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OS Gemini Twin Rear Walbro Gas Conversion

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OS Gemini Twin Rear Walbro Gas Conversion

Old 03-12-2022, 02:33 PM
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mitchilito
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Thanks for the kind words guys.

Speaking of more projects, it just happens that I ran into another old OS 4-stroker in a drawer today that I honestly forgot I even had. And in the same box with it is a Walbro carb that came off a 4-cycle weed wacker. Could be a marriage made in heaven!

Stay tuned. . . .




Last edited by mitchilito; 03-12-2022 at 02:40 PM.
Old 03-12-2022, 05:25 PM
  #27  
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Originally Posted by mitchilito View Post
Thanks for the kind words guys.

Speaking of more projects, it just happens that I ran into another old OS 4-stroker in a drawer today that I honestly forgot I even had. And in the same box with it is a Walbro carb that came off a 4-cycle weed wacker. Could be a marriage made in heaven!

Stay tuned. . . .



Nice "find"! Do you have a number from that Walbro carb?

Last edited by Jesse Open; 03-13-2022 at 05:24 AM.
Old 03-14-2022, 08:58 AM
  #28  
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The carb that's came off the weed whacker is a WT456. Smallest venturi I've ever seen on a Walbro. The butterfly orifice is .450 and the venturi in the middle of the carb is TINY. Probably in the neighborhood of .180 to .200!
Old 03-14-2022, 05:37 PM
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Here ya go:
Old 03-15-2022, 04:12 AM
  #30  
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Looks like I'm headed in the right direction!

Thanks Jesse.
Old 03-25-2022, 02:44 AM
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Default Some corrections

I was just looking through this thread and realized there are a couple corrections I’d like to make. Ability to edit these posts only lasts about 5 minutes so I have to do it waaaaay down here at the end where virtually no one will see it!

Firstly, I think the factory recommended nitro content was 10% not 15%. Not that it matters now!

Secondly, I had a little trouble bending up some stainless pipe after I wrote this thread and came to find out I wasn’t heating it up enough. Stainless needs to be anealed at over 1000 degrees. This is way brighter than the dull red I mentioned. Bright yellow is what you want, then it will bend like butter. No need to quench - this pipe air quenches.
Old 03-27-2022, 10:53 AM
  #32  
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Thanks for the updates, still looks good.
Yes, takes some good heat as well as a slowed cooling to aneal stainless.

Regarding the nitromethane content :
That applies primarily for glow ignition, actually some are using as low as 3% with no real problems, especially with larger Saitos.

When using CDI with methanol, you can just leave it out

Will be watching for the conversion of your 180 Please keep us posted.
Old 03-27-2022, 03:55 PM
  #33  
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When I first installed the C&H ignition I did indeed run straight methanol - no nitro. We used to call it "FAI fuel."

As for annealing stainless, it's my understanding that after bright yellow heat a quench is actually better. Something about preventing carbon precipitation blah blah blah. With this thin wall tubing it air quenches and bends like butter.

Hey, speaking of my next gasoline conversion, I had the good fortune to find a Gemini 300 at a great price and will be receiving it next week. THAT will be my next conversion and I will chronicle it carefully here. I can't wait!
Old 03-28-2022, 08:44 AM
  #34  
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Your work is too good. You make me look like the hack I am. I H*te you.
Can not wait to see your next project!
Big thumbs up
Old 03-29-2022, 11:02 AM
  #35  
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Ha ha ha! You're cracking me up Meyers. I'll bet you're not THAT big of a hack!!

Thanks for the kind words-
Mitch
Old 03-29-2022, 11:34 AM
  #36  
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UPDATE!!

I thought I was done with the Gemini 160 - but NO! As I mentioned I have a Gemini 300 coming (I can't wait) and I'm getting geared up to do a gas conversion on it. First thing I want is a suitable Walbro carb for it. Well, it turns out that the perfect carb for the 300 (I believe) is on the 160. As I discussed a few posts above, I have a Walbro with a truly TINY venturi - the WT456 - that I was going to use on a single cylinder OS120. However it should really be perfect for the Gemini 160.

