Distributer, vacuum advance, and a broken piston...
03-26-2010, 11:21 AM
#1
Senior Member
Thread Starter
Distributer, vacuum advance, and a broken piston...
Long story.
My LG4 is in the machine shop getting the block and heads rebuilt. When I took it apart I found the reason for it's failure: cracked piston that was falling apart fast around the rings and one of the skirt tabs at the bottom had broken off already. The other pistons had stress cracks on the lower skirts as well but hadn't broken apart yet. I asked the machine shop about it and they suggested that the distributers timing advance may have been off and caused detonation, which over a period of time would have destroyed these pistons.
I looked at the distributer and the one that was on the car does not have a vacuum advance. It had a Holley and GM HEI with mechanical advance only.
Now the questions.
-Should this engine have an HEI with vacuum advance as well, or is that just to fine tune or get more advance beyond the mechanical? (If machine shop is right I had too much advance to begin with, right?)
-Does it sound like the mechanical advance was malfunctioning (sticking) or maybe the wrong spring weight (or whatever GM used in these)?
-What else might have caused piston failure like this?
-Could improper carb tune have caused it? It didn't seem to run too bad, other than burning a lot of oil since I bought it.
I don't really want to buy another distributer, even though they are only around $100. I prefer to reuse this one since I have already put a new module, cap, rotor, and coil in it last year.
My LG4 is in the machine shop getting the block and heads rebuilt. When I took it apart I found the reason for it's failure: cracked piston that was falling apart fast around the rings and one of the skirt tabs at the bottom had broken off already. The other pistons had stress cracks on the lower skirts as well but hadn't broken apart yet. I asked the machine shop about it and they suggested that the distributers timing advance may have been off and caused detonation, which over a period of time would have destroyed these pistons.
I looked at the distributer and the one that was on the car does not have a vacuum advance. It had a Holley and GM HEI with mechanical advance only.
Now the questions.
-Should this engine have an HEI with vacuum advance as well, or is that just to fine tune or get more advance beyond the mechanical? (If machine shop is right I had too much advance to begin with, right?)
-Does it sound like the mechanical advance was malfunctioning (sticking) or maybe the wrong spring weight (or whatever GM used in these)?
-What else might have caused piston failure like this?
-Could improper carb tune have caused it? It didn't seem to run too bad, other than burning a lot of oil since I bought it.
I don't really want to buy another distributer, even though they are only around $100. I prefer to reuse this one since I have already put a new module, cap, rotor, and coil in it last year.
03-26-2010, 12:55 PM
#2
Supreme Member
Re: Distributer, vacuum advance, and a broken piston...
All LG4 engines came stock with a computer controlled HEI. No vacuum advance canister, just an electrical plug to allow the ECM to adjust the timing.
If you have gotten rid of the original computer controlled QJet, you need to ditch the computer controled HEI for an earlier non-computer controlled one (which will have a vacuum canister).
I doubt this is what caused the engine damage, but it needs to be fixed if you expect reasonable performance from the new engine.
If you have gotten rid of the original computer controlled QJet, you need to ditch the computer controled HEI for an earlier non-computer controlled one (which will have a vacuum canister).
I doubt this is what caused the engine damage, but it needs to be fixed if you expect reasonable performance from the new engine.
03-26-2010, 09:28 PM
#3
Senior Member
Thread Starter
Re: Distributer, vacuum advance, and a broken piston...
Yeah, one of the previous owners ditched the QJet. I guess I will order one from Summit with mechanical and vacuum advance.
Thread
Thread Starter
Forum
Replies
Last Post
Bohemian
History / Originality
24
08-15-2017 05:11 PM
