Challenge Xtreme petrol mower hard starting

7 Jan 2009
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United Kingdom
Hi people,

I have acquired a lawnmower from my deceased grandad. It is a Challenge Xtreme XSZ40E with a Chinese Sumec S475 engine. It’s causing me issues when trying to start it and I wonder if anybody has any idea what the issue could be. I have made a small video which I have linked to.

Basically, it’ll take me a good five minutes of trying to start it to get it running, and that’s only if I over prime the engine so that petrol starts leaking out of the air filter housing. It’s had a new plug, gapped as per the manual at 0.8mm, confirmed a good spark and fuel is freely getting to the carburettor bowl. Once it’s running, it runs fine but even after its warmed up, if I stop it then it takes me just as long to start it again.

the issue appears to be with the governor / throttle setting. With the engine off, the throttle is half open with the tension from the governor. If I disconnect the spring from the governor and close the throttle it starts instantly, however the engine then idles at too slow a speed to mow the grass without stalling out as soon as you mow grass that’s anything more than a few inches long.

Is it normal that the governor is keeping the throttle open so much? If not, what’s supposed to happen! The carb is completely factory set with no mixture adjustment screws. The only screw i have sets the minimum speed and this never comes into play because of the governor.

thanks in advance.

Last edited:
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I’d say the governor is working as it should, and I presume this engine has no separate throttle control on the handlebar?
Does the primer bulb, and pipe, connect to the plastic elbow on the left of the carb?
It’s only really feasible to start correctly with the primer connected and the carb, with air filter, bolted up to avoid air ingress.
If that checks out ok, the carburettor needs to be stripped out and the main jet and emulsion tube above it blasted through with carb cleaner and compressed air, if you have it.....this is an easy enough job, no special tools needed.
John :)

Thanks for the reply. Yes the primer bulb is usually connected to the plastic elbow and air filter housing nicely bolted up with gasket to the carb. I just removed it for the purpose of the video. There is no throttle control or choke no.I shall have a look into cleaning the carb as you suggested.

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