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OS Gemini Twin Rear Walbro Gas Conversion

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OS Gemini Twin Rear Walbro Gas Conversion

Old 01-09-2023, 05:44 AM
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Good work on the so called "rebreather" .
I have used them off and on for years on methanol fueled, glow igition engines with absolutely no negatives. Theoretical con-jecture aside, speed, idle and transition are not affected.
I have always introduced the vent products upstream of the carb intake. Works well .

Hub-spigot joint

swedge punch

Swedge forming the hub

Tube end before swedging

Intake pssed thru bulkhead

Originally Posted by mitchilito
There are few more enjoyable things than working on one's metal working machines (other than gas engine conversions ). I spent years upgrading my Chinese equipment. Now they're pretty darned decent.
Yes, metal working is a blast, the planes are a good excuse to use the machines

I have owned several lathes over the years. The current lathe is a Clausing 4900 10" with 24" between centers. A good size for most everything I care to do. My first Bridgeport was a "M" type, the current one is a "J" model with hard chrome ways and 42 inch table.
Never owned any Chinese made machines but I have repaired a few
In this area there is a lot of very nice machine equipment often available in the used market.
A few years ago many of the public schools shut down their Industrial Arts departments. This also put a lot of clean, low time machines into the market.
Rather than using an electromechanical phase convertor to generate 3 phase power, they are both fitted with solid state Variable Frequency Drives.

My son has a huge, very well equipped wood shop with industrial level machinery and my wife's brother runs a custom cabinet and door shop. Both live nearby so woodworking machinery is a cinch.


.




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Old 01-10-2023, 01:46 AM
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Originally Posted by Jesse Open
Good work on the so called "rebreather" .
I have used them off and on for years on methanol fueled, glow igition engines with absolutely no negatives. Theoretical con-jecture aside, speed, idle and transition are not affected.
I have always introduced the vent products upstream of the carb intake. Works well .


Yes, metal working is a blast, the planes are a good excuse to use the machines
Wow Jesse, that is real dedication to make an intake tube! Is that a Saito 150 or 180? As I said somewhere earlier in this post I had a Saito 150 that I plumbed the case discharge DIRECTLY to the intake manifold (in between the carb and head). I expected it to destroy the required manifold vacuum but instead it made absolutely no difference and ran like a top. It was the neatest and coolest mod I ever did. The airplane I put it in stayed clean as a whistle.

I bought my Grizzly machines years ago and just love them. I used to tell people that quality-wise they were best thought of as mostly assembled "kits" because all the final finish work needed doing! The basic cast iron processing was/is fine and dandy. And I converted the mill to variable frequency drive a couple years ago - so cool.

Thanks for chiming in. Its fun to see this old thread continue on!

Old 01-10-2023, 03:55 AM
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Originally Posted by mitchilito
Wow Jesse, that is real dedication to make an intake tube! Is that a Saito 150 or 180? As I said somewhere earlier in this post I had a Saito 150 that I plumbed the case discharge DIRECTLY to the intake manifold (in between the carb and head). I expected it to destroy the required manifold vacuum but instead it made absolutely no difference and ran like a top. It was the neatest and coolest mod I ever did. The airplane I put it in stayed clean as a whistle.

I bought my Grizzly machines years ago and just love them. I used to tell people that quality-wise they were best thought of as mostly assembled "kits" because all the final finish work needed doing! The basic cast iron processing was/is fine and dandy. And I converted the mill to variable frequency drive a couple years ago - so cool.

Thanks for chiming in. Its fun to see this old thread continue on!
The engine is an older Saito FA-50. Actually it only takes about 1/2 to 3/4 of an hour to fabricate a stack like that one.

Most of the older home hobby type lathes like the typical "South Bend" had fairly soft, plain iron ways. Add to that, they were rarely treated to proper waylube and it gets hard to find one that is not badly worn. The Clausing at least has very nicely finished, bright, flame hardened ways and holds accuracy well. In the meantime, I got to learn a bit about hand scraping and flaking in order to get decent accuracy with those old Logans and Southbend lathes from decades ago.

