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Sherco 17 engine rattle on cold start


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Hi everyone. 
Sherco ST300 17, 60 mh. Engine rattle on a cold start on medium rpm. When warmed up, after 40-60 seconds the rattle is disappears.
The clutch does not affect the sound. This sound appeared at about 50 mh. The warm engine is working well. 
Please help identify the problem.
Edited by Simon-
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I never had a two-stroke engine, the sound is as if there is a metal ball in the cylinder. I removed the exhaust pipe and looked at the condition of the piston. The piston looks good, no scratches.
If the little end bearing in a bad condition, how will the sound of the motor change?

the sound is like this https://www.youtube.com/watch?v=vQrdyM3Nwrc


Edited by Simon-
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  • 2 weeks later...

My 17 Sherco Factory would would rattle sometimes on startup for 30 seconds or so. I didn’t dig into it to much but it sounded like detonation. I thought it was the fuel in the float bowl had lost octane while sitting and would cause the rattle until the bowl was replaced with new fuel. It seems I’ve read about this happening to dirt bikes before. 

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Thanks everyone for their answers. The problem has been found. When I removed the cylinder head, I saw about 2 mm of carbon deposits on the piston. Also, the cylinder head was covered with a huge amount of carbon deposits. The piston rings did not move. Also, the upper connecting rod bearing was slightly loosened, about 0.3 mm. Piston wear was 0.02 mm just opposite the exhaust port. The piston is still in good condition, but I am planning to install a new piston with rings and an upper connecting rod bearing.
For the last 30 hours I have used synthetic Shell Oil in a 33: 1 ratio. About 200 ml of black liquid poured out of the exhaust pipe. I've always thought a lot of oil is good for the engine. On the advice of the forum, I will go to 100: 1.

Cylinder condition:



Piston condition:





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The larger of the 2 measurements is the 1 to use, .02mm of taper is next to nothing which is mainly due to being a watercooled motor. As a comparison I measured a TY175 piston last week that tapered closer to .25mm from the base of the skirt to just under the bottom ring

With a good synthetic oil a ratio of 70-80:1 is good, won't clog your exhaust packing & still safe if you do any trail riding between sections

You will need to repack the muffler, the packing will be saturated & you'll keep getting oil out the end of the exhaust for quite some time

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I would reuse that piston and if the ring end gap is still OK, reuse the rings.

For the little end bearing, do a visual inspection on the rollers, the pin and inside the conrod hole with a magnifying glass. If it all looks OK, reuse them too.

You will find that trials two strokes running modern oils gum up the rings at an alarming rate, especially if there is too much oil in the fuel.

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