Tecumseh HH70 starting help, please

It's an HH70-140038C, serial # 8296C on my rototiller. The engine starts and runs great, but the problem is that it darn near rips the pull rope out of my hand, and, although I'm not 100% certain, I think it is getting worse. So, first question, am I correct in thinking that it's a timing issue? Second, I have removed the flywheel to check the key and it is good, but where do I go from here, as far as timing? My old reliable Chilton's small engine manual has proven irreplaceable any number of times over the years, but I haven't worked on Tecumseh's that much and the timing procedures seem a bit too general, causing me to wonder if I should follow them or not. So if I'm on the wrong track and should be looking at something else I would appreciate hearing that, but if I'm on the right track, if anyone can tell me how to proceed with verifying the timing that would be greatly appreciated. Again, it starts fine, usually on the second or third pull, and runs great.

 

Could be timing if the cord ripping is due to early ignition (before TDC) resulting in the piston upward movement attempting to reverse causing the cord to pull against the hand. Tu correct this, verify the ignition module gap to the flywheel magnets is correct (I use a business card of about 0.010 inch). If the ignition module mounting holes vs the mounting screws leave any gap, move the ignition module as far as it can go in the direction the flywheel is moving when starting. If the cause of the cord rip is compression, you will have to live with it unless it has a compression release that needs repair. Hope this helps.

 

Good to check the flywheel key to make sure it hasn't sheared. That's a little more common with mowers since they hit solid objects and shear the key, but the cord jerking back is typical of a sheared key.That gives you advanced timing.

 

I am familiar with using a business card to set the coil gap on B&S engines, and typically go the next step to make the final adjustment with brass feeler gauges to manufacture specs, but not sure if that actually changes timing, and it’s not applicable to this engine anyway as the coil is in board of the flywheel, not outboard like a Briggs. I did mention that the first thing I did was check the fly wheel key, and that is all good. Didn’t mention this in the first post, but I also checked the recoil starter itself when I first took it off, and it is working exactly as it should, no wear or anything detrimental in that regard. Thanks.

 

The flywheel key determines timing unless you have the older points/condenser ignition. So if that's good, you're good on timing. If you have points (pre 1986) you need the correct point gap.

Another check you could do is for carbon build up which could be raising the compression or hot carbon, if the problem shows up in restarting.

Also you might try a different gas. On the higher compression jobs you may be getting preignition.

 

You need to remove the key from the flywheel to really check it. If you have not done this, you have not checked it. Even a partially sheared key will cause what you are experiencing.

 

I removed the nut and did a visual of the key from the end first, and it looked fine, so I removed the flywheel, then the key, checked the fit of the key in both the flywheel and crank, and the key is fine. Had hoped to not pull the head off, but will do that this evening and check for carbon buildup.

 

Unless it's hot start, carbon buildup should not not cause early ignition. And yeah that pull back hurts. I installed an early version electronic ignition, and I had kickback, so I removed it. Later on I saw a new version of the elect ign, that had a anti kickback feature, so I installed it, and it works great, have had it 15, or 20 years.
Sid

 

The concern on carbon build up is raising the compression.

 

Adjust the points, if yours is old enough to have them. As they wear, they close sooner and open later than they should, changing the timing.

 

Thanks, Cheese. I think that's what I was looking for. I had gone back yesterday afternoon to look at it and figured the points were the only thing that could change much, so went back up to the house for a bit and was glad to see your post. I wasn't sure if that could be significant enough until I saw your post. Dwell as I think we used to check on automotive engines, not sure if that's how it's referred to on small engines. Anyway, the gap was pretty close, except that there was a spot at the back where they had obviously arced pretty good because a chunk out of one side that had welded itself to the opposite side, so I ordered a new set last night, along with a head gasket as I pulled that off too to check for carbon as Bob suggested. No significant carbon, but figured why chance the old gasket as long as I was ordering the points. So we'll see, but am thinking the points are going to help the situation.

 

Good deal, when points wear, the dwell is longer and that technically would retard the timing a bit but with your arcing signs, there's no telling what it was doing.

 

To correctly set the timing on a Tecumseh adjustable points ignition you must set the stator to break open at the correct distance BTDC:

https://imgur.com/WHOwh1Y

The correct setting for your engine is 0.080 BTDC

 

Well, installing new points and setting the timing per manufacturer's recommended procedures has not cured the problem. So, upon a little more research, it seems that there is a centrifugal mechanism on the camshaft that holds the exhaust valve open a bit more at low RPM, as in starting. Is anyone familiar enough with these to know whether this could be as simple as the mechanism being gummed up? Or is there something in there that wears down? It's not a big deal to pull it apart at this time, but no sense if it's of no benefit. And if anyone wonders "why doesn't he just replace the engine", yes, I could do that, but the unique thing about this engine is that it has a crankshaft of course, which turns in one direction, and another shaft that turns in the opposite direction, so replacing the engine could be easy enough, and the tiller would go forward, but, short of getting stupid and engineering an auxiliary gear box for it, I would lose reverse. Not that reverse is used real often, but the machine is heavy, so there are times when it is real nice to have.

 

Too bad you didn't check for a compression release operation when you had the head off. It was suggested as a possible cause of the pullback in post #2. To check its operation, you observe the exhaust valve closing as you rotate the crank by hand (in the run direction). When, the exhaust valve is almost closed, the downward motion of the exhaust valve stops for a few degrees of crank rotation and then finally closes as the crank continues to be rotated.

 

I did rotate the crank by hand with the head off, while watching the valves, and did not see any delays or whatever, but it's one of those things that's a bit hard to eyeball with little published details by Tecumseh and nothing tangible to measure, so that's the reason I'm revisiting it. I'll probably open it up and see what I can find. Thank you.

 

Compression release (counterweight device) is part of the camshaft and is not repairable. If the compression release mechanism is worn the camshaft has to be replaced.