Holley Carb guru needed.
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Laughing Gas
Joined: Jul 1999
Posts: 1,047
Likes: 22
From: Lacey, WA
Holley Carb guru needed.
Hi,
I have a Holly LIST-6830 4bbl on my 1970 F-250 4x4 with a 390. I rebuilt this carb and have a problem I can't fix. The carb back fires (burps) at mid throttle only. Shoot a flame a foot high out the carb, scary! If I push the pedal past mid way it runs VERY well. I have checked everything I can think of to fix it. The floats are correct, idle mixture's fine. Cannot find any vacuum leaks. I am sure the problem is in the carb as I borrowed another carb and it runs fine on my truck.
As a side note, I have discovered that I failed to install the Diaphragm check ball in the secondary vacuum canister. What is the purpose of this ball. From the diagram of the carb on the Holley web site this ball looks like it is PREVENTING the vacuum needed to operate the diaphragm. ?? I will install the ball tomorrow but I personaly don't think it will fix my problem.
Thanks in advance for any suggestions,
Lee
I have a Holly LIST-6830 4bbl on my 1970 F-250 4x4 with a 390. I rebuilt this carb and have a problem I can't fix. The carb back fires (burps) at mid throttle only. Shoot a flame a foot high out the carb, scary! If I push the pedal past mid way it runs VERY well. I have checked everything I can think of to fix it. The floats are correct, idle mixture's fine. Cannot find any vacuum leaks. I am sure the problem is in the carb as I borrowed another carb and it runs fine on my truck.
As a side note, I have discovered that I failed to install the Diaphragm check ball in the secondary vacuum canister. What is the purpose of this ball. From the diagram of the carb on the Holley web site this ball looks like it is PREVENTING the vacuum needed to operate the diaphragm. ?? I will install the ball tomorrow but I personaly don't think it will fix my problem.
Thanks in advance for any suggestions,
Lee
Laughing Gas
Joined: Jul 2001
Posts: 849
Likes: 14
From: Fayetteville, NC
Holley Carb guru needed.
I'd check your timing just for the heck of it. I remember when I rebuilt my 351 that I had put the dis. back in WAY too far advanced and the truck would shoot flames out the carb. I retarded the timing back to where it was supposed to be and never had a problem with it again. Just something you might want to check.
1978 F-150 flareside. Primitive by todays standards but she was a jaw dropper back then. :-staun
1978 F-150 flareside. Primitive by todays standards but she was a jaw dropper back then. :-staun
Senior User
Joined: Oct 2001
Posts: 113
Likes: 0
Holley Carb guru needed.
Well mid throttle is about when the secondaries start to open so it could be part of the problem. If they are hanging up or flopping open all at once it could cause a temporary lean out and would stumble the motor pretty bad. Timing is usually the culprit for a backfire through the carb though. Its caused by not having enough advance which allows the spark to fire way too late. By then another intake valve is starting to open and you get fire up through the intake and carb. It may be that you have a secondary stumble coupled with the advance not advancing enough or fast enough. If you have had any significant backfires into the carb you will probably need to consider installing a new power valve unless you have the power valve check ball protection.
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