Wrong Spark Plug

14 years ago

'73 Simplicity 3410S Landlord Model #990756 (10 HP)

In an effort to save a dollar, I recently installed a spark plug with a longer threaded stem than the one that was in there and boy, have I got problems. The engine is turning over, but won't start.

A friend told me to remove the head (something I've never done before), and evaluate the valves. I noticed a fair amount of debri from the smooshed spark plug head, so I cleaned that all out, and re-installed the head.

No luck.

It's cranking, but not starting. The friend also told me that you should be able to lift the valve (on this model, I'm pretty sure the longer stem plug was mashed into the intake valve) and inspect it for being bent, but the valve doesn't seem to slide up as he says it should, and I don't want to force it.

Next steps?

I am a newbie to this forum, and know just enough to be dangerous. I would appreciate any help offered. I'd ask my friend to come over (he does this work regularly), but it will be at least a month before he can get away.

Proceeding with caution...

Penny wise and pound foolish in Frederick, MD.

Sort by:Oldest

Comments (4)

baymee
14 years ago
Some friend? Too busy to come for a month??

I don't know if you have an air compressor, but I would place the piston at 1/4" down past TDC on the compression stroke and use an air gun with a rubber tip large enough to seal the spark plug hole. to apply 10 pounds of pressure. Make sure you use a socket and breaker bar to hold the flywheel nut, to prevent the flywheel from turning, because it will. If you hear the pressure coming out through the carb, you know the valve is bent.

If you don't feel confident to do this, don't do it. The air pressure can cause the flywheel to turn if you don't hold the breaker bar.

Experienced mechanics use alot more pressure than 10 pounds and they use a leak down tester.
Greg Goyeneche
14 years ago
I'm having a little trouble following your narrative, so if I am repeating things, be patient.

I gather the intake valve won't move if the engine is rotated? To check, pull the head off and rotate the motor manually. Each valve should open and close every other revolution. First the exhaust will open and as it is closing the intake will open. After intake closes, another full revolution is required before process starts again with exhaust valve.

If you are not getting any intake movement, the collision of spark plug and valve might have broken the camshaft.
Related Professionals
Orange County Landscape Architects & Landscape Designers · Martinsburg Landscape Contractors · Grand Rapids Landscape Contractors · Plattsburgh Landscape Contractors · Hot Springs Architects & Building Designers · Crestview Architects & Building Designers · Valdosta Architects & Building Designers · Winston-Salem Architects & Building Designers · Taunton Architects & Building Designers · Kirksville Architects & Building Designers · Ithaca Architects & Building Designers · San Francisco Architects & Building Designers · Carson City Architects & Building Designers · Redding Architects & Building Designers · Yakima Architects & Building Designers
rcmoser
14 years ago
First Off I hope you got all the metal out of the combustion chamber(would include fine pieces between the piston and bore on top of the rings) otherwise along with the bent valve you will have scoring on the cylinder wall.

Should be mark where the plug hit and got smashed? Probably bent the valve and now it won't seal off. Heck I would just do compression check (which you probably have none that why it won't start?), if I got pressure say 80 PSI or more I would just put the right spark plug in and fire it up. Sometimes if it will start the opening and closing of the valve will straighten it out and seal off better after heat cycle.
iambobo
Original Author
14 years ago
A word of thanks to baymee, ggoyeneche, and rcmoser. I appreciate you taking the time to respond and try to help me out with my problem. After a fair amount of trial/error, and my friend helping me from a distance via telephone, we've determined that the longer spark plug stem more than likely bent the intake valve. Therefore, there isn't sufficient compression ---- gas is blowing out the carburetor anywhere it can find space, and that is atypical. I was going to sell it for parts, but my friend insisted that when he builds his garage this summer (2012) a space may be designated for my mower --- for an engine tear down and rebuild. Putting the wrong spark plug in was a costly error (I'm looking for a second hand riding mower to get me through the season), but I am now poised to learn more about motor rebuilding as a result. That's the silver lining, and happy ending to this sad story of user stupidity. Thanks again ---- happy mowing!!