I have a 1977 L48 I rebuilt this fall and winter (mild performance cam change as well as an intake change), and my concerns started out with a poor idle. The advice I got was to set up 36* mechanical advance and the turned into that the distributor could not get up 36* at an initial base timing set of 12*. Ok, so I put on some lighter springs, no change. Took off the springs, no change. So there is no way I am going to get 24* mechanical advance from my HEI. What is the issue that I am having? Should it not peg out at 36* without spring at a 1500 or so RPMs. I am certainly not getting that much advance with stock or light springs at 2800 RPMs. What should I try? Another issue is that now that I have pulled the distributor to get a closer look, I noticed a lot of play (1/8" or so) at the end of the shaft between the housing and the gear. Logic tells me that this could cause some timing issues with the gear potentially moving up and down as the engine ran. Is there a fix for that or is the distributor need to be replaced. The distributor looks clean and the like with little wear, but I am not sure what my next step is with this thing.

 

There is also a stop bushing in there. It may need to come out or be turned down to allow full advance. .125 end play is too much also. I think it should be less than .010, but look that up to be sure. The distributor can be rebuilt to correct that end play, and also check for wear in the lower bushing.

After 38 years of driving a HD oil pump, I had almost as much side play as end play.

 

Is the stop bushing located between the gear and the housing? I see a bushing down there that has tab that are folded upward.

 

It's in the plate that the weights are attached to.

 

There is no advance stop bushing on a factory or Chinese built HEI distributor. Older points type distributors yes, but not the HEI's.
The amount of advance built into the HEI the distributor is controlled by the advance cam. To change the amount of total advance in your engine you either have to increase the initial-base timing or purchase a different advance cam.

Changing the springs or weights will only get you the the maximum advance faster or slower.

If you've gone to an aftermarket cam the engine is going to want more initial advance anyhow.

I would suggest you check the shaft end play. Most HEI's have a horrendous amount of end play. I believe the correct amount of end play for an HEI is .015-.020. I believe the others can go slightly tighter.
Shimming isn't going to help the advance curve either but your timing will be more stable at speed.

 

There is no reason to try to maintain the base timing at 12 degrees. Set the total to 36 and let the intial fall where it may. If you have a mild performance cam, you want the initial to be in the 16-18 degree range (at least) for best idle and throttle response. Your "short curve" distributor is highly desirable in a performance application - there is no reason to lengthen the curve.

Lars

 

True. The 12 degrees at idle will only make it idle rougher and produce less torque down low.