125-200cc SM 125 Speed

Hi,

Really need some help from you knowledgeable folk - I'm new to two-strokes!

I just picked up an 07 SM 125 this week and Ive been working out little issues after stripping it down and cleaning it. When I took it home I was easily doing around 60mph but since putting everything together Im only seeing 40mph at best and I have the throttle wide open most times... I know these bikes can push much more than that but I don't have a clue whats up!

First thing I found was that the cables to the exhaust valve weren't connected to the servo (the exhaust valve seems to be controlled by an electronic system consisting of a servo motor and CDI which sense RPM from the white/blue spark wire going to the coil. Hooked the cables up again (I have no idea how they got off) and I can confirm the system works as revving the bike shows it opening and closing. The only issue I have is that the valve doesn't seem to move much when I am out riding...

The second thing I found was some aluminum can in the muffler tip! I have no idea why it was there or what the idea was but I thought that definitely had to be the problem, but alas only gave a minor speed improvement. I can confirm the exhaust is clear of obstructions.

The final point I make is that the bike seems to be running rich? Not sure but there was quite a bit of spooge on the bike when I got it and I have since sealed the joint between the muffler and expansion chamber so not sure how it is now...

Just doesn't seem to have a big hit on the power band and the expansion chamber sounds like a limiter with some sort of rattle when I'm going flat out...


Any ideas what I can do?

Thanks

hello and welcome to two strokes I have a bike 2 years older than yours but with a very similar engine and the same servo etc you mention. So when you rode the bike home, it was all running fine, no bogging down, hitting high revs no problem yes? And then you found the powervalve cables were not on. Were they attached to the other end by the exhaust port properly? They slide inside behind a housing to an ''actuator arm'' or some such term, I'm wondering if maybe they are loose and not turning that arm to open the exhaust port up properly. Also check your battery is connected and charged up, when my bike's stator stopped working my battery ran flat and without power it refused to rev properly, it would hit about 5000 then not go any higher, even with full throttle [which I found weird as you'd assume it would work without a battery circuit but hey ho, worth checking. Don't worry about splooge around the engine or the exhaust, 2 strokes do it all the time, you'll start to see it build up on the back of your number plate. And that rattle from the expansion chamber? Before I cleaned my powervalve out mine would get stuck in either the on or off position rather than changing with the revs of the engine [hence no real poweband take off like you describe] and it did make a distinctive rattle near the chamber, like a metallic pinging noise. It also needed to be tightened back up to hold the cables properly, ever since then, no pinging noise, powervalve opens as and when it should. Sorry to not be of more help but others with more knowledge will be along soon I am sure!

Yeah, the "cable holder" (for want of the proper term) was not bolted to the servo arm under the tank, I put a new bolt in and it was fine.

What actually solved my problem:
Believe it or not, a new spark plug! There was an NGK BR10EG installed when there should be a BR9EG in there. Additionally, I dropped the needle to the 2nd notch (where the manual says it should be) and my bike now runs clean, without spooge! Reading the spark plug confirms I am burning correctly (a nice brown/tan color) but I may go up one main jet size for good measure, not sure.

Crazy how the bike would max out at 35mph and now pulls 70+. I guess the "limiting sound" was the spark plug maxing out...

glad to hear it's going well, I got mine to 72 on a gps once and that's with upped gearing. With the sms keep an eye on the oil pump and two stroke level or disconnect it and run premix, some pumps do like to fail at 60mph and lock your back wheel up on a hill [Of course that didn't happen to me! ]