Timing on my big block jumped 20 degrees while running at 4500 rpm. Immediately I reset it and it jumped agian. The second time it was just from the load at about 2500.

I pulled the distributor out and inspected for damage on the gear or possibly a sheer pin and all is fine. Next was disassemble to inspect gears and chain. I found the timing mark on the cam gear is at about 630, and no timing mark on the crank gear. I also found the the port side of the chain was tite while the starbord side of the chain was only loose enough the expose one tooth. I could not find and obvious damage to the gears of chain.

I assume that as long as #1 is at TDC then a timing mark on the crank isn't that important, however it seems that the timing mark on the cam should be at 6. (straight down)

What is my next step? adjust something, replace something, or look at other possible problems?

time for a good quality chain&gearset

Timing mark on the crank is at high noon, timing mark on the cam sproket is at 6. Both dots are together. A little slack on one side is normal.

Are you asking about ignition timing or mechanical timing??

If your mechanical timing was out by 20degress, I doubt that the engine would even run. just because it looked like it was at 630, did you have something inside the engine to make sure it was exactly at TDC, or just a guestimation?? the difference between 6 and 630 is approximately the difference in the amount of time it takes to complete the transition from up to down in the compresion stroke.

What was the indication that there was a problem? The timing light?? or did the engine start to run funny? Did this happen on the trailer? or while you were out running?

Ignition timing was off by 20. Yes I did have something in #1 to be sure I was at TDC. the timeing mark on the harmonic was at o also. When the timing jumped it just started to run poorly at high RPM. imediately I checked and found it was off, then reset it. The boat hardly got on plane and it happend again.

Wierd. It's a little beyond me then.

Broken camshaft????? Is the key still in the crank gear??

It jumped 20 degrees twice, but is still able to run (although runs poorly). IMO, it can't be jumping the cam timing - it would not run with the cam timing off 20 degrees, much less run at 40 degrees off.

I would think you have an issue with the cam / distributor gear. That could jump a tooth, causing arounnd 20 degree of change. Check both gears and the hold down to make sure it's not jumping. Also check the pin in the gear - if it broke, the gear could be riding higher on the distro shaft, allowing the jump.

Edit: Jeff1000man gives another good reason the distro gear could jump (below)

Weak timing chain cover letting the cam walk forward?? If the cam button worn out??

did it still run decent ? if so try another light...what ignition ?

Use the cheapest non digital timing light u can find !!!!!!!!!!!!

I think I had an "ah ha" moment. I have a Thunderbolt ignition with an electronic advance. With no obvious physical damage the timing issue is electornic. Another symtom I completely overlooked was that the tach was jumping. This leads me to the ignition box being bad. I'll get a new one and hope thats the fix.