Basic 390 Timing Setup?
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Freshman User
Joined: Sep 2003
Posts: 37
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From: San Antonio
Basic 390 Timing Setup?
I just got a 72 F100 Sport Custom with a 390/Edlebrock Intake/Holley 4V setup. I'm trying to track down a backfiring-while-decelerating problem. I'm checking the basics first.
Problem is I'm not familiar with the 390.
Which cylinder is #1?
What is a good timing? I read elsewhere on the forum that 6 degrees BTCD is stock. I also read that 10 degrees is best. I'm running regular unleaded.
Matt
Problem is I'm not familiar with the 390.
Which cylinder is #1?
What is a good timing? I read elsewhere on the forum that 6 degrees BTCD is stock. I also read that 10 degrees is best. I'm running regular unleaded.
Matt
Senior User
Joined: Apr 2003
Posts: 426
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From: Knoxville, TN
Basic 390 Timing Setup?
#1 is passenger front.
Timing at 10-12 if it will go that high with no spark knock will probably provide best performance.
A dashpot would help the popping, as would slightly richer mixture in most cases.
Timing at 10-12 if it will go that high with no spark knock will probably provide best performance.
A dashpot would help the popping, as would slightly richer mixture in most cases.
Senior User
Joined: Apr 2003
Posts: 426
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From: Knoxville, TN
Basic 390 Timing Setup?
http://www.holley.com/HiOctn/ProdLin...MSCA/11-4.html
Basically it is a vacuum diaphragm that mounts to the Carb to slow or “cushion” the throttles return to idle. It minimizes the sudden change in manifold Vacuum and helps with the backfiring. Most mid 70’s on carbureted stuff has one. (manual or automatic)
Basically it is a vacuum diaphragm that mounts to the Carb to slow or “cushion” the throttles return to idle. It minimizes the sudden change in manifold Vacuum and helps with the backfiring. Most mid 70’s on carbureted stuff has one. (manual or automatic)
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From: Eureka, Ca.
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Laughing Gas
Joined: Jul 1999
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From: Lacey, WA
Basic 390 Timing Setup?
Matt,
When you say you have a backfire are you talking about 1 large pop, or like my truck did, do you mean that you have many small pops.
Mine would "rumble" when coasting and it turned out to be that the idle was too rich. I adjusted the idle screws in alittle and cured it.
Good luck,
When you say you have a backfire are you talking about 1 large pop, or like my truck did, do you mean that you have many small pops.
Mine would "rumble" when coasting and it turned out to be that the idle was too rich. I adjusted the idle screws in alittle and cured it.
Good luck,
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Freshman User
Joined: Sep 2003
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From: San Antonio
Basic 390 Timing Setup?
Lee,
It's a chain of pops after letting up on the accelerator. The more under load before letup...the louder the pops. It's not a true carb backfire. I've been tweaking the idle mixture to no effect. My Holley apparantly has no dashpot, so I can't try that idea (unless I can retrofit one...I'm just learning about Holleys).
I did make contact with a previous owner. His theory is that the #1 exhaust valve is burnt. I wonder if that would be a cause for the symptom?
It's a chain of pops after letting up on the accelerator. The more under load before letup...the louder the pops. It's not a true carb backfire. I've been tweaking the idle mixture to no effect. My Holley apparantly has no dashpot, so I can't try that idea (unless I can retrofit one...I'm just learning about Holleys).
I did make contact with a previous owner. His theory is that the #1 exhaust valve is burnt. I wonder if that would be a cause for the symptom?
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Joined: Jul 2003
Posts: 2,232
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From: Colville, Washington
Basic 390 Timing Setup?
If you have a vacuum gauge you can check for the burnt valve. The gauge will drop intermittently when the cylinder with the burnt valve operates. The popping could also be a minor exhaust leak. The extra air in the exhaust will burn in the exh manifold causing a pop noise.
