That's what we do... needle spacer...
... but,.. if you're into experimenting,... (which it appears you are) ...
there is an airjet above the rubber diaphragm accessed from the top... (making it larger or smaller will change the amount of vacuum pulling at the slide)...(not sure which way does which)...

Adjustment of enrichment at full vacuum decel,.. is the TEV's job... but by playing with the spring, you are only adjusting when it activates, which will make it activate when it shouldn't...
Not sure how,.. but you need to adjust how much fuel it adds, rather than when...

I believe the TEV is not really doing much incremental fuel control... more like the vac-pet,.. it's open or closed...
...(a tapered needle, is incremental... a flat stopper, is on/off)...

What I find that eliminates backfire is throttle. I've gotten pretty good at controlling the backfire while riding by smoothing out the way I get on the throttle.

As a matter of fact, I can decide pretty much decide when I want it to backfire...cuz sometimes it's just a matter of letting people know where you are...

Awhile back I messed with the TEV and it made my bike run horrible, smoke like a diesel and it did actually get rid of the backfire...i put it back to the stock spring...

I can mostly control backfire when I'm riding alone, but when I'm in a tight pack of 20 bikes hurtling through the twisties, I have to open up or jam shut the throttle frequently and often abruptly. And that's when I get a lot of backfires and some annoyed or startled fellow riders.

Quote:

  I think the yellow warning at the head of this thread needs to be revised. I took the bike out on an hour-long ride that combined highway, back roads, and town driving and tried to simulate the kind of ride that would make it backfire, gunshot style. I got a couple of little pops from time to time, but no gunshots at all while riding. I still get a gunshot crack on shutdown at least half the time, but I think that's a different issue.

I believe this is a good technique to try if simply turning out the idle mix screw doesn't get rid of backfires.

Reading over the earlier, locked thread, I think the problem most people had was using too weak a spring or taking too much off the stock spring, so the TEV was engaged all the time. The 1/8" I took off the length only slightly weakens it, but it seems to allow just enough additional fuel to flow on high-vacuum decels to prevent backfiring. At some point I may trim another 1/16 or so off the spring to see if it helps with after fire, or if the backfires come back on an actual group ride, but for now I'm happy. The bike is much more fun to ride without having to monitor my throttle use to keep from backfiring.

The parts list doesn't help much - somebody with a carb in their hands needs to look closely at the TEV circuit....and see where it pulls the fuel and air from.  It may be that it uses the Enrichment lever circuit.  The parts list shows part 40.....No. 2 air pilot jet?????  What does that one serve...the enrichment circuit?

Been riding for a while now with the slightly-clipped TEV spring and I think this is a pretty good workaround for an S40 that backfires a lot at sea level, for which turning out the idle mix screw is not sufficient and that has no air leaks. It's gotten cold here, but the backfires are still gone and otherwise the bike runs the same.

I'd like to see it back in the tech section. The trick is to remove only a little of the spring at a time until the backfires stop. It might be a different amount for different years or altitudes, but it probably won't be more than I had to clip, since I'm literally at sea level, where it's running leaner than it is for folks in the hills and mountains.

My two cents, anyway. Made the difference between I bike I decided to trade toward something else and one I'll hold onto a while longer.

I bet it would work for you, too. It just makes the decel a little richer. Too rich at 4000,' probably, but only briefly, and only a little longer than the TEV richens the mix anyway. If I ever get up to 4000' I'll let you know if there's a down side to the fix, such as idle dropping too low on decel. Still too early to see the effect on gas mileage, but I'm fine with trading a few mpg for a quieter ride.