You guys that have converted GX31's: Did you modify the throttle configuration on the carb? I'm wondering if there is anything I can do to avoid using a high torque servo for this linkage. The existing lever requires quite a lot of force to open...and then is spring loaded on the return. Should I change anything or just set up a bellcrank linkage to a high torque servo? I'm new at this and would appreciate your voices of experience. Thanks!

I have looked at several carb alternatives for mine. However, I decided to make a new intake manifold and use a WT type carb. Took the engine completely apart to clean it and see what makes it tick. While apart I drilled and tapped a spot in the block for a pulse tap to the WT carb. Just turning by hand produces good pulse. Got the idea from a guy that uses these in boats turning over 10000 RPM. He claims no problems with this setup. If this does not work for me I will just plug the hole and make a manifold to rotate the WY carb. Found several with high needles and 10.5mm venturi. All I need to do is weld the header pipe I just made to the flange and time the engine. As mine sits it is at 4 pounds.

I removed the barrel and clipped the return spring about 3/4 of the way or so and removed the top part with the hook on it. Now it has enough pressure to keep the barrel under tension so it raised the needle but not enough to provide any real resistance to the servo.

Never did source a better carb for mine. What number carb did you find, and where did you find it????

AV8TOR

Thanks for the ideas. Let us know how the WT carb works out...10,000 RPM would be something else out of one of these engines. He doesn't have a problem with cooling?

I am using a WT-434 9.53mm venturi. I use this carb on several of the conversions I do. I like the throttle shaft because it sits up a little higher than most. Also has a choke. IF the WT carb does not work well I will try a WYK-61 a 9mm carb but with a high needle. The low can be adjusted by removing the cap on top of the barrel. The WYK-38 is supposed to be 10.5mm but the book and online shows 12.8 I think. My local vendor is verifying this for me. I hope to run mine this weekend even if I don't have the header done. I hope to get close to 7000 on an 18/8 Zinger. Then I may order the cam and see what I can get out of it. Would be nice to see a strong 7500 but who knows. I plan on using it in a Balsa USA Sopwith Pup 1/4 scale.

Tim

Got the Honda GX31 running tonight. Have not made the complete header yet but ran it anyway. Put an 18/8 Master Airscrew wood prop on it. Fired right up and purred like a kitten. Had to back the high needle out about 3 turns to get it to wind up. Runs right up to high RPM then just snaps right back to a perfect idle. I will experiment with bigger carb tomorrow. I have several 11.11mm. Could not get a tach reading as the lights were messing with the strobe. Ran it for 20 minutes on less than 8 ounces of gas.

Any chance of a pic or three? Where did you put your pressure tap? I'm not sure I understand how it makes a pulse if the vent in the valve cover is open??

AV8TOR

Yes I will get some pictures here. The fuel draw was good just turning the engine over by hand. On the test stand the tank has a 12 inch line or so to the carb. The pulse is created from the pressure of the piston going down and the vacuum created by it going up. I was concerned as well but there is a guy on a boat page that gets tons of power from these. I am not saying that the WYK carb wouldn't work but we all understand the WT and know how to adjust them. Where I tapped the case the oil is a vapor and my pulse line was dry after a 20 minute run.

Here is a picture a friend at my field snapped with the Honda running. Tached it at 6200 with no header at all and an 18/8 prop. I think it will go up some with a header. I will be making the final intake for it now that I know it runs well. I made one out of 2 pieces of 1/2 inch Acetal. One with the bore of the intake on the cylinder and one with the bore of the carb. I will mill a new one out of 1 inch thick Acetal and taper the bore from about 5/8ths down to about 1/2 inch. The air will flow smooth and have no sharp transition edges like now. Maybe a little more RPM with it. I hope the manifold and header gets me to 6800, maybe wishful thinking. After that try different carbs and props and then start all over with a new cam. I will post a video that was shot as soon as I get it from my friend. Sounds good with open exhaust too.

Tim

Here is a link to the video. Resolution a little low on the first so the whole thing can load. Shows the WT carb and the tap in the case for pulse pressure to the carb. Second one is short but sounds and looks better. This thing ran for 45 minutes on 8 ounces of fuel.

http://www.youtube.com/watch?v=mYPAjyKaNLs

http://www.youtube.com/watch?v=eVZsEy1FvfM

hi! sorry if this is out of topic, but does anyone know whats the thread size on the shaft of the GX31?
thanks a alot!
ernest

Mine is an 8mm X 1.25. My engine is one of the first and was made in 1997. The other shaft size is 10mm.

thanks a lot tim! i'll just try to measure it whether its 8 or 10.

