Burned Valve?

Bob_Bainbridge · Apr 30, 2010

While I was gone on a 9-day bus trip my daughter was able to get the 3235 out of the barn for the first time because of the soft wet yard. She mowed twice during that time and got buried in the mud at the rear of the lot. She had to call my son-in-law to help push it out. She got stuck once more but was able to rock it out herself. When I got home I got out to mow again and it sounded like it was running on one cylinder. I expected to find one plug not firing but they were both hot and the right bank plug wire had a big chunk melted out of the boot like it got really hot. Both plugs looked good but I found the engine will run on the left cylinder alone but will not run on the right one alone. There is compression but I have to find my compression gauge to check it tomorrow. It sounds to me like it may be firing through the exhaust valve. The manual looks like the valve seats are not replaceable so I hope it's nothing more than a valve. Is this common and is it possible that getting stuck caused severe overheating of the engine?

Bob B.

valley · Apr 30, 2010

If you have good compression you don't have a burnt valve. Check the plug, maybe a new wire and give it a try.

Bob_Bainbridge · May 1, 2010

I found my compression gauge and checked it today. I read 70 psi on the left cylinder, which seems a little low to me for a supposedly 9:1 compression ratio. I got zero on the right bank so there's definitely a problem on that side. Does anyone know what normal compression is. The sevice manual describes doing a cylinder leakdown test but gives no compression readings. The two cap screws holding the engine lift bracket and shroud on the left cylinder head would not loosen and snapped off. Gad, I love aluminum engine parts. I took the rocker cover off and verified the pushrods and valves are operating OK. I'm debating between tearing into it or calling the dealer to pick it up for $25. It's supposed to pour tomorrow so I temporarily threw it back together and mowed the rear lot on one cylinder. I broke two of the quarter-turn fasteners putting the side panels back on. Maybe it's trying to tell me something.

Bob B.

Bob_Bainbridge · May 3, 2010

The dealer suspected a blown head gasket but said that would cause low and not ZERO compression. I suspect worse things such as a bad valve.He quoted $80 for parts and 4-5 hours labor at $85/hour for head gasket. I decided to tear into it myself. So far I was able to get 2 shroud screws out of the head by rotating back & forth while spraying oil on them. Four others snapped off. They're not critical and will be replaced anyway if it ends up needing new heads. I'm having a problem getting the large plastic shroud loose from the rear of the engine. Not sure what is holding it. It has to be moved back to get access to the intake manifold. Has anyone done this job?

Thanks, Bob B.

I found out it's called the BLOWER HOUSING. Hopefully it can be moved out of the way without pulling the engine.

Bob_Bainbridge · May 4, 2010

I managed to pull the right head today and the dealer was correct - it was a blown head gasket with a 1-inch break. The valves look good. One head bolt was not even finger tight. I'll pull the other head tomorrow to see why that one has low compression. I'd like to talk to the engineers that design these things. The exhaust pipe can be unbolted from the head and slid into the muffler but lacks clearing the studs by 1/2 inch. The grill had to be removed and the muffler mounts unbolted to gain that extra 1/2 inch.

Bob B.

carpenter383 · May 4, 2010

Kinda late now, but may help you diagnose the other head.
You can use compressed air to see what's leaking.
Rotate the engine til it's on TDC of the compression stroke (both valves closed) Find a way to hold the crankshaft from moving.
Use compressed air to pressurize the cylinder while listening and feeling for air leaking out of the exhaust, carb, and crankcase. It's normal for a small ammount of air to exit the crankcase.

Bob_Bainbridge · May 5, 2010

carpenter383 said:
Kinda late now, but may help you diagnose the other head.
You can use compressed air to see what's leaking.
Rotate the engine til it's on TDC of the compression stroke (both valves closed) Find a way to hold the crankshaft from moving.
Use compressed air to pressurize the cylinder while listening and feeling for air leaking out of the exhaust, carb, and crankcase. It's normal for a small ammount of air to exit the crankcase.

I just pulled the head, seeing that I was this far in the tear-down, and everything looked good on the left side. Perhaps my old compression tester isn't working right causing a low reading of 70. The only other cause would be leaking rings which doesn't sound logical considering how everything looks. There's no slop or movement of the piston in the bore. A little oil squirted in the cylinder will prove or eliminate that as a cause. I used a sheet of 150 grit sandpaper to resurface the heads slightly. The tractor dealer didn't answer my email parts inquiry so I'll have to call them in the morning to see if they have the gaskets in stock. From what I've been able to find on the web most mechanics recommend installing the gaskets dry.

Bob B.

Bob_Bainbridge · May 10, 2010

I finished the job yesterday and mowed the yard today with no problems except yesterday I pulled the dipstick out to check the oil level and the handle came out and the dipstick remained in the tube. The yellow plastic looked like it disintegrated and I had to dig it out with a small screwdriver. A new dipstick is $6 plus shipping from the dealer. I put a plastic plug in the hole and started getting clouds of blue smoke while mowing. The plug had blown out and oil was spraying on the hot exhaust. I ended up bending the dipstick for a handle and sticking it upside down in the tube since a piece with the O-ring was still attached.
I ended up with compression readings of 135 PSI left and 125 PSI right. I don't know why the difference but I guess that's within specs. The head gasket kits contained new bolts for heads that use bolts and new flat washers and nuts for the the type like mine. I fired it up without all the shields and let it get hot. I then retorqued all the head bolts to 30 ft. lbs. Each nut turned an additional 1/4 to 1/2 turn. Hopefully the new gaskets will hold. I started wishing that I'd let the dealer do the job for $500.

Bob B.