Tecumseh 5hp short stroke engines

[Carlos]

There was a short stroke crank in the HSSK50 engine for a couple of years or so that for some reason used a 46mm stroke reducing the capacity to 187cc. Apparently these short stroke motors are capable of turning 5,000rpm all day long and with short bursts up to 7000 reliably if used with synthetic oil.

Has anyone run the shorter stroke HSSK50 motor compared to the standard 49mm stroke HS50 motor?

Obviously both motor configurations are short stroke as they share the 71.43mm bore, but very curious as to what kind of max rpm you guys have seen from your either engine configuration ... and any of the tricks to keep the engines from going BANG!

 

[ATKRIDER]

On an HS Tecumseh with the minibike throttle if you put the governor spring at the bottom hole on the throttle arm and the top hole on the governor arm than set the static governor adjustment the engine will run governed at 4600 rpm and that's using a new governor spring. If you were to cut a coil or 2 outta the spring it would be right about 5000. I have set numerous HS engines up this way and check them with a digital inductive tach and they are always the same. I have yet to spit a rod out of one-

IF you were going to spin one up tighter for extended periods personally I would use an aluminum NON lighting coil flywheel and mount it to whatever crank you are going to use than chuck the mess up in a lathe and at least true the flywheel. They tend to wobble more than anything so if you take care of that it should be good-

 

[Carlos]

Thanks I will try that, 4,500 to 5,000 is the exact band I'm aiming at.

 

[Stangrcr1]

Does the higher rpm have anything to do with the looser tolerances of the SK motors?

 

[Carlos]

Given that shorter stroke cranks are stiffer, flex less and piston speeds are lower at any given rpm, they are capable of turning higher rpm.

I wasn't aware the Snow Kings ran looser tolerances, I know the exhaust valve on a snow thrower takes a lot of punishment as the engine is constantly run at around 3,600rpm even when it's not under load.

As the amount of ignition advance is fixed, assuming the amount of advance has been reduced to aid starting therefore is way less than the engine could actual use, there's a flame passing over that exhaust valve the whole time the engine is running.

The exhaust valve/guide does certainly seem to suffer, particularly the guide. Both the HSSK50 engines I own have what I would normally consider a sloppy exhaust valve, but I've been told this was by design, whether this was the same on the H and HS minibike engines I don't know. Can anyone shed light on this?

My main question is whether anyone has experienced running the rarer 46.8mm short stroke configuration that Tecumseh introduced for 2 or 3 years in the HSSK50.

Why Tecumseh chose to reduce capacity by 10cc on their 5hp engine just for a couple of years I don't know, perhaps they were just using up surplus stock of cranks from another less popular engine, perhaps it was the only at introduction of the HSSK50, but it didn't take long for them to change back to the 49.23mm stroke.

It is interesting that Tecumseh's power/torque curve graphs produced sometime in the 80's are only plotted up to 3600 ... yet the later winter engines with the 49.23mm stroke specify the governor set at 3750.

As much as I love this simple, and somewhat crude little engine, Tecumseh's literature, amount of different engine configurations, tolerances and clearances seem to be a complete mish mash and differ depending on what you read, who wrote it, or when it was written ... v. confusing!

But I am still really curious, even if I don't max my engines out ... what kind of rpms do you guys see on your modified engines? And has anyone any experience with running this rarer little short stroke motor?