The machinery is like a hobby unto itself.It can compliment and enhance other hobbies as well.
One of my other hobbies has been Amateur Radio. I took an interest in microphones as part of that hobby, high end ribbon mics in particular.
A bit fussy to build as a typical ribbon is about 6mm X 20mm but only 2.5 micron thick. It is also corrugated and suspended very closely between two strong, magnetic poles. Between the motor element, the enclosure and the base, there are many opportunities to use machinery.As well as exercise a little creativity along the way. Much as we do with the toy airplanes.








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Old 01-11-2023, 02:46 AM
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What a work of art that microphone is! I saw a "How it's Made" on these and that inner ribbon was fascinating. Beautiful work.

I also have an amateur radio license - but only because I wanted to transmit on the 50mhz band back in the 72mhz radio control days. Sure don't need that anymore!
Old 01-11-2023, 04:00 PM
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You may be talking about the video that was put out by Rhode. An excellent video. I got my initial impetus to build ribbon mics from restoring and re-ribboning vintage mics for collectors.


BTW. I still use 50 mHz more often than not. It has been ultra reliable.


Things that work.
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Old 04-07-2023, 06:05 AM
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Hey Mitch, My 28% Jungmeister is getting to the point where I should start thinking about a engine. I have an OS 160T sitting in a box and I'm hopeful that the twin would work. Curious as to the approximate weight of your Rascal? I apologize if you already mentioned it.

Back in the day I too got my Amateur license to be able to fly 50Mhz.
Old 04-10-2023, 03:52 AM
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Originally Posted by JohnHab
Hey Mitch, My 28% Jungmeister is getting to the point where I should start thinking about a engine. I have an OS 160T sitting in a box and I'm hopeful that the twin would work. Curious as to the approximate weight of your Rascal? I apologize if you already mentioned it.

Back in the day I too got my Amateur license to be able to fly 50Mhz.
Wow, I don't check back in on this old thread much, John. I was surprised to see a posting. My Rascal weighs somewhere around 14 -15 pounds. If you're using nitro (which if so I pity your poor Gemini internals) you might have enough power to satisfy you. I just could't subject these beautiful motors to the corrosion that comes with alcohol - especially if you are using any nitromethane. Yes, you can mitigate it to some extent with LIBERAL doses of Marvel Mystery after run oil - but you can never eliminate it. But if you convert to gas this engine just isn't a powerhouse and needs to be put in a lightweight airframe that doesn't require max power. That's why I chose the Rascal. It's the PERFECT airframe for this engine (in its current state) IMHO. I would guess that the gas converted Gemini 160 puts out approximately the power that the nitro powered Gemini 120 puts out - which really isn't that much!

From one old geezer to another: we're all dating ourselves when we admit to having an amateur radio license to operate at 50MHZ!



Old 04-10-2023, 09:24 AM
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Originally Posted by mitchilito
I would guess that the gas converted Gemini 160 puts out approximately the power that the nitro powered Gemini 120 puts out - which really isn't that much!
I can provide some real-world numbers:

At full chat and burning gasoline, the OS Gemini 160 will produce somewhere around 2 hp @7500 RPM (-ish) on practical settings, and close to 2,5 if all peaked out. As measured on the bench, Of those numbers I am sure, I converted one for a friend.
But doing so continuous will eventually fry the engine of course. The exhaust valves will start to act like glowplugs and the engine will lose power (as if it is running lean, but it isn't) and eventually that will lead to damage.

But it does not fry THAT easy. It will without too much trouble accept a decent warm-up run on the ground with subsequently a full 1 minute of full throttle climbing (what I do with mine when it has towing duty). At about 1 min 15 sec I have to reduce throttle a slight bit to not exceed the CHT limit I have set myself (and that limit has a safety margin, of course. I limit myself to a CHT of 130 degrees C, the power loss occurs at or around 145 deg C)
It will also accept "static peak RPM" all day long in flight. Static RPM rises by roughly 10% in the air, so to maintain static peak you will have to reduce throttle a bit, which in real life will result in approximately 80% of peak output, which comes down to 1,6~1,7 hp
My ASP copy does about 7400 RPM on the ground, and about 8100 in the air. If I limit myself to 7400 in the air, it won't overheat, period. It will stay shy of the 130 deg temp limit I did set myself. I have now over 60 hours and 5 seasons of rather intense use on it. It is on it's third set of sparkplugs. Valve lash remains constant (I check every 5 hours), so I am pretty sure everything is OK internally.

