Briggs and Stratton motor surge (Not Diaphragm)

MHunterNZ

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Hi All

I have an old 3HP Briggs and Stratton motor which is surging in revs at full throttle (Can see governor pulsing too).
I've removed the carb and cleaned the fuel filter mesh and replaced the diaphragm and gaskets.

This hasn't seemed to help at all. There is an mixture adjustment screw on the side, but I think that is only for idle, and it makes no difference when I move it.

Any idea what else this is likely to be?

IMG_5930.jpgIMG_5933.jpgIMG_5931.jpg
 

ILENGINE

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You could still have a mixture issue that is causing the surge. Did you remove the mixture screw that you discussed and remove and clean the jet that is behind it. Also with the older engines like that it is not unusual for the valves to get out of adjustment and can also cause some running issues, normally associated with low power and hard to start.

If an engine is running without a load it is actually running on the idle circuit of the carb, and could still be a partially blockage that is causing you issues.
 

MHunterNZ

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You could still have a mixture issue that is causing the surge. Did you remove the mixture screw that you discussed and remove and clean the jet that is behind it. Also with the older engines like that it is not unusual for the valves to get out of adjustment and can also cause some running issues, normally associated with low power and hard to start.

If an engine is running without a load it is actually running on the idle circuit of the carb, and could still be a partially blockage that is causing you issues.

Thanks for the answer.

I did remove the screw and inserted a thin piece of wire through the hole.
Maybe I need to look at removing the carb again and soaking it in carb cleaner for some time?

Could it be caused by the idle screw being set to low, so that when the governor closes it almost stalls the motor or something similar?
(I don't know where the idle screw should be set to)
 

ILENGINE

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Idle set too low can cause stalling because the throttle butterfly closes too far starving off the fuel and air from entering the engine. Around 1800 rpm should be close for that engine. Also if the idle circuit isn't working will cause the same thing.
 

MHunterNZ

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Great. Thanks for the info.
 
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