Ported a Stihl 4 mix 31.4cc for an HT101

[rynosawr]

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.

 

[earlthegoat2]

5 angle valve job next? Haha. It would be tough to get the right tooling up into the cylinder to get at the seats. This could be unfortunate to not be able to experiment with as a valve face recutting is one of the most efficient methods of building power on the cheap in the automotive world. Here only the valve seats could be faced.

Though I have never got a good look at a 4-mix valve seat so maybe they are not bad but since you say there is a ton of room for improvement everywhere else I figure this is also something that was not paid attention to.

I would be very interested in the end results. I would hope the valve lift and seat profile would not be the bottleneck in the entire system making the port work less effective.

 

[Chris-PA]

Interesting, thanks for the pictures. It's nice to see something different!

 

[rynosawr]

I agree that a carbide cutter, multi-angle seat and multi-angle backcuts on the valve faces should be performed, but I doubt there is a normally-available cutter head small enough to reach all the way down the cylinder to get to the valve seats. Also, I took my pin gauges to the valve guides and both measure .137 inches or 3.4798 mm, so I wonder if anyone makes a cutter head stem guide that small either. I will ask my buddy with the serdi valve shop equipment.

I have a lathe so I can at least put a few backcuts on the valves.

I will lap them to the seats as well to verify a good seal.


I haven't checked the quench pad squish yet either.... I am wondering what compression these run at, unfortunately, it would take welding the top of the piston and machining it down to get the compression up.

I was also looking at the spark plug placement not pointed to the exhaust and chamber design lacking swirl. I might could weld the plug hole up and drill and tap it for a better angle, but with it so difficult to reach the chamber, I don't know if I could clean it up nicely.

 

[rynosawr]

Interesting, thanks for the pictures. It's nice to see something different!

Thanks Chris!

I have a really wild and awesome idea for Stihl, but figure I would get told to beat it if I approached them, as they probably don't take outside designs or license fees on patents too often.

 

[thefarmboy21]

Maybe some undercut valves and some LS6 valve springs???

Haha, looking forward to hearing about good results. My buddy does small engine work and he complains all the time about the 4-mix trimmers. He always says "Stick with STIHL saws and Echo trimmers" I like STIHL everything myself, but I don't have any 4-mix engines either lol.

 

[rynosawr]

Yessir!!!

I may buy new OEM valve springs just to make sure they have enough pressure to keep the valves from bouncing on the seats...

I acquired a bunch of junk/rebuildable equipment from a friend before he moved out of state, and several Stihl BR550 and BR600 blowers as well, so I have more of these 4 mix engines to work on after this one.

I really need to get those leaf blowers going ASAP with fall upon us... That and the blowers take up the most room in my shed.

Small engines are/were a hobby for me, but it has become a 2nd job of sorts now to help support my family.

 

[earlthegoat2]

I rarely have problems with 4 Mix Trimmers, Edgers, Hedge Trimmers but I have quite a few problems with 4 mix blowers. That is of course excluding the time I had three string trimmers grenade on me because a valve broke free of the retainer and fell into the cylinder. All three happened the same week. Short blocks were bought.

 

[blsnelling]

Very cool. I'm anxious to see how it turns out for you. I don't think I've ever seen anyone mod a 4-Mix.

 

[rynosawr]

I rarely have problems with 4 Mix Trimmers, Edgers, Hedge Trimmers but I have quite a few problems with 4 mix blowers. That is of course excluding the time I had three string trimmers grenade on me because a valve broke free of the retainer and fell into the cylinder. All three happened the same week. Short blocks were bought.

Sheesh, that is a scary thought on dropping valves down the holes. I wonder if the valves were bouncing on those trimmers because of weak springs, and that is why the retainer disengaged the valve??

Maybe I should look into making some type of retainer upgrade...

What problems do you normally see on the blowers?

Thanks for the input!

 

[rynosawr]

Very cool. I'm anxious to see how it turns out for you. I don't think I've ever seen anyone mod a 4-Mix.