So I ordered a rebuild kit for it, rebuilt it and fit it to the 160. It took a little more work than the existing Walbro because the choke and throttle levers were all wrong for model use. No problem - I machined a set of aluminum arms for it and slapped that baby on there!

It was a really old carb, originally designed for a small 4 cycle weed hacker and I had my doubts that I'd be able to re-purpose it - even with the rebuild. But long story short, I did get it running and running pretty well. I'd say it's an improvement over the first one - but only a little. It's pretty hard to slap a stock carb designed for a different engine and expect it to run exactly right on another, very different engine. Interestingly enough, once I got the needles set a well as I could, the idle was not low enough - even with the butterfly closed all the way. SO: the butterfly disk has a little recess cut out of it to supply air when fully closed (see pic). I pulled the disk out and soldered over the recess to close it completey. The engine would now idle very low (see vid) but still not low enough to quite kill the engine! I don't know where the air is coming from to keep it running but run it does - barely!

One last note before we look at the visual aids: I ran it for a good half hour today getting the needles right etc. I started with about 8 ounces of fuel and when I was done I still had a good 5 ounces! The amount of fuel this 26cc engine uses (or doesn't use!) is astounding.



Look at the top of the butterfly. That little dent is all the air getting in past the disk. And even once I soldered it closed the engine would still run! Don't really understand that. . . .



If you compare it to the first run video I posted you can see it definitely transitions from idle better than the bigger carb did. I'm so happy! I also do a pump test in the vid for those that might wonder how well the pump is working. I assure you the stock set up as received from the factory (nitro) would have PROMPTLY died if I had pitched the run stand up like that at low throttle. So cool. . . .

Last edited by mitchilito; 03-30-2022 at 06:33 AM.
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Old 03-29-2022, 02:47 PM
  #37  
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That is flat out nice. Great transition, super smooth throttling. Did you happen to measure the venturi size. That carb number as you probably know has been discontinued as far as Zi can tell.

Once again, keep em coming!
Old 03-30-2022, 03:06 AM
  #38  
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Throat diameter is somewhere around.180 to .200. Truly tiny. Another interesting feature of this carb intended for 4-cycle use is that it has a large weak spring pressing down on the reservoir membrane in the pump section. Not really sure what that’s about.

I suspect a similar carb could be bought new. They are so inexpensive if I had to I would probably buy a WT45. . . Fill in the blank.

I would try to research what piece of lawn equipment it came from. We’re looking for the 4 cycle units.
Old 03-30-2022, 03:52 AM
  #39  
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Here’s an amazing resource I found on a different thread there on RCU:

RE: DLE 30 carb for 4 st ? an replacement for walbro wt-456-1
Walbro model numbers make no sense. I have no idea what the difference might be. Here is a list of walbro carbs that have an accelerator pump. The pump is a very good thing:

Walbro catalog and here is the list of Wt carbs with accelorator pumps, and hi and low needles and no primers.

6.5 mm with fuel pump return spring (4 stroke)
WT 366-1
WT 456-1

7.95mm
WT 37-1
WT 424-1

9.53mm

WT 35-1
WT 38-1
WT 204-1
WT 357-1
WT 367-1
WT 381-1
WT 385-1
WT 417-1
WT 449-1
WT 450-1

11.11 mm
WT 29-1
WT 356-1
WT 358-1
WT 464-1
WT 516-1
WT 517-1
WT 527-1
WT 613-1

12.7 mm
Wt 513-1
Wt 537-1

13.49mm
Wt 171-1
Wt 172-1
Wt 228-1
Wt 285-1
WT 529-1
Wt 451-1
Wt 480-1
Wt 540-1
Wt 542-1
Wt 555-1
Wt 610-1

If your 30cc liked the carb you have it will be about right for the 1.8 (29cc). If the carb is to big the adjustments get to sensitive, to small and you don't get maximum power.

