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OS Gemini Twin Rear Walbro Gas Conversion

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OS Gemini Twin Rear Walbro Gas Conversion

Old 08-03-2022, 03:09 AM
  #151  
1967brutus
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Ah, yes, I forgot the imbalance.... If I have time, I'll post a pic this evening of what I did to cure that. No idea if it is usable for you, but it's just a thought and fairly easy to execute, also easy to remove if it does not work or adapt to finetune if it does.
Old 08-03-2022, 07:01 AM
  #152  
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Originally Posted by mitchilito View Post
I'm re-breathing the (significant amount of) crankcase discharge oil and
Is the crankcase discharge black in colour? I mean, maybe not right now while running a rebreather... but if you were to disconnect the rebreather line, and let the engine run for a few hours to flush the old dirt out of the crankcase, is the oil discharge black and opaque or brown and translucent?

Because if it is black and opaque, it means it is polluted, and then you recirculate it to pollute it some more.
Old 08-05-2022, 02:20 AM
  #153  
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Originally Posted by 1967brutus View Post
Ah, yes, I forgot the imbalance.... If I have time, I'll post a pic this evening of what I did to cure that. No idea if it is usable for you, but it's just a thought and fairly easy to execute, also easy to remove if it does not work or adapt to finetune if it does.
Yes, I'd like to see that picture. . .
Old 08-05-2022, 10:17 AM
  #154  
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Originally Posted by mitchilito View Post
Yes, I'd like to see that picture. . .
As you perhaps know, the ASP has a fairly standard glow carb with lateral moving throttle barrel, as opposed to the OS Gemini, which has a carb where the throttle barrel stays put when opened or closed.
Most likely the OS designers knew what ASP did not or was too cheap to do something about: At low throttle, the opening is slightly offset to the right (when viewing from behind the prop) and that is where I suspect the asymetry in fuel distribution is caused.

I placed a slightly offset and very tiny "scoop" made from an oblique cut piece of 1/8" alu tubing, it protrudes also about 1/8th, or maybe a bit less, into the central intake directly behind the carb, I just drilled a hole, epoxied in the piece of tubing and plugged the tubing also with a blob of Epoxy,
It forms an obstruction that directs any liquid coming from the carb more towards the center of the induction passage.

I overshot a bit, because after this, the left cylinder was favoured a bit more, and now is the "fattest" cylinder, although the differens is much smaller than what it originally was.
At anything above 3500~4000 RPM both cylinders run more or less equally rich and fat, only at low settings there is still this difference. Not perfect, but much better than it was, and if necessary, it can be tweaked.

I have no idea if you can use this exact same idea, but maybe it resonates somehow with your own thought-experiments.

Mind you: the scoop is placed in the T-connector where the intake branches off to each cylinder, this is NOT the carb.




Last edited by 1967brutus; 08-05-2022 at 10:31 AM.
Old 08-05-2022, 10:39 AM
  #155  
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In the radial (also an ASP) I solved the issue with the unequal distribution in a slightly different way, in that there the carb is attached to the distributor via a single 90 degree knuckle,
Here the issue is that in the 90 degree corner, the air can follow the path of the knuckle, but the liquids carried with that air fly on straight through and collide with the opposite side of the passage, then flow along the wall and do not enter the distributor "centered".

So here I drilled three tiny holes that are so small that they do not noticably affect throttleability or fuel draw, but what they do is form 3 tiny airjets that re-atomize the fuel back into the airflow.
More or less the same working principle, but executed differently.





FWIW: I do not think these methods are interchangable, I don't think the scoop would have worked with the radial, and I also don't think the airjets would have worked for the twin, but perhaps in your set-up, which is noticably different than mine, the airjets might find a place. Not in anyway meant as "this is how you do it", much more as "here are some ideas outside the box"...

Old 08-06-2022, 01:16 AM
  #156  
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This is interesting - thanks for posting. I've always suspected the imbalance in my Gemini comes from different length/shaped intake tubes - but who knows!

