Craftsman push mower, Mod.#917.388300; Techumseh 6hp engine, Mod.#143996004

Mower ran fine about 10 days ago when I parked it in garage. When I tried it again, it started and ran for only seconds and shut down. Subsequent pulls felt like there was no compression; blade turns when I pull rope and I hear a swooshing noise toward the rear of the engine. Took off the muffler and can see valve rod moving when turn the blade; took out the spare plug and can feel toward the left of the hole what I think may the top of the piston moving up and down. When I put my finger over the spark plug hole, I feel no pressure against it.

 

It sounds like the cylinder is gas washed. Squirt a few drops of oil in the spark plug hole & pull the rope a few times. You will regain your compression. Install a new plug.

 

Could be your intake valve is stuck. That could be the woosh you hear, the sound of air coming out of the carb.

 

You can take the valve cover off to check that. You will be able to see the valve stems moving when you turn the blade by hand. TAKE the plug wire OFF first.

 

Hello Jaywalker,

Sounds like the classic case of a chunk of carbon build-up breaking loose and getting stuck under one or both valves keeping them from seating, removal of the head and cleaning the valve faces and seats (cylinder decarbon) will likely restore compression.

I've seen this many times and just finished the same task on a machine yesterday.


Good Luck

 

Thanks for replies from Pulpo, BoudreaxEunice and 31YTech.

Pulpo's suggestion (the simplest, thankfully) did the trick. At first, after pulling several times, compression was not restored, so I took BE's suggestion and pulled the valve cover, turned the blade and saw both pushrods moving, so put it back on. Was starting to take off the gas tank and pull rope assembly, but decided to check one more time for compression and had some, so put the plug back in (after cleaning it) and tested the feel of the pull, which felt normal. Primed it and EUREKA, it started fine. Ran it for a couple hours, mulching leaves and it's like new!

Jaywalker

 

Hey, guys:

Used the mower all day after I got it going. Parked it for about three weeks and same trouble. I just got around to looking at it again today, squirted oil in cylinder again and after a while, got compression, tried to start it and heard a noise. When I took the valve cover off again, I found the spring on the top valve stem loose and dangling on the stem. Apparently something is broken to release the spring compression. What do you think?

 

They usually have some kind of keeper that sits in a groove around the valve stem. A common setup is a split col-let that you seat in the groove with the valve spring compressed. Then you release the compression and the spring pushes up and holds a collar around the col-lets to hold them in place.

To have what you've describing either the spring would have to break or the valve break off the stem.

 

marbobj:
Thanks for your advice and sorry I haven't replied sooner. You were exactly right on...keeper was unlocked and spring, of course, was loose. I haven't gotten it fixed yet, 'cause I'm devising a way to compress the spring without taking the head off, hoping to save additional labor. Cool weather has delayed my efforts, but will update here later.
Jaywalker

 

You can order a valve spring compessor tool on line about 15 bucks for a B and S it works on all small engines.

 

Sometimes putting the spring back on can be a challenge since the valve tries to keep falling into the cylinder. You might try feeding a nylon starter rope into the cylinder and let the piston push the rope up against the valve to hold it in place.

If you want to try it start by bringing the piston up on compression, then both valves would be closed and the pressure from the cam would be off them.

 

Thanks again, guys! I made a tool to compress the spring and used a broken wrench with a slight bend on the broken end that fit through the plug hole and was able to wedge it in on top of the exhaust valve to hold it closed. It was difficult to get the collet positioned so the narrow part of the opening was in a position to slide
onto the valve stem, but I persisted and got it in place. Rotated the crankshaft several times and could see that the spring was seated properly on the collet and the valves appeared to be moving as they should, but again had no compression.
Followed the oil squirts in cylinder procedure and soon had compression. The good news is that I put manifold back on and tested, it ran OK for a few minutes, so I put it the valve cover and muffler back on and tested again. Runs good, restarts after shut downs and even after letting it sit for an hour or so. I'll try it again after sitting overnight and hope for the best. I think I still have some kind of problem causing the repeated gas-wash situation after not using for a time... We'll see!!!

 

OK, I left it overnite Jan. 26, went out next day, primed and pulled one time for successful start, let it run 10-15 mins. and put it back in garage. So, I let it sit two onites, went out this smorning, primed and pulled one time to start. Bad news: It ran for 15-20 seconds and died. Pulled rope and had NO COMPRESSION! Any further diagnosis suggestions? Thanks.

 

If you actually have no compression, that sounds like a sticking valve. Check the valve with the spring problem.

 

I agree. It sounds like the valve stem and/or guide has a varnish or carbon buildup that is making the valve stick. If not, the valve stem may be slightly bent, which will also cause sticking.

 

Want to thank all who helped me with this problem and let you know the outcome.
If I'm right, the final "fix" will come when the weather is warm enough to work outside comfortably, but after multiple tests of clamping off the gas line from the tank to carb and leaving it alone for two days or more, the compression was holding, so I bought and installed a cut-off valve in the line, have done further testing, as above, and it's held compression and started every time! I'm now thinking a dirty or defective float valve might have been the problem all along???

Jaywalker

 

The fuel shutoff is a good idea, but don't count on that fixing the sudden loss of compression. I imagine you still have a sticking valve and it may resurface. If it does, go back to the valve springs and spray a little acetone or aerosol carb cleaner on the valve stems, work them up and down a few times, then repeat. Then wash the stems down with a penetrating oil like PT Blaster.

When you change the oil put a little Sea Foam in the crankcase. It'll dissolve the varnish build up.

 

marbobj:
Sry, forgot to mention that I had turned the mower to a position that the plug was UP, poured a reasonable amount of Marvel Mystery Oil into the combustion chamber and let that sit for a few days, before turning the crankshaft by hand a few revs. After that is when the combustion returned and has stayed with me so far...!
Jaywalker