Rising revs

rising revs

Hi, someone mentioned that the only reason the revs will rise is because of an air leak, this is not entirely correct, if your float levels are out a bit or the float valve is weeping, allowing the carb to overfill, fuel will enter the inlet manifold and be drawn in to the engine - thus the revs will rise.
If thew carbs are out of balance that can have the similar effect, even though the sales have the same amount of slack in them. or if a butterfly is staying open due to week closure springs. this is a known fault.

the reason. the fault manifests it's self after a run, can be due to when the engine reaches operating temp ( like when it has warmed up after a run) it will run more efficiently and even a small amount of excess fuel in the float bowl will be drawn up by the jets and cause the engine to run faster.
If you do suspect it is badly set floats of dripping float valve you might find that after a run if you leave the fuel turned on, the outside of the carb may show signs of fuel coming out of the float bowl overflow ( although this might have blocked up over time


I would also check the valve clearances, again a sicking or badly adjusted valve can sometimes cause the revs to rise slightly and if for whatever reason the valve is not seating correctlty this will also cause accelerated valve seat/stem wear.

For my two penneth I would first check the carbs...Ian
 


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