This is about an 8 year old mower that has been well maintained.

I took it out this morning to use it for the first time this year, primed it 9 times and it started on the first pull, as always! Cut approximately 10 minutes worth of grass and had to move so yard decorations. When I went to start it up again there was a new rattling sound initially coming from the engine but it did turn over and run normally. I had to stop again in 5 minutes but this time when I went to restart, the engine had more of a rattle and sometimes when I pulled the cord it was just stop mid-pull and the cord would yank from my hand and a pop sound would come from the air filter shooting a bit of dust.

Now it rattles a lot, won't turn over and every 2 to 4 pulls it locks and yanks the cord from my hand. I've pulled the spark plug and I can then pull the cord freely without it stopping/locking on me, so I'm suspecting the problem is related to one of the valves not opening. Is this assumption correct, and is there any easy way to check the proper valve operation?

I also wonder if there's something that has come loose or is slipping as this seems to be sporadic and I think if a valve was broken and stuck shut it would be constant.

Though I've never opened one of these engines up so I might be figuratively blowing smoke out my own exhaust port!

 

In the past or in the recent weeks, have you hit a stump or some really hard object like a rock? It sounds like you have a sheared flywheel key. If you hit something hard, it will break it right off and cause it to kickback and yank the rope from your hand as you stated.


By any chance, have you looked under the deck and checked the blade to see if it is bent or if the shaft off the motor is bent?

 

Does it have oil in it, however it does sound like the blade was loose and during the process kicked back and sheared the flywheel key. Have a good one. Geo

 

Check your blade, I'm thinking it is loose. Fix that before looking into the flywheel key. A loose blade can cause the engine to kick back even if the flywheel key is fine. It also will cause the rattle.

 

I agree with cheese that the blade might be loose but if when you check it and it is tight, there is a link below to a video that shows how to replace the flywheel key. In the video, the person says that 90 percent of the time, when the mower kicks back when you go start it, the flywheel key is broken.

YouTube

 

The blade is tight – no play in it. The blade and crank appear ding and bend free. I've not hit anything big in the last year or two as far as I can remember. The blade/crank seem firmly attached to the flywheel at the top where the pull mechanism is housed.

I can pull the cord slowly and it consistently works through the full range of it, but if I pull it at a speed to start it abruptly stops half way and yanks from my hand.

 

That sounds like a broken flywheel key if it doesn't show any signs of damage from hitting a root or large rock. Look at the video I posted a link to and perform the steps described in the video to see if the key is broken.

 

The blade feels tight, just for grins, remove it and replace when they flop around sometimes they stop setting on top of the retainers and seem tight but really aren't. Have a good one. Geo

 

There is no video in the link. Have a good one. Geo

 

Geo, I can confirm the blade is tight.

Ilike2mow, I can confirm with Geo that the link does not connect to a lawn mower repair video.

 

Sounds like the flywheel key is sheared. The noise would be caused by something else though.

 

Well, you guys were right, this looks like a sheared key to me...



Now I need to find a way to get my flywheel off as my puller doesn't come with bolts small enough to fit/jam/cut threads in the holes. Any other ticks to get it off, or am I destined to buy a new puller or take it somewhere to have the sheared key replaced.

 

Could a 2-jaw gear puller remove the flywheel?

 

Just pick up some long 1/4" X 20 TPI bolts at the hardware store and use them with your puller.

 

1/4" you say... those holes look smaller. Any trick to getting the bolts started and cutting threads into the aluminium flywheel?

 

Just get some hard enough bolts, grade 5 or 8, and they will cut their own threads. !/4" is what fits.

 

Thanks cheese, that worked! Got the bolts, snapped the first one off so went back bought 3 spares, came back and added a bit of oil to the holes and threads. I was able to get all three in about 1/4" and popped off the flywheel!

Now I just need to locate the proper shear key replacement. Where would I find the model number for this Tecumseh engine so I can locate the proper part number I need?

 

The model number might be on the front of the shroud. It could also be on the left side (when you are looking at the front of the motor). I would suggest looking on the Tecumseh website, getting the part number, and then looking on Amazon because I have found several times when ordering parts for Briggs and Stratton engines the price on Amazon is usually a lot cheaper (including shipping).

 

The model number is usually on a grey label on the shroud, or use the sears number on the mower itself to look up the engine number. If you go to any small engine shop and ask for a flywheel key for a tecumseh push mower, they should know what to give you. The key has been the same for decades.

 

Okay, I'm at loss now. I replaced the flywheel key, reassembled everything correctly, but still get the kick-back from the rope at about half a pull and it won't start. The symptom remains despite replacing the sheared key, so the problem must be something else. And yes I did take the starter assembly off the top again to see if the new key sheared, but it did not.

Could the problem be a broken valve? It might explain the rattling I was hearing last weekend before it failed. How easy would it be to check them?

 

You would take off the valve cover off and take off the push rods and see if one of the valves are broken.

 

Yep, something bad happened. The flywheel key sheared when the engine came to an abrupt halt. Now we have to figure out what the abrupt halt was. Does the engine have compression? After reading your first post, I almost wonder if the engine isn't blown and it's just hitting the broken rod inside when you try to crank it, rather than kicking back.

Now that a new key didn't fix it, the only thing that makes sense that would also explain the rattling is a blown engine. How "low" was the oil when you discovered the problem? Below the safe zone?

 

The oil was low but still within range.

