[fiftyfivegasser]

First a little history - have a 350 LT1 (1970) engine that I have changed some parts on. Ran fine before, but could never get the engine to run right except at about 4 degrees BTDC. Several months ago, I purchased a new mechanical cam/lifters; purchased different heads (went from 76cc open chamber to 64 cc closed chamber).
Installed the cam and did the breakin with the old heads still installed and everything okay (still time at about 4 degrees). Installed the new heads and the rebuilder screwed up and installed valve guides that were too tight and the #4 intake slapped the piston, bent the pushrod and bent the valve (first time out). Removed the heads and had a local shop correct the problem.

Now for the issue. I can get the engine to fire and run, but it stumbles under acceleration until the rpm's get to about 3500 in any gear and engine smoothes out. Checked static timing and the balancer mark is nearly 5" away from the timing cover marker at idle with vacuum removed and plugged(engine starts fine and idles fine). Set #1 at TDC, valves closed and balancer lines up with -0- on timing tab. If I retard the dist., the timing marks move closer together, but the closer I get to -0- degrees, the engine will not idle without a lot of idle screw adjustment and if the carb is bumped, it backfires. If I set the timing by ear (good idle, etc.) I get the stumbling and hard to pull off issue (straight drive) and the crazy timing mark issue. New plugs, wires, dist. and coil and have tripple checked the wires for correct location (plugs to dist.)

Would changing to a lower cc head cause the timing to get way off or did something get screwed up when the valve slapped the piston? No damage to piston other than a fingerprint of the valve...forged pistons. I'm at a loss as to what is needed to fix this mess. Thanks.

[32Essex]

Did you degree the cam? Did you check true top dead center to the timing mark on the balancer to the tab for alignment ?

[sqzbox]

Have you checked the vacuum reading at idle? If the needle is shaking pretty good it could be rocker arm adjustment.

[fiftyfivegasser]

Vacuum is running 8" at idle. If I advance timing further (can get 12"), the engine starts detonating. The cam was installed at TDC (#1 up and valves closed). Ran okay until I reinstalled the heads and this is when the screwed up timing started and engine stumbling under a load during initial acceleration.

[sqzbox]

What kind of compression are you running now with the new heads?

If I remember right, 1970 lt1's had 11:1. Thats high for todays gas!

[a1supersport]

My initial thought was you dropped your distributor in one-tooth-off.

Did your local machine shop check all the valves/guides to make sure there were no other tight guides? Since you had everything running with the low-comp. heads...and all you did was swap the heads...it has to be something that you removed/installed while changing heads (heads, intake, carb, dist.). And since your timing marks were so far off...it made me think of the dist.

[sqzbox]

[QUOTE=a1supersport]My initial thought was you dropped your distributor in one tooth off.[QUOTE]

Simple rotation of the dist. would compensate if you didn't stab it exactly where you had it when removed. All you need is the rotor button pointing at the tower with #1 spark plug wire at TDC.

The timing chain is another story. One tooth off and your screwed!