Ignition Timing Question
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Ignition Timing Question
I have a 79 4x4, 4speed 150 with a 400. It has a 4bbl holley that I recently rebuilt. Prior to the rebuild: choke didn't work, vaccuum secondaries didn't work and it idled like crap. The rebuild was a complete sucess, and it runs great EXCEPT: A carb backfire twice now, and it has "dieseled" briefly once or twice when turning it off. I suspect a timing adjustment. I haven't had a chance to check it, but I'm wondering if anyone knows, based on symptoms which way I need to go?...retard or advance. Thanks.
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Joined: Aug 2003
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From: Crestline, California
My limited experience and understanding is that if it diesels (or pings under load), it may be too far advanced. I just had to readjust the timing on my Edelbrock and it was 4 degrees retarded. At first I advanced it too far (hoping for better mileage...Ha!!) and it did diesel after, but when I brought it back to the exact mark all fixed. Don't know about the backfire though. The experts here can probably answer that one.
Dak
Dak
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Joined: Apr 2004
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From: Austin, TX
Dieseling can be caused by a number of things, but if timing is the culprit (which it sometimes is), it's usually due to retarded (late) timing. This is because late timing increases the combustion chamber temperature, which can lead to carbon hot spots. However, dieseling is much more often due high curb idle or a vacuum leak. However, advanced timing can lead to dieseling as well, but it's not as common. Technically speaking, combustion taking place any other time than the ideal cylinder position can increase heat on piston surfaces. So, the general answer would be that either advanced or late timing can cause dieseling.
Try not to think of it as "do I need to advance or retard." Set it to the specification and then take timing out of the equation. People chase their tails trying to time by ear and it's the wrong way to do it in the first place.
A carb backfire means combustion is taking place with an intake valve open, which can very easily be timing related. A lean condition (faulty accelerator pump, vacuum leak) can cause this problem as well.
Try not to think of it as "do I need to advance or retard." Set it to the specification and then take timing out of the equation. People chase their tails trying to time by ear and it's the wrong way to do it in the first place.
A carb backfire means combustion is taking place with an intake valve open, which can very easily be timing related. A lean condition (faulty accelerator pump, vacuum leak) can cause this problem as well.
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Thanks! Carb settings are spot on...I believe I replaced all vaccuum lines at time of carb rebuild, so my gut is still saying timing. I'll check it this weekend when I have the chance, and update the post. Thanks for the insight.
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ericsmith32
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
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Nov 23, 2006 01:21 AM
