OS Gemini twin rear carb gas conversion

[mitchilito]

No glow. The first thing I did is install the C&H spark ignition and I’ve been VERY happy with it.

 

[awake]

Thanks! I didn't think one could run gasoline with glow plugs (can one - ??), but as I mentioned I wasn't sure if I was seeing a spark plug in each cylinder.

 

[SPOTTER]

What percentage of oil do you use?

 

[SPOTTER]

Ok, it's not clear from the photo

 

[mitchilito]

Thanks! I didn't think one could run gasoline with glow plugs (can one - ??), but as I mentioned I wasn't sure if I was seeing a spark plug in each cylinder.

There is such a thing as glow ignition for gasoline but I’ve never seen it.

 

[mitchilito]

What percentage of oil do you use?

I would normally mix my gas at 20 to 1 for this engine. But because I recirculate crankcase discharge oil I mix 15:1.

 

[Vietti]

Ratio seems backward? Wouldn't you need less oil in the fuel if recycling some from the cc??

 

[petertha]

Interesting mods, thanks for sharing.

My (limited) understanding of the Walbro style pump is they rely on the positive & negative pressure pulses of the 2-stroke engine crankcases to act on the diaphragm. I assume you fed a crankcase line or maybe what was your drain to the pressure inlet port of diaphragm? But isn't a 4-stroke pressure profile quite different than 2-stroke (net positive & lower?). I'm not even sure what opposed twin would be. I mean it obviously works, I'm just curious how. Have you ever pulled the fuel line off to see if it squirts? (ie. fuel pressure strength).

I don't think any of the RC methanol multi-cylinder opposed or radials had exhaust pressure taps feeding fuel tank the way the singles did to addfuel pressure boost. My own opinion is that fuel starving in aerobatics is more related to low / lack of fuel pressure. As the model changes orientation so does the tank & thus reduced hydrostatic head. Usually if anything smaller venturi's draw fuel a bit better, but obviously not enough to overcome the flight regime. That's why the YS systems were so nice in both 2S & 4S.

Do you happen to know roughly how big the venturi orifice of the original OS methanol carb was & how that compares to the gasoline Walbro style?

I vaguely recall reading about the Iron Bay accessory. Not sure but I think maybe similar principle to the Cline regulator? Anyway I found an old link. It says compatible to both methanol & gasoline. Anyways I assume you did not run it on the original OS methanol carb at all? You plumbed it to the Walbro (with its pump/regulator unit removed?)
http://www.ironbaymodelcompany.com/ibmco recreational website_010.htmhttp://saito-engines.info/pumps.html

 

[mitchilito]

Ratio seems backward? Wouldn't you need less oil in the fuel if recycling some from the cc??

You are absolutely correct, Vetti! I'm confusing my percents and ratios - but even that isn't correct because 20:1 is actually 5%. Funny how the mind works - or doesn't!. Thank you all for making me stop to think about it! I Haven't actually mixed any reduced oil gas yet or I might've figured it out on my own

So to get really confusing: I would reduce the oil content in my 20:1 fuel mix 10% to account for the recirculated oil or about 4.5% oil to gas which adds up to 5.76 ounces of oil per gallon.

Thanks Vetti. I'm glad I figured this out before my mix my next gallon!

 

[mitchilito]

Interesting mods, thanks for sharing.

My (limited) understanding of the Walbro style pump is they rely on the positive & negative pressure pulses of the 2-stroke engine crankcases to act on the diaphragm. I assume you fed a crankcase line or maybe what was your drain to the pressure inlet port of diaphragm? But isn't a 4-stroke pressure profile quite different than 2-stroke (net positive & lower?). I'm not even sure what opposed twin would be. I mean it obviously works, I'm just curious how. Have you ever pulled the fuel line off to see if it squirts? (ie. fuel pressure strength).

I don't think any of the RC methanol multi-cylinder opposed or radials had exhaust pressure taps feeding fuel tank the way the singles did to addfuel pressure boost. My own opinion is that fuel starving in aerobatics is more related to low / lack of fuel pressure. As the model changes orientation so does the tank & thus reduced hydrostatic head. Usually if anything smaller venturi's draw fuel a bit better, but obviously not enough to overcome the flight regime. That's why the YS systems were so nice in both 2S & 4S.

