I've been using my new Toro snowblower and I thought I had my other old 1982 Craftsman 826 sold, but as I went to fire it up for the potential new owners in advance of their arrival, it didn't seem to be running right. It had a tough time starting, idled horribly rough and later backfired like a shotgun (after I played with the carb a bit).....all of which was news to me. So after scaring off those buyers, I figured I would post on here since it took me a little bit to figure out what the real problem was.
The enigne on this beast was a good old Tecumseh HM80 (which was the early version of the L-head motor found on snowblowers today now called the HMSK80 in the newer models......the design really hasn't changed in the 40+ years the motor has been in production). So naturally I first assumed it was a carb problem since the engine ran perfectly during the summer, so I tried to adjust the main jet on the carb to see if maybe that was the issue. All that seemed to do was make it backfire worse and the idle jet obviously had little or no effect. Next I assumed that since the carb was so old, maybe the carb float needle was not closing properly and gas was flooding the carb. So I went down to the store, got a carb rebuild kit (TEC P/N 631978.....see attached pics) and proceeded to rebuilt the carb which was in fairly good shape when I actually got it all torn down (lucky me, since I wasn't in the mood to drop $100 on a new carb TEC P/N 632334A). The carb float (TEC P/N 632019) was pretty worn so I replaced that too. Nope, still had issues and it seemed to be getting worse.
Good link for carb rebuilding if anyone needs it:
http://www.jackssmallengines.com/faqrebuildcarb.cfm
I then was a bit concerned maybe it was having a mechanical problem. So I pulled the spark plug and use a compression tester to verify what kind of compression numbers the motor was seeing. After pulling it over by hand a few times, I saw about 75 psi compression which should be more than enough for an older motor like this to run fine (I think the minimum is 60 psi). I sprayed a bit of carb cleaner in the plug hole and let it sit there for a little while. I then turned it over again and cleared the carb cleaner and came out about 2-5 psi higher on the compression readings. This was definately not the problem.
Next I wanted to see if the flywheel key might have begun to shear (thus ruining the timing on the plug firing) so I first decided to doublecheck the ignition just in case and to make sure it was firing properly. So I pulled the plug boot, stuck a screwdriver in it and held it near ground while I turned over the motor.....whack, whack, whack.....ok that works. Then I pulled the plug, put the boot back on, held it against a ground and turned over the motor.....a few tiny sparks and then nothing. Finally alas, the culprit was a stupid spark plug. I remember back in the summer throwing some kids toys towards the back of the garage and more than likely the plug got hit by something and was damaged. There were no cracks on it but something was clearly not right with it. I tried grounding in with the plug boot attached on my new Toro and the plug was still not making a spark.....definately dead.
I then went to the store and got a new RJ19LM plug which I found out (from Tecumseh) replaces the RJ17LM & RJ8 plugs that came stock on this motor from the factory. Some guy at Home Depot just wanted to sell me any old lawnmower plug the same size.....close enough he says, it'll work fine......I said um no thanks, I'm trying to troubleshoot, not cause more headaches for myself. I installed the new RJ19LM plug and it fired immediately leaving me to simply tune the main and idle jets once the engine warmed up.
Lesson is.....don't forget the simple stuff.
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