Compression Problem on Riding Mower

[Native87]

My riding mower has lost its compression. My son was using it and hw turned the engine off without throttling down and it backfired. Now it wont start and spins easy. Flywheel key looks good. I am thinking a stuck valve maybe? Its a 18.5 B&G engine OHV. How do you go about repairing a stuck valve if that is the case? Any other ideas about this problem would be helpful. Terry (Native87)

[chrisser]

Is this a Briggs & Stratton engine?

I found these pics on the web that show a bit of the valvetrain

http://faqs.custhelp.com/rnt/rnw/img/enduser/16-parts_of_the_ohv_small_engine-front_view.jpg

http://faqs.custhelp.com/rnt/rnw/img/enduser/17-parts_of_the_ohv_small_engine-side_view.jpg

Looks to be pushrod operated with rockers to actuate the valves.

I'm not familiar with common problems on this engine, but there's a fairly small list of parts that could be damaged/stuck.

Could have:
- blown head gasket
- cracked/broken piston
- stuck/broken rocker arm
- stuck/broken valve
- stuck/broken pushrod
- broken piston rod or wrist pin (or even crankshaft)


I can't tell from the pics how the pushrods are actuated. Probably a cam somewhere. The cam lobe could be worn off or the camshaft broken.

What other symptoms are there? Is there any resistance at all? Any noises/rattling? Can you see the output shaft - does it turn when you try to start the engine?

The quickest determination is probably going to be to take the head off. That way, you'll see the valvetrain and the piston.

How much of the engine can you access easily? A leaking head gasket will probably push enough air out that you can feel it when turning the crankshaft.

If the piston is moving in the bore and it doesn't have a hole in it or something, than even with a valve stuck open (for whatever reason), you should feel at least some air movement in/out of the exhaust and/or intake (depending on what's stuck where) or you should feel some air movement through the spark plug hole.

If you're reluctant to take it apart, you could put something (piece of stiff, heavy wire or a stick, etc.) into the spark plug hole. Carefully turn the crankshaft and you should be able to tell if the piston is moving. You could probe the piston surface for holes. If you used wire and bent it into an "L", from the pics it appears you should be able to feel the valves moving (or not moving) if you can orient the wire properly. The pics aren't all that clear, but if you plan on taking off the head, you are likely going to have to remove the carb and exhaust. At that point, you should have access to the intake and exhaust ports and will be able to see at least some of the valvetrain.


A lot of times, you can find online manuals for smaller engines like this. Might be worth digging around - it always helps to have reference materials. You can probably get parts online as well if it comes to that.

Good luck!

[backlash]

Check the valves.
Mine was hard to start and I thought the battery was bad.
New battery same problem.
I searched google and found the answer.
http://en.allexperts.com/q/Small-Engines-Lawn-1746/2008/4/valve-adjustment-ohv.htm

This fixed 3 motors that I thought had serious problems.
If it doesn't correct your problem I would bet that site can help.

I just reread your post.
Had the same motor in our mower. I think my wife hit something like a big rock while mowing.
Take the spark plug out and put a stick in the hole then turn the motor by hand.
If the stick doesn't move the piston isn't moving and you have big problems.
That just happened to our mower.
Took the motor apart and the connecting rod was broken into a lot of pieces.
Looked like it exploded.
Motor is shot.
New motor is about $600.
Not really worth the cost and hassle of replacing it when you can get a new mower for $999.
I took one off an old mower and put it on the newer one.
Good luck.

backlash

[WRTN]

I have seen more than a few exhaust valves get stuck in the open position due to warpage, wear, and/or carbon build up. Try to get to the valves in question and check for a stuck open valve. Many time a little pentrating oil and carefully actuating the valve up and down will free it up.......at least until the next time it sticks.

[johnny]

18.5 is probably a twin--right?

If o/h valves then access should be no prob and much the same as bikes. Most likely has an access plate or cover that facilitates the adjustment of the valves and by removing it you can readily see by spinning if they are working properly.

Check rods the easy way--yank plug and put thumb over hole and crank it over--no push on the thumb--piston aint moving. Even stuck open valve will have a small push.

[Native87]

Thanks Chrisser, backlash, WRTN, and johnny. I am going to pull the access cover today. Start simple and work up. When i turn it by hand or even spin it with starter there is no loud noises etc. I don't think its some kind of catastrophic failure. It is getting fire but when it is turning it will "pop" every now and again so I am thinking a valve problem also. I will update about this. Its a fairly new mower and when my Dad was living it was taken care of. So, I am hoping it isn't nothing too bad.
Thank you all for your time and advice. I don't get to post much here anymore but winter is coming. I always turn to the forums here before I go anywhere else for sound advice. Again, thank yall and I will let you know what I find out today. Have a great Labor Day weekend. Native