Here is a link to walbro, click on the series you want and look around.

http://wem.walbro.com/walbro/family2.asp

Look around EBAY for a carb that is 10mm or under and is New Old Stock. They are cheap. You may find your WT 456-1 there. Shipping may be an issue for you though.

Remember you need to run a lot of oil in your mix on a glow conversion, 15 or 20%? Maybe someone can comment on this?
Old 03-30-2022, 04:11 AM
  #40  
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Also: here’s a Hobbyking carb they claim is for 26cc

https://hobbyking.com/en_us/turnigy-...EaAuZAEALw_wcB
Old 05-18-2022, 05:04 PM
  #41  
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May I ask, which are your preferred plugs?
Old 05-21-2022, 12:12 PM
  #42  
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Originally Posted by mitchilito View Post
Here’s an amazing resource I found on a different thread there on RCU:

RE: DLE 30 carb for 4 st ? an replacement for walbro wt-456-1
Walbro model numbers make no sense. I have no idea what the difference might be. Here is a list of walbro carbs that have an accelerator pump. The pump is a very good thing:

Walbro catalog and here is the list of Wt carbs with accelorator pumps, and hi and low needles and no primers.

6.5 mm with fuel pump return spring (4 stroke)
WT 366-1
WT 456-1

7.95mm
WT 37-1
WT 424-1

9.53mm

WT 35-1
WT 38-1
WT 204-1
WT 357-1
WT 367-1
WT 381-1
WT 385-1
WT 417-1
WT 449-1
WT 450-1

11.11 mm
WT 29-1
WT 356-1
WT 358-1
WT 464-1
WT 516-1
WT 517-1
WT 527-1
WT 613-1

12.7 mm
Wt 513-1
Wt 537-1

13.49mm
Wt 171-1
Wt 172-1
Wt 228-1
Wt 285-1
WT 529-1
Wt 451-1
Wt 480-1
Wt 540-1
Wt 542-1
Wt 555-1
Wt 610-1

If your 30cc liked the carb you have it will be about right for the 1.8 (29cc). If the carb is to big the adjustments get to sensitive, to small and you don't get maximum power.

Here is a link to walbro, click on the series you want and look around.

http://wem.walbro.com/walbro/family2.asp

Look around EBAY for a carb that is 10mm or under and is New Old Stock. They are cheap. You may find your WT 456-1 there. Shipping may be an issue for you though.

Remember you need to run a lot of oil in your mix on a glow conversion, 15 or 20%? Maybe someone can comment on this?
if you want to go deeper into modifying your OS Twin. you may want to mount some Carbs with Accelerator Pumps in them. click my link. HP make 2 sizes of Carbs with accelerator pumps. 1 size is for a .61 size engine and the other is for a 120. yes they are 2st carbs, but they should work good on a 4st with long Itake Manifolds, as most all 4st car Carbs have accelerator pumps.
this is something I would like to try on my Saito 100 Twin.

Model Engine Company Of America - America's largest producer of Model Engines for airplanes boats and cars (mecoa.com)

Jim
Old 06-02-2022, 01:14 AM
  #43  
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I couldn't find said carbs on this link?
Old 06-07-2022, 02:12 PM
  #44  
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Well gang, here she is - right where she belongs: in a sweet old-timey model airplane. This is the second Sig Rascal 110 she has been in. I lost the first one due to radio failure and it nearly broke my heart. Thank goodness the Gemini was unharmed. I've always loved this engine/airplane combo and now it has all the cool engine mods. So fantastic.







