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OS Gemini Twin Rear Walbro Gas Conversion

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OS Gemini Twin Rear Walbro Gas Conversion

Old 07-23-2023, 10:33 AM
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Originally Posted by 1967brutus
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..
I think you are correct, Brutus. Unfortunately, the Walbro isn't really atomizing the fuel perfectly because even when set on the lean side BOTH exhaust valves are carboning up badly - but the right one is much worse. We already know that cylinder runs richer.

I honestly think I'm going to put a "alcohol kit" in my Walbro and just run straight methanol and oil. No big downside with methanol other than a tiny bit more tendency to corrosion - and quite a bit of a price increase. But next to no carbon. And methanol would give me more power.

Last edited by mitchilito; 07-24-2023 at 06:10 AM.
Old 07-23-2023, 10:37 AM
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Originally Posted by Jesse Open
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
Yeah, it's a Grizzley, Jesse. It's a D1-3 spindle and I do have a full collet set (up to 1 inch) for it. Wasn't really all that helpful for this little project though because of the way I had to approach the stems.
Old 07-23-2023, 12:45 PM
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Originally Posted by mitchilito
I think you are correct, Brutus. Unfortunately, the Walbro isn't really atomizing the fuel perfectly because even when set on the lean side BOTH exhaust valves are carboning up badly - but the right on is much worse. We already know that cylinder runs richer.

I honestly think I'm going to put a "alcohol kit" in my Walbro and just run straight methanol and oil. No big downside with methanol other than a tiny bit more tendency to corrosion - and quite a bit of a price increase. But next to no carbon. And methanol would give me more power.
That is why I use no Walbro... their atomisation is horrible, and basically leans on the crankcase (they are predominantly used in 2-strokes) doing the atomisation and evaporizing for them.

I use the bogstandard glowcarb combined with the Stihl solenoid, and I ain't got no sticking valve trouble. Maintenance is limited to regapping spark plug after 25 hr, replace when performance deteriorates, new engines need a valve clearance check every 5 hrs, but so far every one of them basically stabilized in that aspect after about 30 hrs and now I basically do not bother anymore on "older" engines.
Works for me, and so far, now about 4 years in (first tests ran in April 2019, currently 13 engines running, amongh which a Mk I OS Wankel) I yet have to have the first one fail, solenoid, electronics OR engine....
Went from primitive "fuel curve in the transmitter" to "atmospheric compensated" and every step brought some improvement, to the point where the only improvements are "smaller and compacter"I see no real possibilities for further improvement WRT performance. System has matured.
Wished it was marketable, but it isn't. But it IS doable for anyone that wants it, and the info is out there.
But back to Methanol will ONLY happen when cars change over to Methanol (meaning: when the infrastructure for buying fuel becomes availlable for methanol). Because that's the whole point of the excercise. Fuel that I can get at any and every streetcorner. I refuse to travel 100 miles roundtrip to buy fuel that will rust my engines and I don't like to have to keep a large stock of fuel because the purchase is a chore.

Last edited by 1967brutus; 07-23-2023 at 01:07 PM.
Old 07-23-2023, 01:11 PM
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Originally Posted by mitchilito
Yeah, it's a Grizzley, Jesse. It's a D1-3 spindle and I do have a full collet set (up to 1 inch) for it. Wasn't really all that helpful for this little project though because of the way I had to approach the stems.

Yessir,

I was thinking more along the lines of a slit bush held in a collet large enough to pass the valve's head. Not a big deal I like to avoid chuck jaws when working close
Very healthy looking valve otherwise.
Old 07-24-2023, 06:13 AM
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Originally Posted by Jesse Open
Yessir,

I was thinking more along the lines of a slit bush held in a collet large enough to pass the valve's head. Not a big deal I like to avoid chuck jaws when working close
Very healthy looking valve otherwise.
You are right, Jesse - hand working around a spinning chuck is a real hazard. But I still do it anyway!

And I could've easily sized my little slit disc to fit in a collet! Next time for sure. . .

Last edited by mitchilito; 07-24-2023 at 06:19 AM.
Old 07-24-2023, 06:18 AM
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Originally Posted by 1967brutus
That is why I use no Walbro... their atomisation is horrible, and basically leans on the crankcase (they are predominantly used in 2-strokes) doing the atomisation and evaporizing for them.

I use the bogstandard glowcarb combined with the Stihl solenoid, and I ain't got no sticking valve trouble. Maintenance is limited to regapping spark plug after 25 hr, replace when performance deteriorates, new engines need a valve clearance check every 5 hrs, but so far every one of them basically stabilized in that aspect after about 30 hrs and now I basically do not bother anymore on "older" engines.
Works for me, and so far, now about 4 years in (first tests ran in April 2019, currently 13 engines running, amongh which a Mk I OS Wankel) I yet have to have the first one fail, solenoid, electronics OR engine....
Went from primitive "fuel curve in the transmitter" to "atmospheric compensated" and every step brought some improvement, to the point where the only improvements are "smaller and compacter"I see no real possibilities for further improvement WRT performance. System has matured.
Wished it was marketable, but it isn't. But it IS doable for anyone that wants it, and the info is out there.
But back to Methanol will ONLY happen when cars change over to Methanol (meaning: when the infrastructure for buying fuel becomes availlable for methanol). Because that's the whole point of the excercise. Fuel that I can get at any and every streetcorner. I refuse to travel 100 miles roundtrip to buy fuel that will rust my engines and I don't like to have to keep a large stock of fuel because the purchase is a chore.
Brutus, it's pretty easy for me to get methanol - mail order. I think I'll just live with the carbon for now. I hate to think about changing out the carb gaskets and all the fuel tubing (to silicon) when the engine itself is running GREAT. It's super simple, has plenty of power, great handling etc.

