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engine question
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Ray White



Joined: 02 Dec 2014
Posts: 3993
Location: Derby

PostPosted: Sun May 10, 2020 2:54 pm    Post subject: engine question Reply with quote

Please bear with me but I have a question about a lawnmower engine. It's a 4 stroke Briggs and Stratton. It seems to have developed too much compression so I can't hardly turn it over. With the plug removed it's free again.

I can't think what's caused the problem which
has just started after a year's hibernation in the shed.

Any ideas pleas?
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Keith D



Joined: 16 Oct 2008
Posts: 1006
Location: Upper Swan, Western Australia

PostPosted: Sun May 10, 2020 3:09 pm    Post subject: Reply with quote

A friend overfilled the oil in his Briggs and Stratton engined ride-on mower. The starter wouldn't crank the motor over. When I checked it out, he had overfilled the oil. I drained it down to the level mark on the dip-stick and it started instantly with a massive cloud of blue smoke!

Keith
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Kenham



Joined: 12 Mar 2012
Posts: 195
Location: Kent

PostPosted: Sun May 10, 2020 3:44 pm    Post subject: Reply with quote

Is it electric start,some of those Briggs engines have a decompression device built in, I had the same problem. It is made so the tiny starter motor can turn it, without it working it can't get it over compression. Not easy to mend I think you have to strip the complete engine.
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Peter_L



Joined: 10 Apr 2008
Posts: 2524
Location: New Brunswick. Canada.

PostPosted: Sun May 10, 2020 4:41 pm    Post subject: Reply with quote

As per Kenham's reply, it's all to do with compression release. Is it electric start?

https://www.youtube.com/watch?v=ggV0wS6VMSs

YT videos explain how it works and also that the centrifugal cam is deep in the engine.

If you can get it going and spread the oil around inside it may "self mend"
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Ray White



Joined: 02 Dec 2014
Posts: 3993
Location: Derby

PostPosted: Mon May 11, 2020 12:29 am    Post subject: Reply with quote

No luck with the mower. It is a cord pull and I have not had this problem before. I checked the oil and it is at the correct level. I checked that the brake lever cable is connected and not holding on but it's not that.

I wondered if the exhaust valve could be stuck shut and how that might cause the extra compression??? but I don't suppose that would happen.

Unfortunately, I suffer with painful joints and I can't pull on the rope for long before giving up.
It started raining so I just put it back in the shed.
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Farmer John



Joined: 18 Feb 2010
Posts: 178
Location: Manawatu NZ

PostPosted: Mon May 11, 2020 4:55 am    Post subject: B&S compression lifter Reply with quote

Hello Ray. If your B&S is an OHV then it will almost certainly have a compression release. Remove the rocker cover and turn the engine slowly by hand with the plug out. Watch the inlet valve. As the piston nears TDC on the compression stroke the inlet valve should open briefly, it is easy to see.
If it does not then you may have a simple valve clearance issue so that the compression lifter does not actually lift the valve off the seat which means resetting as per the manual. Otherwise there is a strong chance that the mechanism is broken which means dismantling the engine and maybe a new camshaft.
I went to a lot of trouble to get a Cub Cadet to start with new battery, new heavy duty cables, automotive solenoid, and then remembered the compression lifter! Adjusted the valves and away it went! That is, until it blew the bottom out of the crankcase, a fault which USA knew about several years before we stopped importing them. The simple fix was to finally grow out of "fixer upper" mowers and buy a new John Deere. Perfect.

John
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MVPeters



Joined: 28 Aug 2008
Posts: 745
Location: Northern MA, USA

PostPosted: Mon May 11, 2020 11:34 am    Post subject: Reply with quote

Ray White wrote:
No luck with the mower. It is a cord pull and I have not had this problem before. I checked the oil and it is at the correct level. I checked that the brake lever cable is connected and not holding on but it's not that.

I wondered if the exhaust valve could be stuck shut and how that might cause the extra compression??? but I don't suppose that would happen.

Unfortunately, I suffer with painful joints and I can't pull on the rope for long before giving up.
It started raining so I just put it back in the shed.

Ray
See if there's anything here that will help - look for the Briggs videos:

https://www.youtube.com/user/mustie1/videos

If you've got spark, the likelihood is that the carb needs cleaning. Modern fuels don't have a long shelf life.
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D4B



Joined: 28 Dec 2010
Posts: 2090
Location: Hampshire UK

PostPosted: Mon May 11, 2020 12:33 pm    Post subject: Reply with quote

Mine almost broke the pullcord because the bearing between the engine and the blade was seized. With big garden gloves on and the plug lead off, rotate the blade and lubricate and or clean the bearing until it rotates easier....

