Timing Question
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Joined: Mar 2017
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From: Opelika Alabama
Timing Question
Good Morning, I have a recently acquired 78 with a ZZ4 engine and an Edelbrock carb. I've been dickering with the timing and have a beginners question. The engine starts hard, idles smooth, "chatters" on acceleration and tries to continue running when it's shut off. I have the paperwork from when the engine was installed which says for the timing to be set at 10* btdc at 800 rpm with advance plugged and 32* total at 4000 rpm. I'm at 10* at 800 but still have these symptoms. Do I need to advance or retard the timing to alleviate this? Also, the vacuum advance is connected to a port on the carb that has suction at idle. I thought that it should be to a ported source but my reading shows that some run to full vacuum. Any tips are appreciated, thanks in advance!
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Joined: Mar 2017
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From: Opelika Alabama
Thanks for your reply...Timing with the VA plugged back in is 10 btdc at 800 rpms. Haven't tried to drive it with the VA disconnected. I assume that would rule out a bad vacuum advance?
Melting Slicks
Joined: Dec 2009
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From: Thunder Bay
It certainly sounds like your timing is too far advanced, although 10 degrees is hardly a lot. You should probably check the timing mark on the balancer to make sure it coincides with top dead center. The balancers have been known to fail and for the outer ring to rotate on the inner ring, throwing the timing marks off. Also what cam, heads, and compression are you running? These are the things that will affect your timing requirements. As for the ported vs manifold vacuum argument, most guys (myself included) run manifold vacuum.
Thread Starter
Advanced
Joined: Mar 2017
Posts: 73
Likes: 1
From: Opelika Alabama
It certainly sounds like your timing is too far advanced, although 10 degrees is hardly a lot. You should probably check the timing mark on the balancer to make sure it coincides with top dead center. The balancers have been known to fail and for the outer ring to rotate on the inner ring, throwing the timing marks off. Also what cam, heads, and compression are you running? These are the things that will affect your timing requirements. As for the ported vs manifold vacuum argument, most guys (myself included) run manifold vacuum.
Melting Slicks
Joined: Dec 2009
Posts: 3,195
Likes: 632
From: Thunder Bay
thanks for your reply...the engine, as far as I know is a stock ZZ4 which according to the book that came with the car says that is has aluminum heads with 1.94" intake and 1.50" exhaust valves and 58cc combustion chambers with a 10:1 compression ratio. Has a steel roller tappet camshaft with .474 intake and 0510 exhaust valve lift. Makes 355 hp at 5250 rpm.
Melting Slicks
Joined: Dec 2009
Posts: 3,195
Likes: 632
From: Thunder Bay
I finally found some camshaft specs for your engine. You only have 208 degrees of intake duration. Your initial advance should therefore be in the 8-10 degree range, so you should be in the ballpark if your timing marks are correct. regardless, a couple fo degrees either way is unlikely to cause the kind of problem you are having. I think you are going to find you are out farther than that for some reason. Either that or the problem is unrelated to timing. The next thing to check would be mixture related, vacuum leak, etc., possibly fuel quality.
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Thread Starter
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Joined: Mar 2017
Posts: 73
Likes: 1
From: Opelika Alabama
I finally found some camshaft specs for your engine. You only have 208 degrees of intake duration. Your initial advance should therefore be in the 8-10 degree range, so you should be in the ballpark if your timing marks are correct. regardless, a couple fo degrees either way is unlikely to cause the kind of problem you are having. I think you are going to find you are out farther than that for some reason. Either that or the problem is unrelated to timing. The next thing to check would be mixture related, vacuum leak, etc., possibly fuel quality.
Dan