Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

weird off-idle hesitation

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Marx3:
stock 170 hp 8.2:1CR chevy 350.
Qjet 17059276
All airbleeds .070
71 jets, 44 rods, tip-in perfect.
.036 tubes and .050 DCR.
acc.discharge .027
Car ran perfectly. Perfect response, no hesitation anywhere.
Then I put on better iron heads ( should be hitting about 280-300 hp at the crank ), a 218/224@050 flat tappet cam, an edelbrock dual plane and built a 2.5" dual exhaust.
CR is 10:1
Car still had great response everywhere.
Timing was mid to high 30´s ( cant remember exact number ), so I backed it down a bit because it would ping from the the higher CR.
Backed it down about 5 degrees to low 30's.
The timing is now 15 initial, 32 total plus 14 degrees vacuum.
Now that I have backed the timing down about 5 degrees, the engine has developed an annoying off-idle stumble.
I dont understand ?
I have been runnign proted vacuum and tried full, just to see if it would help. It didn't.
Figured the engine might needed a bigger shot from the acc. pump so I upped the discharge holes from .027 to .031. Didn't change a thing.
I can always put on another airhorn, if I need the discharge to get smaller.

77cruiser:
Can you shorten your advance curve so you can put your initial back up?

Marx3:
Yeah I guess I could easily put in the stock advance center piece and weights, just to try if this would solve the problem. That should put my curve down around 10-12 degrees again.
I prefer it up around 15-18 degress though.
I just dont get why I get a noticable hesitation, just because I back the timing down from 20 to 15 initial.

Cliff Ruggles:
The accl pump will only effect off idle or transition if you are getting into the throttle quickly, or going to heavy/full throttle.

A hesitation caused by reducing the timing tells us in most cases that the mixture is pretty lean and requires a lot of timing to burn it effectively.

You can at this point add timing back in or fuel, or a little of both to correct the issue.

I'd start by raising the APT screw, to richen it up some in that range.....Cliff

Marx3:
re-checked timing today. It turned out that was a bit more retarded, than I thought.
It was at about 5 BTDC initial.
Got it up to around 15, like it should. That helped a bit.
Then I re-checked pirmary calibration and it WAS a bit lean... ( did both the tip-in and put in a wideband too )
I was at about 16:1 at cruise.
backed the APT out a bout 1 full turn. Tip-in was better and the wideband now said 14.5-14.8:1
Ideal. Car feels a bit stronger and has much less of a stumble / hesitation.

The hesitation is noticable on of-idle take off.
The stumble is not really there, once the car is moving.

I have a feeling that the 214/224@050 cam ( 112 lsa ) is struggling a bit with the 2.41 rear end and the stock converter?

Do you think .031 discharge could be causing this? I know .031 is propably a couple of sizes larger than needed, but do you think it will cause a stumble?

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