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OS 50cc Gemini Twin Gasoline Conversion

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OS 50cc Gemini Twin Gasoline Conversion

Old 12-23-2022, 02:22 AM
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mitchilito
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Default OS 50cc Gemini Twin Gasoline Conversion

I converted the smaller OS Gemini 160 to gasoline in a different thread here on RCU and it has been a fantastic success. You can see that thread here:

OS Gemini Twin Rear Walbro Gas Conversion

I bought this 50cc engine a year or so back specifically to convert as well. It appears to be in great shape with the exception that a previous owner put plier marks on the prop driver (I can't fathom doing that to this jewel of an engine) and one of the exhaust tubes is damaged. But a brief examination looks VERY promising:



Old 12-23-2022, 02:32 AM
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So the first thing to do is clean it up for partial disassembly. I'll take off the carb and engine mounts and cap everything for a kerosine bath. Externally the engine is in great shape but it's very dusty/dirty.



That prop driver needs to be fixed!


After I got the stock carb off I started the process of figuring out where the Walbro will go. This looks promising!

I want the carb intake facing forward so the stock carburetor mount won't work. I'll have to make my own mount/intake manifold.

Last edited by mitchilito; 12-23-2022 at 02:54 AM.
Old 12-23-2022, 02:44 AM
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I couldn't stand that prop driver one more second so I pulled it off, made a mandrel and spun it up in the lathe to skim a few thousandths off of the rim to clean it up. Fortunately the gouges weren't too deep and it cleaned right up. SO much better:



Looks like new again
Old 12-23-2022, 08:32 AM
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Scrolling down the pics first seeing that walbro in place of the glow carb, I thought how quickly and neatly you had it done. Next pic that carb is just sitting there. I would bet you are going to do a great job, can't wait to see it!
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Old 12-23-2022, 10:23 AM
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Looks promising, Mitch!
Wishing for a smooth process of converting, and of course also for a merry Christmass and happy new year!
Old 12-24-2022, 01:27 AM
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Originally Posted by 1967brutus
Looks promising, Mitch!
Wishing for a smooth process of converting, and of course also for a merry Christmass and happy new year!
Thanks, Brutus! Same to you and yours. Glad to see some of the "old gang" over here on this new thread. I promise - we'll have some fun.
Old 12-24-2022, 01:44 AM
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I took the engine down to the case with the cylinders (and pistons/rods) still in place. Everything else came out. It was an "interesting" disassembly. It had never been apart and OS had the bolts STUPID tight. So tight one of the cam case bolts would NOT release so I had to put vice gips on it (I hate that). I eventually had to replace it and it is a RARE bolt: 3.5mm. I actually found one in my hardware collection. And once the cam case cover (and tappets) were removed the cam shaft/gear would NOT release from the little front bearing. Long story short, it fought me tooth and nail but I finally prevailed! That's weird cuz I've disassembled a number of the small Geminis and this part was alway easy. Once I got it out I had to burnish the mating end that goes into the front bearing for a nice slip fit.

Everything looked darned good with one notable exception:



Say. . . that doesn't look right. :-0. These are the left side push rods!

Last edited by mitchilito; 12-24-2022 at 02:06 AM.
Old 12-24-2022, 02:02 AM
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I checked the rest of the engine inside and out to find out what may have turned the left side push rods into limp spaghetti but every single thing looked pristine. The lifters, cam lobes etc were all in great shape as were the valves, rockers etc. Absolutely no visible damage anywhere else on that side. I will make a new set of rods and go on about my merry way. Here's some pics I took for your dining pleasure.



Lobs look great.

Pristine cylinders

Right side rods didn't suffer the same fate

Nice!

This is fun for me


Old 12-24-2022, 07:30 AM
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Originally Posted by mitchilito

Everything looked darned good with one notable exception:



Say. . . that doesn't look right. :-0. These are the left side push rods!
Dang... I thought they were licorice strings... If you hadn't told, I never would have guessed they were the pushrods... really...

Last edited by 1967brutus; 12-24-2022 at 07:33 AM.
Old 12-24-2022, 02:33 PM
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So what are you thinking Mitch, over revved? I mean it doesn't seem to have enough castor accumulation on the valvetrain to stick rockers or valves. Might not be a bad idea to check the rod big ends for roundness?
Old 12-25-2022, 01:47 AM
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Originally Posted by Glowgeek
So what are you thinking Mitch, over revved? I mean it doesn't seem to have enough castor accumulation on the valvetrain to stick rockers or valves. Might not be a bad idea to check the rod big ends for roundness?
To be perfectly honest, I have ZERO idea how the rods could wind up in that condition without any other damage although the rods and lifters DID have significant castor accumulation. I will say this: the rods are very soft (and THIN) so it wouldn't take much.

