I've read many posts on tuning the carb on the S40 to eliminate backfiring. As my carb had a stripped idle mix screw thanks to the previous owner's attempts to adjust it, I had the local shop drill it out and, as I also had a jet kit, asked them to rejet to eliminate backfiring. I am trying to find out exactly what they did, but from the writeup it looks as if they replaced the pilot jet with one size up and adjusted the idle mix. The bike still backfires. Less often, but still almost every time I either snap the throttle off at any speed or roll it down or off from high speed (as when preparing to exit the highway or stop at a stop sign). What's the most reasonable next step to try in order to eliminate backfiring? It now seems to occur only when the throttle is at about 25% or below.

There should be no exhaust leak, as they also replaced a leaking head plug at the same time the rejetted the carb. Their claim is it will continue to backfire unless I install a less restrictive exhaust, but I'd like to keep the bike stock.

Thanks,
David

More than I wanted to (I think it was $150, but I don't have the receipt handy) but less than the cost of a new carb. I was not confident I could drill the screw out myself without wrecking the carb, nor was I confident I could get the carb bottom off without stripping the screw heads. The carb had not been disassembled before. Now that it's already been taken apart and put back together, it should be safe for me to further tinker with it myself as needed.

Gunshots. Haven't noticed a significant change when the humidity changes. Should I have?

If backfires get worse on humid days, or when riding in the mountains it indicates that you are on the rich side...
If backfires are reduced,.. you are lean...
Either, will cause backfires and popping...
Constant backfires in all conditions usually indicates an exhaust leak...

Some locations have very little variance... Where are you located?...

DavidOfMA wrote on 11/19/12 at 09:58:21:

Yes, it do...
A bigger pilot might help...  if adjusting idle mix to 3 turns out don't do it,.. you might try a step larger on the pilot...
Also, make sure that you idle speed isn't too low... but, I'm assuming that's okay because the dealer set it for you...

Too true... ...

Aside from making sure there is adequate oil pressure to the cam bearings, a higher idle speed will reduce the afterfiring and any tendency the bike has to stall when you come to a quick stop. You can prove it to yourself by closing the throttle slowly and perhaps not quite completely on deceleration instead of snapping it shut. Ever hear of anyone complaining about the afterfiring except when they are decelerating from speed with the throttle completely shut?

So... verified that the pilot jet was upsized to a 55 from a 52.5 and that the TEV is working fine. But, the bike still backfires, generally a loud gunshot crack but sometimes two quieter ones, as I'm decelerating and get below about 25% throttle. Opening up the idle mix screw another half turn dropped the idle speed and made the bike feel a little boggy, while only very slightly reducing the backfiring.

Any other modifications I could try, from a carburetor point of view, that have been shown to fix this problem? I did look at the "trim the TEV spring" thread, but it seems the results there were inconclusive.

Would drilling out the muffler have any effect on backfiring? I'm not interested in doing a muffler replacement at this time, and I don't particularly want to make the bike any louder, but I'd trade some increased volume for fewer backfires.