Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Chasing lean condition at WOT

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Marx3:
.118 MAB's, 75 jets and 50M rods sounds exactly like any smog SBC 350 would be calibrated... Your 500 deserves better! Nice setup btw. I remember your thread on cadillacpower.

If I were you, I would at least get a hold of some of Cliff's 44 primary rods. 
Dont think you wil ever get full control over the afr's, with that large mab calibration.
Are you able to drill and tap the MAB's, so you can install screw ( or press-in ) air bleeds, to re-size the MAB's?


I rebuilt the Qjet on my buddy's stock ( pure stock ) 1976 500. I bumped up the timing some and at WOT, the engine would ping like crazy, even with DA rods, untill I got them high enough ( bent the hanger upward ).
Cliff later confirmed, that these huge Caddy engines need ALOT of fuel.
If I recall correctly, I ended up with .070/.070 MAB's and 72 or 73 jets with Cliff's 44 rods plus DA's. With the advance curve made longer ( stock HEI ) and total timing around 30, that big old Eldorado does 0-60 in about 8 seconds. With just at carb and timing tune. ( wouldnt even spin the tires with the stock setup )

77cruiser:
You might have to open up the seat to .140. Can you up your fuel pressure a bit?

Frank400:
M rods have the large .036 tip for single main air bleed carbs.  If you have dual .118 air bleeds, you need different primary metering rods, the ones with .026" tip.  That's why you're running lean while under heavy load. 

    Frank.

Cliff Ruggles:
Correct, "M" series metering rods are a poor choice for those carburetors, and for sure it needs more fuel everyplace.

The "stock" set-up for many of the later large MAB carburetors used 77 main jets and 48 or larger "K" metering rods, and they were pretty lean at part throttle on the pathetically anemic smog engines they were used on.

If I were building one of those carburetors here, it would get smaller MAB's and a completely different calibration, to effectively feed the big 500cid Caddy engine......FWIW.....Cliff

Jeffs68:
Just to satisfy my curiosity, did you have any drivability issues, or was the A/F ratio the reason you started making changes.

I wouldn't be too concerned with the motor going lean at WOT, how long do you stay at 5K + Revs?

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