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OS Gemini Twin Rear Walbro Gas Conversion

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OS Gemini Twin Rear Walbro Gas Conversion

Old 06-18-2022, 12:50 PM
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mitchilito
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This is a standard Walbro carb off of the RCGF 15cc gas engine. I just happened to have taken a couple pics of it this morning. I even took a venturi shot. I’d estimate the venturi to be .375 give or take a few thousandths. I took a pic of the serial number.

I cannot overstate how great this carb is working on the Gemini 160. It is well behaved in every respect. I'm so happy!





Last edited by mitchilito; 06-18-2022 at 01:36 PM.
Old 06-20-2022, 02:45 AM
  #52  
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For those that might want to know, I put a Xoar 18x6 prop on it and it seems perfect for this application: tons of power but certainly not overloaded.
Old 06-20-2022, 03:23 AM
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How does the Xoar compare to the APC wideblade series? I run an APC 18 x 6 Wideblade on my ASP 160 and it does about 7400 on the ground, 8200 in the air and pulls stumps. I use mine for glider towing.
Old 06-21-2022, 02:01 AM
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Originally Posted by 1967brutus View Post
How does the Xoar compare to the APC wideblade series? I run an APC 18 x 6 Wideblade on my ASP 160 and it does about 7400 on the ground, 8200 in the air and pulls stumps. I use mine for glider towing.
I have not tried the APC wide blade. I think I'll get one and let you know.

I had used my first rascal (with this gemini - not converted to gas yet) for glider tow. That was a lot of fun!

I will say this about converting a glow engine to gasoline: horse power is going to be reduced slightly. However, I can't really tell much difference with my conversion so it can't be much. She puts out a ton of power.
Old 06-21-2022, 05:25 AM
  #55  
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Originally Posted by mitchilito View Post
I will say this about converting a glow engine to gasoline: horse power is going to be reduced slightly. However, I can't really tell much difference with my conversion so it can't be much. She puts out a ton of power.
It does, and "chemically" (the amount of energy per cylinder charge, basically) it is less by about 20% for gasoline, which if that number is used as rule of thumb, seems to be fairly accurate in predicting what prop can be expected to produce the same RPM as on glow, if RPM/prop on glow fuel is known. But of course, that also depends on where the engine is operated in its RPM range, because engines become more tolerant to lugging down after conversion (in fact, they prefer it, it helps keeping temperatures down a bit).

Since AFAIK the output in HP is not stated for both your OS boxer as well as my ASP, I don't know either what the exact reduction in power is for this engine. the only indication I have is that the vendor that sold me my ASP told me what he had seen these engines do on glow fuel (about 2,3 hp between 7500 and 8K), and apparently the numbers he gave me were "straight from the box" and before break in, because although initially mine was producing significantly less than that (first runs showed something like 1,7 hp), after about 15~20 runhours it turned out that it only missed that 2,3 hp by about 0,1...
According to my prop-calculator (Pe reivers' propcalc), mine puts out in the neighbourhood of 2,1~2,2 if tuned for peak on the ground. (I tried different online calculators, with virtual identical results by the way).
That is not a tune that is usable for flight (too lean) but in "flight tune" there is still about 1,9~2 hp on the ground, which increases in the air due to unloading. For transparency: I tune about 300 RPM rich from peak for flight.
My guess is there is well over 2,2 HP availlable in flight. But that is just that, an educated guess. I have not yet found a propcalculator that can work with "in flight RPM" so there's no way to be certain...

FWIW: I have run a converted OS FT160 side by side with my converted ASP (I did the conversion on both, so there are no significant differences in the method of conversion), and there is no real measurable difference in performance. The OS is (much!) nicer made, but not measurably stronger or weaker in performance. Can't comment on the durability differences, my ASP shows zero deterioration, neither does my clubmate's OS, but he flies his a lot less than I do mine, so no way of telling either way.

