fixbroke

Hot Exhaust Melting Plastics

19 posts in this topic

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!

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You never mentioned the fan or coolant.

 

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

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Sounds more like you have ran it hard, and the carb was set up for trials. They will overheat. You need to confirm your jetting and also if the impellor is actually moving fluid. If it was mine, I would pull off the front pipe and check out the piston to see if you nipped it.

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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?

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Just to confirm, the exhaust could of just burned up the old oil. Myself with a new bike I would check it over pretty good. Good luck.

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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.

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Check the amount of transmission oil... it its really low (you'll have to drain it to check on the Sherco) you could have blown a seal which would suck oil, run really lean and get v hot with lots of acrid smelling smoke. When you pull off the pipe check the piston top - if it's wet and oily that might be the cause.

Edited by paulmac

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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!

Edited by fixbroke

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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.

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Easiest thing to check is the timing, there should be a mark on the stator that lines up with the edge of 1 mounting lugs. 

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If the timing has been altered too much it can cause overheating

When you repack the muffler apart from the difference in noise level you should find that you get an improvement in low end power

You don't mention any abnormal noises from the engine so jetting doesn't sound to be any issue, 125 is fairly standard for trials & I've never had any problems trail riding with the same jet even when working the motor hard enough to burn all the oil out of the packing which is normally what you'll see when you stop the motor & it smokes for a while afterwards

Photo of the piston looks pretty normal, any damage you would normally see will be further down the skirt, usually scoring is what you'll see

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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.

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Normally when the hall effect sensor fails you get no spark at all or very intermittent spark at best.

This comes from experience.

If the timing marks are in the right place then it wouldn't hurt to check the stator readings against on splatshop's site

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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.

Edited by fixbroke

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Two stroke timing retards as the revs are increased (Opposite to a 4t motor)

 

Edited by suzuki250

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