Looking for larger model carb design

[flatbelter]

Hi All,

I’m looking for a simple carburetor design suitable for a moderate (25-100cc) gas RC airplane engines.

I have been using Zama and Walbro carbs but was hoping to find something small and light, suitable for flying. Preferably something to get away from those pesky diaphragms.

I have been looking at some twin needle and air bleed carbs, but was wondering if someone had a design that worked well for them.

Thanks in advance.

FB

 

[mu38&Bg#]

Quite honestly I wouldn't stray from Walrbo in an airplane. You have a fuel pump built in and a good metering system. What sort of problems have you been having with Walbro carbs?

Greg

 

[flatbelter]

I guess the biggest problem is the carbs are older units salvaged from "dead" equipment. So they are in varrious conditions ranging from ready to go to nearly hopeless. I haven't found any good local suppliers of carb parts so it's all mail order.

I'm waiting for the latest order of parts to arrive in the mail.

Since I'm waiting anyway and I've never tried building a carb from scratch I thought would post and see what turned up.

 

[mu38&Bg#]

Yes that is a problem. I usually look for new ones off Ebay, but getting a part number suitable for your application can be tricky as well. Searching the Walbro site is difficult.

 

[Rocket Man]

Check out the venturis I have built. If you extend the .125" tube all the way across the air intake hole with 2 tiny .040" holes on each side of the .125" tube as the air travels across the tube it produces a pressure drop that sucks the fuel out of the 2 holes. If you drill and tap 4-40 threads on the exact opposite side if the fuel inlet tube you can build a needle valve that will screw into the tube to the 2 holes. Put a tiny ink pen spring over your needle valve then adjust it to get the fuel you need. I built one for my model jet engine but I don't have a close up photo.

 

[flatbelter]

Hi Rocket Man,

Those look pretty neat, and within my machineing capabilities. ;D

Looks like the venturi replaces the original carb setup?

What is the center diameter of the venturi?

What is the ID of your 0.125" tube?

Do these have any throttling action? If I understand what I'm looking at you only have control of fuel flow. Is that correct? If not, can you elaborate a little more on how they work?

I like the extended venturi, I was looking at the fuel cloud that develops over a Zama carb in operation without a velocity stack. I could see where the cloud could get blown away during flight possibly leaning out the motor.

I think I will try building a couple of these on the weekend, wish me luck

FB

 

[Rocket Man]

The small venturi is for an OS25 model airplane engine. The larger venturi is for a OS40 model airplane engine. The hole throught the brass piece is about .075". I was using these with the original inline fuel needle valve that comes with the OS engine. The hole diameter is 10% larger than the RC carburetor.

I made some others that I drilled and tapped a hole with 4-40 threads where the brass piece is. I bought a OS carburetor needle valve assemble at the hobby shop and screws it into the hole. I did not have to make the brass piece and the metal needle valve that I bought is better than the plastic needle valve that is attached to the back side of the engines.

Here is a pic of the airplane and engine I son was flying. It is speed limit combat 75 mph. With a different engine the airplane does 140 mph.

I use the same needle valve idea for my jet engine.

[ame]http://www.youtube.com/watch?v=s5KInr3C9vQ[/ame]

 

[flatbelter]

Thanks Rocket Man,

Nice sounding pulse jet, has it flown yet? I love seeing them fly.

Nice looking plane, good to see it's a family afair. Just got back from spending the weekend with my father and son flying an assortment of RC planes. Trying to get some airtime in before the weather turns cold.

Well, my Walbro repair parts arrived in the mail today, I'll rebuild the carb tomorrow and with a little luck, I'll have a nice 59cc plane engine ready to fly. (once I build an airframe to hold it)

I still want to try making an air bleed carb and a glow engine to go with it, thanks for the great ideas.

FB

 

[robcas631]

Rocketman, Wow did that scare the beans out of me! Have you measured the thrust? If so what is it?

 

[Rocket Man]

Rocketman, Wow did that scare the beans out of me! Have you measured the thrust? If so what is it?

Static thurst is 4 lbs running on propane, 5 lbs running on gasoline, 5.5 lbs running of gas/kerosene, 5.5 lbs running on gas/diesel, 7.5 lbs running on 100% Methyl Alcohol, 7.5 lbs running on 95%/5% Alcohol/water. With 200 mph Ram Air and a second fuel tank designed to give the engine more fuel as it picks up speed its 7 lbs on gasoline and 10 lbs alcohol.

 

[Blogwitch]

Anyone interested in pulse jets, here is a nice website that I have followed for a few years.

http://www.pulsejetengines.com/


John

 

[flatbelter]

Alright so the Walbro carb is a pretty neat little bundle of engineering. I went through an SDC (21-85??) carb today. New gasket kit off flea-bay for $5 including shipping. I converted from external pressure tap to the internal to the throttle plate. I put a temp plug in the original tap hole (I'll make a better plug tomorrow), dremeled a new slot to the pump membrane inside the carb plate. About 20 min to rough tune and now it's time to find a suitable prop.

It's still a heavy, blocky, lump of metal, but it has possibilities.

Anyone try converting these carbs to glow (alcohol) fuel? (now that I've asked, I'll go search for previous answers)

 

[mu38&Bg#]

Alcohol conversion, usually involves enlarging the jets to about twice the area. Changing the pump diaphragm to the tan one, and maybe even opening the passage behind the inlet needle. I've never done it though. I've been converting glow engines to gas and diesel.

Greg

 

[flatbelter]

Alcohol conversion, usually involves enlarging the jets to about twice the area. Changing the pump diaphragm to the tan one, and maybe even opening the passage behind the inlet needle. I've never done it though. I've been converting glow engines to gas and diesel.

Greg

Hi Greg,
Yea, finding alcohol compatable internals seems to be where most of the fun is. Good to know that there are alcohol compatable parts available.

Doubling the area of the jets is easy if you remember that the squareroot of 2 = 1.414 so you just multiply the original jet diameter by 1.4 to find the new diameter.

You might be able to get away with adjusting the inlet needle arm height (raise it) to get a longer inlet pulse too.

I hear that the E-85 fuel works pretty good for glow fuel, I haven't found anyone selling it in Northern NY state but I keep my eyes open.

How well does the bottom end hold up on a diesel conversion?

 

[mu38&Bg#]

The metering diaphragm will have to be replaced regularly with alcohol. The tan pump diaphragm is the one recommended for alcohol. The blue one is also compatible, but doesn't pump as much fuel. Depending on how big of an engine you put it on it might not keep up with the required fuel flow. Adjusting the needle lever influences mid range mixture. Chances are everything will need to be checked, including testing other springs under the lever. Fortunately alcohol is much less critical of mixture than gasoline, E85 not as much.

Bottom ends are fine. I've had no problems with the O.S. four strokes on diesel.

To optimize E85 on glow you'll likely have to modify compression ratio. I had an FS-52 setup for spark and E85. I put an OS F plug in to try and it wouldn't run at high throttle without plug heat. I didn't put much time into it though. I've heard that the large two stroke Super Tigres are just fine on E85 and the OS F plug.