Your engine might easily pick up a couple of hundred RPM by the way, when the running surfaces re-mate using the different lubricant. That is at least invariably my experience with engines that were run on glow fuel previous to conversion, that they pick up ever so slightly after 5 or 10 hours of operating time, especially if the glow fuel used to contain significant amounts of Castor oil. Apparently the type of lubricant has some effect on the mating of the running surfaces, IDK, but I see that slight gain every time again.
Old 06-21-2022, 09:25 AM
  #56  
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The power differences are far less noticeable when flying typical, high drag type planes.

Very noticeable on the more spirited, sport and pattern type planes.

Even more pronounced in both types however when CDI ignition is used with the methanol, even when the nitomethane percentage is rolled back.
The run time takes a boost as well.

A fellow RCU member did some real fine, well documented work at adapting Saito glow engines to CDI. With proper tuning he was getting real power gains and substantial boosts in run time.


Last edited by Jesse Open; 06-21-2022 at 09:37 AM.
Old 06-22-2022, 01:19 AM
  #57  
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I'm really enjoying this line of discussion but I have to admit that I'm much less analytical about this conversion. The C&H doesn't have a tach lead and I don't have a hand tach so I can't even tell you the R's! Since I fixed every main concern I had with running on glow fuel the only real concern I have now is how much power did I lose and will it affect the durability. . . .

I've been flying it quite a bit lately (with the Xoar 18x6) with the mixture just a tic off of max lean - that's where it wants to run best - and I'm pretty impressed with the available power. The engine is so pretty it's easy to forget that it's a real little dynamo!

Last edited by mitchilito; 06-22-2022 at 02:32 AM.
Old 06-22-2022, 04:51 AM
  #58  
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Originally Posted by mitchilito View Post
I'm really enjoying this line of discussion but I have to admit that I'm much less analytical about this conversion.
my apologies for that... I guess that is a bit of job deformation, never do anything without measuring and comparing the "before/after" and that kind of stuff.
In no way intended for anything else than to provide as much as information as possible for "just in case". They are my experiences with converting these boxers, and if anyone can use it, then that is great, if not, still nothing lost. Your engine won't run any different for it

Originally Posted by mitchilito View Post
the only real concern I have now is how much power did I lose and will it affect the durability. . . .
Power loss; if you assume 20% you're on the safe side, actual loss probably is a little less as in part it is compensated by the slightly lower RPM (better prop efficiency).
Durability depends on your fuel mixing ratio and whether you're light or heavy handed on the throttle.
I can guarantee from personal experience, that if you mix 10:1, keep full throttle limited to max 1,5 minutes (for me that brings a glider to 600 ft, which is where my eyes start to fail me), and make sure you do not exceed 9K in flight EVER, there are no durability issues, not even with the cheap slag-ASP

Originally Posted by mitchilito View Post
with the mixture just a tic off of max lean - that's where it wants to run best - and I'm pretty impressed with the available power. The engine is so pretty it's easy to forget that it's a real little dynamo!
What can I say? Identical experiences to the last letter here: touch rich for best running, and it has indeed plenty power. OK, in all fairness the ASP is quite a bit less pretty (and their blackening on the cylinders fades to some brown/purple hue under influence of heat) but still pretty enough for me, and the sound still gives me goosebumps every time I run it up.

But what I like most is how frugal it is and how easy it starts.

Old 06-22-2022, 07:56 AM
  #59  
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Originally Posted by mitchilito View Post
I'm really enjoying this line of discussion but I have to admit that I'm much less analytical about this conversion. The C&H doesn't have a tach lead and I don't have a hand tach so I can't even tell you the R's! Since I fixed every main concern I had with running on glow fuel the only real concern I have now is how much power did I lose and will it affect the durability. . . .

I've been flying it quite a bit lately (with the Xoar 18x6) with the mixture just a tic off of max lean - that's where it wants to run best - and I'm pretty impressed with the available power. The engine is so pretty it's easy to forget that it's a real little dynamo!