I run 10:1 fuel/oil ratio. I do not have hard info on whether the engine will run cooler on 20:1, but I would expect so.

Just for completeness: Regardless of what OS claims about the engine, make sure you stay well clear of 9K RPM in the air...
Prop for about 8K in flight or 7500 on the ground, tops, and throttle back when in a dive. The conrods are harder and harder to find, crankcases even harder, and the damage can be expensive.
Old 04-11-2023, 01:58 AM
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Great info for us, Brutus! Thanks.
Old 04-11-2023, 04:53 AM
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Thanks Mitch and Brutus. I'm hoping to come in at <15lbs. I'm at 5900' so maybe I should sell this NIB 160 and purchase Saito's new 40cc twin...Thanks
Old 04-11-2023, 07:35 AM
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Originally Posted by JohnHab
Thanks Mitch and Brutus. I'm hoping to come in at <15lbs. I'm at 5900' so maybe I should sell this NIB 160 and purchase Saito's new 40cc twin...Thanks
It won't be a powerhouse, but should fly without trouble. If you want unlimited vertical, buy the Saito, otherwise, your OS will do... My BigLift weighs 15 lbs, and is used for towing.
It is WAY overpowered for normal flight and takes a 7 lbs glider in tow to 650 ft in about 1 minute, give or take.
If I reduce power by about what I described earlier (slightloy over 7000 RPM), I can still easily maintain a 300ft/min climb with that same tow, at above 600 ft altitude.

At your locale, power loss is still acceptable by about 15%, BUT... this power loss also translates in less heat production, so your engine most likely will hold full throttle better (don't take my word for it, use telemetry and sensors to check this if you have the option).
So you should still have that 1,6~1,7 hp availlable is my estimate... Enough for 15 lbs all-up weight

Last edited by 1967brutus; 04-11-2023 at 08:31 AM.
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Old 04-12-2023, 01:51 AM
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Originally Posted by JohnHab
Thanks Mitch and Brutus. I'm hoping to come in at <15lbs. I'm at 5900' so maybe I should sell this NIB 160 and purchase Saito's new 40cc twin...Thanks
I like the 40cc twin idea better, John. You'd probably have some welcome reserve of power.
Old 04-12-2023, 04:12 AM
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No doubt about that, but planes can also have too much power or too much weight in the nose. More is not always better...
Old 04-12-2023, 06:43 AM
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ok, Thanks for the input...you've got me thinking...Mitch, looking forward to the Jungmeister completion.
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Old 05-27-2023, 12:46 AM
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I thought I would touch base back here with what I've been doing lately with my little Gemini. It's still in the Rascal and working great with one exception: it wants to carbon up the exhaust valves - especially the right one. I'm adding de-caronizer (Sea Foam) to my gas which helps but hasn't eliminated the problem. I need to find a fuel that doesn't carbon up the exhaust valves I guess. It's an easy fix when they stick - I just apply a little Sea Foam directly to the valve stem an it's instantly free.

Anyway, I've always wanted to put the Rascal on floats and take it to the Joe Nall floatplane lake and I finally did it this year! It was so fantastic. I built the floats from scratch (foam and fiberglass) and they and the Rascal flew fantastic. It was a real bucket-list week for me and so much fun! See for yourself:


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Old 05-30-2023, 02:30 AM
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Lemme guess: You're using 20:1 fuel oil ratio?

10:1 makes that tendency go away. In the 7 years that I have been messing with small fourstroke gasser conversions, I still have to encounter the first carboned up or sticking exhaust valve in about 10 regularly flown fourstrokes and about 35 gallons of fuel.
Heck, 15:1 might allready cure it, IDK, never went there.

EDIT: Really cool vid of that Rascal on floats, and those floats really seem to perform perfect. Remarkably little spray coming from the "bows" and tracking really smooth in the water.
Also, pretty cool to catch a (brief) glimpse of the man behind these beautiful projects.

Last edited by 1967brutus; 05-30-2023 at 02:38 AM.
Old 05-31-2023, 02:48 AM
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Originally Posted by 1967brutus
Lemme guess: You're using 20:1 fuel oil ratio?

