Chris,
I've just been through this, let's see if I can help. The stutter or
stumble is due to more fuel/air being delivered by one carb than the
other. The carbs need to be balanced so they deliver the same mix under
the same conditions through the whole throttle movement range. This is
done by balancing the throttle openings and the fuel mixture (assuming
the carbs are working properly).
A few questions:
1. I'm assuming we're talking stock Zenith downdraughts, yes? Are the
carbs WIP or WIA? It should be stamped into the top cover of the carb.
S2 should be 36WIP2 or 36WIP3, but someone may have swapped them.
2. Has the stutter developed quickly, or was it always there? If it's
just developed, it could be the coupling between the two carbs is
slipping, or an air leak has developed in on of the carbs.
3. Are the linkages in good order? No elongated holes or sloppy bushes.
This isn't going to cause the stutter, but it does cause you to make
adjustments to the carbs to get around the linkage problem. In turn
those adjustments can contribute to the stutter.
If it idles OK, and you can adjust the idle speed using the throttle
stop screws, the throttle shafts are probably OK. You can check them by
disconnecting the flexible coupling between the two carbs and wriggling
the throttle shafts side-to-side and up-down. The trick here is how much
slop is acceptable. Your mechanic should have a feel for this. Too much
slop means air leaks past the shafts. This usually shows up as an uneven
idle, and an idle speed which is hard to adjust. If the throttle shafts
are worn I have been told of a company here that has some oversize
shafts left on the shelf.
If the carbs are WIA type, check that the 'economy valve' diaphragm is
OK. It is under the little 'top hat' on the top cover of the carb.
Typically the diaphragm cracks or the spring (in the float bowl of the
carb) sticks. This area of the carb can pull quite a vacuum under
acceleration, so make sure everything is sealing up nicely.
Pretty obvious, but check all the bolts and screws are tight. These
motors have a reputation for vibrating bolts loose.
Finally, if you need any parts let me know. I collected about 6 carbs
worth of bits to fix mine, before Tom W. found me two good carbs :-)
I hope this helps.
Cheers,
Paul.
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