2 Stroke engine seizure

[770]

So I was riding at top speed at the highest rpm for roughly about 2-3 minutes. Suddently when slowing down the bike died. (The clutch was already pulled in) Then the next time I started the bike, it made this terrible noise and it died on idle. The piston and cylinder was fine, but the plug was extremely black. There was also some very small metal shavings on the piston and the top. I can also now lift the conrod a little up and down. So I think the bearing has seized.

Why do you think this happened? Is it bad to run 2 strokes at very high RPMs for extended periods?

husqvarna sm 125

[MattWadz]

yeah not great to run them at top speed for 2-3 minutes!

Have you taken the top end off yet or are you assuming the pistons okay?
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[770]

[stevo as b4]

I wouldn't say it's at all bad to run at maximum load and peak power rpm for extended periods, providing you can keep the motor cool enough, the jetting is safe and correct for full load conditions, and there is plenty of lubrication. These things are not always easy to achieve in the real world without instrumentation and monitoring, and it's also quite likely that under moderate load and speed if your jetting is not spot on it won't overheat/detonate/sieze.

In your case it sounds like the big end bearing has got too hot and been damaged, which is likely to be from either overheating due to a weak mixture, insufficient lubrication/lubrication failure, over revving causing extra heat, or the bearing itself to fail due to stress.

When you came off the throttle after your top speed stint, did you slow down gradually or for a long period on a fully closed throttle? Does your bike run on pre-mix or autolube?

If there is no signs of heat damage or detonation on the piston or cylinder head, or smeared alloy on the bore, then I would be leaning towards lubrication being insufficient, unless you have over revved it downhill etc for a long time.

If your running autolube, then you need to check the system for air bubbles, leaks, and check the delivery rate is correct on the pump. If pre-mix, you need to check if possible that you were mixing at the correct ratio, and I take it you are using a top spec fully syth oil in a high output 2stroke like yours?

[Kickstart]

Hi

If it is running premix (think as standard they have a pump) then I would suspect you just slowed down with the throttle closed. If using a pump then check it is set correctly.

As to running flat out, I would expect it to cope with it fine.

all the best

Keith
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[770]

[Kickstart]

[770]

[Ariel Badger]

for some reason you ran sans oil and took the big end out. The crank may be ok but find out what caused the problem, as to running flat out I have run an RD350LC on the edge of the red for 100 miles with no grief.
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[770]

https://www.youtube.com/watch?v=I_dWF7EuxUA&feature=youtu.be

[stevo as b4]

[Kickstart]

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[_Iain_]

50:1 on the road

My CR is reccomended to run at 40:1 and isn't really designed to be pinned flat out for any length of time. On the road I run around 35:1 & even then dont hold it flat out for more than a few seconds in top as it's done everything it'll do by 65mph.
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[Vincent]

I'd stick the oil pump back on....when you've rebuilt the motor that is
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[MarJay]

[Kickstart]

Hi

Aprilia RS125 runs an oil pump that is quoted as giving between 0.9% and 2%.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[stevo as b4]

I think there's a lot of truth in the argument that the more load a motor is under and for longer, the oilier it needs to run. I read somewhere about 500cc NSR V-twins that a team experimented with different oil ratios and found no extra performance from going much over 25-30:1, and the only big difference at leaner mix's was reduced reliability.

It's right what people say that trial's enduro and MX motors especially are under a much different kind of loading than a hard worked road bike or even a track racer. Road bikes spend a lot of time running part throttle too, and race and MX bike far less. Smaller motors need to be jetted differently to bigger motors as they are under more load for longer. Perhaps oil ratio's need to be richer as well for small hard worked engines, than big 250/300cc soft and lazier trials and enduro bikes?

[Bendy]

[Hokum]

So is it correct you don't want to use engine breaking on a 2 stroke as your putting too little lube into the engine?
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[_Iain_]

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[Bendy]

[Kickstart]