10:1 makes that tendency go away. In the 7 years that I have been messing with small fourstroke gasser conversions, I still have to encounter the first carboned up or sticking exhaust valve in about 10 regularly flown fourstrokes and about 35 gallons of fuel.
Heck, 15:1 might allready cure it, IDK, never went there.

EDIT: Really cool vid of that Rascal on floats, and those floats really seem to perform perfect. Remarkably little spray coming from the "bows" and tracking really smooth in the water.
Also, pretty cool to catch a (brief) glimpse of the man behind these beautiful projects.
You could be right, Brutus. But there are other differences between our operation, no? You are using different gas aren't you, I'm using the ethanol free gas we have here. And also I'm using Red Line oil. However, I think I will change oils and increase to 15:1. You convinced me.

As for the "man behind the projects". I'm sorry to have scared the viewing public but it couldn't be helped!
Old 06-02-2023, 11:12 AM
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Originally Posted by mitchilito
You could be right, Brutus. But there are other differences between our operation, no? You are using different gas aren't you, I'm using the ethanol free gas we have here. And also I'm using Red Line oil. However, I think I will change oils and increase to 15:1. You convinced me.

As for the "man behind the projects". I'm sorry to have scared the viewing public but it couldn't be helped!
I have indeed been using ethanolated gas, E5 at first because that's what was on tap, and now E10, but I do not think that is what makes the difference between carbon depositing or not. I would not know how that would make a difference or why E-free gas would deposit more. I am convinced the sheer amount of liquid oil is washing (maybe "blasting" the crud off. I haven't pulled any of the heads in ages. Another difference is that I reroute the oil effluent from the crankcase through the rocker covers, but I do not always do that for customer engines (some people don't like me drilling holes in their engines, for some weird reason?) and those engines also don't foul.

But rest assured, you did not scare me...
Old 06-04-2023, 01:43 AM
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Originally Posted by 1967brutus
. Another difference is that I reroute the oil effluent from the crankcase through the rocker covers, but I do not always do that for customer engines (some people don't like me drilling holes in their engines, for some weird reason?) and those engines also don't foul.
I have been seriously considering doing this exact thing but like your customers I'm apprehensive about drilling holes. I'd be really curious to see how you go about that, Brutus. Some day would you post pictures?
Old 06-05-2023, 01:08 AM
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Originally Posted by mitchilito
I have been seriously considering doing this exact thing but like your customers I'm apprehensive about drilling holes. I'd be really curious to see how you go about that, Brutus. Some day would you post pictures?
how about "right now"?

It basically comes down to drilling 2 holes (I typically use 3 mm, I guess you guys would go for 1/8") and epoxy in two short stubs of Aluminium tubing of the corresponding size.
For Epoxy I usually take Belzona 1111 because I have about 2 lbs of the stuff laying around, but JB-weld, UHU Plus E300 (probably unknown on your side of the pond but very similar to JB) or Araldite 2011 also have given exellent results.
Location of the drilling is a bit depending on the application, and is not critical.
If possible, I try to aim the incoming oil to the valvestem of the exhaust, "just because", but I do not believe, nor have seen this to make a significant difference.

.52 fourstreoke in pusher config. Here the drain is aimed at the exhaustvalvestem, because the supply was not possible, and the breathing action combined with the short discharge line still has some cooling effect. At least, that is my guess and hope. Could be wishful thinking but regardless, the engine shows excellent compression and valve-seal despite running pretty hot (cowled and pusher, slow flying plane, what could go wrong? ) About 3 years of use, checked valve clearance once...

.30 fourstroke. I could not get the supply aimed at the exhaust valve stem, because of the location of the crankcase vent at the backplate (muffler prevented the direct routing of the tube). Same story as with the pusher for the location of the drain. 6 years in use, LOTS of runtime (estimated over 100 hrs, it's our clubtrainer) Valve clearances have not needed readjustment the initial 20 hrs of runtime. It is on it's third sparkplug if that is an indication.

Engine in inverted position. Reason for using Tygon is that this engine showed poor ring-seal, and I wanted to know if there would be metal debris in the oil, hence the small magnet tie-rapped to the tubing. No particles detected yet.

