YSM8 Revs up Dangerously

[-OvO-]

I'm not sure you understood my post -- I was deflecting suspicion from the governor.

If the fuel cutoff works, it should stop the engine regardless of the state of the governor. Likewise, if the fuel cutoff works, it shouldn't matter if the fuel injector is bad.

Either (a) your fuel cutoff is not working or (b) you're burning something that isn't supposed to be fuel, ie, motor oil.

Can you verify whether the fuel cutoff works or not?

[Faster]

Originally Posted by -OvO-

I'm not sure you understood my post -- I was deflecting suspicion from the governor.


Can you verify whether the fuel cutoff works or not?

Even if his fuel cutoff has become disconnected or has a broken cable that should just mean he couldn't stop the engine that way.. it shouldn't cause runaway RPMs.

By the sound of it the engine starts quickly and easily, so bad compression likely isn't the issue, but motor oil fumes could still be the problem.

It does seem that if indeed the governor was the problem it would be unlikely to settle down at any time.

Return spring on the throttle linkage?? is the throttle arm (or it's internal mechanism) floating on it's own?

Disclaimer: not real familiar with that particular engine..

[-OvO-]

Originally Posted by Faster

Even if his fuel cutoff has become disconnected or has a broken cable that should just mean he couldn't stop the engine that way.. it shouldn't cause runaway RPMs.

Absolutely agree - just backing up and breaking it down into little steps.

[casey1999]

Originally Posted by Harborless

Just finished getting the engine system connected all up. Started it up and cranked on the first try. Ran it ten minutes putting t in and out of gear at the dock.
All of sudden after I revved engine slightly and was throttling back down the engine started revving up uncontrollably. It would not shut off from pulling the choke and was speeding up so fast it would have blown up had I not went into the engine box and pushed the compression valve shut.

Waited a few minutes and piddled around. Started back up on first crank. Ran for two minutes. Same issue.

Engine is 1979 YSM8 Yanmar. Any Ideas?

Don't quite follow. Do you mean engine will rev up and down with no problem in neutral but you have the problem when in gear? If this is the case, make sure the gear selelector cable does not interfere with the throttle positioner- the cables move around quite a bit when changing gears.

[84westy]

Reading the OP's description makes me think that the engine isn't burning fuel when it runs away.

Not related to sailboats, but to a VW diesel in my camper van:
The engine ran well and was regulated by the governor in the Bosch VE pump. It would randomly run away at very high RPM. The pump limited to 5200 rpm. When running away it would go to about 7800 rpm. Long story short, the EGR pipe (no valve in these engines) had gotten spongy and soft with years of exposure to engine oil and had swollen internally. The engine was sucking oil and burning it for fuel. I replaced the hose and the problem went away.

I would check the engine oil level to ensure that it isn't over filled and then check the EGR circuit for clogs or damaged hosing.

Most injection pumps are internally mechanically governed with a stop. It is unlikely that the pump would allow run away with a mechanically controlled engine and I would be looking for other fluids that the engine could burn.

[Capt Len]

If it's not runaway on crank oil/fuel mix is it possible the control wire over centers on the lever.when you rev up. Chevy v8s can do this with a broken motor mount.British Fords and vdub rabbits (pathfinder?) are known for crank oil runaway and the best way to stop them is to check the problem before it happens and you have to stuff your shirt down the air intake before it throws a rod thru the hull.

[FORCOL]

I currently own a YSM8. I have owned 2 other YSE8's, and have pulled apart all of them for repair or maintenance.
These engines require high compression and a precise injection spray of fuel oil to get combustion.
Knowing how hard it is to get one started without these two key facts tells me it is impossible to run on lubrication oil.
Another urban myth.
Remove the throttle cable
The spring attached to the throttle lever on the motor must be the original Yanmar part. This is very important as it gives the governor the exact resistance to keep revs uniform.
I bought the ysm8 with two weak springs put on by previous owner and the revs went all over the place, including shunting.
I replaced this and the engine went very smooth after that.
If this doesn't work, remove the injector pump and overhaul it.
You can do this yourself if you find a copy of the maintenance manual online (if you don't already have one).

[SloopJonB]

Originally Posted by FORCOL

I currently own a YSM8. I have owned 2 other YSE8's, and have pulled apart all of them for repair or maintenance.
These engines require high compression and a precise injection spray of fuel oil to get combustion.
Knowing how hard it is to get one started without these two key facts tells me it is impossible to run on lubrication oil.
Another urban myth.

I'm not so sure. I bought a YSM8 new on the pallet years ago. When I was examining it I put the crank handle in and spun it over with the compression release held open. When I let go of the C/R the engine fired up - on the pallet. It must have been running on the assembly lube. I hit the C/R and it stopped but it must have fired at least 4 times before I got my wits about me.

The only time that engine was hard to start was when it was extremely cold out - dead of winter. I started it by hand NUMEROUS times over the years when I had let my battery go flat.

[FORCOL]

The poster states his engine runs away for no apparent reason.
If people replying to this post think it (OP's problem) could be related to lubrication oil being driven through the rings causing such a perfect combustion that: a highly precision built; one cylinder; high compression; 3200rpm revving; with very delicate timing; and a precise injection system; 8hp diesel engine could run out of control: then you all need to TAKE ONE APART.
Seriously, the bloke with the Comby motor had five run-down cylinders, pushing away a very tired 6 cylinder motor. He attributes his claim that diesel engines run out of control because of an oil leak through the rings!
DIESEL ENGINES/MOTORS DO NOT RUN ON THEIR OWN LUBRICATION OIL. FACT!
Take one apart. No, take 3 apart, like I have, and get them to start, AND RUN.

I know what the problem with posting on a sailing forum is: you people change the oil in your engines as much as you change your underwear.
If you can't read between these lines then I will help.
The rack and pinion fuel distributor is within the oil sump (connected to the fuel pump). I you do not keep this item religiously clean( through changing the oil), it foul, and cause irratic behaviour with your motor

[FORCOL]

Secondly, if i went out to my shed and hand cranked my YSM8 without the key in it or the fuel on, it would run for about a minute.
YANMAR would not distribute an engine without fuel oil in the pump, injection line; or injector nozzle, as new. WHAT MANUFACTURER WOULD. My YSM8 can run on the 200mm of 4mm fuel line disconnected from the fuel filter for well over a minute.