Mitch,

Sure runs.... and looks great. Your efforts are earning a fine return!


The durability, in most cases should improve with gasoline.
Gasoline itself has a bit of lubricity, methanol .... relies on the oil as it imparts little if any. Gasoline is far less corrosive than methanol based fuels, especially when nitromethane is involved. Of course, you can easily do without nitromethane when using spark ignition.

With proper care and handling, wear is really a small issue with either fuel. I have several methanol fueled engines that date back 40 years or more, Wear has not been an issue. Yes, that care and handling is a bit more specialized with methanol, but we do generally enjoy taking good care of the entire plane anyhow

Overheating however is a factor with gasoline, especially some conversion applications. Many converted glow engines do need their time spent at full loads limited.

As you know, starting either should be about the same. The broader rich/lean mixture tolerance makes methanol, especially CDI sparked methanol very tolerant and easy to start.
The high vaporization rate of gasoline usually makes for easy starts, especially in colder weather.

With CDI ignition, with similar engines you will burn about 1/2 as much gasoline for the same RPM with a given prop on methanol with zero nitromethane. Keep in mind, that zero nitro fuel costs considerably less than the typical 10%~15% fuel. Still not as cheap as gasoline

You will however have the potential for about 20% more power, plus the ability to maintain that power for extended periods.

With planes like the Rascal, a little extra power is seldom missed anyhow.

Great job!



Last edited by Jesse Open; 06-22-2022 at 08:53 AM.
Old 06-23-2022, 12:35 PM
  #60  
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QUOTE: With CDI ignition, with similar engines you will burn about 1/2 as much gasoline for the same RPM with a given prop on methanol with zero nitromethane.

Jesse, this is something I hadn't mentioned so far but I have a teeny tiny gas tank in the Rascal (375ml or 12 oz) and it's just crazy how little fuel this thing uses. I'd estimate that I use 6 ounces for a vigorous 10 minute flight. Just amazing - especially compared to alcohol.

I also have a Saito FG60 three cylinder gas radial and that's where I first noticed how little fuel these gas 4-strokers use. It's 60cc and barely uses any gas at all.

Last edited by mitchilito; 06-23-2022 at 12:42 PM.
Old 06-23-2022, 01:25 PM
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Originally Posted by mitchilito View Post
I have a teeny tiny gas tank in the Rascal (375ml or 12 oz) and it's just crazy how little fuel this thing uses. I'd estimate that I use 6 ounces for a vigorous 10 minute flight. Just amazing - especially compared to alcohol.
I have 250 ml (8 oz) in my Big Lift. I can do 5 safe tows on a tank with some spare. A tow consists of about 1 minute elevated idle for warming up (I don't open the throttle above 3500 RPM unless CHT is appr 70 deg C (appr 160 F), then a full throttle run of about 1~1,5 minute, and subsequent return and landing, basically at idle apart from the overshoot for linedrop. So I'd say that is about right, 6 oz for 10 minutes of "vigorous flight".
For normal relaxed flying at 5500 RPM with the occasional climbout however, the 8 oz lasts at least 40 minutes.
Old 06-23-2022, 01:41 PM
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Originally Posted by mitchilito View Post
QUOTE: With CDI ignition, with similar engines you will burn about 1/2 as much gasoline for the same RPM with a given prop on methanol with zero nitromethane.

Jesse, this is something I hadn't mentioned so far but I have a teeny tiny gas tank in the Rascal (375ml or 12 oz) and it's just crazy how little fuel this thing uses. I'd estimate that I use 6 ounces for a vigorous 10 minute flight. Just amazing - especially compared to alcohol.

