Holley question...Smaller carb or jets???
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Le Mans Master
Joined: Apr 2000
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From: Cresskill NJ
Holley question...Smaller carb or jets???
I dynoed my car last week at ECS and found that the car is running way too rich, possibly in the range of and 8:1 mix when I get on it. The car is currently running a 402 with an Stealth intake and an 850 Holley double pumper. I was wondering if I can get away with just using smaller jets or should I just go to a 750 or possibly any smaller? Any input would be appreciated. I have about 50 or so HP that I need to stop from going out the tail in black smoke :yesnod:
Thanks,
Chris
Thanks,
Chris
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Re: Holley question...Smaller carb or jets??? (DngrZne)
You should be able to jet it down, It's probably a little big for what you are doing but it still shouldn't run too rich. Something is out of whack. Are you sure the air filter isn't restricting it (too short?), fuel needles leaking? etc?
A 750 will probably be about right for you.
JIm
A 750 will probably be about right for you.
JIm
Thread Starter
Le Mans Master
Joined: Apr 2000
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From: Cresskill NJ
Re: Holley question...Smaller carb or jets??? (427Hotrod)
You should be able to jet it down, It's probably a little big for what you are doing but it still shouldn't run too rich. Something is out of whack. Are you sure the air filter isn't restricting it (too short?), fuel needles leaking? etc?
A 750 will probably be about right for you.
JIm
A 750 will probably be about right for you.
JIm
Thread Starter
Le Mans Master
Joined: Apr 2000
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From: Cresskill NJ
Re: Holley question...Smaller carb or jets??? (Taijutsu)
I saw a similar post where the exhaust was opened up and that cured the rich problem and added big hp! Could this be your problem?
Le Mans Master
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Re: Holley question...Smaller carb or jets??? (DngrZne)
what is the LIST # including the - # on your carb air horn? also make sure the carb bowl vents are at least 1/2" below the air filter top cover. :chevy
Thread Starter
Le Mans Master
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From: Cresskill NJ
Re: Holley question...Smaller carb or jets??? (clem zahrobsky)
what is the LIST # including the - # on your carb air horn? also make sure the carb bowl vents are at least 1/2" below the air filter top cover. :chevy
Le Mans Master
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Re: Holley question...Smaller carb or jets??? (DngrZne)
the numbers are stamped on the front of the choke housing,the part that holds the choke plate. look above the front float bowl. :chevy
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Re: Holley question...Smaller carb or jets??? (DngrZne)
Pull the back bowl off and see if it has had the power valve plug removed and a power valve installed. Most carbs come jetted a tad on the rich side. If the only time you go rich is with your foot in it then the front jets are OK. Check the jet sizes front and rear anyway so you'll know what you have and check the power valve sizes. Over size jet in the rear and/or the addition of a rear power valve is what I would look for.
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Le Mans Master
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From: Cresskill NJ
Re: Holley question...Smaller carb or jets??? (Dryseals)
Ok, well I just pulled off both float bowls and this is what I found. The carburator is number 4781-1. Number 80H jets all around. If the power valve is a spring with a diaphram then it has them on the primaries and secondaries and they are both number 6.5. The carb bowl vents have plenty of clearance, and the air filter has an open top so there would be no restriction anyway.
Is that enough info?
Thanks again,
Chris
[Modified by DngrZne, 6:29 PM 7/13/2002]
Is that enough info?
Thanks again,
Chris
[Modified by DngrZne, 6:29 PM 7/13/2002]
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Re: Holley question...Smaller carb or jets??? (DngrZne)
80 is correct DP are very rich because they are race carbs to be used with open exhausts. i would go to 78 in the pri and 78 in the sec. also take some 76s to the next dyno test. installing some .015 diameter wires in the idle feed restrictions in the metering bodies will lean out the idle and leanout the over all fuel curve.
[Modified by clem zahrobsky, 7:36 PM 7/13/2002]
[Modified by clem zahrobsky, 7:37 PM 7/13/2002]
[Modified by clem zahrobsky, 7:36 PM 7/13/2002]
[Modified by clem zahrobsky, 7:37 PM 7/13/2002]
Thread Starter
Le Mans Master
Joined: Apr 2000
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From: Cresskill NJ
Re: Holley question...Smaller carb or jets??? (clem zahrobsky)
installing some .015 diameter wires in the idle feed restrictions in the metering bodies will lean out the idle and leanout the over all fuel curve.
Thanks,
Chris
Le Mans Master
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From: delmont pa
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Re: Holley question...Smaller carb or jets??? (DngrZne)
a 750 would be better street carb but a 850 would make the most top end HP when dialed in. if you are going to drag race with open headers i would stay with the 850. the idle feed restrictions are the little brass inserts in the metering bodies. chevy high performance magazine a month or two ago had a good article on this. like i said a 750 would be a better all around carb for every day driving. :chevy
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Re: Holley question...Smaller carb or jets??? (QuickVet)
Double pumpers need bigger jets than single pumpers because they are set up with much less power valve flow. The difference between cruise mixture and power enrichment mixture is very small. They are intended to run at best-power A/F ratio at all times. If you try to size the mains down for a leaner cruise mixture, it will have poor part throttle (hill climbing) power because the power valves flow too little and it will be too lean.
You really want to keep the jetting the same all the way around and just jet for best power. If you have an interest in gas mileage, step away from the double pumper. They just don't work that way. You can mess with the power valve restrictions, but I'd just get a 750 vacuum secondary if you care about mileage.
You really want to keep the jetting the same all the way around and just jet for best power. If you have an interest in gas mileage, step away from the double pumper. They just don't work that way. You can mess with the power valve restrictions, but I'd just get a 750 vacuum secondary if you care about mileage.
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Re: Holley question...Smaller carb or jets??? (DngrZne)
If the rich condition only exists when you have your foot in it then the best thing to do would be to take a manifold vacuum reading at the rich condition. The power valve (that diaphram thingy) is set up to open at 6.5" of vacuum. Most of these carbs are a tad on the rich side and with the power valve opening up can put you a little too rich. This is a big carb for the engine but can be made to work. The key here is to figure out if the A/F mixture is correct for you application. If the manifold vacuum reading is not below 6" when you go rich then look at smaller jets. If the reading is below 6" when the rich condition hits then the power valves are opening and adding more fuel.