Do you happen to know roughly how big the venturi orifice of the original OS methanol carb was & how that compares to the gasoline Walbro style?

I vaguely recall reading about the Iron Bay accessory. Not sure but I think maybe similar principle to the Cline regulator? Anyway I found an old link. It says compatible to both methanol & gasoline. Anyways I assume you did not run it on the original OS methanol carb at all? You plumbed it to the Walbro (with its pump/regulator unit removed?)
http://www.ironbaymodelcompany.com/ibmco recreational website_010.htmhttp://saito-engines.info/pumps.html

The Gemini twin is horizontally opposed with the rods on their own journals so the piston motion is perfectly opposed. Since both pistons achieve bottom dead center together a substantial pressure pulse is generated. Perfect for powering the Walbro pressure pump.

I just measured the venturi on the stock OS carb: .250 or 6.3mm. So small. The venturi on the walbro is substantially larger, say .375 (just an estimate) but obviously not too large to function well in my application.

I picked up the crankcase pressure pulse from a port on the back of the crankcase (that's the "U" shaped brass tube). If you look at my manifold you will see where the mounting face of the Walbro picks up the pulse. As for how strong this pulse is, I have no idea. This is what matters: when spun over, the engine pumps the fuel readily from the fuel tank and while running I pitched the entire test stand to better than 45 degrees to the sky and it had no effect on the mixture setting. That's all I need. Now some of the more observant folks looking closely at my setup will note that the crankcase is actually open to atmosphere because I have the oil drain open to the carb inlet. Long story short, after experimentation I found out that the pressure pulse generated by the Gemini is strong enough to power the Walbro pump even with the drain open to atmosphere.

As for my original mods while still running alcohol it went like this:
I converted to spark ignition (using spark plugs. . . ) and then I added a walbro pump section (machined off of a walbro carb) and an Iron Bay regulator to the stock carb. I ran this on my own mix of methanol glow fuel/5% oil with no nitromethane (aka FAI fuel). It ran great and pumped fuel well.

I hope I answered your questions Mr. Petertha.

 

[Makin chips]

Looks like the spark plugs and ignition box to me.

 

[petertha]

I contacted Adrian at C&H Ignitions and ordered his wonderful ignition conversion for this engine.. This is running on straight methanol, no nitro, and 20% Redline two stroke racing oil.

@mitchilito

Can you tell me more about the C&H conversion:
- aside from mounting the timing pickup magnet, was it pretty much plug & play?
- assume those are 1/4-32 spark plugs
- what are the specs of your battery & about how long will it run between charges?
- when you ran straight methanol on spark how would you compare the power level to prior nitromethane/glow?
- what is your ignition timing setting & did you have to fiddle with it much?
- did you consider gasoline or Avgas? I've heard that head & exhaust temps typically rise but not sure if a problematic level. Any comments on that aspect?

 

[mitchilito]

-The C&H conversion was easy-peasy. One could also use the Moris Mini Motors conversion. Straight drop in.
-Yes, spark plugs are 1/4-32
-This ignition I got from C&H is marked "6volts" but they vary. Expect somewhere around 300mah draw.
-Sadly, I didn't do a thorough analysis of the specs before and after conversion to gasoline, however I think you can expect a slight drop in performance using gasoline. Still plenty of power.
-Timing: 30 degrees BTDC is typical. I purchased C&H's "timing kit" which is a handy setup for this process. I'll be using it plenty in the near future. (See pictures)
-A glow engine will run hotter on gasoline. One big reason is that you pump more than twice as much fuel through the engine when using methanol (6:1 vs 14:1). All that alcohol being vaporized as it flows into the cylinder head is an extremely effective refrigerant. With that said, the materials and processes designed into our little glow engines are up to the task of running on gasoline AS LONG AS: you recognize the fact that they will require measures to mitigate the extra heat. I run a slightly smaller propeller after gas conversion to reduce the load slightly and would be sure to add cooling baffles on installation where I probably wouldn't using glow. I can't really speak to long term durability on gasoline - I simply haven't run these enough, but I believe they will work well for a long time. One has to decide if they are willing to give up that last little bit of power for the benefits of running petrol. For me, when running these Gemini twins the answer is a resounding YES for all the reasons I stated in the beginning.

