Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Qjet 17057253 on a 380 Olds stroker
Seff:
Car: 72 Cutlass
Engine: Olds 380 stroker 9.5:1, Ebrock performer RPM intake, stock 7a heads with 2" intake valves and a bowl blend port job. HEI connected to manifold vacuum.
Cam: 221/228@.050 on a 112lsa, hydraulic roller cam.
Idles at 13" at 750 RPM.
Carb: 17057253 rebuilt by the third recipe. Soft PP spring.
5 speed manual transmission.
Symptoms: (With APT at 2.5 turns up from bottomed out)
Cruises lean (16-18:1 AFR), both in 4th and 5th gear. More so in 5th, where vacuum is lower due to lower RPM.
Gets rich (12:1 AFR) when engine braking/letting off the pedal.
When going up hill and maintaining speed (as in, a slight increase in throttle opening), the engine leans out.
The opposite, going downhill with less throttle opening, fattens up the mixture.
Symptoms: (With APT at 4 turns up from bottomed out)
Cruises at a perfect 14.6-14.8:1 AFR.
Leans out worse when going up hills
Even fatter when engine braking
My idea is that the jets are too small, forcing the APT to keep the needles too far up to maintain a good cruise AFR - this makes the PP have too little travel to work properly - this means the needles can't bottom out under high vacuum during engine braking, and that they can't rise enough to fatten properly at a moderate throttle increase.
Am I totally off?
Thanks.
Frank400:
Numbers are one thing, but the question is: Does it run good ?
Leaning on slight acceleration (power piston still down) is normal from what I've always seen, at least on all the Qjets I've built, all of them dual main air bleeds like yours. It's something you never notice until you have an AFR gauge.
Out of curiousity, what size are your jets and primary rods ? Have you measured your main air bleeds ?
What is your "heavy load" AFR ? while staying on the primaries.
As for the richening while deccelarating, two things that I can think of; throttle opened a bit too far at idle, exposing a little too much of the transfer slot, where it can pull lots of fuel on decceleration. You can give the motor the air it needs at idle by enlarging the idle air bypass and closing the throttle a bit with the idle speed screw.
Not pretending to be an expert, but third recipe for such a cam at 112 lsa might be a little on the rich side ?
Seff:
No, I don't feel it runs right.
MABs are 0.070".
I built it to the third recipe based on the vacuum at idle.
Heavy load AFR is up there in the 16.8-18 range.
73 jets and 44 tapered rods.
I have idle bypass air currently, though it's not terribly large.
Cliff Ruggles:
I've built hundreds of 1976-1989 Olds carburetors with that jet/metering rod combo with perfect success, so suspect you may have other issues.
Float level is a big player, as is fuel pressure and size of the fuel inlet seat.
I don't like or use manifold vacuum to the advance for most well thought out engine builds and you need to look at how much timing it's adding and how strong the spring is in the vacuum unit.
Might be dropping out timing as you increase throttle angle and engine load which effects engine performance in the "normal" driving range.
Also look at the mechanical advance curve, should be adding nothing at idle but start right off idle and all in by about 2800-3000rpms or so.
For the later Olds units I seldom tough any of the airbleeds, so if you opened up the lower MAB you may need to add a little more jet to make it happy but it still wouldn't be nearly as far off anyplace as you are seeing with a .070" bleed in that location......Cliff
Seff:
Float and seat adjusted. Throttle blades are nigh totally closed at warm idle, do I dobt think the transfer slots are active?
Timing starts at 13 btdc at idle and totals at 36 at 3000 RPM. Vacuum adds 18 degrees.
Will test now.
Unrelated question: I live in a cold climate, and drive year round. Would hot idle compensator help my cold carb drivability?
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