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Very odd problem with Tecumseh engine.

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Arcstarter

Wed Nov 03 2010, 11:21PM

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

For the past couple months i have been making modifications on a Tecumseh h50 5hp OHV engine. So far i've ported, port matched, polished, lapped the valves, removed the governor, and made some new springs for the valves. After all this, i ran it many times, totaling to at least an hour of run time.

Well, one day, after running many times in the past flawlessly, it stopped running, due to a sheared flywheel key. I figured no biggy, so i bought some keystock. As soon as it started to run (which it ran fine until it died), the key sheared again. This happened again, then i got another key that is made specifically for this engine, and it sheared before i could even get it to run.

The flywheel is iron, and very heavy compared to the other flywheels i have... I have checked all around it and there is nothing that protrudes hitting the magneto or what have you. The crankshaft is in fine condition, and there is no scoring on the piston or cylinder. The crank (including the piston and all other parts) spins freely, which i tested with a drill. There is no external load.

The only thing that could be causing the key to shear that i can think of is the engine suddenly stalling, but if something where to collide inside the engine, i think i would know. There would be a loud noise or broken parts or some such i would imagine.

I am really puzzled! Does someone have experience with an engine doing this? Have i completely overlooked something that could be causing this? I could solve any problem i had with all the engines i have worked on in the past, but this is one problem i cannot solve on my own.

IamSmooth

Wed Nov 03 2010, 11:39PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Is it possible that it is not the flywheel stopping the engine, but the engine is freezing causing the flywheel to get sheared from the momentum?

doctor electrons

Wed Nov 03 2010, 11:43PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Try to take some weight off of the flywheel

The key is being sheared by the inertia at the shaft being too great to move the flywheel.
You are getting more bang for your buck with all the work you have done. The porting alone will cause a larger reaction in the cylinder. Whatever you
do, dont put a stronger key in place of the old one, thatll be the death of it! The reason the key does not shear instantly is the timing of the cylinder
firing. Think of it like snapping a bolt with an impact gun, it takes a bit but repeated shocks will cause the failure.
edit: What i mean by the inertia being too great is, the failure point of the key is below the inertia it takes to move the flywheel.
I maybe should have said that first!

Arcstarter

Thu Nov 04 2010, 12:44AM

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

IamSmooth wrote ...

Is it possible that it is not the flywheel stopping the engine, but the engine is freezing causing the flywheel to get sheared from the momentum?

That is what i am suggesting.

@Doctor Electron

Well, after all the mods, the engine ran fine for about an hour in all, and never sheared a key. Why would it start now? Not to mention, the torque gained is likely around only 10% according to assorted things i have read about engines that have had those tune ups. Also, the low end torque is likely about the same considering there is more time for the cylinder to fill with the air/fuel mixture, so it is less constricting than at higher speeds.

It seems like what pinging would do... But i am sure that can't be the problem, nothing has changed that would cause that. It is still stock compression and timing.

Time to tear it apart... I have not done that yet, because when you take the head off, it destroys the head gasket, which is an 8 dollar loss. Considering my lack of funding, that's 8 dollars too much!

What else could possibly cause the engine to cease, but then spin freely right after? I would know if something was in the combustion chamber between the piston and head, the momentum of the piston alone would be enough to crack the head. It never ceased while spinning with a drill. It would suck if it did though, a broken wrist sounds painful!

Im not going to give up on this! I had to completely rebuild the abused engine. That only involved cleaning and a complete disassembling, because all the parts where still in good condition.

BTW, here is the engine running

Don't mind the almost 2x rated RPM, the only risk is the flywheel exploding (that's a comforting thought...) Men don't use mufflers.

Edit: Well, i tore the crankcase apart, and i did not find anything that would cease the engine. I did find that the gear which some weights sit on to for a centrifugal governor was messed up, some gear teeth broke off and where sitting at the bottom of the crankcase. It was plastic, and the teeth are whole, so this leads me to believe that it is insignificant.

I took the valvetrain off (cam and pushrods), and took the spark plug out, and spun the engine at about 800rpm for quite some time and it seems to be just fine... There is nothing in the combustion chamber and the compression is fine.

Something i did forget to mention, was when the engine was running before all the failures due to the sheered keys, it seemed like the engines timing went off because i got fire out the carb, then the spinning stopped, and the flywheel started spinning on the shaft until it stopped. One of the times, the flywheel started spinning on the shaft, and the crank was not spinning, then the flywheel must have gripped onto the shaft, and made the crank start spinning again. The engine ran for a few cycles with bad timing, then died. So, the engine did not completely cease, otherwise it would not have started back up like that.

Renesis

Thu Nov 04 2010, 01:48AM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

The grooves for the key needs to be tight, for if not the key would be impacted by the flywheels inertia everytime the engine fires. Being that your engine is much more powerful than a stock engine, with a heavier flywheel, could it be that the grooves on the axle and flywheel are worn out?

Btw that thing is begging for a turbocharger

Arcstarter

Thu Nov 04 2010, 02:22AM

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

I have checked that, as a matter of fact, and it is as tight as i could ever hope for it to be... As far as i can tell that's not the problem.

And yes, i have totally been looking for a tiny turbo :). I was actually making one. I found a turbine that spun up pretty fast with the exhaust, and an impeller that made a good 1-2PSI of boost, but due to a lack of materials i abandoned the idea. It would be about as big as a small 50-120hp turbo due to the fact that it would be made of parts i found

2Spoons

Thu Nov 04 2010, 03:56AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Could you be getting severe pre-ignition?

Steve Conner

Thu Nov 04 2010, 12:05PM

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

Here are some other possibilities I can think of.

It's simply making so much power that it destroys mere steel. :)

Removing the governor allows you to rev it to a speed where the crankshaft has a torsional resonance with the flywheel. The resulting huge forces destroy the key, not to mention everything else. This is one important reason why engines have rev limiters.

You screwed up the valve or ignition timing while assembling it, or they slipped. If the timing suddenly slipped by a whole lot, maybe it could make the engine try to run backwards while it was going forwards. How are the camshaft and magneto driven on this engine? Chain, gears? Did you mess with them? Are they both driven the same, or is the camshaft driven one way and the magneto is built into the flywheel?

It's not cease, it's seize.

Arcstarter

Thu Nov 04 2010, 08:28PM

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

A 10% increase in power would not shred all my keys. I have seen people push 20-25 hp from a 5hp engine with stock keys (at least, there was no mention of a new key).

When the key gets sheered it was running at idle speed. It did not run for long enough to rev it, i always let it warm up before i rev it. Not to mention, if it was a torsional resonance thing, why would it have just started? It would have started when i first made the modification.

I checked the timing, because that is what i thought was the problem at first considering the fire coming from the carb. The lifters, pushrods, and lobes are all fine. The cam is driven with a gear, which was still aligned like it should be, so the valve timing is fine.

The ignition is the simple magnet on the flywheel, and a magneto... The only way it's timing could be off is if the key sheered, so it can't be what is sheering the key. The magneto is just a coil, it's not the type with the darlington transistor and trigger coil.

Pfft, i knew it was seize, i was just making sure you knew!

I am done trying to find the problem, it's time to weld the flywheel to the shaft! If it breaks that, im not sure what!

doctor electrons

Thu Nov 04 2010, 09:19PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Maybe a sticky bronze bushing on the crankshaft? Spun bushing on the crankshaft? Could be why it fixed itself?
Just fill-er-up with racing gas and point it at the neighbors lexus!!

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