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Discussion Starter #1
So, the latest issue in my TW200 Rescue Project is that it has started missing. It already has a new carburetor and is jetted properly based on my wideband O2 sensor testing. It has a DG exhaust, XT225 head pipe and a TwinAir air filter.

I hadn't really ridden the thing long enough before for it to really be noticeable. It almost seems like it's temperature related. It only happens at part-throttle... from idle up to maybe 2/3 throttle. Above that, it goes away. Like if it starts doing it while I'm riding, and I open it up, the missing stops.

When it happens, the carburetor does not move; not like how my old carburetor was bouncing around when it would pop (and it did it much worse).

I've had three ignition coils on it... the one that came with it, a Chinese knock-off (which missed constantly) and now another OEM used coil. The used one I just got today acts exactly like my original coil. I'm wondering if I have yet another bad coil, or if it's something else. I suppose it could be the carburetor, but I thought I'd ask here before I tear into it again.

I shot a quick 15-sec video of it missing at idle when I got back from my test ride tonight.


Thanks!
 

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that there sounds like a lean miss to me, mine does the same thing when cold, so I would say you have a carb issue theres my .02 for ya,,, good luck:)
 

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I am on the opposite side with a miss. 1/3-1/2 throttle(taller gears) under load and it pops, backfires and blows a blue flame out the exhaust.... I will be checking my carb first...... Anything after that is real expensive....
 

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Discussion Starter #5
I guess I'll go through this morning and add a half turn to the AF screw and then raise the needle with a shim and see how it goes. Maybe I'll pull it and doublecheck the float height while I'm at it.
 

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Discussion Starter #6
Back... float height was only 21mm. Spec is 26-28. I adjusted it to 26. I also went up one MJ size, went to 3 turns on the AF screw, and shimmed the needle again. The result? It's worse. It starts missing at idle almost right away. It misses so badly now that it will actually stall the bike. And when I rev it, the revs stay high for a while before coming back down.

So, short of undoing what I just did, I'm not sure what to do. If the problem isn't in the carburetor, as Lyterx suggests, what could it be? Timing?
 

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Making the mixture richer and the miss increases? Going back induces it miss less again?
Doesn't sound like lean misfire. Look at plug? Fouled plug? Hook up timing light to check CDI.
Loose little parts?
 

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Discussion Starter #8
So, here's the latest since I posted that.

I hooked up my wideband O2 sensor again. I saw that idle was very lean... lambda values of 1.05 to 1.08. I turned the AF screw out... somewhere between 3-1/2 and 3-3/4 is my best guess since I did it with the motor running. It's now idling a lot better at 0.92 to 0.98. For those who don't know, the Lambda value is how rich or lean it is, where 1.0 is perfect (14.7:1); under 1.0 is rich, above 1.0 is lean.

I noticed when it would miss, that the Lambda value would jump into lean, usually around 20%.

I posted all this in a new thread on ADVrider. One guy said it was carbon deposits. I suppose that's possible; the bike has had a lot of short rides, and nothing real long. So using his advice, I sprayed water into the airbox a few times with the RPMs up (with the motor good 'n hot). I killed it a few times, but after that I rode into town... up and down a big hill between here and there, so it got plenty hot. It missed quite a bit on the way there, but only once or twice on the way back.

I'm starting to think I may try raising the needle again. I noticed it get quite hot on the way back, hotter than I've ever seen it on my Vapor. This is at the spark plug, btw, not oil temp... but I saw 408 deg. So it may still be a bit lean. As for why it ran worse earlier today, I dunno.

I already changed the spark plug this morning, rechecked the valves, etc. How do you check the CDI? I have an inductive timing light. I'd like to give that a shot.

Rob
 

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Dude — this is a TW you’re talking about here — not a Formula One racing car

Just get on it — ride it — and thrash the tits off it …………
 

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Are you sure you don't have an air leak causing the lean condition? I've never heard of anyone having to open the AF screw that far.
Usually if the pilot screw needs to be 3 or more turns out, you should go to the next larger pilot jet size.