The recoil I'm feeling when attempting to start it doesn't feel abrupt like it's mechanical, but more as if compression builds to a point and never gets exhausted, as if the exhaust doesn't open. Which maybe explains the backfires through intake air filter.

I'll test compression this morning.

Are there any "take apart" or service manuals available free or for purchase online that may give me a bit more detailed direction? I know as I get further into this I'm going to have to be more concerned with order of disassembly, bolt torques and gasket replacement.

 

I'm getting 70 psi compression.

 

Okay, head off and valves seem tight and appear to operate correctly when hand turning flywheel. There is a bit of buildup on the piston, but I'm not sure that's normal or not...



As I turn the flywheel there seems to be a bit of oil that remains in the bottom of the cylinder on the downstroke. I'm assuming this is the engine normally lubricating itself, but want to confirm its not a case of too much oil in the crankcase. Dipstick reads full.

There is also build up on the head but its thin and doesn't appear to be something that is causing my problem.



There is also little to no scoring on the cylinder walls. This is the worst of it and it does not feel rough...



Could it be crankcase pressure buildup that I'm feeling as the kickback?

 

Is the blade worn down really short?

 

Check the valve stem clearance. To do that take off the carburetor and remove the breather cover, turn flywheel till both valves are closed and then turn 1/4 turn more and valve clearance should be .006 to .008. If not valves need to be ground to get proper clearance. Also is the compressing release working? One valve will open just slightly, and the close back, if it doesn't then a camshaft is needed with new compression release. Lots of work involved here.

 

Nope, new blade last spring.

 

I'll have to check that later. Could they deteriorate that quickly? Fine for 15 minutes and then unable to start with a kickback? Also not sure why the flywheel key sheared or when. I know nothing had been hit in at least 2 or 3 years.

 

The valves won't cause your problem on a flathead engine. There isn't much left to check here. Maybe the crankshaft is bent badly and binding.

 

..........................................

 

I doubt it's a bent crank since I haven't hit anything and it was working, without issue and wouldn't restart after I moved the stuff on my lawn. It's very weird.

Could it be I didn't torque down the flywheel nut enough after replacing the flywheel key? It was tight enough to hold it in place, but it certainly wasn't to the specs I just found here on the interest about 30 minutes ago.

The valves seem fine. I'd even suspect missing teeth on the camshaft gear train, but the valve actuation seems in sync with the piston motion and there's no slop or looseness to them.

It has me baffled!

 

It would help if you could post the engine model, type etc maybe the mower model would help.
If you didn't tighten the flywheel nut properly you probably sheared the key again. It still sounds like the blade is loose, maybe the blade adapter has sheared it's key or is cracked, some blade bolts bottom out if a washer is missing. Have a good one. Geo

 

It's a Tecumseh TVS120-66020C with a Craftsman engine part number 143.006202

The flywheel key is still intact, but I will check the blade again. I didn't know there was a key on it too.

I'm going to have to pick up a new head gasket this week before I can reassemble everything because half stuck to the cylinder and half to the head when I took it apart.

I tried calling a local repair shop today, but they gave me the impression they thought they could take advantage of me and charge me more than what it's worth because I'm a clueless woman! Jerks!

 

Before you put this engine back together scrap all the carbon build up off the piston. Also before you put flywheel back on inspect the key area of the flywheel because they are bad about cracking in the key area. If it is cracked, replace it. I really don't understand why a loose blade would cause a kick back of the pull rope when starting,,need better explanation?

 

The blade is a necessary component as it acts as a flyweight. It helps carry the engine past the compression stroke and dampen against the combustion of the engine. Try starting one without the blade and you'll see pretty quickly what I mean. Be prepared for it to kick hard and possibly break the pull rope.

Ever wonder why push mower engines on pressure washers have cast iron flywheels and the ones on push mowers have aluminum ones? Because the pressure washer has no blade, or heavy component to swing around as a flyweight. It needs a heavier flywheel to compensate.

 

Thank You for explanation,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

 

The engine may or may not have a keyed crank but it is worth checking. Also you really need to remove the flywheel to check the key, check out the pics below to see what I mean. Have a good one. Geo
Here the key looks fine as if you were looking down at it.


However once the flywheel was removed.

 

I still haven't figured out what the problem with this engine is. I've picked up a new head gasket and will get it back together this weekend hopefully.

I did pull the flywheel again – it and the key were fine and intact. I will ensure the flywheel is torqued on to spec this time and will also check that the blade is tight.

Valve operation seem correct and the slight compressing release is there on the one valve as you mentioned mowerdude. I'll try to take a peek at the valve cam this weekend and get some measurements.

Otherwise I can't see why else it could be giving me this kickback when starting... since nothing was hit when this started to fail (kickback on when starting). It's very weird because the key had sheared (slow failure?) and the flywheel had turned about 20°, which obviously would throw off the timing causing the kickbacks, but once the key was replaced and the timing was hopefully returned to 'correct', the kickback symptom remained.

 

An abrupt halt is the only thing that will shear the key. The key isn't there to hold the flywheel in place except just long enough to tighten the nut. It holds the flywheel in time so that when you tighten the nut, it is timed properly. So slow failure isn't an option. If the flywheel moves, the key shears to keep from damaging the flywheel or crankshaft. If the flywheel is properly torqued, it can't move unless it has a lot of inertia forcing it to turn faster than the crankshaft. Maybe the initial kickback was strong enough to shear it? A good kickback qualifies as an abrupt halt I suppose.