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Leics. LE7 3RE
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  Seizures

First be sure that if a bike has been rebored, that the clearances are correct.

For Scott pistons in an iron bore for normal road work, the clearances are:

0.009” at ring land and 0.004” at skirt

For fast sustained road work give 0.010” at ring land

For racing 0.0125” at ring land and 0.0045” at skirt

Tolerances plus 0.001” minus 0.000” ie allow more clearance rather than less

Note. Silk pistons have incorrect taper for use in iron barrels. As they are lighter they are good pistons that keep loads on bearings down, but they must be re ground to give correct taper and clearance if used in an iron barrel.

If clearances are correct then check the following:

I suggest retarding ignition advance a little. I had the same trouble on a Rep I had years ago. Also make sure plug is not too soft as lawnmower plugs tend to overheat become incandescent and thus light the fire before the spark is due. This excessive effective advance of the burning process will very quickly cause overheating and thus seizing and holes in pistons. Look carefully for signs of small aluminium globules on spark plug end.

If you are only using standard 190 main jet, the needle / needle jet arrangement should pass required fuel at full opening. To prove this, remove main jet completely and go for a run. Up to 2/3 throttle all will be the same. Over 2/3 opening, you will get on to the main jet. As there will be no main jet, if you open it up fully, it should be over rich and start to four stroke and stutter. If it does not do this, then it proves that insufficient fuel is passing through the annular space between needle and needle jet at these openings. This is a common problem where engines have been tuned and want more fuel, but only the main jet has been altered. A carb acts like a relay race in that the opening transition is through three main stages and all must be as well aligned to their neighbour as possible so as to give a seamlessly progressive opening where the air / fuel ratio is maintained.

You can, of course, check that your fuel filter is not partially blocked, as it has been known that if it is partially blocked, then it will flow enough for moderate throttle openings, but insufficient for full opening. Result is that as output of fuel from the float chamber is exceeding input, the level falls, the mixture becomes weak, and overheating and seizure follows.

On rare occasions I have known excessive vibration to cause carb frothing and thus incorrect carburation, with all that can ensue. Do check for air leaks, but to be honest, these are more likely to cause trouble on starting and low opening, where the ratio of extra air admitted to the "official" air coming in via the carb, is significant. This ratio diminishes with opening.

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