OS Gemini twin rear carb gas conversion

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Dec 2, 2013
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Really couldn't find a post category that this build fit so I'm going to post this here in the engines area.

I've always loved the Gemini twin but the actual operation of it in a model aircraft has real drawbacks:
1) When run on the factory recommended fuel, methanol with 15% nitro, the combustion byproducts are hideously corrosive to the engine internals.
2) The oil that blows by the rings in the combustion process is needed to lube the internals. As the engine runs, this sludge discharges all over your beautiful aircraft.
3) The glow fuel carb has an amazingly tiny venturi which CANNOT draw fuel in anything but static, level, conditions. Engine dies in even mild aerobatics.
4) Glowplugs are wonderfully simple but for max reliability require heavy onboard glow equipment.

SO: over the years I devised a fix for each and every one of these shortcomings and I will present them here for your edification :cool: . First let me give you a picture of the final, fully developed product.
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So the first thing I addressed was the glow situation. I contacted Adrian at C&H Ignitions and ordered his wonderful ignition conversion for this engine. It works great. Here it is in a Sig Rascal 110, which was a wonderful engine/airplane combo. This is running on straight methanol, no nitro, and 20% Redline two stroke racing oil. This eliminates the nitric residues in the engine case and added TONS of reliability:

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But I still had the tiny venturi/fuel draw problem. What I needed was pumper carb - Walbro style- which pumps and regulates fuel. Since I couldn't readily find one to fit this installation, I manufactured it out of Walbro and other parts.

First I machined off the pump section of a Walbro leaving just the pulse-operated pump. Then I happened to have a alcohol-friendly regulator already laying around. This was manufactured by a company named "Iron Bay" which is most likely out of business now. The whole contraption actually work PERFECTLY and provided a endless supply of regulated fuel to that little carb and ran like a watch in any attitude. The combination of pump/regulator/venturi perfectly mimics the simple Walbro carburator. The downside was it was hideously complex! See for yourself:
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And it's just starting to get interesting! So far I've addressed the Glow Plug drawback and the stock carb drawback. What I still had was ridiculous complexity, sludge down the bottom of the aircraft and some alcohol related corrosion albeit not as bad as when running nitromethane.

To solve the complexity AND the corrosion problems simulatiously I decided to convert to gasoline and instal an actual Walbro carb. I looked at the gas conversions for this engine that are already out there and frankly, I didn't like the way they approached it. These conversions have the carb slung under the engine and I can't see trying to fit all that into an airframe. Don't get me wrong, these conversions run great, I just did not like the look:

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The first step is getting the proper Walbro for this engine. As I've said, the venturi on the stock carb is minuscule so I needed a Walbro set up for a tiny gas engine. Fortunately, Morris (I think in the video above) mentions that he's using a Walbro off of the RCGF 15cc engine. Bingo! I ordered one and had the starting point for this project. Thanks Morris!

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I decided to make a wood model to check form and fit:


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I knew I was really onto something. That Walbro tucks up behind the engine fantastically. Time to cut some metal!


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The Gemini twin has an untapped pressure port already present on the on the upper part of the crankcase. I drilled it through, threaded and tapped for a nipple long ago - so I could inject preservative oil into the cam case to limit said corrosion. So I designed my new induction manifold to tap right there for the pressure pulse that runs the Walbro pump. Convenient! That's the nipple you see above and you can see how it's drilled through to the center of the face where the Walbro has it pressure passage.
With the intake manifold completed it was time to bend up some intake pipes. I had some 8mm O.D.x.8mm wall stainless pipe already so I prepared to bend it up. I'll say now that .5mm wall would be better but I didn't have any. The .8mm wrinkled just a little on the insides of the tighter bends. Not a functional problem but I was going for perfection. Now, let's visit tube bending ABC's:

The first thing I do is heat the entire pipe to a dull red and let cool slowly. Then: I fill the pipe with low melt temperature bismuth metal. This step cannot be skipped or you will surely collapse your tubes. I made my own tubing bender long ago. I just need to make some 8mm particular parts for it.
Nice bunch of hot ridding. Check McMaster Carr for cero bend metal. You just warm it up a bit and pour it into your tube . Ends plugged. Then bend as rod. When done soak in hot water or gentle soldering torch and pour the cero bend out. Save for reuse later. You can also try copper tube 1/4” inch works for models. You need to silver braze as standard solder will not hold up to gas motor exhaust temp . You can get 1/4” copper fittings on Amazon. They are used on heating and screws work. McMaster also has brass and copper fire your flanges if needed.
It's a great job, but I don't understand why to put another pump? the walbro already has a pump inside.
It's a great job, but I don't understand why to put another pump? the walbro already has a pump inside.
Not sure where you got confused but there is only one pump - the one on the walbro. Originally I added a pump to the stock carburetor. But that's all history now.
Here's a couple build pics.


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Here you see the pressure tap off the top of the cam case. Very convenient location.


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Of course with the carb sticking out the back the stock engine mount won't work so a set of standoffs is called for:
So to this point I addressed all the Gemini shortcomings but one: the waste oil discharge out the bottom of the engine. What a mess! Years ago, long before the manufacturers themselves started doing it, I ported this waste oil discharge straight to the carburetor induction manifold. Yes, in-between the carb and the cylinder head. It shouldn't work but it does - fabulously. Viola - no more waste oil running down the bottom of your aircraft!

On this conversion I made a velocity stack and tapped it for the waste oil. This puts the case oil into the inlet although from past experience I know it would work plumbed directly into the induction manifold. The reason I didn't do that on this engine is because I've found that in between runs the crankcase on this engine drains a LOT of oil. l wouldn't want that collecting in the manifold and causing a potential hydro-lock.

This method of recirculating the oil has the added advantage of allowing one to reduce the amount of oil mixed into the gasoline. I'm shooting for a total oil content of 20% so I'll now mix my gas/oil at 15:1.
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And here's the final result. This vid is the first runs. I hadn't got the mixture needles quite right yet but you'll get the idea. I could not have hoped for a better result. Thanks for staying with me to this point. Hope you enjoyed -


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