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.
This happened to a 348 tri power I have and it turned out to be the distributor point plate. I put in another distributor with same advance and now it runs great and all the stumbles are history.
How are you bringing it up to TDC? A positive stop? Could the ring on the balancer have moved? You say the closer you go to -0- the worse it gets? Well it will. Get it set by ear, and put some more timing in it until it clatters under load then take some timing out. I have a feeling your balancer ring has moved. Set it with a vacuum gauge yet? EDIT: reread my post and I sounded like an ***hole. No intention of that. I was in a hurry.
Cam is 30/30 Duntov and balancer lines up with the timing tab (-0- degrees) at TDC #1. Video of engine running before head swap http://www.youtube.com/user/fiftyfiveg***er#p/a/u/0/0fbAlpFNqLM The timing changing after the head swap has me scratching my head...not sure what changed? With a vacuum gauge, getting about 8 inches. If I change timing to get more vacuum, the engine runs like ****. Can get up to about 12 with advance just before pinging. The reason for the 4 degree timing is that's where the engine performs best. Good start, no ping and good power (that is before the timing changed to what I have now).
Have you checked that the timing marks on the timing gears still line up.. If the chain a jumped a tooth or three when the piston hit the valve? Did you use a degree wheel to verifiy the cam is intstalled/ machined correctly.. Just the head swap should not change timeing..
When you modify and engine with cam a such you improve the upper RPM effficiency. When this happens it requires less timng or less total advance. In a SBC typically less than 40 degrees total. (38 would be good place to start. )BUT if you set the timing total at 38 at 3000 RPM then you will now have retarded timing at startup and low speed. (below 3000) Now the engne will run poorly, bogging and lazy at low speed. if you are one of those people who have a fetisch for running vacuum advance as well you will really be in trouble. Forget the vacuum advance . You no longer need it. Recurve your distributor for about 2/3s the mechanical advance it came with. Get a advance curve kit if it is a Gm distributor. I weld up the end of the advance slot or bronze it up 1/3 then clean up the job with a file. this usually brings it in line . Forget the intial. Set the timing at 38 total advance @ 3000 RPM(no vacuum ever hooked up. ) to start. Use the medium springs in the advance curve kit. Re-adjust idle speed etc when done. Your engine will now feel like it gained 300 HP or more. It willl be crisp responsive and fast. As long as it can start without straining it is fine. Do not be afraid to experiment a bit . Some will stand as much s 42 degrees total , others wont even like 38 and will settle at 36. Play with the advance springs till you are happy. Lastly dont argue about this . It is fact and is not disputable. The hp that is available from fancy boxes and double yoked kryptonite coils is questionable at best but the HP and perfomance that is available from matching the advance curve to the engine is down right astonishing. Turn those ponies lose! git er done Don
I have the intake/carb off and will be replacing with a different setup tomorrow. I'll try with the no vacuum advance and medium springs to see waht happens. Right now (as of last nights trip), it pulls pretty hard but is constantly bucking when trying to cruise (any speed). Accelerate, and it goes without hesitation...ease back and starts bucking again. Probably need lower gearing also since I'm running a 4-speed. Thanks for all the suggestions.
They are not sugggestions, it is how you do it. Been doing these set up and similar since 1966. Try it , you will like it. Don
Got engine put back together, set initial timing and tried connecting vacuum (just to see) and engine started running like ****. Advanced timing a little more (12 degrees), plugged vacuum and took it for a spin. Runs great but a little warmer than usual. Didn't floor it (yet), but seems to have great response. Again, thanks.
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