Thread: distibutor advance and ignition timing

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Ok here it is ....I have about 12 intial on the engine at idle with the vac cannister disconnected....I bring the engine up to 3000 rpm with the can still disconnected I get 22 degrees advance and finally reconnect the vac to the can and I get another 12 degrees for a total of 34 degrees.The engine seems to like this setting in nuetral in the drive way.
My question is the distributor was set up before a similar engine.....from these numbers it looks to me that the distributor is set up for about 16 degrees intial timing on the engine to get to close to 45 to 50 accumalative timing at part throttle or cruise.???????????

Thnx Denny ,I just went out and re-established what was actually going on....
Ok I have 10 degrees intial with no vac
I have 12 degrees mechanical
I have 12 degrees on the vac can
Soo at idle with the can hooked up I have 22 ,
When I mash it to the floor it drops the vac cans 12 and replaces it with the mechanical 12 so the total on the timing light never goes over 22.
Should I run more initial timing{ that looks like what this distributor was set-up for}?I have read you want about 36 degrees to 50 total.Am I leaving that much timing on the table or am I not understanding what I am reading ...{which may very well be the case } or am I on track?
Better yet if this was your engine ..and you had established this baseline as to what was happening,where would you go from here ?

Originally Posted by shawnlee28

Ok here it is ....I have about 12 intial on the engine at idle with the vac cannister disconnected....I bring the engine up to 3000 rpm with the can still disconnected I get 22 degrees advance and finally reconnect the vac to the can and I get another 12 degrees for a total of 34 degrees.The engine seems to like this setting in nuetral in the drive way.
My question is the distributor was set up before a similar engine.....from these numbers it looks to me that the distributor is set up for about 16 degrees intial timing on the engine to get to close to 45 to 50 accumalative timing at part throttle or cruise.???????????

As you've indicated, most small block Chevys like around 10* to 12* initial and most of them will run best with 35* to 36* initial/centrifugal. If you're reading only 22* initial/centrifugal, then you need more centrifugal in the distributor to come up to the 35* to 36* initial/centrifugal, all in by 2,600 to 2,800 depending on chamber design. Some fast burn chambers will like a little less initial/centrifugal, maybe 33* to 34* initial/centrifugal. Vacuum can be whatever the motor likes best at cruise, but as you alluded to, most will run best with 45* to 50* initial/centrifugal/vacuum under light load.

The choice between manifold vacuum and ported vacuum has been addressed on this board many times. Here's a good article by c9x and is based on his first-hand experience. Do a search on here for "ported vacuum" and you'll find 74 threads dealing with it.
http://www.clubhotrod.com/forums/sho...=ported+vacuum

Thnx guys,I think it is close enuff untill I can actually road test it{its a manual ...so a load test in the drive way is probably not a good idea }.Then I will mess with the springs and weights with the timing light.I also have the vac hooked to the full manifold vac below the throttle blades on the carb.

If I understand this correctly.. the initial timing can vary depending on what the motor wants to start well,my question is is there any advantage to having the initial timing set one way or the other?

Is the mechanical timing advance amount set on the distributor {the 12 degrees that I get now}or is that also going to change with the springs and weights?

I understand pretty good from reading how the vac can works,it helps keep the engine cooler at idle and helps emissions,it also helps fuel economy at cruise,is there any advantage to running more or less vac advance.

I understand that detonation is pretty much the limiting factor on timing advance and is not alway audible.........still keeping the motor out of detonation is there a advantage to running a little more of one than the other ?
I appreciate the answers so far and I have read alot of info but not pertaining specifically to these questions..thnx

Originally Posted by shawnlee28

Ok here it is ....I have about 12 intial on the engine at idle with the vac cannister disconnected....I bring the engine up to 3000 rpm with the can still disconnected I get 22 degrees

Sounds to me like you have the idle of the engine too high. This will cause the mechanical weights to be in an advanced position at idle. Setting the initial timing this way will cause the maximum mechanical advance to be at a figure less than what the distributor has "built in". Most GM distributors have a built in mechanical advance of 20 degrees. Since you are only getting 22 degrees Total mechanical advance at 3000 RPM, then your actual initial would fall to 2 degrees BTDC at an idle of 600-750 RPM. This would cause your engine to run hotter than "normal" and you would more than likely see your exhaust near the manifolds get cherry red at idle.

Changing the springs and/or the weights will NOT change the amount of available mechanical advance. It will only effect how quickly the advance weights begin to move under centrifugal force. The usual reason to change the springs and/or the weights is to achieve the full mechanical advance around 2500-3000 RPM.

Try setting your Total Mechanical Timing (vacuum hose disconnected and plugged) to 32-38 degrees at 2500 RPM. If the timing increases when going above the 2500 RPM, change the springs to weaker ones so that the timing will max out around the 2500 RPM range. Lower your idle RPM to 600-750. Check the initial timing. It should be in the 12-18 degree range depending on what you set the Total at. If it isn't, then the mechanical weights are not closing completely. Check for rust or burrs on the mechanical linkage.