I'm running a 6AL w/ MSD start retart box and a locked Comp 9000 Distr. The retard gives 10 degrees retard @ start while starter is engaged and then 0 retard. I have my timing set @ 36.5 degrees and all's well until I rev it to 5000 RPM or above and then my light shows the timing retarding to 34 Degrees or so. Slow it down and it is back up to the set 36.5 degrees. I've check my distributor and the lockout can't move. I'm using a craftsman timing light which has a dial back feature which some say may be the problem.
What do you guys think?? Is there anything else that could be going on??

I have a dial back and a standard light. For total timing with my MSD box, I use the standard light because the dial back jumps around. FWIW, MSD has a light that they claim is the only light that will be accurate with MSD boxes.

Im going threw the exact same thing....I am starting to think the cam is moving. I have a solid roller with a cam button. It was set up with .008" of clearance. I will have the machine shop check it out soon

So if the cam walks alittle it will effect timing?? I would think it would still be on the lobe so it wouldn't effect the timing. Can you explain??

The helix of the distributor drive gear as it matches the cam gear would engage differently if the cam moves...I think...lol..thats my theory anyways

if the cam walks back it will push up on the dist. causing the timming to retard.....if the cam walks forward the timming can advance but in most cases this dont happen because the movement of the cam pulls down the dist.(takes up the slack) just the way the teeth mesh......but if there is some slop in the dist. for upwards movement which in most cases there is timming retards, to what degree? depends on how much the cam walks back and how much slop in the dist. for up and down.....most the times cams want to walk back and thats why you hear sometimes "the timming chain and gear ate into my block".....depending on how important that few degrees are you can try and control cam walking with a roller bearing set behind the cam gear and cam button..........

Can timing chain slack be a cause?

what is the start point on your timming

At Idel 10 ? and what is your Idel

Do you have a total 36 at what rpm ?

I don't think timing chain slack is a possibility as it is a new cloyes tru roller. The timing is locked @ 36.5 so @ start it would be 26.5 and then goes immediatly to the 36.5. I like the slack in the dist theroy as the dist. is the only used part on the engine other than the block and bare heads!!It seems to have some up and down movement. I'm going to take my dist to a shop and have it spun on a dist machine and see what happens. I'll post the results.