Thanks Brad! I enjoy and appreciate your posts. I hope to learn something about these little engines and see what kind of outcome the mods have...

 

[earlthegoat2]

A little background on the trimmers grenading. I took over a very poorly equipped and maintained shop that was responsible for multi-million dollars worth of equipment. The management knew they had to invest more into their equipment maintenance and that is the reason I was hired. I had shop management experience and I knew what had to be done to turn it around and I was given a sufficient budget to do so.

These trimmers came from the previous shop manager's maintenance practices so who knows what went on with them in their past. Most likely, he just kept them running and didn't fuss with anything else on them such as valve clearance adjustment. Even so, I had adjusted all the valves on the 4 Mix equipment we had soon after getting there. Everything was fine for 6-8 months when this happened. It seems too much of a coincidence that I had three go from the same problem in the same week but I have no other explanation unless improper mix gas got into them somehow and all three were being filled from that tank. To complicate matters, this equipment is gassed up at one of 3 different fuel sources. I only have control over one of those fuel sources....so who really knows. All I know is that it is unlikely a valve lash problem and more likely a fatigued spring problem. This all happened several years ago now and I have not had one of those types of power heads fail in any way since.

Onto the problems with the blowers: I admit, I am no expert on Stihls or 4-mixers though I have a decent amount of experience with them. The problems with the blowers go something like this. One day, they just wont start. Sometimes they have lost compression. (valves) Sometimes they are not getting fuel. (carb adjustment usually) I have had the cam frag on the blowers only never the trimmers or edgers. What happened these times was the cam gear wore out and broke. Its a real bugger to get to the cam gear on a BR600 as taking the fan impeller off can get dicey depending on the day or how you hold your mouth.

Of these problems the two most likely problems are going to be the loss of compression and fuel problems related to the carb adjustment. The carb one confuses me a little. How is the machine losing its adjustment on the carbs? Its several different machines too so it is not isolated to one bad carb. Is it because the valves are out of adjustment so the carb has to provide more fuel to an improperly opening valve? No because I always check the valves whenever there is a problem and even though the valves have come out of adjustment a tad, I don't think it would be enough to completely kill an engine probably just run sluggish.

Ive only diagnosed one compression problem and that was a cracked valve. Had to revalve the cylinder. All the other valve problems related to compression were fixed under warranty. Yeah, the BR600s, within one year, developed compression problems related to the 4 mix system. I have a good working relationship with my Stihl tech and he told me the problems were cracked valves, improperly seating valves, camshaft lobe wore down, etc. I am not a fan at all. I started phasing out the BR600s in favor of BR430s and Echo PB770s. The 770s have their own little BS problems but they never lose compression arbitrarily. The 430s are pretty solid just don't have the power of the 600. Oh well. As a maintenance tech, I prefer reliability over max power even if the operators do not. Either are cheaper to purchase and maintain.

 

[blsnelling]

How old were these BR600s? Earlier 600s were very problematic, due to valve and seat issues. Once they got the metallurgy right, BR600 issues went away for my dealer.

 

[earlthegoat2]

They were purchased in 2011-2012

 

[rynosawr]

Thanks for that info!

I have several BR600's and a BR550 an I am wondering what year they are.... I wonder if I have the early crap-tastic ones...

 

[blsnelling]

They started putting the Magnum decal on them at the same time they made all these changes.

 

[ANewSawyer]

I just read about porting four strokes this way a couple of weeks ago. I don't come from a racing or car crazy background, so not real surprising. Back on topic. I am not surprised that this can be done because four mix engines seem to be cheap. They really should have a removable head, IMO. There are lots of other things, design choices I should say, that lead me to not purchase any more 4-mix engines. I just got a KM-130 used because the price was less than half of MSRP. And dealers around here very, very rarely discount belowe MSRP. Can't argue with the torque though.

Hey, on those BR's having crazy carbs, do the BR series carbs have accelerator pump too? I know my KM-130 does. Somebody said they do strange things when the pump goes.