So you are introducing fresh air into the intake tract after the carburetor? You must've had to richen your mixture quite a bit.

I had thought of putting an adjustable air bleed on the intake pipe of the rich cylinder but that really would be a last resort. I'm going out to fly the Rascal this morning with the iridiums. If the left one doesn't foul all of this will be moot (fingers crossed).
Old 08-06-2022, 08:53 AM
  #157  
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Well I did fly a bunch this morning with the iridium plugs and it did oil foul on me. Wouldn’t start and run up the second flight. So I cleared the plugs (it was mostly the right one this time). I also disconnected the crankcase rebreather. I got three more flights after that without a problem.

Bye-bye rebreather!

Old 08-06-2022, 09:17 AM
  #158  
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I would imagine with crankcase oil drops happening at regular intervals rather than continuously, introducing it into the intake drop by drop would cause the engine to go oil rich every time a drop is introduced. Seems like that would be difficult to tune the mixture for a happy engine all the time.
Old 08-06-2022, 09:52 AM
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Originally Posted by mitchilito View Post
This is interesting - thanks for posting. I've always suspected the imbalance in my Gemini comes from different length/shaped intake tubes - but who knows!

So you are introducing fresh air into the intake tract after the carburetor? You must've had to richen your mixture quite a bit.

I had thought of putting an adjustable air bleed on the intake pipe of the rich cylinder but that really would be a last resort. I'm going out to fly the Rascal this morning with the iridiums. If the left one doesn't foul all of this will be moot (fingers crossed).
On the bold: ONLY on the radial. I did some math and the airjets have less than 25% of the cross section of the airslit of the carb's idle opening, so it did not affect fuel draw too much, and since on that engine the idle adjustment was rather sensitive, it made things easier rather than more difficult. But mostly, since it made distribution per cylinder a lot better which made the engine respond much more consistently to the changes in needle settings, therefore made tuning a lot easier.

Before:

After:

The differences are not so much audible (but if you play the vids side by side, there is a few hundred RPM more in the 2nd vid and the throttle response is slightly better), but VERY clearly visible on the temeletry (I have 5 separate cyl temperature readouts, an average temperature, and for each cylinder the deviation from average, and with that info and a trendlog, differences are extremely visible ).
I believe the gain in RPM results from 5 cylinders all runing the same (optimal) mixture, instead of 2 cylinders running too rich and 3 cylinders slighly on the lean side but each one on a different mixture.

The Boxer, that is NOT an airjet. No hole, it is plugged off, it is just the piece of tubing sticking out and forming an obstruction, creating a bit of turbulence and deflecting the fuel/oil droplets a bit towards the middle of the T-branch in order to correct the imbalance.
On the underlined: I would not do that, because the airbleed is static while the throttle is moving, making the imbalance shift as a function of throttle movement. Personally, I strive to find a solution that is such that it remains the same regardless of throttle position. With that deflector I did not 100% succeed in that, but it is way better than the standard set-up.

Last edited by 1967brutus; 08-07-2022 at 12:42 AM.
Old 08-06-2022, 10:01 AM
  #160  
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Originally Posted by mitchilito View Post
Well I did fly a bunch this morning with the iridium plugs and it did oil foul on me. Wouldn’t start and run up the second flight. So I cleared the plugs (it was mostly the right one this time). I also disconnected the crankcase rebreather. I got three more flights after that without a problem.

Bye-bye rebreather!
Dang... I hate to say it, but... (sorry, just pulling your leg a little, Mitch)

Just happy to see you solved this, and I am fairly sure you will have good results with the standard plugs now as well, so what's not to like?

Now, I am sure given your metal working skills, you will out-do my hideously ugly valve lube system with some beautifully crafted piping arrangements (please do so I can in return copy that design...)

