Okay, It has been a big learning curve but thanks to all of you who have struggled with similar projects and shared your considerable knowledge and experience this one is finally ready to go. This will summarize my experience and hopefully save anyone else who is interested in using this engine some frustration and expense. It cost me more to do this due to experimenting with various carburators. In a thumb nail you need the following:

1. Lots of patience and desire to make it work.

2. one Briggs & Stratton 40cc, four stroke gas engine.

3. One 11mm Walbro WT (butterfly style) or ZAMA carburator.

4. one glow plug adapter.

5. one OS A3 glow plug.

6. one glow plug driver. (simple 1.2v rechargable battery and a switch)

7. Make an external pulse line.

8. mix pump gas and 33% nitro glow fuel for R/C cars. (1/3 nitro, 2/3 gas).

Here are some pics of what to do with the carb spacer/manifold. First you need to cut the (carb end) of the spacer off for two reasons, 1. get rid of the pulse slot, 2. be able to enlarge the hole to match the carb. I used a tapered grinding stone in my drill press. You don't want to go all the way through. Just deep enough to get the right diameter to match the carb. This will leave a nice taper from the round carb hole to the oblong engine port.

Next, you need to tap the holes for the pulse line fittings in the engine manifold flange and the carb fuel pump plate. Here are some pics that show what it looks like when done. It has been suggested that a spring should be used inside the pump which I tried but decided to leave out for now since I wasn't able to get it to work right and so far the engine has run fine without it.

These engines are not loud because they are low compression so I decided to run a straight exhaust header. I had this one custom made by K.M. Model Aircraft Components. It is a nicely made aluminum header and was very reasonable. I messed up on the measurement and had to have a spacer made but it should be all one piece. Anyway, because of the low compression the best way to get the most both performance wise and cost wise is to run a gas/glow mix. Many may disagree with this but I know what it has cost to build this engine and I didn't have to spend a lot on an EI and there is no timing adj. required to be able to flip start this engine which was a goal for this project. The total cost of converting to glow (not including the nitro fuel) is about $40. which includes the adapter, plug, some batteries a switch and a locking glow plug cap. For bench testing I just left my Hot Shot glo-driver attached to the plug. It will take some time to get the needles adjusted right and these engines need to warm up a bit before they will take more throttle but after a little priming with the choke closed it starts after a few flips and pops and goes right to idle. I am turning a 20x10 MA Schimitar prop but hopefully it will turn a 22x8. As soon as I get some tach readings I'll post them.

Well, so far I am getting around 2,000 rpm at idle but I know my Glow Bee tach isn't the best. It seems to be at the right place and I can open the throttle up without any hesitation but I can't run at full throttle. Once I get to about 1/4 to 1/2 full throttle it shuts off. Might have to try the spring in the pump again. Any and all comments are welcome.

Got the idle down to around 1,000 rpm give or take a little since this Glo-Bee tach isn't the most accurate. Put the spring back in the carb pump but still can't open the throttle past half way. Tomorrow I'll try a smaller prop and get some high rpm tach readings or at least find out what it is turning as far as I can open the throttle.

Decided to try a smaller carb. and had a brand new ZAMA, C1M from a Poulan chainsaw (the Poulan people tell me the venturi size on this Zama is 8mm & throttle bore is 14). I tapped it for the pulse line and stuck it on. (I made a mistake earlier thinking the Walbro I was using was 11mm but it was 13mm). I was able to get higher rev's but still couldn't get full throttle so I took the carb off and put the spring in the pump and also mixed up some more fuel. Started out the same but after running for a few minutes and fluctuating the revs it came alive and went full throttle. Seems the engine was not fully broken in and I noticed all along that the exhaust was pretty black indicating to me that the ring was still breaking in. I got the tach out and was able to push the engine to 7300 rpm. and this is with a 20x10 MA wood Schimitar prop. I had to take a break at this point. I'll start her up again and see if it gets any better. Still want to do a little tuning to make sure it doesn't stall when the throttle is released. So far it just slows down to a nice putt,putt,putt.

JohnF,

I noticed that my "used" 40cc ran stronger than my new one,so a breaking in period may help.Very interesting results.It may be enough to make me tinker with the compression on one of mine to see what that does.

J

Okay, I decided to try a 20x8 prop same type MA and this thing is really coming on. I got 9,000 rpm on the tach. and even if it's off some I doubt it's off that much. I have some vibration but I think that is to be expected with a 4-stroke and I haven't balanced this prop and I think it's still breaking in. I may still try a 11mm Walbro WT391 carb but the bottom line is this engine likes less air and more fuel. Oh, the best part is one or two flips and popps and she's running. Almost sounds like a real LeRhone. To be continued....

