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 .
Before going further, the vacuum advance is considered separately from the "total timing" when setting up a performance advance curve.
You need to set the timing curve up correctly. To do that you need to confirm TDC
is correct on your damper/timing tab. If you don't have a timing light that shows advance, make a timing tape
to use for setting up the curve.
Most of the improvement you are feeling is coming from an increase in initial timing. That is not the least bit unusual when a performance cam is used. But if you use a lot of initial timing, that may mean limiting the amount of mechanical advance to keep the total advance (initial plus mechanical) from exceeding 36-38 degrees. There are advance weight and spring kits (don't use the weights, just the springs) to adjust the RPM when the mechanical advance is all in.
Once the timing curve is sorted out you can go about setting up the vacuum advance. You may find using about 10-14 degrees of vacuum advance is about right. There are adjustable vacuum advance cans for the early points-type distributors that can help you set up the vacuum advance. You will often see as much as 50 degrees BTDC at light throttle cruise conditions. Any more is unnecessary, and if the engine surges w/that much vacuum advance it can be lowered to suit the engine and conditions.
On the HEI distributor page in the wiki there's a section on tuning the advance curve
that will help.