Hi, I have a Hayter Hawk with a B&S Spirit 375, which is running rough (revs from medium to low intermittently but does not cut out). Both my local service centres have a 6 week wait time so I have tried the following myself. New spark plug, new oil, fresh petrol, cleaned and re-oiled air filter. This is the limit to my "service" capabilities. on removing the air filter I ran the mower to view the choke assemble below. as it revs from low to medium the butterfly choke and the arm, to which it is linked, move as the engine revs up and down. My question is is this movement causing the reving? or is there another cause to the revving which results in the movement?The butterfly part and the arm are connected by a solid pin, The idle Govenor spring runs from the loop on the solid pin to a static loop towards the front. the main govenor spring runs from the manual choke adjuster to the same loop in the solid pin. Moving the manual choke has little if no effect on how fast the engine runs, all it does is very slightly add tension to the main govenor spring but not sure this does much. Is this setup correct? Is a part missing? something more solid between the manual choke adjuster and the butterfly choke or arm to which this is connected? I have a photo of the assembly which I can send if it helps. Hayter Hawk manual and Briggs 375 manual do not help. thanks Nish

Nish, April 2009

Thanks for the quick response, taking apart the carb seems a little beyond me. Is this definitely a job for the service centre (6 week wait) or can I do this with some sort of guidance from manuals and/or advice? Also If I have to wait for a service is the mower still okay to use? ie using it won't cause any further problems or longer term damage? thanks PS put a new spark plug in yesterday and checked it today after running mower for less that a couple of minutes all told. The plug already showed burn marks and some carbon build up, is this normal or related to my main problem?

Nish, April 2009

The throttling on these engines is entirely governed by linkages and springs that connect the engines governor to the carburettor. The butterfly valve in the carb is moving its position because the engine revs are not constant - so the engine slows down, the governor senses this and opens the throttle to try to increase revs. Once the engine starts to rev, the governor closes things down again. Anyway, this is more than likely to be a slight blockage in the fuel system, and the carburettor needs to be taken apart for a thorough clean using carburettor cleaner and compressed air, preferably.

John, April 2009