Honda Lawnmower Engine Problem

[Nick]

I have a Mountfield lawnmower with a Honda GCV135 engine. I tried to use it the other day and although it started immediately it wouldn't run at a constant speed. It slowed to a stutter and then speeded up then slowed down and so on. There is a linkage between the carburettor and a spindle projecting out of the engine casing. This is oscillating backwards and forwards. I know what it's connected to at the carburettor end but have no idea what the spindle is connected to inside the engine. Please can anyone suggest what might be going on? I've done the obvious things like renewing the spark plug, cleaning the air filter, checking for fuel line blockages. Next on the list is to strip down the carb. but I thought I'd post here first. TIA, Nick.

[usenet]

That's the speed regulator, there's a flap inside the cowling which gets pushed by the flow of air as the engine speeds up, this closes the throttle and (if it's all working correctly) should prevent the engine from over-revving.

It sounds like fuel starvation, so double check the fuel lines first, then I think your next move is probably what you said, there may be a blocked jet in the carburretor.

It may also be worth cleaning out all the accumulated grass and gunk that's in with the regulator flap too.

[Nick]

Thanks for that. Please can you give me a clue as to where the regulator flap is? I heve removed the petrol tank which exposes the magneto and a 'fan'. Is it somewhere in this area? Nick.

[PJ]

I agree entirely with usenet's statement.

Also, the fuel pick up in the petrol tank has a mesh filter which tends to block.

[bof]

In message , Nick writes

Mine (with a Briggs a Stratton) will do this after it's been running a couple of minutes, if the fuel tank has been filled right up; if I unscrew the petrol cap it's all OK again, so I guess it's due to a vacuum in the tank.

[The Natural Philosopher]

Supposed to be a pinhole in it..

[usenet]

This is it (the speed regulator that is).

See above, it's whatever is on the other end of the "linkage between the carburettor and a spindle projecting out of the engine casing.", the engine end of it will almost certainly have a flap or something which is 'blown' by the wind from the magneto fins.

[Dave Liquorice]

My B&S developed a similar fault last year, took the tank/carb off to look for gunge, nothing significant found and it worked fine after assembly. This year it started doing it again, dismantled again, noticed that one of the flap valves formed by the thin "rubber" gasket between the carb and the tank was tucked into the hole it was supposed to cover and was a little frayed on the edge. On assembly I made sure that the gasket was positioned so that this flap covered the hole as much as it could. Works fine now, might be better than it was as well.

These small engines are simple but very cunningly engineered. That simple gasket together with channels and cavities on both sides and one spring form the fuel pump, "float" chamber and required valves. Drive for the pump comes from the vacum in the carb acting against the spring.