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Lawn Mower Fan
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681 Posts
Discussion Starter · #1 ·
I've got my co-worker's old Craftsman II mower with a small Tecumseh on it. I fixed this mower for him a few years ago; he keeps it at his beach house in Charleston, so it seems less than optimum maintenance. Common symptom: won't start. So I took the bowl bolt out, cleared the varnish from the jet, put new fuel line on it (brittle), added an inline filter, and also put in a new plug and air filter. The primer bulb looks brittle, but still works.

The problem is, it won't run at full speed without randomly bogging down close to the point of stalling. At full speed, it'll run-run-run-choke-run-run-run-choke. And it'll do that for ever it seems. If I pull the throttle lever (on the carb) back AT ALL, even just a hair, it'll run fine, albeit at a slightly slower speed and not much power when mowing grass, especially when the drive system is engaged.

It's not like a L-B Duraforce lean surge. This is more like it runs fine, but then just trips all over itself at intervals.

Usually, cleaning the jet is enough to get a Tecumseh running well again, but this one just seems down on power overall, plus the bogging issue. Does it sound like it's running lean? Could I drill that jet out just slightly? I can't imagine a new carburetor is all that expensive for this, and may be worth pursuing just to escape the hassle of screwing around with jetting, only to give up in the end anyway.
 

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Premium Member
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2,567 Posts
Does this engine have a plastic carb on it?
If so, the best way to fix those is to simply replace the carb.
Dave
 

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20,000 +posts!
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I've had coils that acted up that way and fooled me into thinking it was fuel/carb related...try closing up the spark plug gap some ,like down to .015,and see if it now runs good wide open or not (it will idle rougher with a narrow gap !)..if it seems better it could be the coil isn't putting out sufficient voltage or the points & condensor( if it has any!) could need to be cleaned or replaced...weak valve springs and sticky valves can cause high speed missing too..I've had primer button carbs act goofy when I didn't change the primer bulb,I'm cheap and hate replacing things that still look good,but putting a new one on seemed to make the engine run better to me..
 

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Lawn Mower Fan
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681 Posts
Discussion Starter · #4 ·
Thanks guys!

It is a metal carb. I will check the spark and coil gap, etc. It has a CDI on it, no points.

I can watch the throttle, and the governor definitely steps the throttle closed when it bogs. Maybe I should have put that in my first post: the governor is closing the throttle; I don't think it's a true "bog". It's like when you run the engine fast enough, the governor starts to misbehave and over-compensates and closes the throttle. Could that be a governor starting to fail?

Unfortunately, it's a weird engine: there's a horizontal shaft out the side of the engine that drives a belt that runs forward to the front wheels, for the drive. So finding a universal replacement will render the SP mechanism in-op.
 

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Deceased, April 2015
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190 Posts
If it's the engine I'm thinking of, the body of the carb bolts to the intake pipe which in turn bolts to the engine. Check all those bolts for tightness. I had a Tec on a Sears doing the same thing and it turned out to be an air leak at the carb/intake pipe flange.

Charlie
 

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Lawn Mower Fan
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681 Posts
Discussion Starter · #6 ·
Thanks. And you are correct...the air filter, carburetor, and intake pipe are all in a line, and the intake pipe bolts to the engine. I did not replace the intake pipe gasket. I suppose I could be getting an air leak there. I will try some RTV today and see if that seals it up better.
 

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Lawn Mower Fan
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681 Posts
Discussion Starter · #7 ·
Update: I got the mower to run a touch better, but it's still not right. I RTV'd the intake pipe, and I closed the gap on the spark plug a little bit. It still won't run with the throttle lever at full speed. But if I cut it back to just off full speed, it'll run okay. I can even get it to run at what sounds like the right speed. But after a few minutes of running, it's like it gets hot and loses power. This can only be either a fuel or an ignition problem. Could the CDI pack be getting weak, and not providing full spark? Else, I think my only alternative, if the owner thinks it's worth it, is to replace the carburetor. The engine feels like it still has good compression, so I think the engine itself is still good mechanically. Any additional ideas are appreciated.
 

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Jason sounds like you may have a case leak, crank seal leak or a carb gasket issue. Sounds like your are not creating enough vacuum to pull the fuel at high rpms. Usually ignition problems are yes and no...not sometimes it runs and then loses power. Usually it just plain quits and there is no spark.

I'd get some wd40 or starter fluid spray the case, carb gasket area and check your crank seals.
 

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Lawn Mower Fan
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681 Posts
Discussion Starter · #9 ·
I was playing with it yesterday and I think I might have found the problem. If I put my finger anywhere NEAR the primer bulb, even just resting it on the bulb and not pressing it, the engine runs differently. Seems as if this bulb is leaking air. Not enough air to cause a problem at lower engine speeds, but enough at higher engine speeds to lean the mixture out enough to where the mower can't run. I'll get a new bulb today for it and see if that does it. Tractor-Holic suggested replacing it a few posts up, and I think that may be the ticket.
 
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