minor carb problem
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Postmaster
Joined: Jul 2003
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From: iowa
minor carb problem
well, i have a 390 4bbl deal. Its a 670 street avenger, i am using the yellow secondary spring which is rather light. When i mash on the gas pedal i get a bog, then it runs hard. The bog last about 1/2 or 1 second and then goes away.
Is this too from to much air or fuel or what? Normal driving is fine, its just when you mash the gas pedal.
carb specs, 65 main jet, 68 sec jet,.031 pump discharge nozzle, 6.5 power valve.
engie specs, 396, 9.5cr, rpm, headers,crane 901 cam. The engine has 19.5 inches of merc in park at idle.
Is this too from to much air or fuel or what? Normal driving is fine, its just when you mash the gas pedal.
carb specs, 65 main jet, 68 sec jet,.031 pump discharge nozzle, 6.5 power valve.
engie specs, 396, 9.5cr, rpm, headers,crane 901 cam. The engine has 19.5 inches of merc in park at idle.
Last edited by fordeverpower; Apr 2, 2006 at 03:14 PM.
Post Fiend
Joined: Jan 2003
Posts: 8,600
Likes: 4
From: Iowa
Accelerator pump. Take the air cleaner off with the engine off and work the throttle by hand. You should see fuel squirting out into the primary throttle bores, proportionally to how much you move the throttle. If it just dribbles out when you work the throttle, it's not right.
If it works right you might need a quicker accelerator pump cam.
If it works right you might need a quicker accelerator pump cam.
Senior User
Joined: Jan 2002
Posts: 438
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From: Centreville USA
Absolutely nothing to do with the accelerator pump. By your own admission you are running a very light vacuum secondary spring. When you mash the throttle, the secondaries open immediately dumping more fuel into the engine then it can handle, hence the bog. That is also the reason you don't feel it under normal, light driving - the secondaries are not needed. You need to put a heavier spring in the vacuum secondary. Increase the spring tension until the bog disappears. Once it seems gone or at least minimized, try one spring lighter and one spring heavier. Pick the one that feels the best for your driving. My 64 Galaxie with 390 and Road Demon 625 cfm carb uses the heaviest spring due to the weight of the car. Don't mess with the acclerator pump until you get the correct vacuum secondary spring in there.
Post Fiend
Joined: Mar 2004
Posts: 6,056
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From: Union, Washington
I gotta agree with bluesky. In a truck the stock spring in the secondary maybe too light. I usually run my truck with the next stiffer spring than the stock one. Don't ask me which color it is because my memory isn't that good anf I am not pulling the carb down to check. I just finally got it all back together to go on my 460.
Post Fiend
Joined: Jan 2003
Posts: 8,600
Likes: 4
From: Iowa
As did mine. When I ran the Holley 600cfm carb, I ran the yellow spring without problems.
OBTW, if the vacuum secondary spring were too light, it would create a bog by leaning out the engine as the secondaries would open too quickly, as the secondaries dont have an accelerator pump. When they are opened with too little airflow, it doesn't pull enough fuel for the air going in. Hence, bog. If it gets real bad it can backfire through the carb. This *may* be what is happening, but I'd still check the accelerator pump first since it only takes 15 seconds.
OBTW, if the vacuum secondary spring were too light, it would create a bog by leaning out the engine as the secondaries would open too quickly, as the secondaries dont have an accelerator pump. When they are opened with too little airflow, it doesn't pull enough fuel for the air going in. Hence, bog. If it gets real bad it can backfire through the carb. This *may* be what is happening, but I'd still check the accelerator pump first since it only takes 15 seconds.
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Post Fiend
Joined: Mar 2004
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From: Union, Washington
Originally Posted by fordeverpower
my trucks feels like a turd with a heavy secondary spring.
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Posting Guru
Joined: Aug 2005
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From: NE Ga
Just made me think, I'm gonna be putting a holley 600 vac/sec on my 390. Since i haven't installed it and will be this coming weekend, would it be wise to pop the cap off and see what spring i got?? Its going on a 390 in a 1973 F100 weighing in at about 3500 lbs.
Post Fiend
Joined: Mar 2004
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From: Union, Washington
Unless the carb is used and someone could have been into it, I would doubt it would have anything except the stock spring. Your truck weights about 700 to 1200 lbs less than my '68 or my '84 F250s so the stock spring will probably work ok unless you are gonna be doing heavy towing (which isn't a real good idea with a F100).
Post Fiend
Joined: Mar 2004
Posts: 6,056
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From: Union, Washington
How the he!! do you make a 150 weigh 5000#? My extended cab F250 heavy duty only weights in at 4800 with one tank empty. The spring kit is the way to go and in your case, I'd check the spring and if the carb came off a car I'd check for sure.
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Postmaster
Joined: Jul 2003
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From: iowa
well its a 4x4, I usually refer to the weight with me in it cause somebody has to drive it. It used to be heavier, i have since swapped the np203 for np205, got an alum intake, and have taken off my brushguard.
The landfill has scales, and thats where i get it weighed.
The springs i got came with the carb, it was brand new. There is no unknown variables here.
spring breakdown
heavy to light
black
brown
plain or silver
purple
yellow (long)
yellow (short)
white
the carb came with a plain spring installed and a black and yellow one to try. Well the black can go on your vehicles Bear, i am not interested in driving slow. And the yellow one causes my bog.
The landfill has scales, and thats where i get it weighed.
The springs i got came with the carb, it was brand new. There is no unknown variables here.
spring breakdown
heavy to light
black
brown
plain or silver
purple
yellow (long)
yellow (short)
white
the carb came with a plain spring installed and a black and yellow one to try. Well the black can go on your vehicles Bear, i am not interested in driving slow. And the yellow one causes my bog.
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Well, I did my really fast stuff in the past, But I have found that slow I am not, just slower than I used to be. You need to stick the plain spring back in and if that doesn't cure it, get a brown spring. Jumping up or down two levels is asking for trouble. Like changing jets and going up or down 8 or 10 numbers, just not a good move.