Heres the breakdown:

I bought an 84 2.8L bird with a 350 thrown in it. After identifying the block, it came out of a 1977 Corvette, so it's an L48. When I initially got it running, the timing light indicated about 12 degrees of initial timing at idle. Fast forward to last week, I took the heads off to replace the head gaskets, put it all back together being extremely careful to put the distributor in at TDC with the rotor oriented correctly, and now it runs but when I went to time it, I'm all the way out in the 40s at idle for the advance, so something has obviously changed. The engine is stock, has the stock cam, etc. Fairly new to engines so I'm extremely confused as to how this happened. Any insight is greatly appreciated.

 

:shrug:

Who cares. Give it what it wants. It's not looking at "numbers", all it cares is when the spark occurs. It knows what that is better than you ever will.

If it RUNS good, it IS good.

 

You don't want to over do it with spark timing advance since it can start to become hard on the rod bearings. Some of the 4th LT1's in the 90's were commanding upwards of 25-27° at idle (and those are on production engines that are supposed to run well over a 100k miles)... so you can get fairly aggressive. As Sofa said, what the engine wants does have a lot to do with it.

But 40° is kinda gettin' up there... I'd be a little suspicious if you're saying it needs that much timing to run properly.

I would verify true TDC with a piston stop, then go look at where the mark on the balancer is vs the timing marker. See if that 40° is real or not.

 

I wouldn't bother with "mark" and "tab" or "stop" or "TDC" or ANY of that. Sounds to me like, even if you did verify where the "mark" system is NOW, tomorrow it might all be somewhere else.

Set it to where the engine runs best. If it doesn't ping, it's not in trouble, nothing to worry about. Shut the hood and drive it.

 

The car would "run" but idles pretty rough. Once it got some throttle applied, it ran great. I took the distributor back out today and found that the weights were stuck open. The harmonic balancer marking was in line with TDC on the engine, which is why 40 degrees made absolutely no sense to me. Ordered a new distributor since its probably overdue for this engine anyways.

 

'Looks like you may have figured out the problem...