I put the Marvel from my BN on my A. It still doesn't idle great but mid/full throttle is much improved. The idle screw in the Marvel does change how the engine runs. I should add that this carb probably could stand being rebuilt too, the needle leaks a bit and I bet it hasn't been touched in over 20 years, that being said I'm happy to leave well enough alone with it at the moment. At WOT with out any load there's still a miss every now and then but from what I've seen these small engines tend to do that, it's not like my SM that's super smooth no mater where the throttle is.
For the fun of it I stuck the Zenith on the BN. The the idles like a dream, not a miss super smooth and it did respond to the idle screw (ie turning it all the way in changed how the engine ran). Anything above idle though was crap, just like on the A.
So first off I don't think there's an idle circuit problem in the Zenith, it's something to do when it switches over to the main jet. That still leaves me wondering what the heck is going on.
The other factor here is the BN has a battery ignition and the A has a magneto. Some of the roughness at idle might be just the fact that the timing is running fully advanced as a slow speed where as the distributor advances the timing based on RPM. Just saying it's kind of hard to compare the two.
Anyway, if I could get my Zenith to perform like my Marvel I would be happy. I would still love to get it to idle better but that might be somewhat the nature of the beast.
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