Further Adventures In The Carburetor Zone

[Kit]

Yeah, that thing's horrible, isn't it? Whoever did that, did the same thing to the one on the final drive too. Thanks for the heads up on True Value. I haven'treally looked for new ones yet, but I'll check them out.

[Kit]

Well, it wasn't the sync or the timing. So it must be my idle mixture, right?

You can try turning out the pilot SCREWs a 1/2 turn each and retest the ride and the decel popping,

It's not really on decel much, mostly on acceleration. Also, I noticed this morning that it was doing it at idle when it was still cold, but smoothed out (mostly) after it warmed up.

So check my thinking on this: backfires are caused by the mixture being too rich, is that right? And turning the screws in leans it out, right? I have a ColorTune, but the color stays the same (whitish-blue) no matter where I have the screws set, so it's of no help.

[TopCatGr58]

Hey Kit,

I don't have any experience with the colortune device, can't recall what the colors indicate...blue or yellow for rich or lean, but if you're not able to see any color change, then it could be that you do have too rich OR too lean a pilot jet since you can't see a color change. Could also be that your pilot screw tips are broken off in the carbs?

Backfiring can also be poor/intermittent ignition, not firing on normal sstroke cycle, fuel builds up in exhaust, then wasted spark hits during exhaust cycle and lights fuel in pipe via open exhaust valve and spark plug firing.

Hopefully others with Colortune experience will chime in with more precise tuning/adjustment advice.
T.C.

[motoman]

but the color stays the same (whitish-blue) no matter where I have the screws set, so it's of no help.
__________________
Did you replace the teeny-tiny O-rings under idle-mix screws and make sure the teeny-tiny washers are in place and located between spring and O-ring? When replaced and sealing properly you should be able to kill motor when turning screw all the way in. Each screw SHOULD cause the SAME reaction from motor if all is sealing good.......JAT

[cywelchjr]

but the color stays the same (whitish-blue) no matter where I have the screws set, so it's of no help.
__________________
Did you replace the teeny-tiny O-rings under idle-mix screws and make sure the teeny-tiny washers are in place and located between spring and O-ring? When replaced and sealing properly you should be able to kill motor when turning screw all the way in. Each screw SHOULD cause the SAME reaction from motor if all is sealing good.......JAT

Ah, crap, I forgot about those. Yes, if those are not in good shape you can have an almost impossible time tuning the carbs on the newer carbs, cause apparently air is leaking into the passage past there if that o-ring is bad. You can get those o-rings (and the spring and washer as a kit) from MikesXS as a kit pretty cheap, and the ability to tune the carbs can be like night and day between that o-ring being flattened and it being good. On the XS400's with the same carbs it can be literally impossible to get a good stable idle without replacing those o-rings.

[Kit]

then it could be that you do have too rich OR too lean a pilot jet since you can't see a color change.

Cy mentioned this as well, and it's what I'm leaning towards at this point. I'm at the stock 42.5, and he suggested going up one size to the 45s. Pilot screw tips should be fine. Just installed new ones, and I've been very careful not to over-tighten while making adjustments.

Did you replace the teeny-tiny O-rings under idle-mix screws and make sure the teeny-tiny washers are in place and located between spring and O-ring? When replaced and sealing properly you should be able to kill motor when turning screw all the way in. Each screw SHOULD cause the SAME reaction from motor if all is sealing good.......JAT

Yup, those were installed (and in the right order) when I replaced the screws. And they do kill the motor when all the way in, though it didn't occur to me that that's what it meant.

[Kit]

Well, I'm cautiously optimistic. After another 110 miles, everything seems to be squared away.

I replaced the 42.5 pilot jets with 45s. When tuning, I discovered that when the mixture screws were turned all the way in, the motor wasn't quitting. Last time I tuned, all four would kill it when turned all the way in. So I pulled the screws and discovered that for some reason I had put the washer between the screw and the spring, instead of between the spring and the o-ring (as noted above when I responded to motoman, I could have sworn I installed them in the correct order...dunno what I was thinking, but good call on that). As a result, the end of the coil on the springs had cut into the o-rings and severed them. All four of them. I read somewhere that Harbor Freight sells a kit of 225 o-rings of various sizes for $6.99, and there are 10 of the correct size in that kit. Replaced those. Somewhere during all this, I noticed that the seal for the gas cap vent was literally falling apart, so I replaced that. Found a thread discussing that, and someone had discovered that a thermostat gasket from a Toyota Camry works, so that's what I got. And lastly, even though I'm not superstitious, I decided to do something about the gremlin bell that the PO had left on the bike when I bought it. It was dull, a little rusty, and hanging way too high (up high on the forks in front of the horns). So I polished it up a bit, then hung it down low on the frame. Can't say if it did any good or not, but I figured it couldn't hurt.

I've taken her out on two short trips now, and I have power through all ranges again, with no sputtering, backfiring, or bogging down. Keeping my fingers crossed, but I think I can finally close this particular chapter.