Stihl 025 Performance Build Part7: Porting, First Start, Cutting

Part 7 of a broken down build series for using basic hand tools to machine and port a Stihl style aftermarket clam shell cylinder and piston for the 025/Ms250 family of saws. these machining processes include cutting the squish band, lightening the piston, knife edging the crank, and flowing the cylinder, among other things, all with very basic tools. These videos will be uploaded as the editing is done, the saw is currently built with several hours of work on it, trimming trees, and test cuts in a 14" square hardwood test block. This saw is now sporting a diy pipe and I hope to load a short of it doing work soon. As of this posting, 6 full tanks have been ran through the saw and it is on the bench to be torn down for assesment
I am not a certified arborist nor a Stihl tech, views expressed are to the best of my knowledge at the time of recording, and based on many years of Making, Creating, Repairing, and Fabricating whatever may be needed at the time. That said, I am always open to becoming further educated, and furthering skills.
Thank You for your time.

[@HayChaffandSawdust1]

Pretty nice!! Will be anxious to see with adjustable carb and seated rings. I expect there are some smiles in it??

[@Archer_82]

Oh yeah! The smile factor with this one is strong. I’m 6 tanks in on the saw now, and going to make a few changes, that carb being one for sure. I’ll put together a teardown overview on it. I’m going to see if I can get one of those boat motor plugs you had showcased to work in this case. Maybe a few more things down the road. It really turned into a great saw for that tree field management. Limbs very nicely with the quick throttle response and torque. Makes short work of the volunteer mulberry.

[@Kevin.L_]

Impressive work as usual. I hadn't even considered the added volume from stock removal. I was only thinking weight and flow.

[@Archer_82]

Thank You, Sir. I had not gone in with volume of stock removed in mind at all, just kind of weighed that cylinder on camera because I was weighing everything, then It dawned on me that we could ascertain an approximate volume from the removed weight. Now! How do I make that volume make more torque at higher RPM? Or higher cut speed?

[@Kevin.L_]

@@Archer_82 Sounds like it's holding rpm well. Do you have a tach?

[@Archer_82]

I do not, but it’s now ordered. This wood is a harder wood, Camphor Burl. It’s listed as 1800 on the Janka scale, but that seems too high for this individual pice. Also have ordered a carb with High and Low adjustments, this carb was low adjust only.

[@Kevin.L_]

@@Archer_82 Not sure how you got a saw to run after mods without a high jet adjustment. Agree that carb needs to go. Which Tach did you get? I need to get one as well.

[@Archer_82]

Well, I don’t think this one was that tough, I turned in the throttle screw maybe 1/2 turn over where it ran pre mod, low in to stop, then out 1 turn, got it to fire and run on high idle. Then on idle, leaned the low to raise rpm, lowered idle screw, back to low screw, then back to idle screw, in search for the highest running rpm at a low screw setting, with the throttle screw low enough the clutch is not engaging. (That’s where I was grabbing the rim sprocket with it running, checking for engagement, or lack there of. Got low enough it died out, then bumped the idle screw up a touch. I did it in the video, but probably cut about a minute of it out. Have not messed with low or idle since then. It definitely needs high jet adjustment, I can not get it to 4 stroke in free rev, so it could pick up more power still. The tack.. a black on off eBay about $20 sold by Blue Saws. Com

[@cunnings309]

I see that you have not polished the surfaces that you have enlarged; won't this cause friction on the gas flow?

[@Archer_82]

Great question, do you mean the ports, or in the lower case? The exhaust port is the only one I spent time sanding to a higher quality finish. The case and intake.. there are a few different schools of thought here, one being that a rough surface helps keep fuel and oil in suspension in the air charge, the other being that the roughness will hinder flow at the boundary layer of the air. So yes, I do believe that in total the work could flow more air if all work would be finished to higher level of smoothness. However for making power it is also important to keep the fuel atomized during flow. I think where we are here is a compromise of much greater flow than the factory shapes, while still providing roughness for the air fuel mix. I am open to thoughts or sources of research on the subject, as I definitely have more to learn here as well. Thanks for stoping in!

