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4 X Stihl 4-mix 36CC plane. An uncertain ambition

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Old 02-18-2018, 07:50 AM
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paton3k
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Default 4 X Stihl 4-mix 36CC plane. An uncertain ambition

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Hello all,

I have been browsing this site for over a decade... dreaming of the day I'd be able to take on a build. There's a lot of inspiration and knowledge in this community and I have a big appreciation for it. As a career arborist I have lots of chainsaw experience and I teach chainsaw operation/maintenance at a professional level. I've come into many engines that I've held onto in the hopes that one day I'd be able to use them. At the moment I have (among others) four healthy and running HT131 engines (4-mix 36cc) that I was hoping to use on a 4 engine plane. To quote AV8TOR... "Just because it's not a good idea doesn't mean it won't be fun!"

I was planning to give them the mill and lathe treatment until they're as light as possible. The goal here is mostly for bragging rights AND a signature sound, as I think 4 of those screaming away would be unique indeed. I have no idea what style of plane I'd build but I'd like it to have some nice big wings for slow flight and ease of control. I've had a tough time finding multi engine craft on this site. I'm unsure on prop sizes - how they relate to engine capabilities and plane requirements, but I'm still reading. I've seen plenty of the BR600 4-mix being used around here, but none of the smaller ones like these.

Questions:

Am I out to lunch?
Will these provide enough power to lift themselves, let alone the rest of the plane?
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Old 02-18-2018, 09:01 AM
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I say "Go for it"! (I suspect you knew I would.). ;~)

The Stihl Four Mix engines make very good power for a four stroke; almost the level of a similar size two stroke. Yes, the weight is higher, but there is virtually no doubt they will fly. ("Lift their own weight".)

You propose a quite ambitious project. Scratch building huge plane and converting four engines for it. How about starting out with just a single engine plane to get your feet wet?

Props are selected via engine power and design, and plane design/use. Then they are finalized by flight test. Generally, a larger diameter/lower pitch is better for takeoff and climb at the expense of speed, and smaller diameter/higher pitch more suited to higher speed planes, at the expense of static thrust. Your 36cc FM's would work well with 18 to 21" props with pitches from 6 to 10 or 12. (Note that the 4 Mix engines like to rev more than many other four strokes.)

Lose the mag and convert to electronic ignition! (Trigger off the crank and don't worry a bit about the lost, wasted spark on the exhaust stroke.)

Have fun, and welcome to the forum!
AV8TOR

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Old 02-18-2018, 12:19 PM
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Originally Posted by av8tor1977 View Post

Props are selected via engine power and design, and plane design/use. Then they are finalized by flight test. Generally, a larger diameter/lower pitch is better for takeoff and climb at the expense of speed, and smaller diameter/higher pitch more suited to higher speed planes, at the expense of static thrust. Your 36cc FM's would work well with 18 to 21" props with pitches from 6 to 10 or 12. (Note that the 4 Mix engines like to rev more than many other four strokes.)

Lose the mag and convert to electronic ignition! (Trigger off the crank and don't worry a bit about the lost, wasted spark on the exhaust stroke.)

Have fun, and welcome to the forum!
AV8TOR

This is excellent feedback. Thank you. I'm working on a single engine foamy build - to run one of these as a testing platform

Is advancing the timing/weight savings the big reasons for the upgraded ignition? Is there a prop hub/ignition kit that you would recommend for these motors?

I like the look of surveillance style planes, with their long wings and distinct features. I'd like to do a high-wing design if the weight and lift requirements work out correctly. Something like the photo shown here but with a narrower fuselage and a conventional tail.

Also, I've read around here that the crank seals on the FM's is a big deal and if they leak it can lead to major problems. Is it worth tearing them down and doing the valves and seals while I'm at this?

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Old 02-18-2018, 01:36 PM
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Well, it's true that on the Four Mix engines, total crankcase, (and valve cover, cam cover, etc.), sealing is a must, as they operate like a two stroke, with the air/fuel entering the crankcase. They tend to run a bit hot in their intended operation, and seals, valves, and carbon can be a problem. However, we can provide for better cooling and richer operation in our models. As far as overhauling the engine, I always completely rebuild any engines I am going to use for myself or sell. It is just not worth the problems one can encounter with a less than fresh engine on our models in my way of thinking.

