Got an L78 (396/375) that I can't seem to get dialed in. From what I know its a spec numbers matching numbers type rebuild with very low miles on it. When i got my hands on it, it had like 24* degrees initial timing and a big vacuum leak under the carb. The vacuum advance was hooked up to manifold vacuum. Got the vacuum leak fixed, put the right gasket under the carb. verified TDC #1. Set initial timing at 4* BTDC and it just does not like it. I can't keep the throttle plates closed under about 18* initial timing. and even then the motor is producing bupkiss for vacuum at idle. I've been banging my head against this thing for a couple of days and its time to step away for a minute and collect my thoughts, and see if you guys can think of anything I'm missing. Any Ideas? Thanks in advance.
Mechanical advance is stuck advanced. Vacuum advance diaphragm is blown out creating vacuum leak. Brake booster is bad creating vacuum leak. Harmonic damper has spun.
Yeah, see if the timing mark & pointer is telling the truth or not. If it is OK, then check valve timing, n.b. this is different thing than ignition timing.
I would check the timing chain and gears then the harmonic balancer first. Sounds like cam advanced too much.
All good and very possible ideas and suggestions there. Simple things first: Check the advance weights and the advance diaphragm. You can just pull the hose off the carb end and suck on it to check the advance diaphragm. If you get resistance it should be ok if you suck air though the hose it's bad. From there I'd check to make sure that the timing mark is right and if it is right pull the front cover off. You may be battling someone's "performance mod" where they advanced the cam way more than the advised amount.
Yes the 375-396 is solid lifter have a look at valve adjustment, although we don't know for sure what cam is in it now. Lippy
did you check total timing? should be around 36-40. Without knowing what distributor is in it, and what changes have been made to the advance curve, your 4 degrees initial is probably part of the problem.
If everything else checks out, set the initial timing for the highest vacuum reading, and/or the highest rpm, but backed off just slightly.
Most stock Chevy distributers that I played with should have about 22 degrees of machanical movement, so at 4 degrees initial total timing all in would be only about 26 degrees. At 18 degrees initial + the 22 machanical movement the total "all in" should be about 40 degrees, How does it run and drive at 18 inital ?
This all started because it was pinging. I'm trying to track down the engine builder to see what cam is in it, but at low advance we have real low vacuum. also not sure about cam timing. With the throttle plates all the way closed and the idle screws all the way in, its still idling at 850 RPM and running rich. I'm pretty sure the power valve is open at idle. and that's what's giving us grief.
The Jalopy Journal
