Stuck throttle.. cracked cylinder?

[NeverRest]

Hi All,

Very unfortunate that this is my first post... I am new to the forum and also new to trials (enduro background); recently purchased a 14 Gas Gas 300cc. Had a few rides at my local club and I am already addicted!

Problem is, this morning the bike took a bit of a dive and somehow the throttle cable came up from the carbi and got stuck on full throttle for what felt like an eternity (about 10-15 seconds).. tried everything (fuel off, lanyard off, leaned it to one side) and couldn't stop it, then it finally turned off.

Of course after this happened, it wouldn't start again.
I pulled it apart today but the only issue i could find is a very tiny crack(?) in the cylinder (right at the top of the combustion chamber just above where the piston rings stop. Also looks like some sort of leak on the cylinder surface? Is this the Nikasil plating coming off?

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Would really appreciate some advice.

Cheers,
Cristian

[TRIALTECH]

Christian

Based on the information you've provided, it does appear to have a crack allowing coolant into combustion chamber. It appears the effect of steam from the ingested coolant has cleaned the bore directly above and below the cracks. However, is this the reason it would not start? Hard to say without first troubleshooting that fault. It may have had this crack before you're mishap.

There's a couple of options here now you've already stripped it:

1. Send the cylinder away for crack testing and repair
2. Replace the cylinder with a new one.

Both options 1 & 2 will at minimum also require replacing of piston rings and possibly piston (subject to inspection).

It's prudent to inspect for this 'steaming' on the other components eg rings, piston, head etc to get a full picture of what's going on in there. It appears as though coolant under pressure may have caused the no start symptom but that's not guaranteed. What do the piston and rings look like?

3. Take the bike to a competent mechanic for further inspection/opinion

Regards
Denis Lovett
Trialtech Motorcycles
Brisbane
07 3040 0231

[NeverRest]

Denis, thank you very much for your reply.

I've also spoken with Paul from the Hell Team today. Ordered new piston/rings/gaskets and plug just in case.

Once the parts arrive, the cylinder will be off to get replated (at Electrosil, Vic).

I've also learned the valuable lesson that the only way to stop the auto-ignition on these engines would be by placing a hand or foot over the exhaust for a couple of seconds. Expensive lesson though..

Cheers,
Cristian

[David Lahey]

blocking off the exhaust outlet is good for stopping it in an emergency.
You can also invert the bike to starve the jets of fuel (not just lean it over, it has to be inverted)