General Category > Quadrajet Carb Talk and Tips

off idle stumble/lean mixture

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forkitman:
  I have an off idle stumble until about 2,000rpms.  APT adjustment does very little.  I notice that the power valve is not mov ing at 1500rpm.  Is this right?  It has 14hg. at idle.  Do I need a stronger spring under the piston?  Carb #17084227.  75 jets, 47 rods. XE262h-10 cam.  10/1 compression, big valve angle plug heads.  350/4speed 81 vette.

Cliff Ruggles:
What 47 metering rods are  you using?  They should have a secondary stamp on them after the numbers....Cliff

forkitman:

   The rods have 51M stamped on the main body.  The taper runs from 43 to 51.  I don't know why I thought they were 47.    This carb originally ran good on the 81 L81 motor with 73 jets. I put 75 jets in and it runs better, but the plugs show it to be lean still.  How do you know if you have the right spring in the power valve?  Thanks----Bob

Cliff Ruggles:
M series metering rods are correct. 

We NEVER read plugs to determine rich/lean, impossible with this new fuel.

We tune for results, smooth off idle, and at light heavy throttle, thru transition, etc.

RPMS have NOTHING to do with the movement of the power valve, it moves with throttle angle/engine load, which you can not duplicate without actually driving the vehicle. 

A stronger spring lowers the cut-in point, but seldom the correct course of action to tune for light part throttle issues.  The PP spring is for tuning heavy part throttle and quick movements of the throttle which require increased fuel delivery.  The accl pump also helps these scenarios.

For light/part throttle operation and light load tuning do it with the APT system.  I prefer to determine the correct jet size FIRST, by doing some heavy part throttle driving, then go back and fine tune the part throttle/light load range with the APT.

If you can't come in range and the jet size is fine, they go with smaller metering rods (assuming too lean).  If it's rich, go with larger metering rods.

The camshaft you are using is not a user-friendly cam.  It may be contributing or even causing the low rpm issues.  I've had a number of engines in here with multiple issues, and the XE cams have been a common denominator more times than not.

The are on 110LSA, with short seat timing events and excessive opening/closing velocity for the valves.  "Chop" or irregular engine operation below 2000rpms is not uncommon with them, and is caused by the early intake closing point, combined with the short seat timing/quick valve events (bouncing the valves on the seats and some noticeable "reversion"). 

This certainly produces a nice sounding idle, if you like a little "lope", but I've seen those engine run rough up thru and past 1500rpm's or so until things would smooth out, with the only real contributor being the camshaft.....FWIW.....Cliff

forkitman:
 
  Thanks for the info.  I am going to try and richen it up some more with jets and/or rods and see what happens.  I have had a hunch since I started this engine up that this cam was junk, but you would think that Compcams would know that.  WTF?  I appreciate your help.    Thanks
Bob

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