312-8 cutting out.

[ranger 1,798]

Gave the 312-8 a run today, cutting grass in one of the horse paddocks. I had modified the silencer I made a few years ago to reduce back pressure and hopefully heat build up at the exhaust. When I first made the exhaust I took apart the rusted original and copied it in stainless even down to the number and size of holes in the baffle tubes! The issue is, it will run for a while then cut out. When it cools slightly it will then re-start and run again for a while. Carb was a Walbro, fixed jet unit, hence the reason for copying the silencer exactly. I had changed just about everything, carb, electric fuel pump, all wiring,connections, safety switches, etc. I have suspected the exhaust valve since reading sometime ago about early “Magnum” engines suffering from binding exhaust valves due to stem/guide clearance being too small. Apparently a “recall” was initiated with dealers reaming the guides to provide more clearance. Today it cut out twice, no compression when turned over via starter, but after cooling for a few minutes, I heard a load “Metallic Click”, engine started immediately and I carried on, until it cut out again, same scenario, loud “Click”, as valve closed, started up, and away again. I managed to finish the mowing before the heavens opened. Next job, head off and valve out!

 

Doug.

[gwest_ca-(File Mod) 11,472]

Have you tried some Marvel Mystery oil or 2-stroke oil in the fuel? Sounds like the valves could use some lubrication.

 

Garry

[rmaynard 16,293]

If it not a stuck valve, it could be a clogged vent in your gas cap. Next time it stops, loosen the cap. Try starting it. 

 

[ranger 1,798]

34 minutes ago, gwest_ca said:

Have you tried some Marvel Mystery oil or 2-stroke oil in the fuel? Sounds like the valves could use some lubrication.

 

Garry

Yes, tried all this before Garry, nothing worked, I add ZX1 additive to the oil, I use this in all my engines, ( Non P.T.F.E. Friction reducer), also  added to fuel. If the ambient temperature is low, it rarely happens. As I said before, this was a known issue with early Magnum engines. It’s got 800hrs on the clock, 1987 or 89, I can’t remember, probably time It had a decoke anyway.

I’ve had it for quite a few years now, It came free of charge from an ex customer of mine, a local racehorse stud, owned by a Middle Eastern Gentleman, whose fortunes will probably take a tumble when all autos are electric! It stopped working one day, so it ended up on the scrap heap. They weren’t allowed to sell it, so they gave it to me, which they were allowed to do. The repair cost more than the purchase price, (Just), a new fuel pipe!😃

Doug.

[ranger 1,798]

59 minutes ago, rmaynard said:

If it not a stuck valve, it could be a clogged vent in your gas cap. Next time it stops, loosen the cap. Try starting it. 

 

No compression when it cuts out, then after it’s cooled slightly, you hear the click as the spring closes the valve and compression is back.

[rmaynard 16,293]

No compression or low compression? Kohler engines have ACR, automatic compression release. You won't get a proper compression reading until the engine reaches about 600 rpm.

[ranger 1,798]

5 hours ago, rmaynard said:

No compression or low compression? Kohler engines have ACR, automatic compression release. You won't get a proper compression reading until the engine reaches about 600 rpm.

Definitely No compression, when it occurs the engine spins over as if there is no piston inside, This is on the starter. On releasing the key, it spins many times before coming to a standstill. After the “Click”, ( Valve closed, compression restored), and operating the starter, without trying to actually start the engine, the crank stops turning in a couple of turns after key is released.

Doug.

[lynnmor 7,813]

1 hour ago, ranger said:

Definitely No compression, when it occurs the engine spins over as if there is no piston inside, This is on the starter. On releasing the key, it spins many times before coming to a standstill. After the “Click”, ( Valve closed, compression restored), and operating the starter, without trying to actually start the engine, the crank stops turning in a couple of turns after key is released.

Doug.

 

So you have a sticking valve.  Remove it and check that it is straight and clean the valve guide bore.

[ranger 1,798]

1 hour ago, lynnmor said:

 

So you have a sticking valve.  Remove it and check that it is straight and clean the valve guide bore.

Yes, that is my intention.

[richmondred01 2,307]

Reaming the guides you may find difficult depending on the ream you use. 
Many reams will need to pass through the guide and hit the tappet. 
If there’s a guide issue I always knock it out and replace it with a new one then ream while the tappet is out.

However, some may consider this too much work because everything including the cam must be removed.

If you go this route remember to mark your tappet as exhaust and intake.

Edited May 20, 2021 by richmondred01