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Need help choosing carburetor

Old 03-17-2019, 07:09 AM
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GeneralBones
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Default Need help choosing carburetor

Hi. I have an old .40 size nitro engine that came with a ROYAL AIR 40H. I wanted to convert it to a gas engine so I got a gas glow plug. That's given me a good start as I've been able to run it off gas and 2 stroke oil but it stops running at full throttle if I don't open the carburetor enough. But if I open it more it runs too rich and shuts off as well. I want to buy a carburetor made for a similar size gas engine because I've read that nitro carburetors can't work properly for this in some glow to gas article.
Someone in this article says that adding more oil to thicken the gas to make it be able to run with the nitro carburetor should work but I've filled the tank with 1/3 of oil and the rest gas but it ran the same. Seems like a lot of oil lol.
https;//www,rcgroups,com/forums/showthread.php?2496156-Glow-to-gas-conversion
Oh yeah I also grinded a little bit of the head inside to lower the compression because I dont have anyakind of shim solution but it feels about the same. Maybe I should remove a bit more material?

I was wondering if I could get help choosing a gas carburetor that could fit this engine. And maybe any other tips.
Old 03-17-2019, 10:47 AM
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Never done it, but it appears you have the compression thing backwards - to lower compression, you need to add shims to raise the head. Taking material off will increase compression, which in all likelihood will make it even harder to run. Having it stop while running at full throttle would seem to indicate being way too lean, and the heat build up is causing it to seize. You may have scored the piston/liner, which will make it even more unlikely to run right. Royal were "affordable" engines, so probably not the best candidate for conversion. Highly unlikely you will find a plug in carb for that size in gas. Be prepared to hunt down an OS GGT 10 carb, and then machine an adaptor to get it to mount. They are only $135 at Tower Hobbies right now.
Old 03-18-2019, 08:32 PM
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GeneralBones
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Uhh are you sure about the compression thing? Ik adding shims would increase the volume of space inside and removing material would have the same effect. I already have the os G5 gas glow plug that fits and runs. Also, it isn't scored I've checked each time I increase the the space inside the cylinder. Thanks for the csrbucarbu suggestion, that helps a lot. I could probably 3d print a couple adapters for short tests lol but I'll probably end up getting one machined if I can't find a cheaper gas engnee solution. Thanks again.
Old 03-19-2019, 04:36 AM
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If I understand you right you're grinding material away from the combustion chamber in the head? If so, you are changing the shape of the combustion chamber and thereby changing the efficiency of combustion. That comes to a change in power and since these engines are optimized for power, I doubt you'll see any improvements. Find some shims!
Old 03-19-2019, 06:39 AM
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I missed that, that you were grinding away the combustion chamber! Yikes! That would reduce compression. And as Appowner noted, change the efficiency. You're well on your way to ruining the engine! Normal glow plugs work by catalytic action between the platinum wire and the alcohol in the fuel. Makes heat, which sets off the explosion. I'm unsure exactly how OS works it magic for the gas glow plug, but probably mostly dependant on bigger parts to retain heat through to the next power cycle. And an almost (but not quite) level of diesel-like compression to heat the fuel/air mix. Which would mean that engine design would be very critical to the ability to run - mass, compression, porting, etc., etc. Rather than chase a very expensive carb at this point, see if you can find a real spark plug that fits, and electronic ignition module. As to oil, glow fuel tends to be around 20%, or a 4:1 ratio. Besides lubrication, it serves a huge purpose in taking away excess heat via the exhaust. The engine probably will be just fine with a much, much lower lubrication...but given the higher heat production of gasoline, you'd give it a double whammy of much more heat, and greatly reduced way to get rid of it. Hence stopping after a bit at full throttle. Since carbs do have needles, it is probably the least of the challenges at getting the conversion to run and stay running.

I'm sure eventually it is possible to get this to work. But if it was kinda easy, you'd see glow-to-gas conversion kits/services offered, given the terrible rise in glow fuel prices.
Old 03-19-2019, 07:41 AM
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You could mix just a bit of methanol just so the change is not so drastic. Use the corresponding amount of oil. Maybe 1/3 glow fuel to 2/3 gas mixed fuel. There was a gas/glow thread on one of the forums that went into this very deeply. Also there is a Norvel .40 that used a very hot turbo style plug head that works well for some folks and not well at all for others. It has the different head and reworked needle. Some guys ended up with the glow needle,s so experimentation is needed on newer tech stuff. It is good to not modify too much so that you can't go back. Any gas motor will have big end rod wear problems if there is no roller or needle bearings. Lots of oil at the very least is a good idea. Not 40 - 1 at any rate.
Old 03-19-2019, 09:26 AM
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Or just buy one of these, about the same price as an OS carb....

https://www.justengines.co.uk/acatal...ol-Engine.html
Old 03-21-2019, 03:45 AM
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I don't think you've chosen a good candidate for gas conversion. It's not as simple as finding an ignition source. You'll need a roller bearing for the big end of your con rod unless you want to stick with 17% oil in your fuel. Combustion chamber design, as you've noticed, is also a pretty big issue. Since you've already modified it, there's no reason not to keep experimenting just to see what you can do. But I highly doubt you're going to wind up with an airworthy engine this way.
Old 03-21-2019, 06:48 AM
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I am not quite so negative, You will need the normal amount of oil, which is the biggest cost of fuel anyway. You can try E 85 gas if available in your area too. Maybe try running your motor at full throttle to get the mixture right and then adjust for idle. Maybe the taper on the idle needle needs a more gradual taper. Tell us the results. Maybe someone will figure it out. Myself, I would like to at least use less oil, as that is the biggest cost, and also loss of power. As mentioned needle bearings are normally used in two strokes using gas.
Old 03-23-2019, 12:00 PM
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GeneralBones
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Thanks for all the replies! This really helped. I can see that ill probably end up buying that gas engine tedsander suggested. The changing combustion efficiency doesn't help my case lol, ill have to see what kind of shims I can come up with for this experiment. Once I can throw more money at this engine ill experiment some more. Thanks again guys.
Old 03-24-2019, 07:39 PM
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Shims are simple enough. All you need is aluminum sheet. Foil is a thin piece, and a piece cut from a Dr. Pepper can is a thick piece. Hopefully you have a micrometer and can do the math to work out what you actually need.

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