Gray, other gurus, if you have a moment...

I'm having a heck of a time with my '00. It's got a lean spot, bigtime, at 1/8-1/4 throttle, and it stays there, bucking and surging, slowly picking up average revs, but not improving, while I hold my wrist still.

I think this rules out AP issues. It does have a heavy O-ring on the AP timing screw.

I've replaced the fuel screw, but am willing to go back to stock. It's a JD screw.

I have pulled and cleaned the carb (it was pretty darned clean already). AFAIK the vacuum plate is fine. The PAJ is fine (stock, 100). The MAJ is fine (stock, 200). The AP diaphragm is fine, unaltered.

I installed a JD needle (his blue one), fifth clip, and a 168 MJ, which cleaned up the top end dramatically, and left me with this untractable problem down at part throttle.

I have tried, in no particular order, these pilot jets: 40 PJ, 42 (OE) PJ, 45 PJ, 48 PJ.

None of the richer PJ seem to help the problem. The 40 PJ can make it worse, depending on the position of the fuel screw. With the 48 PJ, I can make the idle worse with the fuel screw, to the point of stalling the bike, but otherwise it has no effect on the throttle as I crack it open. No matter what PJ is in it, it pops on decel.

The bike is supposed to be my motard/errand runner and dual-sport (so I can ride it to the OHV area instead of getting my oversized trailer to go ride for a few hours in the dirt, which is a drag). It's going to be unrideable in traffic like this, and just about unrideable leaving corners on asphalt or dirt.

If I rule out dirt in the fuel-screw (idle mixture/pilot screw) circuit, and rule out Pilot Air Jet issues, because of my cleaning and visual verification that carb cleaner shot in the right places resulted in a jet of carb cleaner leaving the right places, I think I have to have either:

1. A vacuum leak. Guessing this would have to be the cylinder-head boot/adapter. Possibly cracked. I think I can check this at idle by spraying carb cleaner at the junction between it and the cylinder head, but I honestly don't know if that's conclusive. If I have no joy with anything else, I'll order one and see if that's it.

2. Valve lash. This would be the valve sucking up into the head and the base-circle of the cam keeping the valve open, right? A lack of clearance?

Before I pull the cam cover off, does this sound right? It's a noisy motor, for sure.

Am I missing anything else? The motor had the YZ stator and flywheel replaced with an '01 WR flywheel and stator. Dunno if that's any heavier, or if it would expose any of the issues I've got. I don't think there's any adjustability in spark timing there, so I don't think that's an issue.

Thoughts, before I pull the cam cover and write further testing off for the weekend?

Basics first:

> Check the valve clearance (just because you should, I don't really suspect it, but you may as well eliminate it as a possibility)

> The slide plate goes in with the square edge with the hole down, and the rounded edge up. It's supposed to have a thin little seal ring on it.

After that, have you tried raising the needle one spot? The throttle range you are talking about is where the pilot circuit hands off to the main circuit, so if the needle's too lean...

You can also try disconnecting the TPS. This will put the CDI in a default, full throttle all the time mode as far as ignition timing. If this clears it up, it may mean the either the TPS or the CDI is faulty. Test the TPS for the right range of resistance, and also, for any spikes or dropouts as it is swept over its full range.

I'm no guru, but have you tried unplugging the TPS (throttle position sensor)?

Cool, thank you. Headed back into the garage...

Very definitely the TPS.

One thing, with it disconnected, I got a hell of a back-kick from the starter. My wife was watching, going "why are you whining? What happened?". I couldn't answer her through the pain and tears...

Then I manned up, and kicked it like I meant it, and it fired up, and ripped through four gears (on the 40 PJ), terrorizing my neighborhood.

I got Infinite between blue-black, and pretty normal .76-1.8k before infinite on the black-yellow when moving the throttle. Book says 4-6k on blue-black.

Will order one tonight.

Thanks a ton gents, and yes, I'll check valve clearances tomorrow after riding (the TTR).

Edit: MRCycles says the TPS is $133.41. Are there alternatives?

Edit: MRCycles says the TPS is $133.41. Are there alternatives?

Not that I know of.

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