I've added a picture of the C&H instructions that came with the kit I bought from them. In the kit you see a compass wheel, the electronic sensor and two TDC rods for different types of engines. Very
handy!

 

[petertha]

Very useful info, thanks so much. So when you run gasoline on a conversion, is it premium gasoline off the roadside pump, or any additives to boost?
I'm never clear what people are trying to achieve here - is it about making up some lost power vs methanol or more about running characteristics? Why do people run AvGas? It looks elevated temperature is the thing to watch out for in any event.

In a glow to ignition conversion, I assume the oil % content might be about the same whether gasoline or straight methanol (or maybe just buying FAI fuel at hobby shop). I can understand fuel availability & maybe internal corrosion difference, but from an exhaust residue perspective, is there any appreciable difference between gasoline & methanol fuel base? Or maybe the oils themselves are different, but I'm assuming none or minimal castor in the methanol fuel, just synthetic.

 

[mitchilito]

Fuel: In ALL my gasoline powered model engines, both 2 and 4 stroke, I use 96 (or is it 97?) octane ethanol-free pump gas. The alcohol has a detrimental effect on the rubber parts of the walbro carburetor. As for the octane content, my understanding is this is directly tied to compression ratio. High compression engines will detonate aka "knock" if the octane is too low to slow down the flame speed. Adding octane to a lower compression engine (such as most of our model engines) serves no useful purpose. As for your avgas question, avgas does have higher octane (100 octane low lead) but as stated not useful in our engines.

Oil content: Now this is an interesting question in this application. As I stated earlier, gasoline is only flowing through this engine at the rate of 14:1 vs 6:1 for alcohol so along with less fuel, less oil is flowing through the engine as well when running gasoline. However, based on what others have experienced (Morris Mini Motors uses 20:1 oil mix) along with my own experience with the waste oil discharge from these model 4-stroke engines, I've found that the same basic oil mix is plenty adequate. As a matter of fact, oil discharge at a 20:1 mix while running on gasoline is a huge MESS. That's one reason I recirculate back to intake. Doing this has ALL the oil discharge going out the exhaust where it can (mostly) blow away.

Type of oil: I use nothing but Redline Racing 2-stroke oil. I also add 1 or 2 ounces of SEA FOAM fuel additive to the mix. Why Sea Foam? When I first started running gasoline in these model engines I started seeing sticky exhaust valves. One of the exhaust valves immediately stuck on this Gemini 160 conversion because I was running my two stroke mix (doesn't require sea foam). I dabbed a drop of Sea Foam and oil mix on the stuck valve and it popped loose right before my eyes. Sea Foam is amazing stuff and just instantly MELTS carbon build up.

 

[mitchilito]

I wanted to add the final conversion pictures for your viewing pleasure. After the conversion I thoroughly cleaned the engine and didn't like the way the blackening looked on the OS cylinders. These engines are famous for fragile blackening here. So, long story short, I used a process called "Cerakote" to apply an ceramic-like coating to them. It came out FANTASTICALLY. After running it a little more I took some Glam shots:

 

[petertha]

The Cerakote came out nice. After cleaning did you apply Cerakote to the cylinders in their stock form, or did you also prep with media blast? What did you use to heat cure after that?

 

[propclock]

Very Interesting . Are there any part numbers on the Walbro
or can they only be purchased from the engine producer
RCGF?

 

[mitchilito]

The carburetor is readily available. Google: Walbro WT962

 

[mitchilito]

The Cerakote is a very interesting process. It's not really "paint" per se, but more of a "wet powder coat." It sprays fantastically but requires a lot of very careful part preparation. Processing goes like this:
-Hot degrease the parts
-Bead blast
-Acetone soak - at least two hours
-Bake parts at 300 for at least an hour to de-gas
Only then are you ready to spray, and it sprays very trouble free. Or at least my cylinders and manifold did. After spraying (I used my paache air brush) bake at 300 for on hour.

Here are the pictures. First pic is the cylinders bead blasted. I baked the parts in a toaster oven and for some reason this site decided it would look better sideways! Last picture is the Cerakote kit as received. It's not cheap (what you see was $65+ but well worth it in my opinion.