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Old 06-15-2022, 12:47 AM
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Here is the maiden flight. You'll have to put up with my new iPhone 13 going in and out of focus (I would've though by now they'd have that fixed) and my fooling around with the roll/pitch trim and engine response (it was a maiden after all )

Here is a noteworthy thing: for this maiden it had the small Venturi carburetor on it which ran great on the test stand. However, during the initial flights it refused to behave. During the flight you see here I was actually running it with the choke half way on! That was the only way to get it to behave. SO: I went back to the RCGF 15cc Walbro carb and in subsequent flights it now behaves MARVELOUSLY. It's a true gem. Lots of power, simple to operate, amazingly clean and perfectly well behaved. What more could you ask for?


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Old 06-15-2022, 04:40 AM
  #46  
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Originally Posted by mitchilito View Post
Here is a noteworthy thing: for this maiden it had the small Venturi carburetor on it which ran great on the test stand. However, during the initial flights it refused to behave. During the flight you see here I was actually running it with the choke half way on! That was the only way to get it to behave. SO: I went back to the RCGF 15cc Walbro carb
That is something I have observed many times in glow-to-gas conversions, that a carb that has basically flawless response on the ground, becomes a pig in the air. I cannot give a real explanation, I can only confirm that it happens a lot.
Something similar happens when you run a Zehoah G230 with the carburettor as used in buggies and planes, in a heavy scale helicopter. Light trainer helicopters no problems, heavy scale ships and suddenly you're in trouble (flame-outs).
No idea why, it just happens.
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Old 06-16-2022, 09:49 AM
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Originally Posted by mitchilito View Post
Here is the maiden flight. You'll have to put up with my new iPhone 13 going in and out of focus (I would've though by now they'd have that fixed) and my fooling around with the roll/pitch trim and engine response (it was a maiden after all )

Here is a noteworthy thing: for this maiden it had the small Venturi carburetor on it which ran great on the test stand. However, during the initial flights it refused to behave. During the flight you see here I was actually running it with the choke half way on! That was the only way to get it to behave. SO: I went back to the RCGF 15cc Walbro carb and in subsequent flights it now behaves MARVELOUSLY. It's a true gem. Lots of power, simple to operate, amazingly clean and perfectly well behaved. What more could you ask for?

https://youtu.be/I3FnWAaJgY8

Marvelous! Great runner, you are right; not much left to ask for. The Walbro is performing flawlessly. A very elegant conversion to say the least.
I can appreciate well the work you have done. A very tidy, thoughtful installation.


BTW,
Sounds awesome!

Last edited by Jesse Open; 06-16-2022 at 09:52 AM.
Old 06-17-2022, 12:43 AM
  #48  
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Originally Posted by 1967brutus View Post
That is something I have observed many times in glow-to-gas conversions, that a carb that has basically flawless response on the ground, becomes a pig in the air. I cannot give a real explanation, I can only confirm that it happens a lot.
Something similar happens when you run a Zehoah G230 with the carburettor as used in buggies and planes, in a heavy scale helicopter. Light trainer helicopters no problems, heavy scale ships and suddenly you're in trouble (flame-outs).
No idea why, it just happens.
Yep, this is a common complaint. We always chocked it up to varying static pressure in the cowl caused by airspeed. I'm not convinced that's the reason in this case though. I think this old carb has other issues.

I'm so impressed with the Walbro off of the RCGF 15cc that I immediately bought another for the upcoming Gemini 300 conversion I'm saving for a rainy day!



Last edited by mitchilito; 06-17-2022 at 12:58 AM.
Old 06-17-2022, 12:50 AM
  #49  
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Originally Posted by Jesse Open View Post
Marvelous! Great runner, you are right; not much left to ask for. The Walbro is performing flawlessly. A very elegant conversion to say the least.
I can appreciate well the work you have done. A very tidy, thoughtful installation.


BTW,
Sounds awesome!
Thanks Jesse!
Old 06-17-2022, 07:45 AM
  #50  
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I'm sorry, but, what is the venturi size that is working best? Is it a vacuum pump with a return spring or the positive negative pulse type?

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