ASP Twin. Crankcase vent split to both cylinders via T-connection. No issues with unequal oil supply, visually they seem to receive the approximate same quantity of oil. Drains rerouted to the centre, to prevent oil "spreading all over the plane". I have kept an eye on whether the length of tubing affects oil drainage from the crankcase, not been able to determine any ill effects. No oil pooling, no crankcase pressure build-up.

and YAY! FINALLY I managed to aim the supply directly at the exhaust valve stems (Alu tubing is the supply, Tygon is drainage. Tygon was chosen there to be able to observe oil quantity and also t get an idea of the ratio between oil and blow-by gasses).

Saitos and in general engines with separate rocker covers like the early OS 120 singles are more difficult to fit with valve lubrication.

For what it is worth: I have repaired holed rockercovers with Belzona 1111, and those repairs generally hold without being really visible, so a drilled rockercover that in hindsight you want to undo should not be a problem, at least not from a technical POV. And I do not care too much about the cosmetics...

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Old 06-05-2023, 02:29 AM
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Wow, great information, Brutus. Now I know what to do if I go that route. Thanks for all the pictures!
Old 06-05-2023, 04:21 AM
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You're welcome! It ain't pretty maybe but I have always been a function over form kind of guy, and there are lots of ways to make it prettier if it has to be for whatever reason.
Opinions differ of course, but prefer to be able to do a complete conversion, including the curing of the Epoxy, in about 2 hours from unboxing, provided I have all the materials at hand. 1 hour of that is the actual curing . The passtime for me is in the running of the engine, not in the modding process, so I don't want to spend ages doing that. I lack your machining talent, so nobody is going to look at my engines in awe. Means I have to make do with people thinking they run nice...
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Old 07-23-2023, 03:14 AM
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Well, something came up that I thought I'd add to this old post. I have a Moki 300 that's going into a warbird. The 300 is famous for sticking exhaust valves which manifests itself by bending/throwing pushrods so I decided to fix the problem. It requires reaming the too-tight exhaust guides and installing a lube manifold which make lubing the exhaust guides a dream. You can see that mod here:

3W Bearcat with Moki 300

A couple days after getting my Moki fixed up it occurred to me that I actually have the same problem with the little Gemini. The exhaust valve guides on this little gem are sized perfectly for methanol, which doesn't carbon up. But the carbon generated while running gasoline produces carbon which rapidly carbons up the exhaust valves making them stick. So I decided to fix this situation same as I did my Moki (mostly). I didn't have a reamer for this application so I decided to lap the valve stems. They started out at .1175 (inches of course) and I lapped them to .116. After the mod I ran the engine and it's a real screamer! Runs like a top and maybe I'll get a lot more service before I have to decarbon the valves like I've been forced to do a LOT lately.

I still need to address what's causing all this carbon. Running hydrocarbon fuel is going to create SOME carbon but I need to reduce this excessive amount. I'm seriously considering converting the Walbro to alcohol gaskets and running methanol/oil for fuel - but I'd rather not if I can figure out how to reduce it.


Here's an exhaust valve straight out of the engine - before modification. That's a LOT of carbon.

This is what I cleaned out of the cylinder head exhaust port area.

Pretty valve after cleaning and lapping to size

Here you see the little fixture I made to hold the valve for lapping

Worked great.
Old 07-23-2023, 03:48 AM
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Looking good now. Nice bright contact area, right down the center of the face.
Is that a Grizzly lathe? I wonder what type spindle it has? A good collet set can be a very handy thing.

Can't say how many times we hear people claim that gasser valves don't stick. At least the two of us know otherwise

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Old 07-23-2023, 05:00 AM
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Carbon when running gasoline simply is caused by a mixture richer than stoichiometric. No ifs and buts. Stoichiometric or leaner=>no carbon formation possible.
If you cannot lean the mixture because of ignitability reasons, that means that part of the fuel is arriving in the cylinder in liquid state. Liquid does not take part in the equation for ignitability limits. Tthen the solution is to improve evaporation/atomisation which will allow you to lean out the mixture. There is only a very narrow margin of fuel/air where NO carbon is formed and then there is a fairly usable margin where carbon formation is "not really a problem", The width of that "not really a problem" is a function of the oil content of the fuel. More oil widens that range.
So either improve "carburation"(more specific: evaporation", or add oil to wash off the carbon.
Reaming the guides only extends TBO, but provides room for carbon to build up, and carbon is pretty abrasive..

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