Have you tried the "thick washer" removal from the diaphragm needle? When I first got the bike it was popping like crazy, I adj. the flow screw out to about 2 3/4 turns and replaced the thick factory washer with two 4mm washers from lowe's. It stopped a lot of the backfire and the pop on shutdown.

Thanks. I didn't find the post I'm seeking in the Tech section or when I searched using "TEV" and "spring" so I posted here, hoping the OP will see it and respond, or someone else remembers. What was appealing about the attempt I'm remembering is that the person who did it found a substitute spring in a hardware store. Easy to try without messing anything up or ordering a spare spring to cut down.

Yes, I did do the white spacer mod about six months ago. Greatly improved midrange performance but had no effect on backfiring. Also torqued down the exhaust header bolts in case there was a subtle air leak; no change.

I hope the original poster of the substitute spring idea will eventually see this thread and respond. It wasn't that long ago.

Went back over last years post, found it by putting in search "valve spring" selected for the "past year", it was about nine or ten posts down.

Started by apache snow | Post by mechanikhan

I got rid of 95% of the backfiring by changing the spring in the TEV to a shorter spring. The length of the stock spring has two negative effects on the valve's operation.  
Having measured the pocket that the spring resides in, I came to the following conclusions:
1. There was not much room for the diaphram to move or open during the high vacuum present at full throttle, thus limiting the enriching effect the valve is supposed to provide.
2. The stock spring was stronger than necessary and was closing the valve too abruptly during the low vacuum present at deceleration, thus the backfire.

Instead of cutting the stock spring, I chose to find a shorter one. I'm satisfied with the first spring I chose, a #90 spring I bought at Ace Hardware. Listed below are the dimensions of both springs:

                                  Stock         #90
Free length      1.400 in.         0.715 in.
Wire Diameter      0.032 in.         0.035 in.
Outside Diameter       0.483 in.         0.597 in.

My bike is a 2002 model with just under 4000 miles. I installed the #90 spring at around 2800 miles. The bike is stock with one exception: a previous owner drilled out the baffle. The bike is much more pleasant to ride, and I would recommend this modification before I changed anything else.


Hope this is what you were looking for....

Thanks, old_rider. This is exactly what I was looking for. ACE hardware, here I come!

Serow, the bike is completely stock. I do live at sea level (literally -- a block from the ocean), which is probably contributing to the problem. Goes away completely with a 55 pilot jet, but then the bike won't idle when cold. Gunshot cracks on decel and on shutoff, not pops and blats.

What I really need is a jet halfway between a 52.5 and a 55, but they don't make one, and I can't think of a way to slightly bore out a 52.5 that would give a smooth bore. If the spring works, that's a better solution anyway.

As I said, I'm able to somewhat moderate the decel cracks when I'm riding alone by easing the throttle to a partly-closed position rather than letting it shut. But I can't do that when I'm closely following someone else, as in a group ride. When I'm decelerating, it cracks when the throttle is less than about 1/8 open.

So I'm trying the TEV trick next, because going to the next size pilot jet was too rich for the bike to idle when cold, even if I started with the choke all the way in.

Serow, I see you are in Tucson. I don't think my bike would backfire at 2000+ feet, either. Even where I was in New York State, where the elevation was about 1000 feet, if backfired a lot less.