Originally Posted by yragat
The engine is a 390 Cadillac it always ran good,but had ton's of blowby and would just spit&sputter from stop to WOT .So had a total rebuild done all new internals and added a performance cam. 478-786 lift 268-274 duration 114-106 lobe center. I'm running 2 Stromberg 97's simultaneously,have pertronix igniter 3 ignition and headers. It ran like a 6 cyl!! Same thing from stop to WOT and even going down the highway if you stomp on it would spit & sputter then try to take off,but not too good. So I took it out yesterday and drove it a little,then advanced the timing a little--it helped ,kept advancing and the more I did the better it ran. Had to stop because Vac can was hitting manifold. Today I checked where the timing was. At 900 rpm with vac disconnected it was at 18 btdc --rev it to around 2000 ad goes up to 26 hook the vac up and rev it and it goes to 43. Starts fine even when hot and is not burning lean----should I leave it there or is that too much timing???I think if I would advance it a little more it would be scary fast. Oh, factory timing was 5 btdc .
With the vacuum advance disconnected and plugged and the engine idle at 900 RPM, you are already getting some mechanical (centrifugal) advance. This means that when you see the timing at 18 degrees at 900 RPM the actual timing at idle is lower. This is proven when you say that the timing only goes up to 26 degrees @2000 RPM. This is not too much timing and in fact may not be enough with the cam you are running.
Set the idle to 800 RPM or less. This is so no mechanical timing is being introduced. Be sure the vacuum advance is dis-connected and plugged off. Set the timing to 18 degrees BTDC at idle. Increase the engine RPM to the point where the timing no longer advances when looking at the mark with your timing light. Make a note of the RPM when that happens. That figure should be in the 36 degree range (+ or - 2 degrees) That will be your Total Mechanical Timing. For best street performance it should happen between 2500-3000 RPM. If you were to have the vacuum advance hooked up and performed the same check, the timing numbers will be about 10-15 degrees higher. Do not set the timing with the vacuum advance hooked up. If you do, the overall timing will be retarded and performance will be as you described above. In a word, lousy. This is because the vacuum will drop to 0 when going to WOT under load. Hook the vacuum advance to full manifold source.
You mentioned that you were unable to advance the timing any higher because the vacuum canister would hit the intake manifold. This can be corrected by lifting the distributor up and then rotating the distributor shaft one tooth in the opposite direction and then re-installing the distributor. You will have to rotate the oil pump shaft as well to get it to line up with the distributor drive.