power valves in racing carbs

Old 06-04-2008, 11:24 PM
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Your welcome Art, anytime we can be of help dont hesitate to make a post.
Probably 83 square would be a good starting point if you go without the PV's, just keep playing with it going up or down on the jets 1 or 2 jet sizes at a time and see what makes the car the most consistent.
Note: make only one change at a time and always run what you brung making changes inbetween rounds is a no no unless its absolutely necessary.
I prefer to have mine square and a tad on the lean side.
If you bracket race like I do and especially if your race at night where it gets cooler as you get into the later rounds a fat or rich carb will give you fits trying to keep it dialed in because as the night air cools it becomes more dense which helps the engine makes more power so it will usually make the car really start picking up between rounds, but if you have it a tad on the lean side it doesnt do it as much.
To help with a possible off idle stumble on a carb thats alittle on the lean side I always used a 50cc acc pump on the primary only which will give you super crisp throttle response.

I hope this helps and goodluck, Charles
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Old 06-05-2008, 04:47 PM
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CPEX: I am confused about power valves too, but I have a very healthy street motor. I have a QF 950 with primary 78 & 3.5 PV. Can't remember secondary set up. Anyways, I want the motor to be a little better for street, because I will go to the track one or twice a year. The motor was in the car when i bought it & it is pretty healty... the trouble is, I just want a little better low end performance/ better idle w/ less loading. Do you think a vacumn advance would help? I don't have one now... its all MSD & 6AL. Would you mind telling me about low end efficentcey? Its ported & polished, 10.2 comp, Team G, BBC all roller w/ solid lift Lunatti. It has 3500 Stall & 488's. Any feed back appreciated. BTW- I drive it on the street occasionally as a toy only.
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Old 06-06-2008, 11:32 PM
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Default power valves

hey cepx111 gao your last post.learned another thing, which is contrary to the way i always set up the carbs. you said to use the 50cc accelerator pump in the front, and i assume the 30cc(smaller) in the rear. is this all the time, or just when you set up the carb for one special setup. could you explain. thanks chevy art. PS this forum is a real trip. thanks for the ride art
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Old 06-07-2008, 12:22 AM
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Yeah anytime you mix big cams and single plane manifolds a 50cc shot on the primary is almost manditory as with some squirter changes. Now putting one on the secoundaries can be a preference or a tuning aide, like I stated before I prefer a leaner carb set-up and thats why on that particular set-up I didnt go square on the pumps not to mention I was running a nine inch slick where traction was marginal at best and I didnt want to take the chance of braking the tires loose.
A double shot off the line just might make you go from a lean stumble to a rich stumble, so make little changes one at a time seeing what works and what doesnt, keeping waht works and removing what doesnt.
And dont make the mistake of trying to cover up something that doesnt work by changing something else, theres a happy medium to everything and finding it is the key to tuning correctly, again to your particular set-up.

Goodluck, Charles
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