I also have a Saito FG60 three cylinder gas radial and that's where I first noticed how little fuel these gas 4-strokers use. It's 60cc and barely uses any gas at all.
Yes, that sounds about right. The CDI Saito FA-82 will burn about 3/10 ounce per minute of 4% nitromethane fuel. 20 minute flights with ~6 ounces of fuel.
My Clipped wing Cub with an OS 160 Gemini II CDI iirc had a 12 or 14 ounce tank and it was good for about 20 minutes with fuel to spare.


Old 06-25-2022, 10:45 AM
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Well since I posted last time I've flown and flown this little gem and it continues to run like a watch. HOWEVER - I did have an interesting anomaly happen today. It's actually the second time it's done this although I think it had the old 4-cycle walbro on it when it first happened.

So I had flown it vigorously one flight (maybe two) and it ran great as always. But when I went to start it the next time it started right up but would not accelerate. Full throttle would bog and begin to die. Long story short I used the choke to finally coax it back to full throttle and once it cleared it was right back to normal. Matter of fact I had re-adjusted the needles again and when cleared it was actually running even better. The transitions to full throttle are fantastic! I flew it a couple more times and could not believe how good this little thing runs.

So this is my theory: It was a pretty warm day, maybe 87 or so, and I'm thinking I had a vapor lock. Once I coaxed it into running up it cooled the inlet tract thereafter. I don't have a big carb insulator on my conversion: about 4 millimeters of nylon. I'm thinking I might make one out of phenolic and see what happens.

Bottom line: I'm still LOVING this thing!!

Last edited by mitchilito; 06-26-2022 at 01:07 AM.
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Old 06-25-2022, 11:23 AM
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Originally Posted by mitchilito View Post
Well since I posted last time I've flown and flown this little gem and it continues to run like a watch. HOWEVER - I did have an interesting anomaly happen today. It's actually the second time it's done this although I think it had the old 4-cycle walbro on it when it first happened.

So I had flown it vigorously one flight (maybe two) and it ran great as always. But when I went to start it the next time it started right up but would on accelerate. Full throttle would bog and begin to die. Long story short I used the choke to finally coax it back to full throttle and once it cleared it was right back to normal. Matter of fact I had re-adjusted the needles again and when cleared it was actually running even better. The transitions to full throttle are fantastic! I flew it a couple more times and could not believe how good this little thing runs.

So this is my theory: It was a pretty warm day, maybe 87 or so, and I'm thinking I had a vapor lock. Once I coaxed it into running up it cooled the inlet tract thereafter. I don't have a big carb insulator on my conversion: about 4 millimeters of nylon. I'm thinking I might make one out of phenolic and see what happens.

Bottom line: I'm still LOVING this thing!!
Yes, that has all appearance of a heatsoak in the carb during standstill, given that your entire crankcase and carb are under the cowl, and the carb is "tucked away" behind the engine, that could very easily happen despite the insulator. Personally, I don't think changing the Nylon insulator for a Phenolic one won't make much difference. Letting the engine idle for a minute or so most likely would have cooled the carb down sufficiently to solve the issue, choking it only accelerated that cooldown by having more fuel evaporating (evaporation = absorbing heat)
FWIW: don't think too much of it, these things sometimes happen, and as long as things are stable in flight, what happens on the ground does not matter too much. In full scale aviation they have procedures for these kind of things, and back in the days of carburated cars, even on hot days, we also never drove off full throttle, because we all knew that was not the right thing to do. IMHO, this is no different.

And yes... Running gas REALLY changes the characteristics of an engine and the whole experience in general, doesn't it?

I have no idea how much fuel you have through it now, but my expectation is, that it will keep improving (while also needing minor needle tweaks) for about the first two gallons after conversion. Not shocking much, just a little every day, keeping that smile on your face fresh every day.

Last edited by 1967brutus; 06-25-2022 at 11:26 AM.
Old 06-25-2022, 03:48 PM
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I'm with you on this Brutus: I'm not worried a bit about this. It ran so great two more flights I can't get too upset!

