gmcfan: Could be one of several problems, or possibly more than a couple going on at the same time. You will have to check to eliminate the sources. Trouble could be from late valve timing (chain), but unlikely. You can verify with a degree wheel. The most likely source of your troubles, are the items that have been adjusted/disturbed/repaired as of late.
Possible vacuum leaks, in either manifold gasket area, or in carb body/manifold mounting. This would cause the off idle"loaping",and the backfiring, and would make tuning impossible because of air leak, leaning mixture at idle, then richen as RPM increase (as vacuum lessens). So the mixture "bounces" all over the place, as the air to fuel ratio cannot balance.
You stated that PO had distributor repaired, possible troubles here also. Need to check advance weights for sticking/binding, and condition of springs. Check vacuum diaphragm for leaks and function. Also check breaker plate for free movement with advance. Check all electrical connections also.
With cylinder leakdown test, and vacuum tests, you should be able to determine if you, in fact, have vacuum leaks, and be able to locate them.
Need to set dwell, then time with timing light.
If you have vacuum leaks in carb body, can/will throw off balance idle mix, both ratio and volume and this will make run lean, and hard to idle. Check fit of horn gasket to float bowl housing for nicks, burrs. Need a good seal. May have to disassemble, place on sandpaper/on glass, lightly smooth burrs/irregularities. Coat gasket with bootgrease, or chapstick for better sealing.
Also if you can get to shop with scope, can set up plug firing in parade/stack patterns to verify spark firing/voltage amounts and check condition of ignition components, to verify all are good working order.
Repost your findings. Good Luck!
Think Chevy!
:rolleyes:
