Having problems with the timing on a 390, When I have it set at 10 degrees the engine runs terrible, I have to have it way advanced for it to run decent but then it turns over slow when warm. The distributer shaft has some side to side play in it other than that I can see nothing else obviously wrong. Not sure what to do next

THANKS

 

First you need to confirm that 0° on the damper is actually TDC for the engine.

 

Yes, those pesky dampers can slip and put your timing off. Take out ALL the plugs so the enigne is easy to rotate by hand, and insert a piston stop tool in #1 cylinder. Gently turn the engine over with a wrench on the front crank bolt until it stops. Make a mark on the damper where the pointer lines up. Now rotate the engine the other way until it stops. Your damper TDC timing mark should be 1/2 way between the mark you made and where the pointer is now after the reverse rotation you did. It amy be off by a tiny bit due to timing chain slack, but your looking for something quite wrong, not a 1/2 degree problem

I'd also check your vacuum advance for a leaky vacuum hose, a sticking mechanism or a ruptured vacuum diaphragm. This may not help your idle a whole lot if you are using ported vacuum, but you never know.