I was just in the shop looking at it though and I notice my ignition box (which in under the engine in the cowl) mostly blocks a big air outlet at the bottom of the cowl. I cut a little air inlet in the cowl under the spinner just for the purpose of cooling things off in there but I fear I'm wasting it.

I'm going to see about raising the ignition box up a bit. It wouldn't take much to make a nice difference.

Last edited by mitchilito; 06-26-2022 at 01:11 AM.
Old 06-25-2022, 10:46 PM
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Can you post a pic of the underside of the cowl installed? Indeed I think the ignition is obstructing outflow a bit, but a small "spoiler" in front of the exit opening, deflecting the propwash downward, has a huge positive effect on flow-through. I did that with my FW190 and it almost doubled the cooling efficiency.
But raising the ignition box by 1/2" should absolutely not hurt anything is my guess.

Personally, I am no fan of having the ignition box in front of the firewall, sooner or later it will get dirty and I dislike dirty electronics. But other than that, that is a tidy installation. Mine usually look a lot less "well planned"

Last edited by 1967brutus; 06-26-2022 at 01:16 AM.
Old 06-26-2022, 08:03 AM
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In the past I had often used a over sized refective heat shield. Made from thin aluminum pie plate material and installed between the carb and insulator block.Form as needed to block out carb from heat radiated from the engine. They can do an excellent job at keeping radiated heat away from the carb body.
BTW, The carb mounting screws should also have heat insulation measures at ALL contact points. Don't underestimate the heat transfer that takes place

Last edited by Jesse Open; 06-26-2022 at 08:20 AM.
Old 06-26-2022, 04:42 PM
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Originally Posted by Jesse Open View Post
In the past I had often used a over sized refective heat shield. Made from thin aluminum pie plate material and installed between the carb and insulator block.Form as needed to block out carb from heat radiated from the engine. They can do an excellent job at keeping radiated heat away from the carb body.
BTW, The carb mounting screws should also have heat insulation measures at ALL contact points. Don't underestimate the heat transfer that takes place
Thanks Jesse, this is good info.
I’m gonna see how my cowl vent works before I do anything drastic

Yes, I put the rascal on the bench today so I could move the ignition box a good half inch above the cowl cooling air exit. Now my little cooling hole will actually Do something!




Here she was yesterday- flying to the moon.
Old 06-26-2022, 06:46 PM
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That exit hole is very ineffective to the extreme. A spoiler of 1/4"will increase airflow through the cowl hugely.
And looking at how much of the engine is under the cowl, I am actually surprised you're not allready having heat issues,,,
Old 06-27-2022, 06:17 AM
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All I saw in the pic was the vertical mill. I'm so jealous!

+1 on air dam to create low pressure area and help outflow at exit hole.
Old 06-27-2022, 06:22 AM
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Originally Posted by kmeyers View Post
All I saw in the pic was the vertical mill. I'm so jealous!
some people have everything... Yeah, I'd definitely like to have one of those as well...
Old 06-27-2022, 12:25 PM
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There’s only one thing to do if you REALLY want a mill - or lathe. BUY ONE 😆

As for the exit lip, I WILL put one on for sure.

Last edited by mitchilito; 06-28-2022 at 12:37 AM.
Old 06-27-2022, 12:29 PM
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Originally Posted by mitchilito View Post
There’s only one thin to do if you REALLY want a mill - or lathe. BUY ONE 😆

As for the exit lip, I WILL put one on for sure.
Better still,

Buy both!

Old 06-27-2022, 12:52 PM
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Originally Posted by mitchilito View Post
There’s only one thin to do if you REALLY want a mill - or lathe. BUY ONE 😆
I have a lathe...

I have a drill press too but no place to install it, and that rules out the mill... Fortunately I have friends with great equipment and skills...
Old 06-27-2022, 12:53 PM
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I bought my machines 16 years ago and they have allowed me to do all kinds of cool things - not the least of which was this engine conversion.

Would not want to be without them.

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