And now I should get pretty good service interval before I have to clean the valves/stems.. .. . ..
Old 07-30-2023, 03:25 AM
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This old thread just keeps going and going! I was talking to a knowledgeable friend the other day and he told me he runs nothing but ALKYLATE GASOLINE in his engines. He says it produces much less carbon etc. I immediately bought a gallon of it at a yard machine supply business here. It looks to me like it's only made by a company called ASPEN - and it's NOT cheap (1.3 gallon=$40). But in the little amounts we're using I'm okay with that.

I mixed my Redline Racing oil in it and filled the tank up yesterday to see how it runs. I could tell no difference in power/handling but the crankcase discharge oil definitely showed less carbon and was much lighter in color. I think with my lapped exhaust valves and this new fuel my sticky exhaust valve might be a thing of the past. YAAAAYY.

On a side note, if you follow the RC Groups engine classifieds like I do you may've seen a Gemini 160 for sale lately. Well I bought it. I wanted a back up in case a connecting rod lets go (a known problem with these engines) or any other irreplaceable part failes on my little beauty. I just can't be without one of these fantastic engines.







Old 07-30-2023, 03:36 PM
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Many of the tree trimmers around here have been moving away from gasoline and using alkylate fuels. The fumes from both the exhaust and fuel vapors are for more tolerable.

By the same token, the electric saws are really becoming popular. Smooth, clean and quiet. They can often start work earlier without noise complaints and far less overall fatique at the end of the day. A lot less maintenace as well.
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Old 11-20-2023, 03:09 AM
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I was out flying the Rascal/Gemini yesterday and it moved me to come back to this old thread and add an update.

I have a Moki 300 radial and the exhaust valves are famously tight on that engine leading to stuck exhaust valves which leads to lost pushrods and crappy running. The fix is to purchase a 4.02mm reamer and ream them to the proper size, which I did. No more problems!

Well my gasoline conversion of this little Gemini revealed that these exhaust valves tend to gum up (with carbon) also which, like the Moki, leads to stuck exhaust valves. These valves don't gum up while running glow fuel because they (a) run cooler and (b) methanol doesn't cause nearly as much carbon.

So I decided to fix it. Instead of buying another hideously expensive reamer (they're like $80), I made a holding fixture to hold the valves, stem forward, on my lathe and lapped the valve stems down. I took about .0015 off of each one. It doesn't take much nor do you want to remove much either!

Well, I've been flying the crap out of it ever since (probably 10 or 15 flights) and between the Aspen fuel and the skinnier stems it no longer gums up. And I can't believe how great it runs. I used to gum up a valve about every 3 flights - so what a great improvement.

I just have the most fun flying this particular airplane. It flies fantastic, sounds so cool and those two cylinders sticking out the front just get me every time.



At Joe Nall. I had to de-gum the valves twice there!

Last edited by mitchilito; 11-20-2023 at 03:13 AM.
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Old 11-20-2023, 07:13 AM
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I have discovered that like 95% of issues that people bring up with gasoline can mostly be solved by running non-ethanol gas and using a high quality oil with preservative.

That sure is a beautiful plane though.
Old 11-20-2023, 09:47 AM
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Originally Posted by RadialWacko
I have discovered that like 95% of issues that people bring up with gasoline can mostly be solved by running non-ethanol gas and using a high quality oil with preservative.

That sure is a beautiful plane though.
It is not in the Non-Ethanol gas, it is in the oil. Some oils do this, some don't, although I am not aware of any issues as used in the Aspen fuel.

I run E10 nowadays, and where in my bike I use an additive to combat the effects of the Ethanol content (mainly a slight lean stutter and bumble, the bike is still carburated), I do NOT use that additive in my converted glow engines.
I have never had a gummed up valve in any of my now 15 or more converted engines from the get-go (now more than 7 years in). I use and have always used Castrol PowerRS 2T for my conversions. Most of my engines have runhour counters, and of those the one that ran most has some 65 runhours (about 400 flights, give or take).
I have engines with more hours, but they do not have counters, so I cannot make statements about those.
Old 11-20-2023, 09:58 AM
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Yeah, I'm sure the mileage varies for everyone. It's just something I noticed after I was having issues my first season of starting with gas engines. After my engines sat over winter I had troubles. Switched to non-ethanol that summer and all of the issues I ever had went away.

It's just something I experienced personally and wanted to share. But in my opinion, if you have access to non-ethanol gas, there is really no reason not to use it.
Old 11-21-2023, 04:52 AM
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Originally Posted by RadialWacko
But in my opinion, if you have access to non-ethanol gas, there is really no reason not to use it.
that opinion is absolutely valid, especially for the low volume as used for flying model airplanes.

The stinger is in the "IF you have..."... in a 50 mile radius there are maybe 3 or 4 pumps that sell E-free, and for me the whole idea of going gasoline was to have fuel availlable on virtually every streetcorner.

So far, for me it has caused zero issues whatsoever. But I have to say, I use electronically supported, pressure, altitude and temperature compensated carburation, which allows a very accurate mixture and a very lean and clean burn.

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