I have Mountfield Laser with a Briggs & Stratton engine and, following replacement of the blade (by me) it is now very difficult to start and the cord whips back when I try.

Any answers please?

Yorkie, September 2008

Pleasure to have been of assistance. Saved a lot of un-nesessary work by simply checking the blade.

Good luck.

pleasant, September 2008

"You may also want to consider the fact that you haven't done the blade bolt up tight enough- the blade acts as a flywheel to kick the engine over when starting- it it's loose the this will lessen the flywheel effect and cause kick back through the recoil".

I'm embarrassed to say that this was the problem. I took the blade off, put it back on again - tight - and my mower now starts fine.

Thanks to pleasant and all who wrote for their help.

Yorkie, September 2008

Thanks pleasant, you're a star and this site is great, so good to have easy access to helpful people who know what they're talking about. It's raining just now but I'll try this as soon as it stops (probably about November!) and post a note on the site.

Regards

Yorkie, September 2008

Ignore Micky-g's answer as that is impossible to happen during normal mowing use.

Previous two answers are correct. Most likely is the flywheel key has fractured causing the flywheel to slip- thus advancing or retarding the ignition timing.

The only way the timing gear within the engine can be altered is by physically stripping the engine and manually advancing or retarding the valve timing gear- none of which can happen through some external force.

You may also want to consider the fact that you haven't done the blade bolt up tight enough- the blade acts as a flywheel to kick the engine over when starting- it it's loose the this will lessen the flywheel effect and cause kick back through the recoil.

If you know you never hit something prior to the belt coming off and it was still starting fine before you replaced the blade then it's the attachment of the blade I would look at first.

pleasant, September 2008

"unfortunately the only way to do this is to disassemble the engine and reset the timing gear mark on the crank shaft and the mark on the cam so they are synchronized"

Wow! What do you think, should I just buy another lawn mower?

Yorkie, September 2008

Thanks very much Phil, and Robert, I knew someone out there could help. Removing the flywheel sounds a bit of a scary process (!!) is it dangerous or just difficult?

BTW I changed the blade because the belt came off the pulley wheel and it seemed worthwhile doing it at the time. Don't know why the belt came off, it just did one day whilst I was mowing the lawn.

Regards,

Yorkie, September 2008

I am aware of the other answers to your question. However if the pull rope on your engine is snapping back hard your engine is trying to fire in reverse. the way to fix this is to physicly retime the engine. unfortunately the only way to do this is to disassemble the engine and reset the timing gear mark on the crank shaft and the mark on the cam so they are synchronized.

Mickey-G, September 2008

I agree with Phil Saunders. If the blade hits somethng, it will stop immediately, but the flywheel will still move slightly, and shear the key. This is OK as it is supposed to shear the key rather than damaging the crank and/or flywheel. Since the flywheel has shifted forward, it advances the spark timing to where it will fire too soon which will make the rope jerk back. Unless you have the right tools, don't try to remove the nut and flywheel. I have done it using a crowbar to hold flywheel, and pipe wrench to losing nut--I don't recommend doing that though

Robert Brown, September 2008

Did you replace the blade because you hit something solid? Chances are the flywheel key has sheared. Iy's supposed to - it's a protection device to prevent more serious damage.

phil_saunders@bigfoot.com

Phil Saunders, September 2008