Ride on lawnmower engine surging

[David_W]

So proper amateur with engines, this was our away last year, running out the fuel. Tried to start it up this year, wouldn't start so took the carb apart, cleaned it in an ultrasonic bath, replaced the seals and the fuel and air filters.
It now starts and runs for a bit, but then surges, going up and down on the throttle on its own. It's got new fuel in it, is there something else that could cause this, or have I not cleaned the carb properly?
Video here in case I'm not describing it well,

 

[Kram]

An air leak would often cause problems, letting it run too fast.

Video shows the govener arm moving to the right to limit the speed which looks to be the cause of the surging.

Check the intake rubber between the carb and engine, if it has one. Did you use a new gasket?

 

[David_W]

Yeah, the governor arm moving is exactly in sync. I did use a new gasket, but lots of room for error in there, I'll check that. Thanks, more than half the battle of knowing which bit to focus on, if it could be air I'll check that all again

 

[james butler]

air leak as said before is common reason for surging on mowers
did all the spring mechanism go together correctly on the speed governor? ive often seen 2 springs working in tandem and briggs and stratton models that gives the mechanism a bit of stability that helps prevent surging

 

[Nomad]

Above answer has the info - the governor is the bit that's causing the fluctuation, reason not yet confirmed.

Look for air leaks, check the air cleaner element isn't dirty or misaligned.
Also look to see if any springs on the governor arm have been knocked out of position.

My ride on mower gave good service for several years but always felt sluggish, after I sorted the aged governor springs it felt like I'd fitted a turbo motor !

 

[Mick Annick]

Above answer has the info - the governor is the bit that's causing the fluctuation, reason not yet confirmed.

Look for air leaks, check the air cleaner element isn't dirty or misaligned.
Also look to see if any springs on the governor arm have been knocked out of position.

My ride on mower gave good service for several years but always felt sluggish, after I sorted the aged governor springs it felt like I'd fitted a turbo motor !

I’ve got an old ride on with a B&S engine, there’re two springs on the throttle & governor, one is such fine wire it’s barely visible, maybe lost missing on yours? I only spotted mine as it broke as I took the carb off.

 

[james butler]

I’ve got an old ride on with a B&S engine, there’re two springs on the throttle & governor, one is such fine wire it’s barely visible, maybe lost missing on yours? I only spotted mine as it broke as I took the carb off.

that extra little one is the one that often breaks or bent when mowers are rammed into low bushes etc when cutting lawns and then start to hunt
had a few from the side of the road that were left out for the scrap man for this reason
or through wear and tear they break or fall apart when servicing
bought the b&s repair kit to replace the springs which often works
quite reliable little engines otherwise if it is a b&s engine

the air leak suggestion can be common with the carb to fuel tank gasket failing or not sealing correctly on mowers with the fuel tank bolted under the carb
not sure the mower in davids video is a brigs n stratton engine but the govenor mech does look a very similar design

 

[jo1010]

+1 for a likely spring problem...

 

[David_W]

It's a Mountfield 1538m mower, which from Google looks like a Stiga? I had a look for a parts diagram to check the spring, looks like there's only 1, but worried now that something pinged off I didn't notice.
Air leak is definitely a possibility, I'm going to check the gaskets are in the right places, when I took it apart the gaskets disintegrated so it was more than a little guesswork

 

[mike 109444]

You could spray sm carb cleaner at the gasket area while running to see if the hunting stops, indicating it's drawing in air and the carb cleaner replaces the air brefily.

 

[David_W]

Huh,

You could spray sm carb cleaner at the gasket area while running to see if the hunting stops, indicating it's drawing in air and the carb cleaner replaces the air brefily.

Oh that's a neat idea, will try that today

 

[daleyd]

I had similar on a countax ride on with briggs engine fitted. I changed fuel and air filters, looked for air leaks, cleaned the carbs - still the same. I eventually found a small bit of plastic had settled into the fuel outlet at the bottom of the tank, managed to fish it out and it ran perfectly afterwards.