OT: Anyone know anything about motor mowers?

I have an elderly MTD rotary lawn mower fitted with a 3.5 hp 4 stroke engine. It has developed a fault, and I would be very grateful if anyone can throw any light on it (before I throw the mower in the skip...)

The engine starts and runs easily enough, but it constantly "hunts" rather than maintaining a constant speed. The throttle control has little effect.

Not a lot to go on I know, but any ideas would be very welcome.

TIA

Mike

Reply to
mikecb1
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If it starts easily then the ignition system is almost certainly ok and the air/fuel mixture is not too weak. Most likely the opposite - the mixture is too rich when the engine is hot. Likely culprits, dirty air filter, badly adjusted or faulty carb or carb choke mechanism. Clean or replace the air filter. If it's just a wire mesh thing then soak in petrol and dry fully. Just removing it altogether briefly would give you an idea if it's the cause.

If there's an idle mixture screw on the carb then note its initial position and have a play with it. Make sure any choke mechanism is not sticking on.

Reply to
Dave Baker

If it has a governor that is controlled by a plate in the airflow from the cooling fan that is counteracted by a spring. You will probably find that it's either bunged up with grass clippings or needs oiling.

I have enjoyed those symptoms, due to that problem, on a couple of mowers.

Mark Rand RTFM

Reply to
Mark Rand

Often one of the fine tension return spring between carby and govenor lever wears through or becomes dissengaged.

Reply to
olmod

Thanks guys - I'll check out the areas mentioned later today.

Incidentally, the governor is as described by Mark, but the vane doesn't seem to be sticking. One of the fine springs mentioned by Olmod had indeed worn through and become detached. I remade the end and hooked it back (but hard to tell which of the many holes on offer it should hook into). In any event, it made no difference to the hunting.

Mike

Reply to
mikecb1

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Update:

I cleaned all the relevant parts, including the air filter, but the problem persisted. I could find no adjustments of any kind on the carburettor. After some searching on the web, I managed to download a copy of the parts diagram for the Briggs & Stratton engine, which showed a further governor spring which was not present on my engine. This is referred to as the "idle governor spring", Obviously I didn't have the correct part to hand, but a rummage in the "useful springs box", plus some cutting, stretching and tweeking, and voila, no more hunting!

Mike

Reply to
mikecb1

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I am glad you fixed the problem because I think that lawnmower hunting is probably illegal. Alan

Reply to
jackary

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-certainly had me foxed for a while!

M
Reply to
mikecb1

I'm moderately surprised that a part with "idle" in its name effects full power behaviour.

Still, live and learn!

BugBea

Reply to
bugbear

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Only if you do it with guns

Reply to
Neil Ellwood

Without the governed idle spring the throttle can shut off virtually completely. This leads to an unstable condition where the engine speed drops close to stalling, the fan operated actuator then fully opens the throttle, which in turn causes the actuator to shut off the throttle, etc, so the engine cycles between high revs and close to stalling.

Mike

Reply to
mikecb1

Thanks for that - I understand!

BugBear

Reply to
bugbear

I think one spring goes between the vane and the carb and the other is a return spring to pull the butterfly shut to achieve idle speed. It is years since I played with these though.

Bob

Reply to
Bob Minchin

No Bob, it's a bit more complicated (on my mower at least). The vane is linked directly to the carb butterfly lever, and when the engine is running the vane tries to close the butterfly. The governed idle spring connects between the butterfly lever and a fixed point. It has a lot of slack in its attachment points so that it only counterfacts the force of the vane at a small butterfly opening, thereby setting the idle speed. The second governor spring also connects to the butterfly lever, and attaches to to a linkage controlled by the throttle lever. When the throttle setting is increased, additional spring tension is therefore applied to the butterfly lever, which in turn causes the engine to stabilise at a speed above idle.

Mike

Reply to
mikecb1

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