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#1
03-20-2006, 05:06 PM
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carb jetting help
hello...i need some help concerning jetting a qjet. going by what my spark plugs are telling me, i am way lean. anyone got any idea a baseline to start with? the carb is a chevy unit, off of a 78 350. the engine it's on is a pontiac 350, small valve #16 heads, .030 over, decked .010. cast iron 455 intake, summit 2800 cam, msd 6a box with gm hei dizzy, iron exhaust manifolds. thanks
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#2
03-20-2006, 05:23 PM
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heff,
Those carbs need to be modified to run properly on a pontiac even though your engine is a 350. From the needle and seat to the idle down tubes and by-pass air circurt all have to be set up for a pontiac..... Cliff Ruggles helped me do my first conversion and I've since done others.. As far as jetting you should hav atleast a 70 jet with a metering rods abot 30 numbers less than the jet size with the correct size needle and seat. |
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#3
03-20-2006, 05:38 PM
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i have the qjet from the 301 i pulled, but it's missing some parts. i guess i could use parts from the chevy carb that are missing, but it's all external stuff gone from the 301 carb. i assume that would work better? start with the 70 jet/40 rod with the pontiac carb?
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#4
03-20-2006, 11:37 PM
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What really helps is if you pull the carb and post the carb ID number (begins with 170), main jet size, primary rod size, and secondary rod size. These are stamped on the parts. Experts like Cliff R. and Kenth can take it from there, if they are listening.
Why do you think it is lean? What are the plugs showing you? |
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#5
03-21-2006, 11:16 AM
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according to pontiac dudes site, the base ring on the r43 plugs, which should indicate jetting, doesn't have color on it at all, and the porcelain is almost snow whtle, and i can occasionally hear some pinging, which from what i've read can be caused by a lean condition? is that correct?
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#6
03-21-2006, 11:30 AM
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A lean condition or too much advance on the timing. I would check the timing first, then go after the mixture.
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#7
03-21-2006, 10:26 PM
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initial timing is set at 12deg right now, i'm not sure what the mechanical advance is, the dist. is borrowed from friend, but he said it would be somewhere around 32-34. i really don't know if that's too much or not enough, i think it's pretty close from what i've been reading
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#8
03-22-2006, 12:12 AM
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It's near impossible to tune by reading plugs. Tune by engine/vehicle performance. Lean primary jetting will show up as "flat" on accelleration, loss of or lack of part for part throttle operation, and excessively lean it may surge and "buck". Most of the later carburetors use APT, which allows for fine control of the part throttle A/F by simply raising/lowering the power piston, which repositions the metering rods in the jets. The later rods are tapered on the second step, or at least tapered between steps, to provide a broad range of control over the A/F ratio for part throttle engine operation.....Cliff
__________________
If you can read this, thank a teacher. If you can read this in English, thank a Veteran! https://cliffshighperformance.com/ 73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile), |
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