Lawn mower engine running unevenly

[Lee Nowell]

Hi All,

A quick update on my mower - thought I would start a new thread in case the specifics helps someone else in the future

The mower started first time - I was amazed. However... The engine runs very unevenly. It doesn't sound like it is likely to stall but runs what I would call lumpy with an uneven rev. As advised here, I did the following before firing it up...

1. Checked the plug. It was clean with no residue. I checked the gap and it was about 21 thousandths inch. The manual says either 20 or 30 but I can't tell which one I have to know for sure
2. The air filter is mostly spotlessly clean but a little but of what looks like oil on the inlet side where the holes of the casing are
3. The petrol tank had a tiny bit of fuel left in it (not enough to pump with the priming bulb) so have put fresh fuel in with the dribble that was there
4. No smoke seen of any colour or noticeable smell

Anyone have any idea where I should go from here?

Thanks

Lee.

[T i m]

When you say 'runs / unevenly' do you mean across the whole rev range?

Many mowers have a simple flap regulator to stop the engine from stalling. It controls the throttle via a flap the sits in it's own cooling air flow, generated by the fins on the end of the magneto / flywheel.

When you set the throttle it is set via this 'regulation' (using a light spring usually) so that it would try to increase the throttle with both at tickover and under power, should it be required.

So, this means they do seem to 'hunt', at tickover especially as the feedback loop is quite slow.

If you give it some throttle and actually cut grass with it and it does so whilst holding a reasonably (but not completely) steady RPM I'd say that was ok? ;-)

Cheers, T i m

[Lee Nowell]

Thanks Tim... Unfortunately my mower is much simpler than that :) It doesn't have any throttle for the user just an on/ off lever and another to activate the wheels.

This is a link to the manual

As you say... I guess if it starts, runs and cuts grass what's not to like :) I guess given I am in "get mower working" mode was thinking I would sort any problems now. Otherwise knowing me I will just leave it as it is with a potential of more aggravation in the future.

[Chris Hogg]

I have had a similar problem a few times in the past. The solution that works for me is to dismantle the fuel tank - carb combination and give it a thorough clean out. It's surprising how much crud builds up on the little gauze fuel filters over the years.

[harry]

It may be fitted with an "air governor" This keeps the motor at a constant speed. Just a flap in the cooling air flow attached to the carburetor. Linkage may be sticking a bit and need freeing off.

[nothanks]

Check the carb jet isn't partially blocked and (as others have said) that the regulator (probably a vane operating the carb butterfly, counterbalanced by a spring) moves freely. There is usually a flap on the carb gasket that acts as a pump, this ages and leads to hunting - easily fixed by replacing the gasket (two or three quid from fleabay).

[harry]

It IS fitted with a governor item 2 on the drawing.

[The Natural Philosopher]

well it prolly has a governor and they can get sticky

look for that and see

[Lee Nowell]

Looks like Harry was correct - thanks. It does have a speed governor. I have tried moving it with the engine running and it does change the revs although at the lowest end it seems like it may stall. Also, the lumpy running is evident no matter what I set it to.

It seems to move freely and the last 25% of the levers movement at the "fast" end doesn't seem to make any difference. Not sure if this tells us anything though

[Lee Nowell]

In terms of the carb, When I remove the air filter, what I believe to be the carb is a plastic looking chamber but no flaps or anything. Is the suggestion here to get some fine wire or something to poke into the holes/ jets to remove any possible debris? For what its worth the carb looks fairly clean.

[T i m]

TBF, Lee didn't say it wasn't fitted with a governor, but that it didn't have a user throttle (except it does, the Tortoise and Hare lever that (probably) manages the governor). ;-)

What it doesn't have is a handlebar mounted throttle.

Cheers, T i m

[Chris Hogg]

With mine (a Briggs & Stratton 4-stroke engine) the carb was partially contained within the little petrol tank. Several nuts and bolts had to be undone before the whole assembly came apart, and it was easily cleaned without putting wires down jets, which I wouldn't recommend anyway. If there is muck in the jet, pushing a wire down it will just shift it temporarily, and after a short time it will get drawn back up into the jet. Take it all apart and flush it clean.

[Lee Nowell]

Thought it might be useful to send some pictures of the carb and the governor thing.

The springs in the picture seem loose when the engine is switched off (can't see them with engine on as underneath the air filter). Almost like they don't do anything. Having said that the governor seems to work but not sure if these are related. Also, having now seen the springs, in the back of my mind I vaguely remember buying new springs in an attempt to fix a (probably similar) issue so maybe I fitted them incorrectly?

[T i m]

I think that's what we would normally call 'a throttle' and that in turn (on lawnmowers) is normally connected to the *actual* throttle via a spring / linkage / governor.

It shouldn't, assuming it's running correctly and the governor is working correctly.

It's difficult to understand what you are describing as 'lumpy' with a single cylinder lawnmower engine Lee. What you might be describing as 'lumpy' is in fact hunting and that can be quite normal, depending on the make / model of the engine etc.

No, it probably wouldn't because 1) it's a very crude machine (in general) and 2) the governor that linkage is probably connected to may not be correctly adjusted or running freely.

No, it doesn't really. ;-)

Recap. The actual carb throttle mech (often a butterfly) is connected via a rod / spring(s) to a paddle / vane that sits in the airflow for the ducted cooling system. If the engine starts to labour though some long grass the revs drop in comparison with the user set throttle and the flap relaxes, allowing the throttle to open and the engine run faster. If the load comes off the revs increase and the flap pulls the throttle closed slightly, managing the revs.

However, as mentioned elsewhere, excessive 'hunting' or poor throttle response could be down to / aggravated by other issues.

If you look closely into the area around the carb you might see a rod going off towards the cooling ducting and if moved carefully should move freely. Often springs can jet unhooked or stretched and that can also impact how well it all works.

If you could find and safely operate the actual carb lever by hand when the engine is running you should be able to hold the revs constant or adjust them more like a conventional throttle.

Cheers, T i m

[The Natural Philosopher]

dirt in the needle valves or a sticky valve then

[T i m]

They speak 1000 words. ;-)

Hmm, I'd have to compare that with a formal manual picture to compare.

That can be right, depending on the throttle position.

They may not at the beginning because 1) the throttle isn't set (off tortoise) and 2) the engine isn't running and the flap isn't pulling against the spring.

As mentioned elsewhere, you should be able to operate the throttle itself (not the lever on the outside) directly with the engine running and that should have a more direct relationship with the engine revs.

They do look quite new and quite possibly (they are often a bit of a puzzle and often not intuitive).

Cheers, T i m

[Kellerman]

Fit a new spark plug.

[Kellerman]

If fitting a new spark plug does not fix it the next step is to fit a carburettor overhaul kit. There are various rubber bits, jets and filters that periodically need changing. Usually found when it does not start after a winter "resting". :-)

[Lee Nowell]

Thanks for the great explanation. Any idea how I find the actual throttle? Is it the silver thing too right in picture 3 which has a "rod" connecting it to the black governor?

[Lee Nowell]

From the pictures which are the needle valves/ valves I should check and how do I clean / fix them