I know the connecting rod big ends are at least very close to very good. I capped the cylinders and rocked the crank shaft and there is zero play. The whole engine looks fantastic mechanically. To be honest, I will have so little money in this project that I have nothing to lose!
Old 12-25-2022, 11:25 AM
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Here she is freshly cleaned/rebuilt with new pushrods. Time to make a new intake manifold. Everything looks really good except with the walbro in the position I want it the fuel inlet pipe is pointing straight forward. Any body know how to rotate it 180 degrees???










Last edited by mitchilito; 12-25-2022 at 11:29 AM.
Old 12-25-2022, 11:40 AM
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Originally Posted by mitchilito
Here she is freshly cleaned/rebuilt with new pushrods. Time to make a new intake manifold. Everything looks really good except with the walbro in the position I want it the fuel inlet pipe is pointing straight forward. Any body know how to rotate it 180 degrees???
Looking good!

I've had several Walbro brass fuel inlet tubes rotate while trying to get the fuel line off for replacement. They appeared to be held in by nothing other than a light press fit. Try turning it?

Last edited by Glowgeek; 12-25-2022 at 11:43 AM.
Old 12-25-2022, 03:26 PM
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That’s kinda what I was thinking. Thanks Glowgeek.

I’ll let you know what happens when I get there. .
Old 12-25-2022, 04:19 PM
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Once again a good looking project.


Last edited by Jesse Open; 12-26-2022 at 02:50 AM.
Old 12-26-2022, 03:08 AM
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Originally Posted by Jesse Open
Once again a good looking project.
Hey, nice to see you here, Jesse. It's gonna be fun for sure. . . .
Old 12-26-2022, 05:00 AM
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Here’s a question for the community at large. For my intended purposes the Walbro carb MUST be mounted “upside down.” That is, the pump section is on the bottom and the regulator at the top. Personally I can’t see a problem but any input from you all out there in gasoline land?


Old 12-26-2022, 09:08 AM
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Carb orientation shouldn't cause any problem.

I went back and read your thread on the FT160 gas conversion. Some serious nice engineering and machining work there, Mitch.

In that thread you asked why there was a spring in the pump section of one of the two carbs. It's a pump diaphragm return spring for use with 4 stroke engines. Unlike 2 strokes, 4 strokes typically have issues with supplying enough high/low pressure impulses from the crankcase to drive the pump section, because the crankcase is normally vented. The spring acts to help with a weak high pressure impulse signal.

Typically you find those springs used in carbs for 4 strokes where the low pressure signal is provided by a passage to the intake tract and no high pressure signal is provided. My understanding anyway. Saito uses this same carb/pump/spring concept on their FG engines.


Old 12-26-2022, 01:43 PM
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Originally Posted by Glowgeek
Carb orientation shouldn't cause any problem.

I went back and read your thread on the FT160 gas conversion. Some serious nice engineering and machining work there, Mitch.

In that thread you asked why there was a spring in the pump section of one of the two carbs. It's a pump diaphragm return spring for use with 4 stroke engines. Unlike 2 strokes, 4 strokes typically have issues with supplying enough high/low pressure impulses from the crankcase to drive the pump section, because the crankcase is normally vented. The spring acts to help with a weak high pressure impulse signal.

Typically you find those springs used in carbs for 4 strokes where the low pressure signal is provided by a passage to the intake tract and no high pressure signal is provided. My understanding anyway. Saito uses this same carb/pump/spring concept on their FG engines.
That's some good info, Glowgeek. I did get that a carb off a 4 stroker weed eater engine but turns out the Gemini makes such a strong crankcase pulse that it works the pump (of a 2 stroke carb) even with the case drain open to atmosphere - in the case of my 160 into the inlet venture, just like this one will.

Thanks again!
Old 12-26-2022, 02:33 PM
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Originally Posted by mitchilito
That's some good info, Glowgeek. I did get that a carb off a 4 stroker weed eater engine but turns out the Gemini makes such a strong crankcase pulse that it works the pump (of a 2 stroke carb) even with the case drain open to atmosphere - in the case of my 160 into the inlet venture, just like this one will.

Thanks again!
I'm a bit curious how strong that pressure signal is. Apparently strong enough for a forward mounted tank, perhaps even strong enough for a tank mounted at CoG. Hard to say without playing with one.

I've not converted a 4 stroke glow to gas using a 2 stroke carb, all mine use Bert's fueling solenoid with Dave's microcontroller. A very slick setup for us mill/lathe challenged folks and having full control of mixture strength throughout the entire rpm range, from the TX, is just pure fun. Temp and barometric pressure compensation is icing on the cake!

