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Collection, Preservation and Display of Old Lawn Mowers

Briggs and Stratton compression release

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I am restoring a Hayterette which has a B & S 10D900 series engine. It has too much compression when trying to pull start. It's has a plastic camshaft gear and lobes and the compression release lever moves freely. The piece of the lever that contacts the lifter is worn. What should this look like? Is it possible that it is too short to lift the valve effectively? The engine will not fire and is very lumpy when pulled. I have adjusted the valves as per B & S instructions.

Any help would be very welcome 

John  T

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wristpin Tue, 30/05/2017

From the way that  your question reads I take it that you have had the camshaft out, so  double check the cam to crank timing.  Then, just to recap on the correct valve clearance setting; both valve clearances should be checked / set  with the piston 1/4" down the bore on the power stroke.  With the correct clearances you should be able to see a minute amount of valve lift as the piston reaches top dead  centre on the compression stroke? 

The ACR may be worn but the effect of such wear will be exaggerated if the valve clearances are too great or even at the top end of their allowances.

Just an after thought , you are not by any chance attempting to start it without the blade disc fitted?

John Trigg Tue, 30/05/2017

Hello Wristpin

Thank you for getting back. I have been a fan of yours on this forum for a couple of years!

I have to admit guilt ref the blade disc???  I had thought that it might have a flywheel effect and then dismissed it ???? Would it be OK without blades?

The camshaft timing was OK... the marks lined up. I note what you say about the valve clearance setting and will have another go but I have to admit to not noting the valve lift you mention.

I have included two pics of the ACR on the cam . I just need to know what this release looks like ??  In the pics is it the right shape or is it broken off?

It is not clear on the pic but the valve lifter has at least 3 concentric circle marks on presumably from the ACR. Is this normal or as a result of some malfunction?

Thank you for any help you can give.

JT

 

wristpin Tue, 30/05/2017

The mechanical compression release on the smaller Briggs engines  is a bit modern for me but this may be useful.

https://www.dropbox.com/s/5u96tz0uhpg7830/Briggs%20Cam%20ACR0001.pdf?dl…

Life was simpler when it was just done by cam profile!

This may seem a bit strange but when  you rotate the engine with the piston exposed, does the sequence of valve opening and closing appear to be correct in relation to the piston's  position in the suck, squeeze, push, F**t sequence? Only reason for asking is that somewhere in the depths I seem to remember that the issue of composite cams failing was raised at a Service Update Meeting.

The concentric marks on the cam follower would suggest that at some time the ACR has been working but if, with the correct valve clearances, there's no sign of exhaust valve lift just at the point of TDC, it would seem that it is not doing so now.

The disc alone should be good enough to take the snatch out of pulling the engine over. Not relevant to the job in hand but worth checking while you are there is the condition of the flywheel key. Even a small indentation / partial shear can upset the ignition timing. I'm guessing that your lump is post early 80s in which case it probably has Magnetron electronic ignition which, unlike the points system,  will still produce a spark, but at the wrong time, with a sheared or partially sheared flywheel key.   

hortimech Tue, 30/05/2017

Looking at that camshaft, it isn't clear, but if you push down the compression relief mechanism, does it spring back again when you release it ?

If it doesn't, then you either need a new camshaft or the spring has come unhooked.

John Trigg Mon, 05/06/2017

The compression release mechanism works fine and the spring is in place. I am just curious to know what shape the foot that contacts the lifter is.

Thanks JT

John Trigg Mon, 05/06/2017

Hi Wrist pin

All interesting.  There is a lot on Google about B&S plastic camshaft gear wheels slipping on the shaft so good tip off.  Think mine is OK?

You have also got me thinking about the flywheel key. The key looks undamaged but is it the right one ?and before the nut screws the flywheel down onto the cone it rocks slightly back and forth. Is this normal?

Thanks  JT 

wristpin Mon, 05/06/2017

Flywheel rock. Ideally no. Are both fascinating clean with no burrs around the keyways?  The function of the key is more to position the flywheel timing wise than to stop rotation , the tapers plus torque should hold it in position.

Fairly easy to check out the cam by watching the valves in relation to the piston and assuming that you've not over gapped the valves you should see the ACR lift the exhaust valve a fraction just before TDC on the compression stroke.