Exhaust Valve Sticking (updated with photos)?
 

My 73 starts easy and runs well for the most part. However, when I am cutting grass after about 30-40 minutes the RPMs will dramatically drop until I either clutch the tractor, or disengage the deck blades. Then the RPMs will pick up again. If I attempt to cut grass again, it will do the same thing. A friend said he thought the exhaust valve might be sticking when it heats up. Could this be right? If so, is there something I can do to repair it, or am I looking at replacing the exhaust valve? If I do need to repair or replace it, are there any special tools that would be needed? Tractor runs fine unless it has the deck running. Thanks,
Jason

Try the following..

1: CLEAN the engine and make sure that there are no obstructions in the cooling path. You can use high pressure air and blow out all the dirt, mud dauber's nests, etc, that may have been blocking the flow of cool air from the flywheel, across the head and engine cooling fins.

2; Make sure the engine is in good tune, points adjusted to spec, carb set, etc. Avoid running lean, it will cause overheating.

3: Try running a little Marvel Mystery oil in your gas. I know, snake oil.... but it actually helps out in the Kohlers.

If all else fails, you may need to decarbonize the piston , head, and valves, and in extreme cases dismantle the valvetrain and run the correct reamer thru a tight guide. Clearance is recommended to be at 4 thousandths.

truckntran,

Try running a little Marvel Mystery oil in your gas.

About how much is that per gallon?:bigthink:

I'll have to ask my Economy owning neighbor..but I beleive he adds a few ounces per gallon. None of mine have needed the MMO treatment.

Thanks Truckntran. I'll look into that when i get an opportunity. I should also note that one time when this occured, it was smoking pretty good from the engine until it cooled down. hasn't done that again. It does seem like it's running pretty hot when I use it. After half an hour the hood is pretty hot to the touch. Jason

Pull the head to see what is going on, It's not a bad job, it may need something as simple as a head gasket, when it gets hot it may leak. You can then inspect the valves, if you have a local shop you could get them ground cheap that would be great if they are in good shape, if not I think new valves are not to expensive. I also agree check the guides.

It may also be the head gunked up with carbon, if it has never been cleaned this would be a good thing to do while checking the valve, I would check the common tune-up items before doing all of this, however cleaning the head, valves etc with a new head gasket is really part of a complete tune-up

Clint

Ok, so I got brave and pulled the head today (never did anything in an engine before) and took some pictures. It isn't as bad as I was expecting, but you guys would know better than me. By the looks of it, could this be causing my above stated problems of losing power and smoking after about 40 minutes of mowing? Thanks,
Jason


http://photos.cubfest.com/albums/use...8Medium%29.jpg

http://photos.cubfest.com/albums/use...8Medium%29.jpg

http://photos.cubfest.com/albums/use...8Medium%29.jpg

http://photos.cubfest.com/albums/use...8Medium%29.jpg

http://photos.cubfest.com/albums/use...8Medium%29.jpg

Ok, so I got brave and pulled the head today (never did anything in an engine before)

Jason thanks for the inspiration!!! I have never done anything like that either BUT my 127 could use a knew head gasket. If that's the head gasket in the fourth or fifth picture, I think that I can do that and change it.:beerchug:

jfinney,
You bet. See the dark places where the head gasket sits? That is where the gasket was leaking. Get a piece of glass about a quarter inch thick and around 14-18 inches square. Tape some emery cloth or 220 sand paper to it and make a figure 8 motion with the head on the paper. When the surface is shiny everywhere the gasket sits, it's flat. Replace the head after cleaning it with some oven cleaner. The type that works cold.

You should check the adjustment of your valves while you are at it. And even better lap them then adjust. If you do everything by the book that repair will last you a very long time ( about 500 hrs).

Ok, here comes a novice question...how do you lap the valves? Thanks for your input Rpalmer.
Jason