carb tuning help

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carb tuning help

Postby katostud » Mon Jul 30, 2007 12:30 am

i have a holley 750 double pumper carb with a proform main body on my 350. it runs good but had some bogs and flat spots. it seems 2 have a little stumble on the primarys and a bog when the secondarys open and theres some black soot in the pipes like its running rich. what jets do u recomend? i have a 350 with 10.5to1 compression,air gap intake stock iron heads with z28 springs not sure of the cam but guessing like a 540 lift,hooker 2135 headers with 2 1/2 in exhaust with turbo mufflers
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Postby 2lucke4u » Mon Jul 30, 2007 2:22 am

Kato
Just did the dyno tuning and settled on 72 primary and 78 secondary,that put the air/fuel just right at 5500 rpm,I had the same problem before we started the tune,,
Charlres
80 monza coupe,black,355,.488 lift cam,performer intake.B&M th350,tci 2200 stall,3.42 posi,street car,
74 GTO 350 4bbl,3sp,3.08 posi,buccaneer red
68 Firebird 400, 4sp,3.08 posi,black
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Postby Sirshredalot » Mon Jul 30, 2007 11:55 am

Where are your bogs?
Are they at idle when you give it gas or under a load when driving?
Just off idle? WOT?

Lemme know more...I love carb tuning.

God bless
-Shred
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Postby katostud » Mon Jul 30, 2007 12:41 pm

its when your driving when u give it some gas. and when its ideling in park and u smash the pedal
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Postby 2lucke4u » Mon Jul 30, 2007 1:10 pm

exactly my problem before the tune,,,change your jets and go from there,,,
Charles
80 monza coupe,black,355,.488 lift cam,performer intake.B&M th350,tci 2200 stall,3.42 posi,street car,
74 GTO 350 4bbl,3sp,3.08 posi,buccaneer red
68 Firebird 400, 4sp,3.08 posi,black
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Postby Sirshredalot » Mon Jul 30, 2007 4:15 pm

Accelerator pump shot.

Youre not transitioning well from off idle to WOT circuit.

Lots of things can help.

First i would take vaccum readings to see where your vaccum is at idle and from there select your power valves.
If your PV's are rated higher than your vaccum reading (big cam) than youll have no fuel to cover the transition.

Also bigger discharge nozzels(squirters) can help as can a 50cc pump setup on the primaries.
The bigger nozzels will flow more fuel faster to help with that bog....you can also change the accelerator pump cams to get fuel to come in faster.

Jets will susally cause you to loose top end or have a high rpm surge.

either way shes wanting more fuel.

God bless
-Shred
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Postby Caveman » Mon Jul 30, 2007 7:32 pm

Shreds is correct. But, I would snuggest that once you are running along you may be too rich on your primary's with the black soot problem also, look at your spark plugs.
John
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Postby katostud » Thu Aug 02, 2007 1:32 am

thanks 4 your help guys. my dad broke out the old holley tuning books and we got it close. we drilled out the squiters a few thousands and got it close. 2 of the squiters that came with the proform main body werent even drilled all the way through. gotta switch jets in the primarys and it will be good 2 go
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Postby Sirshredalot » Thu Aug 02, 2007 4:47 pm

Hey Katostud,

Take off your accelerator pump cam from the primary side and turn it around "backwards"

That will give you some faster pump shot and help out...on the cheap too.
Also if its the kind that has the wide ramp...you can file that off to get it to seat flush.

God bless
-Shred
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Postby katostud » Fri Aug 03, 2007 1:51 am

thanks again. we switched cams and the pumps are working great now and drilled out the squirters and bogs and flat spots are gone. we switched jets in the primarys cuz there was a serge at 2800 to 3000rpms crusing and we went the wrong way. putting in the different jets tomorrow 2 see if it cures it. also now its starting to run out of gas at 5700 rpms with the stock fuel pump so my buddy lent me a bigger one to try out
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Postby katostud » Sun Aug 05, 2007 4:09 pm

switched jets in the primarys and put on the bigger fuel pump. that was the ticket. runs great now. no bogs flat spots or hesitation.
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Postby Caveman » Sun Aug 05, 2007 5:32 pm

Great, You gotta love carbs. EFI is fun too.
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Postby katostud » Sun Aug 05, 2007 11:23 pm

one more question. when i hit about 6000rpms theres a little popping. do u think i might be a little lean on the secondary jets still? i never touched them in the tuning we did
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Postby Caveman » Sun Aug 05, 2007 11:32 pm

Popping could be ignition or is it stalling what they call pushing over. A good rule is about 5 to 7 jet sizes up from your primary's on your secondary's. Could be valve float too.
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Postby katostud » Mon Aug 06, 2007 12:08 am

i dont think its valve float cuz it wants 2 keep reving. put in the jets that came with the proform main body. not sure what they are. had to go up 2 jet sizes in the primarys so im thinking i need to go up in the secondarys as well. im at 74's in the primarys will see this week whats in the secondarys
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