Originally Posted by Fasazu
I have finally installed the 355 sbc in my 74 Nova. It's a mild rebuild 9:1 compression, 882 heads, comp cam dual pattern 427/ 454 lift, edelbrock performer intake w/ edelbrock 600 cfm carb. HEI, jegs ceramic coated headers with 2.5" dual exhaust. What would be a good point to start at with base timing? Where should I start with total timing? It will be a street cruiser, just want the most go without effecting reliability.
Secondly I set the timing at 12 deg advance. I noticed the timing mark bounced between 10 and 12 degrees at idle. What could cause that? I used the original hei that came with the motor. I did install new cap and rotor, new wires, new plugs.
First check it with the vacuum advance disconnected and the port closed to eliminate that as a potential cause. The next more difficult to isolate is the mechanical, which typically at idle shouldn't be engaging but here we make an assumption that this is true, but since this unit is not restrained vibrations, soft springs, excessive idle speed, etc, could be causing it to shake around and affect the base setting.
Worn bearings or drive gear of the distributor can affect the steady timing. The bearings, even with an HEI, can let the shaft orbit which will change the timing relationship of the reluctor on the shaft to the sensor on the distributor's housing. An excessive thrust clearance between the shaft and housing can allow the drive gear to move up and down in its engagement with the cam's gear, this will, also, change the timing of the distributor shaft.
Then consider the cam drive itself, two gears connected with a chain. There is plenty of opportunity for this to shake about, especially when being excited by the fairly slow idle speed which allows the crankshaft to slightly speed up and slow down as power strokes are applied and die away.
So there are a lot of possibilities that will cause the base timing to wander about a little bit. I really don't think a degree or two is enough to worry about. But it does demonstrate why racers go to crank trigger ignitions where possible.