Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Top end only 90MPH, then Carb backfires
wheelz:
Changed my motor from a Pontiac 400-4X with Ram Air Exhaust to an E-Headed 11:1 Pontiac 455 (462) with headers and the 60919 cam.
The 400 had a Cliff's Qjet built for it and it ran great from idle to as high as I ever took it. It just kept pulling. Took the same Qjet and HEI and put it on the 455. Only other change was a new Carter M6907 fuel pump (120gph @5.5psi)
I managed to get the idle setup by adjusting the idle mixture, but it required 16 degrees advance.
Place my hand over the primaries at idle and rpm increases slightly...indicating larger jets required.
Throttle response and mid-range power is OK, however it has no top end. It stops pulling at about 90MPH and will begin to repeatably backfire if kept at full throttle.
Swapped out the secondary rods from DH to DX, but there was no change.
Do not know the current jet size and primary rods used, but I suspect I need larger jets for the 462.
I need some recommendations, as I have a long road trip (Bandit Run) coming up in 2 weeks and need to solve this quickly.
wheelz:
Got it out for a drive tonight and realized that it will not go above 3800rpm while driving. Free revving , it backfires at 3800rpm.
77cruiser:
Might have some broken or real weak valve springs.
wheelz:
Thanks...Guess I will check that next...although they are brand new and this is the recommended kit for the 60919 cam from Crower.
Cliff Ruggles:
I would have suspected running the bowl low or empty (fuel delivery) but that shouldn't happen "free revving" it.
It needs more idle fuel not bigger jets if you had to advance the timing and still a little lean with the mixture screws backed out sufficiently......Cliff
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