Originally Posted by tomthecomic
I think I have a bad or sticking vacuum advance. When I suck on the hose I have to suck really hard to get it to move. No you can not come over. The first time I did this it would not move I moved it with a screw driver and now it moves but takes allot of effort. It does not to have a leek. The reason I ask is at 5600-5700 rpm it looses power pops or backfires and then the power comes back and goes to 6300 no problem. It only does this in gear and not when I rev. The second thing is and this has been going on for some time. I set the idle at 1000 (800 in gear) drive it and some times it is at 1300 rpm when I get back. Next week it is at 1000. Any ideas?
First your carb throttle shaft could be worn and binding the butterflies.
Your irregular idle could be caused by the centrifugal advance not coming back to zero easily. Did you check the spark advance while this is happening?
The centrifugal spring needs to be a tad tighter so the advance does not begin
until 1300 rpm. Using one tight spring and one loose spring set with slack at the post to delay its tension. This gives the overall tension of both springs the same as 2 equal springs so the total advance comes in by the same rpm, but the intial beginning starts later. This should stop the irregular idle if it is advance related.
Vacuum advance cans come in different total degrees of timing, some adjustable, and they also come in different vacuum values for total advance, and different vacuum values to begin the advance. This has to be coordinated to your engine combination.
A person can suck about 3" of vacuum. Most cans don't move at that value. Screwdriver movement should take a lot of effort. There is a stout spring in there for vacuum to overcome.
Popping. Are you sure you are not sucking air at the fuel tank on hard acceleration, and the carb gets temporarily lean when the air bubble gets there. It also sounds like wires or ignition module or coil breaking down.