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Old 07-04-2015, 12:19 PM   #1
Rob.G OP
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Motor is missing at idle and part-throttle

I posted this on the TW200 forum yesterday, but thought I'd repost here since there's an even wider variety of folks with mechanical experience. The bike is a '92 TW200 with around 20k miles on it. I bought it in Feb as a basket case. I've replaced the starter clutch, the regular clutch, the stator, the battery, and the wiring harness. It has a DG pipe with quiet core and a Uni air filter. I also replaced the carburetor with a brand new OEM model, which I then jetted according to my Wideband O2 sensor (with a bung welded into the head pipe).

I've noticed in the past few days that the bike is missing at idle and part-throttle. I hadn't really ridden the thing long enough before for it to really be noticeable. It almost seems like it's temperature related, as in when the bike is warmed up or when it's hot outside. 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 the miss happens, the carburetor doesn't jolt like I've seen in some bikes, like when they backfire through the carb. This isn't that.

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 yesterday acts exactly like my original coil. I'm wondering if I have yet another bad coil, or if it's something else.

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

http://youtu.be/PDN8hYeihBg

Since posting to the TW200 forum, it's been suggested what we're hearing in the video is a lean idle. So this morning I pulled the carb and set the AF screw to 3 turns (from 2-1/2), and added one tiny shim under the needle (one was already there). I went ahead and upped the MJ one size too for good measure. Reassembled and back on the bike, it was actually worse. It started missing almost immediately, and was missing so badly that after a while it'd stall.

I've since undone those changes and put it back together. While apart, I went ahead and checked the valves again. Both were slightly loose (like, maybe 0.001") so I went ahead and set them exactly to the max loose tolerances of 0.004 and 0.006 (I/E). I also put in a fresh spark plug with the proper gap set. When I put the carb back in I made sure it was sealed well against the motor and the airbox, so there's no air leak there. The gas is fresh too, and I've been running Marvel Mystery Oil in it too.

The new result is exactly as it was before the rejet this morning that made it worse. It's rideable, but annoying. I'd love to figure out what's wrong with it... but I'm out of ideas.

Thanks!

Rob
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Old 07-04-2015, 12:39 PM   #2
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is there a vacuum petcock? the vac line needs to be solid, not leaking air. if there is no vac line the manifold needs to be plugged. also, if there is any sign of fuel in the vac line the petcock diaphragm is leaking fuel straight into the engine (my best guess)

this is assuming all the idle/pilot/jet passages and such are scrupulously clean
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Old 07-04-2015, 12:50 PM   #3
larryboy
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Quote:
Originally Posted by Rob.G View Post
It almost seems like it's temperature related, as in when the bike is warmed up or when it's hot outside.


Thanks!

Rob

Sounds like carbon build up to me. Rig up a bottle of ATF with a hose down into the carb throat, keep the revs up a bit and squirt a steady stream of ATF into it. You can also use water, but most people freak out, so I use ATF.

Also, on a very low horsepower engine like a TW you'll be doing yourself a favor to set the valves to the tight side of the spec to get max lift out of the valves for the best perfomance. Just check the gap more often, stupid simple on a TW.
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Old 07-04-2015, 12:59 PM   #4
Rob.G OP
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Breezer:

Petcock isn't vacuum type. Near as I can tell all the inlets into the motor are properly plugged. I can take pix if you want.

I just shot a video of it idling with my wideband O2 sensor operating... waiting for it to appear on my computer (via Dropbox) so I can edit it a little and then post it to YouTube. You can watch as it misses, the AF mixture goes lean. It varies from 5% to around 20% of a jump each time. Also, I adjusted the AF screw out more. Doing it by feel, I'm estimating it's between 3-1/4 and 3-3/4 out and it idles a LOT better now. At first it was idling lean... Lamba was 1.05-1.08 or so. I got it down to around 0.92-0.95 (very slightly rich). It would barely idle with the previous setting, and when it'd pop, it'd just about die.

Larryboy:

Silly question, but you mean the end of the carb that goes to the airbox? Water is fine with me; I've read many times that a little water into a motor helps clean out carbon build-up. I suppose it's possible that's it. This thing has a very weird history from what I've been able to deduce... and by weird I mean ranging from abuse to neglect.

So would the hose from one of my siphon kits (the kind with the pump bubble) work? Gotta find the right amount so I don't drown it.

Rob
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Old 07-04-2015, 01:09 PM   #5
larryboy
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Quote:
Originally Posted by Rob.G View Post

Larryboy:

Silly question, but you mean the end of the carb that goes to the airbox? Water is fine with me; I've read many times that a little water into a motor helps clean out carbon build-up. I suppose it's possible that's it. This thing has a very weird history from what I've been able to deduce... and by weird I mean ranging from abuse to neglect.

So would the hose from one of my siphon kits (the kind with the pump bubble) work? Gotta find the right amount so I don't drown it.

