I would like to thank all of you that have posted useful information on getting rid of the backfire on my wife's S40. I pulled out the spacer on the needle then pulled the brass plug over the air idle jet {apparently 1/8Th turn in gave the highest idle}and slightly backed off the idle screw.

WOW!!  What a difference that simple task made. Well it really wasn't that simple. All the screws I had to back out were extremely tight and I chewed up 3 of them pretty bad. I had to use an impact driver to get three more of them out and had to use a cold chisel to loosen one other. I went down to the hardware store and bought new ones only this time I used Allen head cap screws in the slide valve and hex head bolts on the diaphragm cover to make it easier next time I pull it apart.

Having this forum saved me all kinds of time and probably a bit of money. If any of you haven't taken the time to do the mod well,, your missing the boat. After the mod That bike behaves itself. No more bucking and surging down the road. It's like a totally different bike.

It used to bother me how bad that bike would run. Now I can say it actually is a pleasure to ride. It's nice and smooth and the backfiring is almost non existent. Once in a while I can faintly hear a POOOSH! come out the exhaust and that's it.

Again thanks to all who post here and your positive feedback.

I'm with you. Remove that white spacer alltogether and with a less restricted muffler just change the jet (jets).

Hell, Billy wrote on 12/31/69 at 16:00:11:

Yes it is and it's another bike you have to fix the carb on to make it run right. No I won't chop it but I do like to make mods here and there. I made the mistake of letting my wife ride the Bonnie and now she likes it better than the S40. That was the reason I wanted to fix the carb. I now ride her bike when we go for a cruize wich is OK because I like both bikes. The S40 is much more nimble in tight twisties but the Triumph is still my main ride. We have 4 running bikes and 2 almost completed projects. It's going to be a nice feeling trying to pick out what ride to take for the day. Nothing fancy just fun bike's. Thanks Savage Greg.

I might add the engine used to make a rattling sound when ever you accelerated. Removing that  plastic spacer has all but eliminated that sound. It's so much more quiet now and doesn't sound like it will wear out as fast. when you open your carb you can tell that spacer was an after thought and it shouldn't be there. It only takes about an hour to do and is very well worth it. If you decide to remove the spacer I will tell you I chewed up the two screws that hold the plate over the needle. I ended up using an impact driver to get them out. I used a 1/4" drive adapter and a 3' extention attached to a socket which held a screw driver bit. I put the bit on the screw head turned the sleeve up side down and tapped the impact driver on the floor being careful to guide the extention with my fingers so as not to ding the inside of the sleeve valve. The screws came loose with just a slight tap then were easily extracted. If I had done that to begin with I might have reused the screws.