Just a couple more pictures of the Zama on the engine. I like the way it bolts up better than the Walbro did. You can see that the pulse line is horizontal and I am able to keep it much shorter. I will be able to make a better throttle linkage also. This is the 33% Nitro I am using. There is a note on the top of the can that tells you to "richen up the High speed needle 3/4 turn if previously running 20% nitro". Higher nitro requires a richer fuel mixture. This is a 1 qt. can. Last is a picture of the inside of the head. You need to get the glow plug as deep as possible without hitting the piston. I am told this retards or advances the timing (thanks Ben) This is how it looks with the adapter I am using. Ben (planepounder) has been a tremendous help with this project and it's success. Thanks a ton Ben.

Nice job! I was kida wondering what airplane you may put the engine in? Thanks, Capt,n

Thanks captin, The engine will go in a 1/3 scale scratch built WW1 Aviatik parasol mono-plane. Here's a color profile of it.

Looks like a good choise. Is it a kit or scratch built? Capt,n

Captain, It will be scratch built. Dave Johnson drew the plans for me a long time ago but in 1/4 scale. I have enlarged them to 1/3. I have a thread started under "scale aircraft" titled Aviatik 30.40 it's been awhile since I posted anything new since I am trying to finish my SE5a( McCuddens) first. I just wanted to make sure the engine was going to work so that's why I did the conversion now. The Aviatik should be a fairly quick build once I start the framing.

Well, just an update on the Zama carb., DON'T USE IT! It ran great at first but the diaphram does not like the nitro and deforms. I have a Walbro with the same type of mylar diaphram but it's blue in color so I don't know if it's the same stuff used in the Zama. Got to go back to a Walbro with the other type diaphram I think. To be continued.....

Okay, Have to thank AV8TOR for helping me out with the carb problem. As I mentioned above the ZAMA, mylar diaphram does not like the nitro and AV8TOR tells me he has used the Walbro with a different diaphram material (which I have) with no problems so If you read my "carb search" thread you know what I am doing. I needed to match up the bore of the 11mm Walbro with the manifold I have and could not enlarge the hole in the manifold any further so, I have made a step down adapter from a 40cc Poulan chainsaw manifold. For me this was easier and faster than constructing an entire new manifold. I beveled the inside of the adapter where it fits inside the carb so there is a smooth transition. The ring will be JB Welded to the existing manifold and held in place when the carb is bolted up. Good seal between the two and no interference with the butterfly. Here are some pic's.

Very interesting conversion, John. I have a 33cc B&S 4-stroke that I converted but I could not get it to run right with the stock carb. I didn't know you could use a butterfly carb on them. I have a couple of spares I might try out. How did you determine where to drill the pulse hole in the engine?

Jim

Unlike a two stroke, you do not want to tap a pulse hole in the crankcase. While this will work, at least on the Honda four strokes, it will eventually be problematic due to oil getting into the pulse line.

What we are doing is tapping directly into the intake manifold between the engine and the carb. This provides a vacuum pulse when the intake valve opens, but you need a small spring in the fuel pump chamber to "return" the diaphragm after it receives the pulse.

This manner does not provide a "+/-" pulse like a two stroke setup, hence the need for the spring in the pump.

AV8TOR

Good job on your carb "fix" Jim. I don't know what I was thinking, but I should have mentioned making an adapter to you in our correspondence. As a matter of fact, I had to do the same thing on my Honda. What I did was cut out a piece of 1/4" thick plywood in the shape of the carb gasket. I then made a hole in it, tapering from the carb throat size to the manifold inlet size. I coated it with 30 minute epoxy, and when it was dry bolted it up between the carb and the manifold. Works perfect. Sorry I didn't remember this to advise you earlier. Things have been really crazy for me with our recent move, etc.....

AV8TOR

Hi Jim, I got it running pretty good until the diaphram problem happened with the Zama. I hope I can get the performance back with the Walbro. I still wasn't happy with the transition so I will still be working on that a bit. Hope to make it a little smoother. I also found/fixed a leak at the glow plug adapter. Stay tuned.

Okay, here is the finished manifold with the bore adapter JB Welded on. I ran the tapered grinding stone into just to make sure it is smooth on the inside. I'll use some washers to fill any gap between the carb. and manifold when I bolt the carb. on.

Okay, I got it running again pretty good with the Walbro WT 391, 11mm carb. and the blue colored diaphram seems to be okay with the nitro since I have run it a few times and it is pumping consistantly at high rpm (with the spring in the pump) it seems. Did not use any washers between the carb and manifold since they were keeping too much of a space. I am keeping the high needle rich. I needed to put some pieces of fuel tubing between the head of the needles and the springs to keep them tight. Anyone else ever have to do this to keep the needles from backing out?

Just an update since I now have tried the 22x8 MA, wood, Schimitar. The engine seems to like this prop with the carb settings the way they are. Lots of thrust and still pretty high rpm's . I don't trust this tach but it is showing me 7,000 rpms and at full throttle it sounds like more to me. Of course this will all change when it's in the air. I would like to try a 23A Vess or 23x8 MA since the rpm's are still higher than I need (I think). Nothing left to do now except change the crankcase oil and hang it on a plane. Oh, I didn't mention that the hole in the choke butterfly needs to be soldered close so I can prime the engine when it's under the cowl.