Last edited by 1967brutus; 08-06-2022 at 10:23 AM.
Old 08-06-2022, 10:04 AM
  #161  
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Originally Posted by Raleighcopter View Post
I would imagine with crankcase oil drops happening at regular intervals rather than continuously, introducing it into the intake drop by drop would cause the engine to go oil rich every time a drop is introduced. Seems like that would be difficult to tune the mixture for a happy engine all the time.
That was basically my experience with the factory standard rebreather on the ASP 91 single, where the crankcase breather is connected to the intake header after the carb.

However, AA5BY on RCGroups introduces his "rebreather" in the velocity stack upstream of the carb, and that appears to work fairly well, at least, he does not report any issues with it. Maybe it also has to do with HOW and WHERE the rebreathing is introduced in the intake. Would not surprise me.
Old 08-06-2022, 11:03 AM
  #162  
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Originally Posted by mitchilito View Post
Well I did fly a bunch this morning with the iridium plugs and it did oil foul on me. Wouldn’t start and run up the second flight. So I cleared the plugs (it was mostly the right one this time). I also disconnected the crankcase rebreather. I got three more flights after that without a problem.

Bye-bye rebreather!
What I would find interesting, is if you would have a peak RPM reading with, and without the rebreather. I would expect the RPM to rise by about 100 to 200 RPM, since the rebreather introduces not only oil (which should not make a difference wrt RPM), but also exhaust gas (piston ring blow by) which should displace some of the inducted air. More fresh air=more bang per revolution=more power.

Just curious what the possible difference might be.
Old 08-07-2022, 02:34 AM
  #163  
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Originally Posted by 1967brutus View Post
What I would find interesting, is if you would have a peak RPM reading with, and without the rebreather. I would expect the RPM to rise by about 100 to 200 RPM, since the rebreather introduces not only oil (which should not make a difference wrt RPM), but also exhaust gas (piston ring blow by) which should displace some of the inducted air. More fresh air=more bang per revolution=more power.

Just curious what the possible difference might be.
My new RC Excel ignition does have a tach lead on it so I would be able to check this. Unfortunately it's under the cowl at the moment.

That ASP radial sure sounds good. . . ..
Old 08-07-2022, 03:22 AM
  #164  
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Originally Posted by mitchilito View Post
My new RC Excel ignition does have a tach lead on it so I would be able to check this. Unfortunately it's under the cowl at the moment.
Oh, it was just a thought, no request... No need to pull out that lead if you don't want to.
Originally Posted by mitchilito View Post
That ASP radial sure sounds good. . . ..
In all fairness, that mainly is the ring-exhaust. Without it I don't like its exhaust note one bit. No bark but a shrill yap like a nest of startled puppies... It's a bit clunky too...

But with the ring collector and double endpipe, it really sounds good. Fairly different from a Keleo ring (which have the reputation of sounding good, but this sounds different, as if it is a larger engine than it actually is..

You should hear it in flight. Goosebumps...

Oh, what the heck....


Last edited by 1967brutus; 08-07-2022 at 03:27 AM.
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Old Yesterday, 02:32 AM
  #165  
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Wow, that really does sound good.

Nice landing!
Old Yesterday, 03:40 AM
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Originally Posted by mitchilito View Post
Wow, that really does sound good.

Nice landing!
Thanks... that landing gear was the cheapest piece of servo-operated mechnical crap I have ever seen on a plane, but I managed to do at least 15 or 20 landings with it. Last year I replaced it with a set of decent Electrons but haven't flown it yet due to other priorities.
I hope to be able to take that plane out a few times in the upcoming shore leave.
The cowl is in place in the meantime, and I have flown it with that once. Lot of work getting the baffling under the cowl right, but it now cools better than with the engine fully exposed like in the vid. Plane is faster too, getting more RPM in level flight.

It is my prized posession, that plane and I am extremely careful with it...
Old Yesterday, 09:30 AM
  #167  
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Originally Posted by mitchilito View Post
Wow, that really does sound good.

Nice landing!
Ditto

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