At 3 3/4 turns you are likely to find the pilot screw unscrewing itself, and falling out.

I agree with rbm, likely air leak.

jb
 

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Discussion Starter #13
I rode 82 miles today. All in all it's running better, except that when I got home a few minutes ago, it was missing again at idle. I don't get it.

FWIW I got 63 mpg, all road riding, and lots of 45+ mph too. Not bad I guess. Best I woulda gotten from my KLX250S was maybe 52-55, and that was before the 351.
 

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I went back and read that you have a DG exhaust and TwinAir filter. O2 sensor says lean....did you check for air leaks around the rubber boot?

I don't know if there is a larger size pilot screw for the old style carb, but it sounds like you may need one.

EDIT: according to Irppcer's list, there is a larger pilot jet:

http://tw200forum.com/forum/technical-write-ups/14969-tw-carb-jet-part-s.html

Did you check the resistance of your coil, as per the manual?

jb
 

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Discussion Starter #16
How do you test the coil? I found the spec in the manual.

No air leak around the boot; I'm sure of that. I've checked and rechecked it... well, the boot between the carb and airbox. I was just thinking... any chance of a leak between the boot and the motor itself? Hmmm. I need check the parts fiche and see if there's any kind of O-ring that goes in there and see if it's there. I know the PO replaced that part before I got the bike, so maybe that's it.

I figured maybe I needed a bigger pilot jet. I've already gone up one step on that (the one that came in the ProCycle jet kit). Where do you buy those jets? Rocky Mountain ATV doesn't seem to carry them.
 

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How do you test the coil? I found the spec .. .
Factory Service Manual. Chapter 7, page 9 & 10 (7-9, 7-10), need an ohm meter.

# 42 Pilot jet is at Rocky Mtn under Yamaha '87 XT 350...carb parts fische, part #6

https://www.rockymountainatvmc.com/oem-schematic/2

#48 pilot jet is listed under Yamaha 1990 XT 600, carb parts fische, part #39

https://www.rockymountainatvmc.com/oem-schematic/6

I don't have first hand experience with these jets on the old style carb, and don't know how the Pro Cycle jets correspond to Teikei jets.

What meter are you using with your O2 sensor? (Bosch sensor?)

jb
 

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Discussion Starter #18
Thanks! I tested all three of my coils. The original coil that came on the bike is way out, the cheap Chinese one is even worse, and the one I just got is close, but not quite there.

The manual says: Primary: 1-3 to 1.9 ohm; Secondary 5.3 to 7.9 kohm. This newest coil measures 1.8 and 16.1k. I used two different meters to verify my findings.

BTW I already have the 42 pilot in there; I'd need to figure out where to get the 48. But for now I'm gonna order the O-ring that goes between the carb boot and the motor and see if that helps things. That and try to get a coil that's within spec.
 

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Posted the link to the #48 jet above.

Have you had a chance to try the timing light.

jb
 

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Discussion Starter #20
I went for a long ride today... about 120 miles. Lots of pavement, fair amount of dirt too. In short, I almost didn't make it home.

It ran pretty good for the most part, with a bit of missing here and there. As the day progressed, the missing started becoming more frequent. On the way home (asphalt), it started missing REALLY bad... like I had lost half my power. I couldn't even maintain 55 mph anymore. If I went WOT, it bogged down. I was afraid to pull over because I figured it'd stall and I'd be stuck. To my surprise, when I did slow down to make the turn up to where I live, it actually ran fine at lower speeds/RPM/throttle position.

Engine temp is way up too. I know we've had discussions about how the Vapor's spark-plug-based temp sensor doesn't really mean anything compared to engine oil temp, but I was seeing frequent 480+ deg temps on the road, and on the way home, it actually hit a max of 503 deg. My DR650 has never gotten anywhere close to that. I think I saw 375 once in Death Valley and that was it.

I think I'll be changing the oil tomorrow...
 
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