I bought a NOS BS40 (stock Savage) upped the main  to 157.5 and run a K&N w/ a Supertrapp w/ open reverse cone.
I cut the white spacer down to about 1/2 and fattened the idle screw. I was really on the fence about the 157 or 155 main and went rich hoping to prevent bluing the new Supertrapp. The 'Trapp still colored a little.
It runs OK, but the main feels fat. I am still getting a feel for it. Seat of the pants guess now is to fatten the low speed circuit some and lean the main a size or two. When I get around to it I will drop back to a 155 or 152.5 main, cut more or remove the spacer.
The bike still pops a bit on up shifts, but big boomers are rare, after it is up to temp.
I was not sure which way to go with the pilot jet so I used the '86 size as it was close to the 1st year carb spec which is what I used as a guide.
I never found anything about tuning the jet under the diaphram, I never had to before this one.
The Mikuni is fairly easy to work on. The hard part was finding  the jets. I rode to 10 bike shops and still had to order the 155 & 157 main from the dealer. The main is not going to effect your part throttle as much as the jet needle and that is likely where the backfires are coming from, but hey it's a thumper, it's gonna pop some.
I ordered the '86 jet needle & the needle I got measured the same as the one in my original '01 carb  and after all that I would really like to compare the Dynojet kit to the Mikuni needle. It may have effected the mileage but it still is so much better than my truck, who cares as long as it runs good.

Has anyone ever swapped the pilot jet w/ regard to backfires?

Mr. 650 said
I was really on the fence about the 157 or 155 main and went rich hoping to prevent bluing the new Supertrapp. The 'Trapp still colored a little.
It runs OK, but the main feels fat. I am still getting a feel for it.  


One good way to test your jetting for excessive richness is to run it wide open, then back off the throttle slightly. If the engine accelerates or smooths out, it means your main jet is too rich.

When a carb no longer works as it should I either use it as a paperweight or a baseball. I gave away my 1964 T120 Bonnevilles because they needed carb work (along with other problems).

It's weird, I can tie a size 26 dry fly (about the size of a sugar ant) without a magnifying lens, but CAN'T work on carbs successfully. Too many small parts. Go figure.

I'll give it a try. The plug shows the bike is rich, the pipe shows the bike is rich, but it surges, pops and rings the pipe like it's too lean.
-WD