Tim, it looks like from what I can see on the video, that you have capped, (closed off) the valve cover vent. Is this correct??

Thanks,
AV8TOR

No it isn't capped. Just open to the air. Have about an hour and a half run time on it and no residue has appeared from it. When I mount it in the plane inverted I may see some then. We will see. If I do I will put a fitting in it and run a hose to a velocity stack in the carb. Saw someone here on RCU do that. I made the pipe and flange for the header and tonight I will Alumiweld it together and run it again to see if power increases. Than I will make the intake manifold with a taper and run that too to see if it gains any.

Hey Tim glad to see the engine is working out for you. Makes me think I shoulda got a bigger plane. Sounds great too.

Gary

Gary, thanks for the compliments. I made the header tonight and bolted it on. Left it long until I get it in the plane for fitting. Now I need to make the final intake manifold and start experimenting.

Tim

Tim, it looks great. I have been doing conversions on the Briggs and Stratton four strokes, I want to do a Honda next. We put a Birrelli carb on one of our Briggs last night, its way to much carb for the engine but it did run decent, we need to downsize the carb to make it easier to adjust. I had also planned on putting a four stroke on my Sopwith Pup but didnt want the large jug sticking out the bottom of the cowl, so I went with a 25cc featherlite instead. Here is a video of my Fokker DVII with the Briggs and Stratton four stroke. Its turning an 18-8 at 6500rpm, stock carb. A friend mounted his Honda on a 1/4 scale Fokker DR1, it looks good inverted on that plane. Is the WYK61 the same configuration as the 38 just with a high end needle? Mounting wise.

http://media.putfile.com/Fokker-DVII-Sunny-Day

http://media.putfile.com/Sopwith-Pup

Prop Nut, I wanted to do a Briggs but I talked to someone that blew one up mounting it inverted. How is yours mounted and have you had any issues? The WYK-61 is a 9mm carb and the WYK-38 is a 10.5mm carb. Both have high needles. Did you remove the spring return on yours? I am ordering both today to try them out. I would prefer to leave the WT carb on it but I am getting too much oil in the pulse line now. It was dry for a while but after running it with the header last night I had oil in the line. Ran and sounder better with the header too. I taked to Dave at Balsa USA yesterday about the cowl and he thought the hole in the bottom would need to be small if at all for this engine. I thought he said on the full size plane there was an opening there anyway for engine adjustments. The engine should fit easily left to right in the cowl. How tall are the Briggs engines center of to top of cylinder or rocker cover? If I use the WYK carbs I will need a new manifold to rotate the carb so the throttle shaft is verticle. How did you do yours? The videos were great. What do the planes weigh?

Tim

What's a "Birrelli" carb ? Is it like a Walboro ?

I think he meant Scott from Brillelli. They make conversion engines, mufflers, etc.

Please keep us updated on that WYK-38 carb. I'd like to order one for mine if yours works out ok.

Thanks!
AV8TOR

We ran the Briggs (Not the imperial We, but a friend and I) yesterday for about three hours inverted. There were no ill effects.

The only issue that we had was with running the carb upside down we got poor performace, mount the carb (only the carb, not the engine) right side up and it performed great. I would really like to have a carb that I can adjust both the low and the high, the Brillelli (thanks Tim) carb was just too big to get it to run well.

The Briggs is 8.5 inches from the bottom to the top of the spark plug boot. From the center of the prop spacer to the top of spark plug boot is 6 1/4 inches. I am looking at the plans for my Sopwith Pup, the firewall at the cowl is 9 inches in diamater, so the engine, if mounted inverted would stick out the bottom of the cowl approximently 1 3/4 inches. I would need to cut a large hole in the bottom of the cowl to fit a Briggs. I dont know the specs on the honda, is it shorter? The sparkplug and boot are about 1 1/2 inches tall above the head so even though I would need to cut a large hole, the jug wouldnt show much from the side.

I used a turnbuckle to hook up the throttle, and I did cut the spring down so it only keeps tention on the barrel and slides easy.

The Fokker DVII is 21lbs with the Briggs and Stratton and the Pup is 12lbs with a Featherlite 25cc. Here are some pictures of the Briggs and Stratton engine, we made the custom exhaust so it would exit about the same place as the dummy engine header was.

My supplier of choice is out of stock on the WYK-38 and none on order. I bought a WYK-16-1 and it is 10.5mm. I want to get a 9mm carb too. I will keep you posted on how it runs with it. Should have it by Friday.