720 lost compression

[anesthes]

This is crazy. The engine had 150 psi in both cylinders 2 weeks ago. Took it out today, ran for about 20 minutes - was running absolutely amazing. Then it just died. Wouldn't restart.

Towed it back. Compression test in both cylinders. PTO was 150psi. Mag was 60psi. Assumed it detonated and blew up the piston as I've seen on other 720s ove the years.

Took the head off, piston looks fine, cylinder looks fine, rings look ok.. So I replaced all the gaskets and put it back together.

Still only 60psi on the MAG side.

I can't imagine anything with the RV would cause low compression in one cylinder. This is a single carb so I think that rules out any obstuction.

I'm baffled. I ordered a new piston so I guess I'll pull the cylinder, check it with a bore gauge, and replace the piston and rings but. I just find it really odd.

Picture attached of the cylinder. Head looked fine too, just a little oily in that cylinder, otherwise nothing suspect.

-- Joe

 

[etemplet]

Could be a connecting rod bearing.

 

[anesthes]

How?

 

[etemplet]

Just one of a number of causes. I'd suspect your ring gap is out of spec but you said it looked good. You won't know till you take measurements though. If your connecting rod bearing on the crankshaftfailed the piston won't reach the top of the cylinder so compression will suffer. Check everything and find the root cause so it won't happen again. Good Luck !

 

[anesthes]

Just one of a number of causes. I'd suspect your ring gap is out of spec but you said it looked good. You won't know till you take measurements though. If your connecting rod bearing on the crankshaftfailed the piston won't reach the top of the cylinder so compression will suffer. Check everything and find the root cause so it won't happen again. Good Luck !

Ahh gotcha. The piston travels the full stroke. I guess the rings just decided to give up. I'll do rings on both cylinders, and maybe replace the MAG piston if it looks bad once I take the cylinders off.

Really sucks to happen with only a few good weeks left of the season here in NH.

I don't know what, other than age, would cause the ring to just give up. There is no signs of overheating or lack of circulation in that cylinder, no corrosion, and the plugs looked goot, a little sooty if anything. That cylinder had a bit of oil residue on it so I don't think there was any lubrication issues.

-- Joe

 

[etemplet]

I'd do a complete top end. I guarantee the piston skirt on both are worn. I like WSM pistons because they give you a little extra on the large side as the recommended bore size is +.0045" so you can get away with a slightly larger hone size. If you do a good top end.... the engine will be fine for a longer term. If you change one piston... the other will fail sooner. While you're there.... do it all.

 

[anesthes]

Perhaps the front seal is going.

Front seal?

-- Joe

 

[anesthes]

Maybe front seal causing an air leak?

You mean the crank seals? I did a case pressure test and it held for 5 minutes, but I don't think that would cause a compression loss. The crank seals would leak into the intake area, but once the piston swept past the intake it should compress the charge. The only thing that would cause the charge to escape would be rings or head gasket.

-- Joe