Lawnmower engine

I have a Mountfield (aka B&Q) HP454 with a V35 motor (though the spec probably isn't important). Had it for 4 - 5 years and it's never really performed well, its key issue being that the (fixed) throttle has always felt too slow, so the engine tends to hunt. It's hardly been pampered, but I thought I might give it a service this winter and had a look at the manual to see if I could get a feel for the throttle setting, so I can tickle it up a bit. The manual is very clear that the throttle shouldn't be touched, but doesn't say why.

Does anyone have any idea of the reasoning behind this? It may be that they think any punter is likely to render it FUBAR and never rediscover the right setting range, or perhaps that it would simply use a bit more fuel.

Obviously, a good service might be expected to improve things anyway, but I doubt it, as it's always been like this.

Reply to
GMM
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I guess then that this is a machine with a governor that maintains revs by increasing fuel when the load increases. I have this on a powered wheelbarrow and a water pump.

Reply to
newshound

Well I guess that's the principle, although it has never seemed to respond to any increased load by anything other than slowing down and/or stalling. Certainly it seems to have a sprung governor.

Reply to
GMM

The governor is usually a flap in a cooling air stream. Hunting is usually caused by stiction in this linkage somewhere.

Reply to
harry

Agreed,

It can also be indicative of a wrongly set mixture.

Reply to
Fredxxx

If you do dismantle it, make a note of spring positions and which holes in the throttle and air flap assemblies are being used by the spring and/or other linkage.

Reply to
Fredxxx

In message , Fredxxx writes

In these digital days, I try to remember to take lots of photos as I dismantle stuff. Makes putting it all back together somewhat easier.

Reply to
Graeme

The governors on most lawnmower engines are pretty crude. When the engine isn't running, a spring holds the throttle fully open. When the engine starts and gathers speed, a vane inside the engine cowl - which is connected to the throttle - is blown by the draft from the engine cooling fan in a direction which tends to close the throttle. So - in theory - you end up with a sort of dynamic equilibrium, with the throttle open just wide enough to maintain speed. If the load increases

- as, for example, when the blade encounters a clump of grass - the engine slows a bit, the vane gets blown a bit less enabling the throttle to open a bit more.

That's how it's *supposed* to work, but it's all a bit delicate, with a system of springs and levers which must all be able to move freely as required. A common fault is for grass cuttings to find their way inside the engine cowl such that they interfere with the operation of the vane. It would be worth removing the cowl and cleaning out any debris, and making sure that all parts of the throttle mechanism can move freely.

Is it Mountfield's own engine. I've got a Mountfield mower with a Briggs and Stratton Engine, which I bought at a car boot sale, and it came without any documentation. B&S were very helpful in sending me some information about how to set it all up, and it's now working fine.

I bought a cheap stroboscope (optical tachometer) on Ebay to enable me to measure the no-load speed. You may care to do the same if you can't find anything obviously wrong with your engine, but it just doesn't run fast enough. There almost certainly *is* some adjustment but, if it's like B&S, it involves bending bits of metal rather than turning an adjustment screw - which is probably why they tell you not to do it!

Reply to
Roger Mills

Or the person lives on the top of a mountain...:-) Brian

Reply to
Brian Gaff

My Hayter has a B&S motor, and I always found this ran a bit too slow, and as a result the airflow though the grass collection path was too low and it kept clogging. Judicious application of a short length of copper wire around the spring assembly allowed the spring to be "tightened" just a tad, and that does the trick. The copper wire is easier to bend around than the metalwork of the machine.

Reply to
John Rumm

A complete aside but my mate in the garage was asked to replace a 'noisy' cabin heater fan on an Audi (I think it was) and was provided a replacement s/h (tangential) fan unit to do it with. When he got the old fan unit out and much to his surprise he found a bundle of cable sitting in the fan itself!

It looked like it was a thin coax of some sort, possibly a GPS aerial extension lead but I'm not sure how it could have got in there?

Cheers, T i m

Reply to
T i m

Thanks all.

It seems the answer is simply that the avergae person will make a mess if they try to fiddle with it then, rather than it causing a problem per se. The thought of having to bend things to make adjustments seems a bit crude, but hopefully it's not impossible to get some more performance out of this. Must have a look for the copper wire!

Cheers

Reply to
GMM

GMM posted

A well-known fault on some types of Toro lawnmower (which are based on earlier Hayter models, I think) happens when the front of the mower collides with a solid object such as a tree trunk. When this happens, the metal brace that anchors one end of the governor spring can be bent inwards, slackening the spring and causing the engine to run below full power. The solution is simply to bend the brace back out again.

Reply to
Handsome Jack

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