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post #7 of (permalink) Old 06-29-2009, 03:26 PM
Way Outta Control
Join Date: Sep 1999
Location: usa
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Verified by an air fuel gauge? All the AF gauges I've seen aren't worth looking at. The only way is to measure the CO with an infrared machine. Even then it usually won't help with a stumble.

If you want, look up "propane trick" on here - a sticky - it will tell you in seconds if the stumble is from a lack of fuel. Shoot a bit of propane to it while accelerating, if the stumble goes away it was lean.

But - I doubt it.
You said the pump's working, giving a nice shot - make sure it starts shooting just as the throttle cracks open, not part way open. That's simply a linkage adjustment.

Two carbs, both with the exact same symptoms? What are the chances 10 more will be any different?

My feeling - ignition.
1. The TFI coil MUST HAVE A FULL 12 VOLTS TO IT AT THE + TERMINAL. Check it while running, not with the engine off, key on. The old ignition resistance wire or resistor MUST BE BYPASSED!

2. Check the advance - you said 10 degrees at idle - that must be with the distributor vacuum disconnected! The distributor vacuum MUST BE PORTED VACUUM - near 0 at idle, smoothly progressing as the R's are raised.

3. Use a timing light - vacuum disconnected - it should SLOWLY advance to about the low 20's at about 2000+ RPM. That's the mechanical advance.

4. Connect vacuum, check advance again - it should SLOWLY advance to about 35 degrees at 2800.

If in either case it "jumps" or "slams" from low to high advance - fix that.

5. Pull off a plug wire at the plug - look at the spark as it jumps to ground. It should be a scary sounding, snappy, blue/white spark at least 3/4 of an inch long. If not, find out why.

6. Plugs - make SURE the plugs are the ones the mfgr called out for the ENGINE, not the ignition, not what the parts kid said. USE STOCK ORIGINAL PLUGS designed to work with the heads. Avoid all the aftermarket trash, many of them will cause stumble or internal damage.
Gap at .035 - .040, not at .080 like some fools will tell you.

Stumble is rarely caused by jets - running troubles yes, but not tip-in sag or stumble.

Wrong advance, weak ignition etc are very good places to start looking.
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