Rob

Take the air filter out and snake a hose down in there. That's the ticket, keep it right on the edge of drowning...nice steady stream of fluid. Plenty of vids on the utewbs.
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Old 07-04-2015, 03:02 PM   #6
Rob.G OP
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I gave it a try. The airbox has a stupid screen in the way, so I used my garden hose and sprayer, set to its lightest setting, and turned all the way down. I did manage to drown it twice, but I think it helped a little anyway. It was actually idling worse after that, but I rode it into town... up and over a big hill so it got good 'n hot. It missed a few times going up that hill, and a few times after I left ACE Hardware. But after leaving DQ and riding home, it only missed once, going downhill. It hiccuped a couple of times at idle when I got home but not nearly as badly as before.

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. I wasn't at WOT... 1/2 to 3/4 throttle in 4th and 5th gear running 30-40 mph, with ambient temps in the lower 90's and 50-60% humidity.
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Old 07-04-2015, 09:13 PM   #7
Rob.G OP
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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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Old Yesterday, 03:27 AM   #8
Alexander B
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Not having seen your bike or that model carburator, I suggest you check the setting of the "float height" - the baseline fuel level in the float chamber. If is too low or high, jetting and idle screw will not work properly.
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Old Yesterday, 09:05 AM   #9
Rob.G OP
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Originally Posted by Alexander B View Post
Not having seen your bike or that model carburator, I suggest you check the setting of the "float height" - the baseline fuel level in the float chamber. If is too low or high, jetting and idle screw will not work properly.
I checked that yesterday; float was at 22mm; spec is 26-28mm. I reset it to 26.
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Old Today, 12:54 PM   #10
Alexander B
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Good!
Any difference?
Is the needle and seat ok? (Can it be separated for visusl inspection?) A needle/seat wear can be easy to miss, as it is not very "in your face", and it has most impact when engine is operated with a light load.
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Old Today, 01:03 PM   #11
Rob.G OP
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The carburetor is brand new. As in maybe a month old. So everything in it is in prime condition.

I went for a long ride yesterday and almost didn't make it home. It got to where it was missing so bad I couldn't even maintain 55 mph. So either the coil is hosed or I have a CDI problem. I have a spare CDI I'm going to swap on maybe today and see if it helps. I also tested my coil according to the shop manual, and it's not in spec, so I know the coil itself has some issues. The primary measurement is within spec, but the secondary is double the resistance of what it should be.

Also, the motor seems to be running really hot. I have a Trailtech Vapor computer on it and the temp sender mounted to the spark plug. I know this doesn't indicate oil temp; just the "surface" temp at the base of the plug, but I don't think 490 deg F is quite right. I've never seen over 375 deg on my (also air-cooled) DR650. I know it's not running lean. If anything, it's a tad rich in all areas. I'm glad I'm running synthetic oil, but I think I'll change it again just to be safe.
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Old Today, 01:16 PM   #12
kta
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Float too high would have been my first guess. The float needle could still be leaking slightly. Having the float set a little lower can provide a extra pressure on the needle if it's worn or leaking.

Then valve clearances. Honda XR's have this symptom commonly when the valves are off.

Then vacuum leak at the intake boot (spray wd40 all over the boot with the motor running).

Next check jetting, it could be slightly lean or rich, both could cause a hiccup. We have the same hiccup on our bikes when we're in high elevations. A dirty air filter can make this worse. Once we drop back down in elevation, they idle perfectly again.

You replaced the spark plug already - how did the old one look?
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Old Today, 01:22 PM   #13
Rob.G OP
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The float as it came new was 22mm. Spec is 26-28, so I reset it to 26.

The old plug looked good. It had that nice red tint to it. No white.

Jetting should be a tad rich. I can put my Wideband O2 sensor back on to see. I went up one main jet size, raised the needle with a shim, and turned the AF screw out to around 3-1/2, which they actually tell me is too far. So your thought on an intake leak is what I'm gonna look at here shortly... along with swapping the CDI.

The elevation yesterday when running very poorly is around 200' and 90 deg. The motor tone changed and it felt like it was laboring... not missing anymore; more like it wasn't getting a strong spark. Hard to describe. Yet the tailpipe isn't showing black from being too rich.

I'll report back after I do these latest tests. Thanks for the help btw!
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Old Today, 01:23 PM   #14
kta
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Quote:
Originally Posted by Rob.G View Post
I don't think 490 deg F is quite right.
That sounds WAY too hot. That will cause misfiring for sure, as well as oiling issues. I'm not familiar with the TW200 specifically, but even your headers shouldn't be 500 degrees.

Maybe your electric system is firing erratically. Maybe you are very lean on the pilot circuit. The idle sounds a bit high to me as well.
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Old Today, 01:37 PM   #15
kta
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I was just in the vendors forum. There's a Honda 250 for sale. Here's a relevant quote:

Quote:
It had an occasional engine miss/momentary cut-out in 2013, but a new CDI unit has cured that.
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