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Collection, Preservation and Display of Old Lawn Mowers

Atco Royale additional spring ?

Good evening all I hope someone can help.

I have a 1987 ATCO Royale B20 , Briggs 5hp.

I have just fitted new diaphragm, gaskets and governor spring.

I can’t seem to get the high end revs to settle evenly. The low end and up to half throttle is lovely.  

After half throttle the Governor spring starts to bounce back and forth and there’s what I would call a surging issue going on when the throttle gets past the half way mark.  This gradually gets worse/stronger towards full throttle.

I have read that there was a tech note put out by ATCO years back that explained a fix for this by adding an additional spring.

does anyone know about this tech note or how to solve the issue ? 

any help is gratefully appreciated :)