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Very odd problem with Tecumseh engine.

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klugesmith

Thu Nov 04 2010, 09:20PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

An idea from my own very limited experience with small engines:

Is the crankshaft/flywheel connection a shallow taper?
Be sure the mating tapered surfaces are clean and dry,
without the slightest ding that might have a raised metal rim.
The flywheel should be seated on the taper with a good whack by
rubber mallet, and should require a tool to pull it off.

The primary transfer of torque between crankshaft and flywheel is supposed to
be friction between the mating tapers, not a shear load on the key.
(that's why the connection is tapered, unlike a belt sheave keyed onto a straight shaft).
If the taper is loose, then the key gets an alternating shear load on every rotation,
and won't last long. Especially if there's even the tiniest bit of free play in rotation.

(In my humble opinion, as a lawnmower pusher rather than a karter.)

klugesmith

Fri Nov 05 2010, 06:10AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

As previously mentioned, the flywheel key is not strong enough to transmit the alternating torque of normal operation.
The key is just there to align the flywheel for ignition timing, when the tapered connection is made.
I just confirmed my memory: racing shops sell offset keys for people who want to change the ignition timing. See pictures at:

But you probably knew that, Arcstarter!

Steve Conner

Fri Nov 05 2010, 09:46AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yeah, those are another few possibilities: the taper on the crankshaft is wrecked, or the hole in the flywheel is, or you didn't crank it on hard enough to get enough friction. Or you greased the taper: some of these friction joints aren't supposed to be greased. If you did, try cleaning both surfaces with solvent and cranking the flywheel bolt up really hard.

Having said that, machining tapers is expensive, so I'd expect a cheap engine design to avoid them.

Or maybe a spun bearing as Dr. E. said.

We now know that the fire from the carb was a red herring, it only happened because the flywheel came loose, throwing the ignition timing out.

Arcstarter

Sat Nov 06 2010, 02:11AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Well, i did weld it, and the weld did fine, but it wouldn't start. Then it seized while trying to start it, using my drill to spin it. I had plenty of oil in it, the oil flinger thing is intact, and i was using 2 cycle gas, something like 30:1 gas/oil ratio... Only because that is all i had.

Oh well, i am done with this engine, i did get a bar and put it in the flywheels fins and got it spinning, but once it seizes i am not convinced it is worth screwing with anymore.

Time for the 5hp briggs

. It runs, but i need to find a way to hook a carb to it, and i cannot turn the engine over because of the incredible compression 0_o. I need to fix the compression relief lever. I have never seen an engine that i could spin up to 800rpm, with a flywheel, then let the valve close and not be able to complete even a single compression cycle.

Steve Conner

Sat Nov 06 2010, 05:32PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

OK, so it was more than likely a spun bearing or a seized piston, probably why the engine got thrown away in the first place.

Sometimes an engine can seize up and then free itself off again, but there's more damage done every time it happens.

The neighbours' Lexus has a lucky escape this time

Ryan

Fri Nov 12 2010, 04:36AM

Registered Member #1606 Joined: Fri Jul 25 2008, 02:40PM
Location:
Posts: 71

The flywheel nut wasn't torqued enough, and there usually is a washer or starter ring that goes in behind that nut to prevent it from backing off. Over reving will sometimes cause a back fire, and if you get a back fire while the piston is heading up that CAN sheer the key. Like somebody said above, the key is for alignment only, not for load. The taper is for load, and relativley speaking, the tighter the nut is the tighter the fit between crank/flywheel is, and it shouldn't sheer, provided the fit is clean and snug without burrs. I built racing kart engines, and those offset keys will advance or retard timing to certain specs. If the timing is over advanced, if you have to high compression (which you dont), if you have detonation (overly lean mixture, carbon build up inside the cylinder that pre ignites fuel, low grade or old fuel) that may cause the key to sheer as well. If this was on a lawn mower, chances are somebody hit a stump with the blade and while the engine stops, the flywheel's mass keeps on trying to rotate.

Hope that helps.. even though you've welded it now. Loctite is a less permanent solution on threads or tapers. Use green, its really really tough stuff.

Ryan

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