High-RPM stutter/blubber on '97 ATK260. Jetting?

john_bilbrey · May 24, 2003

My '97 ATK 260 runs great at low and mid rpm, but up top it's blubbering/stuttering. Once the powervalve opens, it runs out of juice real quick - the hit is short-lived. Running a 340 main jet - is this too much, or not enough? I haven't had the chance to do a plug check (WFO and kill it) yet. Maybe it's something with the powervalve adjustment? Any ideas?

Oh, it has a fresh (2 rides) piston,ring, bearing, etc... using Bel-Ray H1R with 93 Chevron @50:1

john_bilbrey · Jun 6, 2003

Just to revisit this one....replaced plug, and it seems to have improved the high-RPM response. However, after one ride it is doing the same thing. Is it possible that I fouled the plug (BR8ES)? Or could the jetting be the culprit? The boys over at the ATK forum weren't sure....Thanks in advance!

atc3434` · Jun 6, 2003

You sound rich on the main jet to me. Too lean and it would run really crisp and maybe bog under high load and high thorttle. Too rich will burble and miss, and not pull cleanly at high rpms. A BR8ES should be hot enough. If the plug was fouled, the bike simply wouldn't run. It's possible the plug is getting lots of deposits on it, and losing perfromance, but usually they just work or don't. Get a fresh plug, and get her up to speed, maybe 3rd or 4th gear. Then go WFO... take it up pretty high, under full throttle, let off, hit the kill, and pull in the clutch, all at the same time. Check that plug out... I'll bet its pretty dark.

john_bilbrey · Jun 6, 2003

Tried the plug test last week - first gear, doing all it would do (throttle wasn't wide open, but I was running it up near the top where the sputter starts). Ran it like this for 10-15 seconds, then hit the kill and pulled in the clutch. It didn't want to die immediately, it just kind of "wound down". Maybe I need to try to get her into neutral while coasting down...Anyway, pulled the plug and it was a light brown color- not white and definately not black and wet. This was on a new plug, too. I'll need a pretty good stretch of road to check it in 3rd or 4th gear, but maybe I can work something out.

atc3434` · Jun 7, 2003

Yeah, a higher gear will ensure you have to use a good amount of power, so you can get a good idea of what the plug is feeling under full load. You want to be WFO open to get a true idea of what the mix is like at full power and high load. As for the motor winding down, the clutch is probably dragging a little, so its not coming to a stop right away as if in neutral. Hitting down to neutral might not be a bad idea. I'm not much for plug chops, I'm a ride an feel guy myself, so maybe somebody else can shed some of their tips for the plug chop. I just want to get something clarified too... you have good power down at lower rpms if you smack the throttle open quick? Like if your just rolling along at a slow clip in second, and you go wide open, it comes on quick and smooth, there isn't a big flat spot, right? I'm thinking if thats the case, you are definalty rich on top. I know my RM was running lean, it would bog pretty bad at lower rpms when you smacked open the throttle. I'm thinking you need to drop a few sizes on the main jet. If its running good at low rpms, and the roll on is good, its probably just rich on top.

john_bilbrey · Jun 7, 2003

Yeah, if you wick it open in any of the first three gears it brings the front end up pretty handily unless you're just almost stopped. In that scenario, it rolls on smooth and then gets squirrelly! ATKs are notorious for clutch drag, I'll try the plug check again and try to find neutral (another trick altogether with it running...) ATK forum mentioned checking power valve and float adjustment - power valve looked good at last top end job (couple of rides ago, new piston, ring, pin, bearings) and the adjustment spring is good. ATKs have adjustable power valve positions, but I've tried several adjustments, so I don't think that's the culprit. How exactly do you check float level on a Mikuni 35mm TMX? I have an exploded view, but not sure exactly how to actually check the level. Thanks for the help!

atc3434` · Jun 8, 2003

I can't help you with the float check, I've never done it myself, although that could be a part of the problem for sure. The fact the the roll on power is really good and strong is just making me lean more to rich on top. I'll bet dropping a few sizes on the main would make the difference... its worth a shot!

john_bilbrey · Jun 8, 2003

I'll see if I can round up a 300 or a 320 main and see what that gives me. Thanks for the helpl!

atc3434` · Jun 8, 2003

A 300 main is probalby going to far, I would go just a couple sizes less. Maybe a 330 to start with, or a 325. And then take it from their, see if it helps your situation. :)

john_bilbrey · Jun 13, 2003

I haven't had a chance to tear into the carb yet. Speaking with the dealer, he said that it could possibly be the coil (ouch) or stator (ouch) going bad, not producing enough juice at high revs. He suggested trying a colder plug to check (B8 to B9). I won't have a chance to work on it this weekend, but I should have some time on Monday.

atc3434` · Jun 13, 2003

yeah, possiblly electronics, but if its starting good, the arn't the likely cause. Possible though. I'm still betting the jetting up taking the problem away. As far as a colder plug helping, I think not.

john_bilbrey · Jun 15, 2003

I believe his thinking with the colder plug (B9) would be that it would jump the gap and fire easier if spark was the culprit. Cheap easy way to check that ($2) instead of spending big bucks on replacement electronics. Jetting check/colder plug will come hopefully tomorrow. If electronics are the culprit, do you think the bike is worth spending the cash on? I found a coil for about $80, but I'm not sure about the price on a new/rebuilt stator...I imagine that it wouldn't be cheap.

atc3434` · Jun 15, 2003

I don't know that I'd dive into it being electronics so fast. Its starting good, and its running ok, just a problem at an isolated RPM range, not general running trouble. I would play around with the main, dropping one or two sizes, seeing where that gets you. I could be wrong, but I don't think different heat range plugs fire easier or harder, as far as the ignition is concerned. Still, for a few bucks, maybe theres something too it, couldn't hurt to check. But my money is still on the jetting, pretty strongly on the jetting! Usually bad electronics start hard.

High-RPM stutter/blubber on '97 ATK260. Jetting?

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