950 hp holley

[cwhubb]

hey, I have a 950hp dbbl pump carb that i need fine tuning advice with. it idles great, is very snappy when you rev the motor in park but it stutters and stumbles under a load almost like when plug wires are bad. It came off a fla car so i had it totally rebuilt and I changed out the 83 jets for 63's I live at 6300ft I checked the plugs and they look pretty good so I know I'm close jet and air/fuel mixture wise, it's an older carb so it doesn't have the plastic linkage adjustment cams, do you think the secondaries are coming in too soon? if I get on it from a rolling start the car takes off pretty hard. thanks

 

[lewtot184]

i would think 63's would be way too lean for that carb, even at your altitude. holley jets those carbs at #79's front and back. this could cause a stumble or stutter.

 

[Meep-Meep]

I don't have a feel for how much venturi area that carb has, but I agree 63 seems like you might as well plug them off.

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You might also try a smaller carb.

 

[cwhubb]

I didnt know that lean jets would cause a sputter situation, I'll try some bigger jets what do you guys think about 73's also do you think I have to mess with the accelerator pumps?

 

[lewtot184]

I don't have a feel for how much venturi area that carb has, but I agree 63 seems like you might as well plug them off.

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You might also try a smaller carb.

those 950hp's have about 6sqin's of venturi. they're just a 750dp with a contoured body and 850 throttle bores. won't flow 950cfm, probably closer to 850. the holley catalog states they came with either 78 or 79 jet on all 4 corners with 6.5 power valves. if it were me and at that altitude i'd start at 76's or 77's. a lot of this depends on the power valve restriction size. going from a 79 jet to a 63 is over 50% reduction in jet area. way,way too far lean. going from a 79 to a 76 is an 11% change, which should be noticeable.

 

[bigaadams]

950HP Holley...when did the rating change from CFM..

 

[Meep-Meep]

those 950hp's have about 6sqin's of venturi. they're just a 750dp with a contoured body and 850 throttle bores. won't flow 950cfm, probably closer to 850. the holley catalog states they came with either 78 or 79 jet on all 4 corners with 6.5 power valves. if it were me and at that altitude i'd start at 76's or 77's. a lot of this depends on the power valve restriction size. going from a 79 jet to a 63 is over 50% reduction in jet area. way,way too far lean. going from a 79 to a 76 is an 11% change, which should be noticeable.

Oh, is that all they are? Then yeah, even as a 750 63 jets ate ridiculous. If he has huge PVCR's then a leaner main will be used, but I doubt that still won't justify a 63. I always start with the factory jetting in any carb and work from there.

 

[cwhubb]

I just ordered a set of jets from jegs, I'm going to try 75's and see where that gets me, the pumps that come with the 950 are 30's. the cam I have isnt radical its the 509 mp so I'm thinking 5.5 power valve

 

[lewtot184]

a 5.5 power valve may not be enough. altitude lowers engine vacuum. i'm thinking a 3.5 or 4.5. you'll have to do a vacuum check to be exact.

 

[daredevil]

.509s lumpier than ya think. 72 degrees of overlap bleeds off some vacuum

 

[cwhubb]

went to 75's front and 76's rear, the car runs way better, do I have to get new plugs every time I read them?