ok heres my combo then i'll tell ya the problem.
355 chevy, 216/228 cam (.454/.480 lift), victor jr intake, hooker super comps, and a holley 750 DP (4779), and 20 degrees timing at 750 with no vacuum advance.the carb has .045 shooters, dual 50 cc pumps with the brown cams adjusted at .017 clearance, 72/76 jets and a 6.5 power valve.
ok heres the problem. the car starts up fine and idles great and all at about 750-800 in P and N. but my plugs are black as anything and also when i go to pull the throttle really quickly the car either backfires through the intake or it will jsut die i need help i can get this thing to run right.
355 chevy, 216/228 cam (.454/.480 lift), victor jr intake, hooker super comps, and a holley 750 DP (4779), and 20 degrees timing at 750 with no vacuum advance.the carb has .045 shooters, dual 50 cc pumps with the brown cams adjusted at .017 clearance, 72/76 jets and a 6.5 power valve.
ok heres the problem. the car starts up fine and idles great and all at about 750-800 in P and N. but my plugs are black as anything and also when i go to pull the throttle really quickly the car either backfires through the intake or it will jsut die i need help i can get this thing to run right.
Supreme Member
Too much intake, too little cam.
You say your pumps are adjusted for "clearance"; you do realize of course that the way you're supposed to adjust them, is such that with the throttle held wide open and the pump lever pushed as far as it will go, there should be that much clearance. There should not be any clearance whatsoever with the carb actually operating, the pump should rest with tension on the lever, which should rest on the cam, at all times.
Your PV is too low value for that tiny cam, since the carb was designed to go on a motor that is set up for performance; you should have around 18" of vacuum at idle out of gear, probably 14" in gear, so your PV should be a 8.5 or a 9.5.
Lean your primary jets down 3 sizes at a time until the car starts to surge while cruising in high gear, then go back up 2 sizes. If it stumbles at tip-in that way, increase the PV number until it cleans up; but don't let the PV number get higher than your in-gear idle vacuum minus about 4". (If your vacuum is 12" idling in gear, don't use any higher number than a 7.5 for example). If that happens, richen the jets 2 sizes at a time until the stumble goes away.
Once you get your primaries dialed in, set the secondary jets for best HP/ best time. Leave teh secondary power valve at whatever it is.
You say your pumps are adjusted for "clearance"; you do realize of course that the way you're supposed to adjust them, is such that with the throttle held wide open and the pump lever pushed as far as it will go, there should be that much clearance. There should not be any clearance whatsoever with the carb actually operating, the pump should rest with tension on the lever, which should rest on the cam, at all times.
Your PV is too low value for that tiny cam, since the carb was designed to go on a motor that is set up for performance; you should have around 18" of vacuum at idle out of gear, probably 14" in gear, so your PV should be a 8.5 or a 9.5.
Lean your primary jets down 3 sizes at a time until the car starts to surge while cruising in high gear, then go back up 2 sizes. If it stumbles at tip-in that way, increase the PV number until it cleans up; but don't let the PV number get higher than your in-gear idle vacuum minus about 4". (If your vacuum is 12" idling in gear, don't use any higher number than a 7.5 for example). If that happens, richen the jets 2 sizes at a time until the stumble goes away.
Once you get your primaries dialed in, set the secondary jets for best HP/ best time. Leave teh secondary power valve at whatever it is.
Supreme Member
F-BIRD'88
Supreme Member
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May 23, 2023
Your black spark plugs indicate a rich condition.
They are carbon fouled and will need to be replaced.
I'd suspect the large shooters and 50cc pumps are part of the problem (too rich for that mild a cam)
Your jetting is fine.
Your power valve is fine as long as it is not leaking. (Check the gasket too).
I would replace all the carb gaskets you may even have the wrong gaskets. (internal leak)
Install .031 shooters front and back. Check for the little check valve in the passage below the shooters. (anti syphon valve)
I would replace the front 50cc pump with the stock 30cc pump.
readjust your accelerator linkage as discribed in above post .
Blow out the air bleeds in the top of the carb.
Your timing is 20 deg at 750. What is it at 4000rpm? Should be 32/36. 20deg at idle is a lot for that motor combo. 12/16 initial better.
Why no vacuum advance? (10/15deg ported vac)
Inspect your ignition system. Plugs, wires, cap, rotor.
After solving this problem consider switching to a dual plane hi rise intake ( performance RPM , Stealth etc) It will be a whole different motor.
The Victor JR. is intended for a more radical motor with a much bigger cam.
They are carbon fouled and will need to be replaced.
I'd suspect the large shooters and 50cc pumps are part of the problem (too rich for that mild a cam)
Your jetting is fine.
Your power valve is fine as long as it is not leaking. (Check the gasket too).
I would replace all the carb gaskets you may even have the wrong gaskets. (internal leak)
Install .031 shooters front and back. Check for the little check valve in the passage below the shooters. (anti syphon valve)
I would replace the front 50cc pump with the stock 30cc pump.
readjust your accelerator linkage as discribed in above post .
Blow out the air bleeds in the top of the carb.
Your timing is 20 deg at 750. What is it at 4000rpm? Should be 32/36. 20deg at idle is a lot for that motor combo. 12/16 initial better.
Why no vacuum advance? (10/15deg ported vac)
Inspect your ignition system. Plugs, wires, cap, rotor.
After solving this problem consider switching to a dual plane hi rise intake ( performance RPM , Stealth etc) It will be a whole different motor.
The Victor JR. is intended for a more radical motor with a much bigger cam.