YZ250 8000+ RPMs at 1/4 Throttle (No Load)?

YZmidLife · Sep 7, 2010

I decided to take my 2001 YZ250 out on the trails to get the jetting straightened out. I'm running pump gas mixed with Yamalube at 40:1, with 2.9 hours on a new top and bottom end. It has never run right, at least in my mind, but I figured this had to do with the FMF Q Stealth spark arrestor I added. Before heading out, I marked the Closed, 1/4, 1/2, 3/4, and Full throttle positions on my grip. It was sounding pretty blubbery, so I went from the stock 50 pilot and 178 main to a 48 and 175.

Just barely cracking the throttle, this thing comes alive, but I get no change after 1/4 throttle, meaning no increase in pull, but no bog either. It's as if that last 3/4 makes no difference. Just before loading the bike back on the trailer, I checked the RPMs using the hour meter, and it was showing over 8000 (maybe closer to 9000) under no load. Assuming my hour meter didn't lie, is this normal?

YZmidLife · Sep 8, 2010

Oh Yamaha, why does my bike want to kill me?

Anyone? Maybe I just don't know jack about 2 strokes. I know I shouldn't expect the power delivery to be linear, but I don't comprehend how I could get that many RPMs at 1/4 throttle. From what I have read, I should expect around 9000 RPMs max, but I would have expected that to occur somewhere around WOT. My butt dyno and the spinning back tire is telling me it is pulling that when I twist the grip about 20 degrees. :yikes:

Since this bike was bought 3rd hand, it may have been screwed up to begin with. Is one of the jobs of my electricals (TPS, etc.) to retard the timing to better sync up the engine speed to the throttle position, or is it just to adjust timing to get the most power at a given throttle position?

Rich Rohrich · Sep 8, 2010

Are you doing this under no load on a stand?

YZmidLife · Sep 8, 2010

I rev'ed it out on the side stand at the end of the day to see what RPM's the hour meter showed. That was only for a couple seconds. I can only guess what the RPMs were when I was riding, but it was screaming and I wasn't nuts enough to bend down to look. In both cases, it seemed like it maxed out at 1/4 throttle, definitely did this under load.

Rich Rohrich · Sep 8, 2010

If the clutch is slipping then the engine rpm and the load at the rear will never "match" up. I'd start there.

YZmidLife · Dec 20, 2010

Well, it's been a long time between rides and I owe an update. Clutch seems to be okay. There's never been an issue with it slipping when I'm taking off or going uphill. Checked all the electrics as per the manual, repacked the silencer, new plug, fresh gas, went down one size on the main jet (although that's probably not having a chance to come into play), measured the float height, and it was a bit colder outside than the last time I was out. She smoked like a chimney for the first 10 minutes, and then only wound up to almost half throttle once warmed up.

I had a suspicion that something funky is happening with my float valve, so I made a tool to check the fuel level similar to the one at the end of this video.
http://www.youtube.com/watch?v=_qAVXneKAkU

The fuel level is showing at about 1/2 an inch above the mating surface of the bowl. I'm assuming the fuel level should be equal to the level of the mating surface and that the float should not be mostly submerged as it sits in gas. Does anyone know what this level should be for a Keihin PWK?

whenfoxforks-ruled · Dec 21, 2010

.63" from the mating surface. Replace the float valve and seat. Hook a timing light up, I would believe that rpm a little more. Is the solenoid feeding the power jet functional? Vintage Bob

YZmidLife · Dec 21, 2010

Wow, 0.63"? I'm sure glad you chimed in before I messed this all up! My manual only specifies float height (6mm), but not the fuel level when the carb is actually upright with the fuel turned on. So, should I expect the plunger of the valve to be fully compressed due to the weight of the fuel in the tank (when everything is put back together), thus allowing the fuel level to be above the mating surfaces? Where can you find the fuel level spec?

I checked operation of the valve by holding it in place as lightly as I could and turning on the fuel. The valve doesn't look worn, but I'll replace it just to be safe. I don't know if the valve seat is a replaceable item on this carb, since it doesn't appear to show up in the parts fiche at the dealer. The float is on order, since none of the dealers around here seem to stock it. Do the valve plungers tend to wear out?

The solenoid clicks when I connect/disconnect 12V. I'm not so sure I believe the RPMs indicated by the hour meter either, so I guess I have a good excuse to go buy a timing light.

Thanks!

whenfoxforks-ruled · Dec 22, 2010

.63 is from the mating surface to the bottom of the float. The measurement is at the point of contact with the float valve. Checking the float level with a clear hose is good, depending on that to see if the valve is shutting off is not good. You can not look at a valve and determine its worth or not. Maybe a microscope, or better yet, when you have some spooge issue or a year has gone by, replace the valve. The seat is not always replaceable. That float gap came out of a 97 CR250 manual, it was one of the few years they ran the power jet carb also. And the solenoid check is the same.

YZmidLife · Dec 23, 2010

.63 is from the mating surface to the bottom of the float. The measurement is at the point of contact with the float valve.

Yeah, I kind of figured that's what you meant after I converted that to mm and found a lot of references to 16mm float height for some PWK's. Both the valve and float will get swapped out, since the float is acting more like a submarine than a boat. No wonder this thing has ran like doo doo. I thought it was just because of the fmf q I was running.

whenfoxforks-ruled · Dec 23, 2010

It does not have to be loud to run good.

YZ250 8000+ RPMs at 1/4 Throttle (No Load)?

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YZmidLife

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YZ250 8000+ RPMs at 1/4 Throttle (No Load)?

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