Might not be your problem but it worked for mine.
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Ray White



Joined: 02 Dec 2014
Posts: 3993
Location: Derby

PostPosted: Mon May 11, 2020 4:00 pm    Post subject: Reply with quote

This is a pretty old machine and was a non runner but it was free so I took it on - mainly because it is self propelled and I thought it would make cutting the grass easier. My garden is on a hill and is a killer with a flymo.

When I rebuilt the carb I found some of the throttle mechanism was missing so by cannibalising another old mower I was able to get this one going. Last year it performed very well with no smoke. It practically pulled me up the slopes!

On extracting the Mower from it's resting place in the shed this year I changed the oil and petrol and made sure the blade was sharp and the belt was O.K. The engine tried to start and ran for a few seconds before dying.

The blade can be turned by hand. I can even turn the engine by moving the fan on the top. It is only when I try to spin it with the rope that it seems to seize but if it was doing that then it would still not be free with the plug removed; which it is.

Something is going on but as yet I don't know what.
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Kenham



Joined: 12 Mar 2012
Posts: 195
Location: Kent

PostPosted: Mon May 11, 2020 8:27 pm    Post subject: Reply with quote

Could it be a timing issue and if so perhaps the flywheel key has sheared, this can cause it to try to kick back on the rope. They have a soft key that can shear, have you hit a rock or something as that can shear the key if the engine stops suddenly. Can't really think of anything else.
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Kenham



Joined: 12 Mar 2012
Posts: 195
Location: Kent

PostPosted: Mon May 11, 2020 8:37 pm    Post subject: Reply with quote

Just another thought if it has the ignition unit outside the flywheel check the gap, a bit of rust over the winter and a bit of play in the bearing could jam things perhaps.
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Ray White



Joined: 02 Dec 2014
Posts: 3993
Location: Derby

PostPosted: Mon May 11, 2020 11:21 pm    Post subject: Reply with quote

Kenham wrote:
Could it be a timing issue and if so perhaps the flywheel key has sheared, this can cause it to try to kick back on the rope. They have a soft key that can shear, have you hit a rock or something as that can shear the key if the engine stops suddenly. Can't really think of anything else.


I can't think of a better suggestion either, Ken.

I just don't want to face up to the possibility that I might have to spend more time on this wretched thing. Any other time I would have thrown in the towel and bought a new one but with no income now or for the foreseeable future I can't risk it.
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alastairq



Joined: 14 Oct 2016
Posts: 1457
Location: East Yorkshire

PostPosted: Tue May 12, 2020 10:56 am    Post subject: Reply with quote

If.....pulling, and investigating the B&S engine isn't to your liking......what about investigating one of the cheapo Chinese 4 stroke engines available? Even from the likes of Amazon?
Maybe a couple of hundred pensionquids or thereabouts?

OK, so some sort of adaptor mounting might have to be fabricated? But, isn't the actual drive connection via belts anyway? That only leaves connection to the gearbox?
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MVPeters



Joined: 28 Aug 2008
Posts: 745
Location: Northern MA, USA

PostPosted: Tue May 12, 2020 2:09 pm    Post subject: Reply with quote

Ray White wrote:
Kenham wrote:
Could it be a timing issue and if so perhaps the flywheel key has sheared, this can cause it to try to kick back on the rope. They have a soft key that can shear, have you hit a rock or something as that can shear the key if the engine stops suddenly. Can't really think of anything else.


I can't think of a better suggestion either, Ken.

I just don't want to face up to the possibility that I might have to spend more time on this wretched thing. Any other time I would have thrown in the towel and bought a new one but with no income now or for the foreseeable future I can't risk it.


It's also possible for the key to twist, rather than shear completely. This would throw the timing off by a few degrees. There's an example in one of the videos linked earlier.
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Ray White



Joined: 02 Dec 2014
Posts: 3993
Location: Derby

PostPosted: Tue May 12, 2020 9:28 pm    Post subject: Reply with quote

MVPeters wrote:
Ray White wrote:
Kenham wrote:
Could it be a timing issue and if so perhaps the flywheel key has sheared, this can cause it to try to kick back on the rope. They have a soft key that can shear, have you hit a rock or something as that can shear the key if the engine stops suddenly. Can't really think of anything else.


I can't think of a better suggestion either, Ken.

I just don't want to face up to the possibility that I might have to spend more time on this wretched thing. Any other time I would have thrown in the towel and bought a new one but with no income now or for the foreseeable future I can't risk it.


It's also possible for the key to twist, rather than shear completely. This would throw the timing off by a few degrees. There's an example in one of the videos linked earlier.


You may well be right. If the key had sheared off I imagine the blades wouldn't turn.?
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