In this Fixit video we’ll take a look at a Laser (by Mountfiled) 46cm Lawnmower, fitted with a Honda GCV135 engine and Keihin BB Carburettor, which would start ok but wouldn’t run consistently – the idle speed was very lumpy and it was constantly hunting on the governor.

There were several faults, some more severe than others: The fuel shut off tap had failed and the air filter was soaked in oily petrol so it didn’t breath properly. The governor connecting rod anti-backlash spring was fitted incorrectly. The crank case breather pipe wasn’t attached to the back of the air box. There was a broken gasket on the engine side of the carburettor, and the heat shield was fitted the wrong way round.

In the video we’ll work through the faults including a strip down of the carburettor (this probably wasn’t needed on this particular occasion) and finally show the lawnmower back up and running.

I’ll put a few Amazon affiliate links to some of the tools I used (or at least the nearest equivalent that I can find) – as with all affiliate links, if you use the link to buy the item, I get a tiny bit of commission but this doesn’t affect the price you pay:

Briggs & Stratton 491588S Air Filter
Hose Clamps
Screwdriver Bit Set
Long Reach Security Bit Set
Watchmakers Tweezers
Olfa Compass Cutter used for cutting the new gasket
Hole Punch Set used for cutting the new gasket

Intro Music
“As Yet Untitled”
by GrumpyTim (available to download from GrumpyTim’s Bandcamp page)




  1. Thanks for this. You tackled a lot of niggling faults that may or may not have caused the symptoms. I'm not so sure about that heat shield being the wrong way 'round. I think the bottom part should bend inwards toward the engine; I think the air atmosphere hole being on the rhs meets up — or should meet up — with the hole in the engine. Could the gasket on it have slipped? I'm yet to see an explanation of the air passage starting at the filter box with the round recess at the rhs transforming over to the rhs hole in the engine block! How does that happen?? Genuine questions 'cos I'm struggling with this right now!
    Thanks again.

  2. The idle mixture screw limiting cap is heldl in place on the screw with a locking adhesive like Loctite, warming the cap wit a hot air gun will enable removing it from the screw below which is slotted so can then be removed with a conventional screwdriver. I recommend replacing the cap once rebuild and adjustments are made as the cap does stop vibrations from shaking the screw completely out of the carb.

  3. Hi Tim / any good citizens 😄, I've just fitted a new carburetor on my Mountfield Honda Gcv135 and it started almost first time, but now it wont tickover even in hot weather. It cuts out when I bring the lever on the handlebars to turtle speed. The outer choke flap seems to be permanently open. Do I need to set this up? Regards Stuart

  4. Good work. My only comment though is that normally you do a screw all the way up counting turns before you remove it rather than count on the way out.

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