires (popcorn sound) alot. I
have repeatedly set the idle to 900rpm, at operating temp, but it
keeps returning to the slow idle. Is it possible that when my
mixture is set correctly this will straighten itself out, or is there
something else amiss?"
I had a similar problem where when warm it would idle slow. Flooring the throttle briefly usually brought back the high idle. In my case it was the large solenoid that shuts off the air to the idle circuit. It's in the very front of the left carb, horizontal. It was sticking closed. I have several of the solenoids and they ALL stick. I finally removed it, put a washer over it, and reinstalled it (so it's about 1/8" farther out now). No more slow idle.