I also convert ALL engines for my personal use to electronic ignition. This is for the many advantages of CDI ignition, including much less weight, more power, easier starting, etc. etc. With the ignition price now down in the 50 dollar range, it is rather a "No Brainer" for me. You mentioned milling and use of a lathe. You should be able to make a prop hub easily. For expedience, I often cut the center out of the flywheel, and then make an adjustable slip ring to go around that with the magnet in it. Then you can just make a much easier prop hub that just screws onto the crankshaft against that mag center section. Slip rings like that are also available to buy fairly cheap, so you could go that way and save a little lathe work.

Good luck and have fun, they are neat engines. If you look at my Youtube video, note that the muffler I installed on that engine was completely open; no baffles and I left off the deflector on the exit so that it was almost like no muffler at all. On my personal 36cc Four Mix, I just use an open length of copper pipe. Neat sound!

(Stihl 32cc Four Mix converted for customer.)

My personal Stihl Four Mix 36cc ignition sensor detail:




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Old 02-18-2018, 01:50 PM
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If you happen to set one of these up to run soon, note that with the exhaust breathing more free, and with no air filter, they run way too lean. And they won't richen up enough by adjustment unless you modify the high speed check valve, or replace it with one stolen from a larger two stroke carb. If you're careful, you can remove the high speed check valve, and drill out the entry orifices with tiny number drills.

By their very nature, these engines have more reversion, or "spit back" out of the carb than other four strokes. This also causes them to run lean when run without an air cleaner and with the prop blast blowing by the carb. Be sure to use a velocity stack on the carb to help with this problem, and/or run an air filter. Nice little air filters are often used by the RC car guys. Have a look at www.davesmotors.com.

I mention you making your own prop hub because since the Chinese invasion of engines, all the companies making conversion engine parts have gone out of business. :~(

AV8TOR

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Old 02-18-2018, 03:42 PM
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This is excellent information and I actually have a lot of other carburetors with adjustable high and lows.
If the sizing was appropriate would it be better to just switch the carb out?

It's a shame to see all the local machinists' talent out of work. I still prefer the bespoke stuff made here
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Old 02-18-2018, 04:08 PM
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I tend to prefer the original carb because it has a fuel pump pulse system designed by Stihl to work best with the unique Four Mix design. Other people however, have used other carbs with success. Your choice, but if you have other carbs available, it is no big deal to swap out that little high speed check valve to one robbed out of a two stroke carb. Rebuild your carb with new diaphragms for best results while you have it apart.

Your original carb does have a high and low mixture adjustment, right? All the 4m's I played with did. If not, then yes, I would say to use a different carb. High and low mixture adjustment provision is a must have deal.

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Old 02-18-2018, 04:13 PM
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Hmmmm... The old memory gets foggy. Now after more thought, I might have had to use the 65cc Four Mix carb on the 32 and 36c Four Mixes because the smaller ones didn't have dual adjustment. Can't remember for sure now. It's been several years and a tough life since I did them.

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Old 02-18-2018, 05:55 PM
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It's funny that you say that because 3 out of the 4 engines have fixed jet screws. Actually they're not even screws, they're just plugs of steel where the screws would be. One of them has regular high and low adjustability. I'll confirm the date of the engine tomorrow but I believe its the oldest one out of them (2009 I think)
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Old 02-19-2018, 09:44 AM
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Yep, I believe it. Smog laws. Many of them don't make sense. A properly adjusted and running engine will have the lowest emissions. A lean misfire will be emitting a bunch of nasties, as will having to play the throttle to get the engine to rev.

"Trust me...I am from the government and I'm here to HELP you...."

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Old 02-21-2018, 07:36 PM
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Originally Posted by av8tor1977 View Post
"Trust me...I am from the government and I'm here to HELP you...."

AV8TOR
Indeed!
OK so 1 out of the 4 is in fact adjustable via H and L but the others are fixed.
I couldn't help but notice in your video that the flywheel side of the engine was shaved down considerably. Was that done on the mill?
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Old 02-22-2018, 10:21 PM
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Yes. I milled down the fins on the flywheel as much as feasible, then re-balanced it. That one was for a customer that wanted to keep the magneto. As mentioned, on all my personal engines, I convert to CDI ignition.

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Old 10-22-2018, 11:19 AM
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Default Stihl 32cc 4 mix

Hi AV8TOR:
I tried to PM you but the mailbox was full. I'm converting my first Stihl 32cc 4 mix to fly a 1/4 Neiuport 28 I'm building.
Can you supply a prop hub and a Ign. sensor mount? Or recommend someone?
Thanks and regards,
John in N. J.
johnlov1@comcast.net
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