Tuning the off idle circuit on a Holley Carb Help needed. - Hot Rod Forum : Hotrodders Bulletin Board
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Old 08-02-2008, 07:15 PM
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Tuning the off idle circuit on a Holley Carb Help needed.

I have a Holley 850 double pumper # 4781 on a 383. Idle is good put I have an off idle stumble then a really nice accelleration! It has 76 jets pri and sec, 45 pv pri and sec (9 to 10 inches in drive), and 31 long tube squiters, 30cc pri pump and 50cc sec pump. I changed the squiters to 37 tube types front and rear and the stumble is much better but still there. The stumble sounds like a lean out condition and when looking down the carb while accellerating the stumble seems to occur just before the boosters start suppling fuel. What I mean is this, I hit the throttle full open the squirters squirt the engine stumbles like almost a backfire, then fuel flows from boosters and the engine revs nice clean and fast. I think I still need more fuel at off idle. What more can I do here?

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Old 08-02-2008, 07:34 PM
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more on the carb...

I have the squirter screws that have the hole in them and the accel cams are pink front and brown rear.
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Old 08-02-2008, 07:58 PM
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This is from the holley library, same as in my holley tuning book.


QUESTION My vehicle has a stumble from a dead stop. What can I do to cure this?
ANSWER The most common cause of a stumble is not having an adequate accelerator pump shot. The first thing to do is to look at the discharge nozzle and make sure you are getting a good strong pump shot. If not then you need to inspect the pump diaphram for a hole or tear. You will also need to make sure that the pump passage is clear from any trash or debris. First you will need to check the adjustment on the pump. To do this you will open the throttle all the way (WOT). Push the pump arm lever down and then adjust the pump override spring to obtain .015" clearance between the pump arm and lever. If you are having a stumble and no black smoke out the tailpipe then you will need to increase the shooter size. If it stumbles and you are getting black smoke from the tailpipe then it will be nessasary to decrease the shooter size.
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Old 08-02-2008, 09:05 PM
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ok...

Ok, the spray from the squirter looks like a pressured spray. The primary sprays first then as I continue to accelerate the rear squirter starts spraying too. It gives a very noticeable ssssiittt sound as they spray fuel. Don't notice any black smoke from exhaust and plugs have a powdered rust color buildup on them. 37 is the largest squirter I have. I just order 40, 42, and 45 tube type squirter sizes. On these squirters that don't have the tube can a guy drill the holes larger?
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Old 08-02-2008, 11:01 PM
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Sounds like you may have a timing curve problem. The squirters you're running should be Ok or too large for many combinations.
What's your total engine combination, vehicle, gear ratio, stall?
You may actually have too much pump shot-I've seen that a few times.
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Old 08-03-2008, 06:55 PM
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got it...

Pumped Power valve from a 45 to 65 and problem solved. I think Im gonna try going down a size or two on the squirters now and see what happens. Why does everyone say the PV should be half of vac reading on automatics in drive. Holley says to take reading in drive and subtract 2 inches for correct PV size!
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