125-200cc 125 SM Won't Rev

Hi all, new member with a couple of "problem" bikes that I would much appreciate your help with!

First up a 2008 125 SM. It will start easily, idle smoothly but just will not rev up past about 2500 rpm, like its hitting a rev limiter.

Checked the carb, reeds, exhaust, all good, but the symptoms are more electrical, like something is breaking down.

Replaced plug, cap, coil, pick up sender, checked stator output, and there's a good 12v going to ECU on two lines (I have the wiring diagram).

Checked earth from ECU, checked all the wiring, earth at coil...... etc etc.

To me the only thing left is the ECU, but am I missing anything obvious?? Oh, no error codes on the dash, but just to check, unplugging oil pump or water temp or throttle position throws up a code as expected.

Help

when you turn the bikes key first time does the powervalve self clean?

No it doesn't...tell me more! I can turn the mechanism so its not jammed.

Hi gazmcfaza can you expand on this please?

Thanks

you can turn the mechanism by hand, that's good, ok, now on that mechanism, the round thing, it should have two small cables attached to it, that then run back up towards the rear brake lever, are they attached to it or broken off?

The mechanical cables are fine, as are the electrical. I was told by an experienced Husky mechanic that this year does not do a power valve self clean and he showed me an ECU reset trick that cycles the PV as part of that. And it did

So the PV looks fine..

these cables are quite small, should be black, like bicycle type, they attach to the mechanic circle piece, they run back to what's called a powervalve motor, which is mounted up above the rear brake lever. When the engine runs, you twist the throttle, that opens up what's called a slide inside the carb, should be a dellorto 28mm. That gives the engine more air, fuel and oil, but to keep on exploding this new mixture, that powervalve motor needs to turn to turn the mechanism, otherwise the motor will bog down, because it can't explode the mixture quickly enough. The sm 125 and wre have a cdi that when the ignition is switched on, the powervalve motor is told to turn, and pull those cables back and forth to make sure it is working. If it doesn't, your engine can't rev very high

ok, only thing I can suggest is the main jet must be blocked, I think, not an expert btw, but when engine comes off idle jet at low revs, it needs more fuel so switches to the main jet, that's the biggest one in the carb, I think you need to take the carb off, turn it upside down, unscrew that big main jet and clean it through with a piece of wire/ carb cleaner, it's probably got a piece of dirt in it stopping the fuel going through

Thanks for the replies. I do know how the power valve works and why, and have been through the carb, exhaust, everything electrical with no luck.

If the power valve was stuck shut or not working the engine would still rev quite high, if not to the limiter under no load, just produce low power.

As stated I am only getting 2500-3000 revs at a guess and it just hits a wall and won't go any higher like the ignition system breaking down.

It might be a combination of power valve and ignition setting

the ignition setting is by my knowledge not something that wanders of that easy out of free will but if all others are checked and found OK it is for sure my next point to look

do you have access to a similar bike that works OK for comparison?

I sense that your are still not sure about the power valve situation better to make sure if that is moving or not

Robert-Jan

If you take the fuel line away from the carb and open the tap does fuel flow out freely?

Hi.... fuel is flowing fine, carb checked thoroughly, power valve irrelevant as the engine isn't getting anywhere near the revs that it opens.... ignition timing fixed on this bike, but it starts easily and idles smoothly, just wont rev over 2500 rpm.

after all checked and more or less eliminated to be a cause its pretty much pointing to the ecu

not that common but not a bullet proof part either

Robert-Jan

Well..... a new ECU and she fired/revved right up! a proper road test in a day or two to make sure everything else is OK but the new ECU fixed the lack of revs. Unusual for an ECU to fail I agree, but this one did....

Have fun with the bike

ECU failure is unusual but not impossible

the way to find out as you went is still the proper way of doing it (maybe a bit time consuming yet not finding out that it was not the ECU but a faulty spark plug after replacing the ECU, that would be a sour victory in my opinion)

Robert-Jan

Well I'm pleased to say that following a thorough road test the new ECU has definitely fixed it

Not actually my bike - amazing amount of power from such a small engine!

And that is still the restricted version of the engine.

a WR and CR has a 30% more output.

Robert-Jan