[@bmoreglass7258]

thinking the larger case volume hurt your intake velocity to much. sometimes less is more

[@Archer_82]

I have also done quite a lot of thinking about that increase in case volume, and its potential benefits and drawbacks. I’m finding with these two strokes, each and every mod or adjustment is a complex balance of compromise. Where I have landed on intake velocity, not saying I’m completely correct, but it’s how im currently understanding it until I learn more, is, I want to brake the intake “velocity” that other porters talk about into a few parts. The volume of air moved by the bottom of the piston is fixed, with bore and stroke, that didn’t change, my most restrictive point in the intake is the carb at the Venturi, that has not changed. This point should be my point of greatest velocity, where velocity plays the roll of fuel volume uptake. What has DEFINITELY changed is, my time of crankcase pressure, it decreased with added intake timing by dropping the cylinder, that’s potentially a negative effect, if that pressure is too low to overcome pressures above the piston when the transfers open. To compound that, and I believe this is what you are referring to, that larger case volume ALSO lowers the crankcase charge pressure. So the two together could cause a poor transfer velocity. To combat this, I have left blowdown high, 24 here, and the exhaust is quite wide, to promote complete exhaust and low combustion pressure before the transfer opens. I believe it is working, as my wash pattern looks fairly strong, reaching the center of the piston. Next upload is a tear down and inspection after 6 tanks. Thanks for the feedback, let me know if there is some part of this I’m overlooking.

[@bmoreglass7258]

@@Archer_82 cool thats good to think about how the blowdown effects things too. been learning to port saws some also. nothing to crazy. i did a 250 with a pop up recently. it sounds cool but dosent really make more power than one with only a muffler mod, timing advance. early non strato 291 liked the pop up tho. id be interesting to T a map sensor in to the to the impulse line to graph crank pressures .

[@Archer_82]

@@bmoreglass7258that’s is an amazing idea! Have you seen any of the two stroke research going on in the HayChaffandSawdust Chanel? He is putting temp sensors in the head, and exhaust, and so many more things all with a saw dyno running custom software. That would be the place to see some testing like this done. I like the idea a lot. As to the popup, I had ordered one for this build also, but it just was not going to give good results for several reasons, I think I have that info in parts 1-3 or so. Thanks for joining in, I think a lot of us are trying to see what we can make these little engines do. Your 291, is that the updated clamshell 290? I have one of those also, swapped to a 390 top end, but the squish is .058, I bet the popup would help, I’ll cut the bearing pockets on that one too.. in time.

[@bmoreglass7258]

@@Archer_82 291 its the newer version that blows up commonly from the stock lean tune and bad crank bearings. yea been fallowing the haychaffandsawdust builds wile doing an 026 build for myself. his videos and dyno numbers has helped alot. need to go back and presure vac check it for air leaks, so havent run it much. 039 i just did a few days ago rips for 62cc. very simple build only widened and raised exhaust, added two finger ports to lowers. plus the usual muffler mod and timing advance. i didnt raise compression but it still pulls a 25 in bar well. one i did a wile back i opened up the intake a bit and it was lazy but wasnt a cheep cylinder with bad timing numbers. hyway with the semi closed port one worked well but i srewed up one bearing pocket trying to lower it and bearing didnt seat well. plan on tig welding and trying to clean up the bearing pocket one day

[@Archer_82]

@@bmoreglass7258ill look for the HyWay cylinder for the 039’s, I think the one we have is Duke’s, not the cheapest, but it still had .058 squish. I hope that tig welding works for you. Curious about the finger ports you added to your stronger 039. I’m rolling around the idea of putting some in one of the 025/250 builds here. Where do they start, did they cross the ring alignment pin? Positioned above the intake? Need to button up a few other little things on the build first, like swapping to the H/L adjustable carb, it has arrived. Also going to do something with ring porting as per that last saw video, I like that idea. The 029/039 I built my son is 0.58 squish 98/98 164* exhaust 79-79 158* intake 26* blow down No extra advance It’s.. okay, needs more work to have any kind of Wow to it.