fixbroke

AHA! I think I found the problem. Sheared flywheel key.

As I mentioned, I'm newish to 2-strokes, but based on a little research it seems the advance should increase up to a certain point and then decrease (while still being advanced). This may or may not have to do with the expansion pipe, which (correct me if I'm wrong) is not present on our bikes. Example 2t ignition curve: I'm finally home and ready to fix this bike! I pulled the flywheel today and it looks to me like the hall effect sensor has been rubbing on the flywheel. Does this look normal to you lot? Should the flywheel have the little ridge shown in the pic, partially covering the semicircular hole, or is this the result of the rubbing?

I managed to borrow a timing light today and checked the timing at different revs. It seems to be pretty close to TDC at idle, trending very slightly (~1 degree ish) towards retarded timing with higher revs. Not what I'd expect to see, but it's confirming my hypothesis about a timing issue. I'll be out of town for a while, but once I'm back I'll delve into this a bit further. I've managed to borrow an oscilloscope which I should be able to use to see what's happening with the hall effect sensors.

Thanks Tony. I'm inclined to agree - it's got to be the timing. I'll check the stator, but I think I'll find that the static timing is set in the factory position. From what I've read it's more likely that the hall effect sensor on the stator has failed, and the CDI isn't advancing timing as the revs climb. The timing light will tell the tale - hopefully I can borrow one this weekend.

I've spent my evening researching this problem, and here's what I've come up with so far. My problem is either from a lean mixture or an ignition issue. I suspect ignition, since a mixture problem should overheat the coolant also, while a late or incomplete spark will send burning fuel into the exhaust and overheat the muffler without boiling the coolant. On the ignition side of things, I need to check: -static timing setting (more advance should mean less heat in the exhaust) -dynamic timing advance (difficult to check when I don't have access to a strobe) -general check of ignition connectors and grounds -stator resistance To confirm there is no mixture error, I'll check: -float level -needle clip position -idle jet (no clogs) -intake air leaks Hopefully one of these reveals an obvious and easily-fixed fault.

I tore into it a bit more today, but I'm left now with more of a mystery than I had before. 1. The exhaust pipe got toasted too, so a muffler fire probably wasn't the cause of the heat. That said, the packing definitely got roasted out of the muffler and it's louder than it was before, so I'll need to cut it open and repack it regardless. 2. I drained the gearbox and got 450 ml of oil, the same quantity I put in there when I first bought the bike. The color is a bit on the brown side, but to me it doesn't look like the milkshake slurry of a failed water seal. (pic) 3. The radiator showed no sign of water loss, but I decided to check the water pump also to ensure it was circulating. Looks good to me. Spins nicely. (pic) 4. I pulled out the PWK 28 carb and checked to see that the main jet wasn't disastrously lean. There's a 125 in there, which I think should be plenty of fuel. I still have to check the needle (but it's bloody cold outside just now!) 5. Looking into the exhaust port, the piston looks to be in good shape (at least to my 2-stroke-inexperienced eyes) (pic) I'm not exactly sure where to go from here. I have some work to do to fix the melted and burned parts, but without some indication of what caused the exhaust heat I'm a bit reluctant to put it back together and take it riding again. Any other ideas or leads would be much appreciated!

Oh, she got a good checking over, to be sure. However, the newness of 2-stroke ownership involved some ignorance over the possibility of mufflers catching fire! is this a problem for MX 2-strokes, too? I'll pull the exhaust and have a peek at the piston, repack the silencer, and I'll probably wrap the whole thing in fibreglass heat wrap as a replacement for the melted heat shield.

Hmm.. Thanks for the replies. I'll pull the pipe off and have a look. By "nipped" the piston - do you mean melted or scored? I would expect if the engine were overheating I would have had some steam blowing out the rad cap, no?

The coolant level has been consistently at the top of the radiator, and the fan and thermostat have been confirmed functional.

Hello everyone. I wonder if you lot might be able to help me diagnose my firebreathing 2006 Sherco 290. The short version of the story is that the exhaust is running way too hot and has melted both the muffler's plastic heat shield and the fender above it. The long version of the story is this: 1. I recently picked up my 290 as my first trials bike and one of my first 2-stroke bikes. It has had the stock carb replaced with a Keihin and I'm unsure about the jetting. I've had it out a handful of times, mostly at the trials club, and it has been relatively problem-free. 2. I brought it out to a very muddy trials competition and throughout the day it got harder and harder to start. After giving it a spray down with water to clean off the mud, it refused to start at all. I chalked this up to accidentally dousing the air filter with water. 3. The next ride I went on, I noticed an awful lot of smoke from the exhaust after a small hill climb. The smoke had an odd chemical smell and continued somewhat smoking after the engine was stopped. Suspecting the air filter again, I removed it completely and took a brief test ride, and there was no smoke. I then cleaned and re-oiled the filter. 4. My last ride was the first trail ride on the bike, which meant sustained throttle rather than the usual short bursts associated with trials riding. Though I didn't notice a lot of smoke (not that I was looking), a burning plastic smell was evident. It turned out to be the melting heat shield and fender next to the muffler. Perhaps unwise, but I continued the ride and proceeded to melt the hell out of things for a few hours. It may be my imagination, but the exhaust may have gotten louder during this ride. I'm left with the following questions: A. Preliminary research has brought up a few threads about muffler fires on these bikes due to oil-soaked damping materials. That may explain a few things in the above four points. Before I go cutting open the muffler to stuff it with new fibreglass, would you agree that this is the likely cause? B. Is it likely that poor jetting on the new carb is causing buildup in the muffler, or is this an unavoidable part of owning a 2-stroke? Wouldn't there need to also be a source of oxygen in the exhaust for a fire to start? C. Is the "clearing out the motor" I hear people talking about (revving up to near redline) to prevent just such a thing happening? Thanks for any help you might have to offer!