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Did it run fine BEFORE you replaced the carb? And you didn't change anything else?
If so, while I certainly concur that it's worth checking all the ignition stuff etc., it seems most likely that something about the carb or your carb installation is the culprit.
The fuel pressure idea is a good one, and so is cleaning the carb -- it's not impossible to get a brand-new carb with crud in the internal passages. Usually I put it on the car and then blow out the jets by pressing a rubber ear syringe against them and giving a few squeezes.
I'm also going to suggest the POSSIBILITY, though, that unless you got your Weber from a Saab specialist who set it up for you, it may have the wrong jets in it, especially the "idle" jet. I put "idle" in quotes because this jet doesn't just control idle -- it controls the whole transition circuit, which is what the car runs on up through 3,000-3,500 rpm or so.
If the idle jet is slightly too small, the symptom you get is that the car runs fine at idle, under light loads at moderate speeds, and at or near full throttle -- but when you try to make the transition from part throttle to full throttle, you get a hard stutter that won't go away until you back off and apply the throttle more gently. What's happening is that as you get to the top of the transition range, but before the carb has switched over to the main circuit, it exceeds the capacity of the idle jet and the mixture abruptly goes lean.
One crude test to see if this might be the problem is to accelerate up to the point where the stutter occurs, then pull out the choke knob a bit. If the stutter goes away, that's a tip-off that the cause is a lean condition.
I hesitate to suggest jetting as a cause because the impulse is always to stick in a hugely larger jet. But don't do it! When it comes to jets in a Weber, bigger isn't always better -- it's easy to "solve" that problem and screw up something else, and eventually you give up on ever getting the car to run decently and join the people who spread the word that "Webers are impossible to tune." That's definitely NOT true -- but you've got to isolate the problem carefully (hence the choke trick above), change only one variable at a time, and work in small steps until you've got the optimum settings.
Some dyno time with an air/fuel ratio meter is supposed to be a big time-saver here, but I've never tried it. Anyone else?
posted by 68.227.170...
, Thu, 11 May 2006 05:38:50
, Thu, 11 May 2006 07:36:59
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