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Jetting Issues From 240 To 280

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I converted my engine into 280cc and kept the same carburetor settings, the bike has plenty of torque but top speed is slower, do the jets on the 280s are different? anybody know the sizes of them? Thankyou

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What is really important is that you increase the size of the main jet . The 280 will have had a larger main jet than the 240.

If you dont do this you run the risk of seizing your piston and wearing your big end bearing.These parts are very expensive, so dont risk it.

It wont make a jot of difference that you are putting the right amount (or more)of two stroke oil in.

Petrol cools the top end of the engine. Less petrol = more heat= seizure.

Its important and the larger jet will give you more top end power.

If you phone the sherco uk importer, Malcom Rathmell, im sure he will remember the jet size. Hes very helpful.

Id be guessing , but the main jet will be about 5 - 10 sizes bigger.They are as cheap as chips on fleabay

Edited by gasgas249uk

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Thankyou, i will call him next week. For the same reason of your explanation i checked the sparkplug to see the air fuel mixture and see if it was lean but it was ok and the temperature was ok as the electric fan was not always on. Could the engine still recieve any faster wearing? Is it still necessary to change the main jet if spark colour seems ok?

Thanks again,


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The sparkplug appearance shows the combustion conditions at the time that the motor stopped firing. If you want to read the sparkplug to check on the mixture at full throttle, you have to stop the engine while it is running on the main jet.

Just because the fan starts and stops normally, that is no reason to think that the motor is jetted correctly throughout the range of throttle positions. The fan responds to coolant temperature, which lags changes in the heat production rate. The thermal inertia of the cylinder and coolant is the cause of the lag. If the lean mixture only cause brief periods of increased heat production, then fan operation will appear normal.

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