I got this Stihl HT101 pole saw all ragged-out looking and not-running, and decided it would be my first experience with 4 mix engines.
It was missing the saw head, so I patiently waited until I found a nice one for $100 on eBay, complete with bar and chain.
Now, I am slowly working the engine over. The exhaust valve was loaded up with crud, but the piston and cylinder look good. The valves had never been lashed and were way off.
Although I have so many builder saws to work on, and shouldn't be modifying anything because of time, I couldn't stop myself.
I kept looking at the engine and coming from a race engine building background and some porting experience, I had to mess with it!
I am sorry, I didn't take any good before pics.
I spent a lot more time on the intake side, as that is typically a place to see more power if the port design sucks, and these do suck, just not enough air.... Haha. The exhaust port is no better design, but is under pressure on the exhaust stroke, so not as much an issue, especially if you can't get much fresh charge in.
I don't polish to a mirror finish because I actually need some surface roughness to create a boundary layer...
Stihl might as well have used plumbing pipe connections for the intake and exhaust port, as it is a straight-walled, 90-degree turn to the back of the valve face for both, and both ports are virtually identical. No short turn radius, no expanded throat under the valve, no special backcuts on the back of the valve heads... These guys didn't do a bunch of research on small engine power and efficiency, in my opinion.
The truth is port design improvements don't cost a thing besides time and R&D and will help with power and efficiency. How do you think auto makers can build 450hp naturally aspirated engines that get 25mpg+?
I did my best to actually create a little short turn radius after lowering the floor and raised the intake port roof and widened the throat. I used to have a flowbench at my disposal, but I moved away.... Still know some tricks though!
Port Velocity and good flow numbers at low lifts with a port that stalls quickly but holds on is what makes huge power in the midrange and much more on top.
I might find a stock cylinder and send it with the ported one to my buddy to get tested.
Either way, I have heard that these HT101's can be a little gutless, so I hope to add some zip to this one.
I will update as I move forward with the build.