I'm not selling anything, I like what you're doing too, less peripheral fueling components required.

Last edited by Glowgeek; 12-26-2022 at 04:17 PM.
Old 12-27-2022, 01:46 AM
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Originally Posted by mitchilito
That's some good info, Glowgeek. I did get that a carb off a 4 stroker weed eater engine but turns out the Gemini makes such a strong crankcase pulse that it works the pump (of a 2 stroke carb) even with the case drain open to atmosphere - in the case of my 160 into the inlet venture, just like this one will.

Thanks again!
Originally Posted by Glowgeek
I'm a bit curious how strong that pressure signal is. Apparently strong enough for a forward mounted tank, perhaps even strong enough for a tank mounted at CoG. Hard to say without playing with one.

I've not converted a 4 stroke glow to gas using a 2 stroke carb, all mine use Bert's fueling solenoid with Dave's microcontroller. A very slick setup for us mill/lathe challenged folks and having full control of mixture strength throughout the entire rpm range, from the TX, is just pure fun. Temp and barometric pressure compensation is icing on the cake!

I'm not selling anything, I like what you're doing too, less peripheral fueling components required.
Most fourstrokes use the carb intake vacuum as pulse generation. That means there is only a negative pulse, and not a positive one. That is done to prevent the pulse side of the pumpmembrane from clogging up with crankcase oil residue, thus restricting membrane movement,
The downside is that a returnspring is needed, in order to change a "negative to zero" pulse into a "negative to positive" membrane movement. There would be no pumping action otherwise: the membrane would draw in fuel but not push it out, if not for the spring.
If the crankcase vent is used as pump actuation, the pulse will be "negative to positive" because the average pressure in the crankcase is atmospheric (open connection to atmosphere). That pulse is strong enough to pump fuel without the need for a spring under the membrane, but as said, the downside is that there is a small possibility for the pulse side of the pumpmembrane filling up with oil residue. Which, IF it happens, should be a very easy fix, but it would ruin your flying day a little bit.

Last edited by 1967brutus; 12-27-2022 at 01:54 AM.
Old 12-27-2022, 04:16 AM
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I'm seeing very little residue in the crankcases of my 4 stroke gassers after three seasons of flying them. Just a very thin film of blowby components adhering to the case walls. I suspect a walbro pump diaphragm would need replacing before enough residue/buildup occurred to cause a pumping problem, but I don't have any practical experience with this setup.

I suppose venting the case from bottom and a pulse line from the top would be the best long term solution.

Last edited by Glowgeek; 12-27-2022 at 05:00 AM.
Old 12-27-2022, 10:12 AM
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Originally Posted by Glowgeek
I'm seeing very little residue in the crankcases of my 4 stroke gassers after three seasons of flying them. Just a very thin film of blowby components adhering to the case walls. I suspect a walbro pump diaphragm would need replacing before enough residue/buildup occurred to cause a pumping problem, but I don't have any practical experience with this setup.

I suppose venting the case from bottom and a pulse line from the top would be the best long term solution.
Like I said, the likelyhood is not all that large, but here's the thing: Oil residue has the tendency to collect in dead ends, not in places where there is a lot of movement. That is why in the crankcase you won't see much residue, or in the ventline (plenty movement there).
But even if the likelyhood is small, it is a possible cause of irregularities.
Old 12-27-2022, 01:55 PM
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Talking

Hey, this is a great conversation. Just what I like to see on my threads!

So it's time to bend some intake tubes. These are the heart of these gas conversions so it has to be done right. This bigger engine uses 8mm stainless tubes so I had to make some new parts for the "Parker Hannifin" style (that's what Jesse Open called it when he saw it ) tube bender I made.

While I was working on it I got to wondering what I first made it for and then realized I made it for the very first engine project I made from castings - YEARS ago. I'll include pics of it here.



The wheel, shoe and the steel end stop (with the two bolts holding it) need to be made for each size tube. I just made these 8mm parts.

Here she is ready to bend

This is the Cerobend metal that HAS to be poured into the tubes prior to bend. Very low melt temp.

My first engine. Made from rough castings (cylinder head, crank case and cam cover). Every thing else I made from scratch. I take that back. The carb came off an OS I think.

I considered this my machinist apprenticeship. You can see it run on my YouTube channel.





Last edited by mitchilito; 12-27-2022 at 04:59 PM.
Old 12-27-2022, 03:30 PM
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That's one very cool looking old canted valve 4 stroke. What is it, Mitch?

Last edited by Glowgeek; 12-27-2022 at 03:34 PM.

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