Tauri Flyer

Any thoughts on which is a better engine to convert, A Honda 25 or 31 4 stroke? I haven't choosen an airframe yet so that's not a consideration that's in the mix yet. I've seen some thoughts that the 31 is a bit of a brick and not a whole lot more powerfull than the 25.. Yes I know that most of the gas 4 strike are somewhat "brickish" I guess what I'm looking for is power to weight ratio.

tim220225

My Feedback: (4)

Well I can say that you won't 3-D them. I am currently working on a Honda GX31 and I am posting in this forum:

www.rcuniverse.com/forum/fb.asp?m=4286595

This project has been fun and different than any 2 stroke conversion I have done. Definately a short nosed plane of some sort for these engines. I am getting 6200 RPM from mine with an 18/8 prop and both the stock carb and a WT series carb. I tapped for pulse but I am scrapping that idea. I am getting oil vapor in the line and it is causing issues I don't want to deal with now. I ordered a new bigger carb for it with a high speed needle and will be making a new intake to rotate the carb for better throttle hook-up. If this carb works well I will be ordering a new cam from a guy that does boats with the Honda and see how that helps.

cheechukranch

Look at Honda's replacement for the GX31, the new GX35.

As far as planes...ones with short noses and lots of wing area. The WWI bipes and tripes would be ideal. Sig Rascal 110's and Bridi Big Bees have flown with these (GX31/GX25) as well.

Tauri Flyer

As far as airframes go,,it will be something short nosed too help the balance issues. Possibly a WW1 fighter. What I'm interested in is power output vs engine weight. I've seen prop recommendations of 18x8 for both the 25 and the 31 while the 25 comes in at quite abit less weight. If these engines are turning the same prop at about the same rpm, then the choice starts to get easy.

tim220225

My Feedback: (4)

Carr Precision lists his engine weights on his web page. The 25 he lists at 3 pounds 14 ounces and the 31 lists at 4 pounds 6 ounces. That weight is with CH ignition. My 31 is the same. For me I would take a 24% increase in displacement over a half pound of weight. Looking at the 2 strokes I have done, the Echo 23.6cc fly's the same plane (Balsa USA Eindecker 90) way better than a 21cc Echo. Turns the same RPM but with a bigger prop. Carr Shows he uses the same 18/8 prop on either engine he offers. I would think the 25cc will turn the prop but at lower RPM. I looked at it both ways too. Went with the 31 as I think the torque and pulling power will be better. I just figured as the car guys think. There is no replacement for displacement. So far I am real happy with mine. New needle carbs should be here today or tomorrow.

Tim

subarubrat

My Feedback: (1)

There is a fantastic replacement for displacement, forced induction. That said, I don't expect a roots blower for these engines. What I would love to see is a higher compression ratio via a new piston. Money says there is some widely made engine out there that has the right bore, right wrist pin height, etc. and we just need the right person to identify it. These engines are designed to run in the worst conditions, high heat, low octane, clogged fins, heavy load etc. and they are not optimized for our use which is the best possible circumstance, clear fins, lots of air, as much octane as you want, etc. We could run a 12:1 CR in these engines and gain a huge leap in power.

tim220225

My Feedback: (4)

That would be nice. I am always looking. Did get the WYK-61 a 9mm carb today. Bolted it on and fired it right up. Gained 200 RPM or so. 6400 now with excellent idle and throttle response. Also got the WYK-138 a 10.5mm I believe but it got dark on me before I could test. So I started cutting on the Echo 61cc twin I picked up. Very light.

Tauri Flyer

Please don't forget to post RPM figures on that 10.5 mm carb! I'd be real interested in what you find there.

Gismo

I have been working on a stock GX25. I have it turning a Zinger 16/8 @ 6800 rpm with the muffler on and just shy of 7,000 with the muffler off. The carb is stock with the plastic adapter bored to 3/8". The impulse port was drilled and enlarged in the plastic adapter as well. It seems to run pretty well so I am thinking of trying it on a 110 Rascal.

I also spent a lot of time on pistons and found that Honda runs a straight aluminum bore and has a special coating on the piston. Most engines have a coated bore (chrome usually) and an aluminum piston. I tried three sources to build up the top of the piston but each time the coating was damaged or the piston deformed. I did run one engine breifly at 10.5:1 but had no notiable increase in power, probably wasn't a conclusive test. If a piston can be found and coated it may prove helpful. I talked to Wiseco but they did not have any forgings small enough to work with.

I looked at the GX 31 but the additional weight seemed like it would offset the power gain. I still am tempted to buy one. Hope this helps

Tauri Flyer

Well I finally finished my GX31 conversion. I had to rebuild the carb (wyl127) Seems the original engine owner opened it up and reinstalled the inlet needle arm upside down [:@] Anyway shes turning a 20X6 zinger at 6600 and has a nice low idle. The sound is just too kool! Needless to say I haven't been able to stop smilin! I've got a GP Super Stearmen and a Uravitch D7 that should make a nice home for this puppy.

davewallace

My Feedback: (29)

Gismo,
A longer rod could do the same thing as building up the piston. If the piston isn't reaching the top of the cylinder wall, you would have some room to increase compressiom. It would be easier to make a rod, then a new piston. You may have to have the edge of the combustion chamber releived a bit to allow the piston to rise higher.

Dave

jtisch

Is the WYK-138 10.5mm? Another thread said WYK-137 is 10.5 I was thinking of getting one for the 40cc Briggs. I'm not sure if the larger bore will help the power but having adjustable mixture at both ends would be nice.

Thanks,

J

keibo

HELLO TIM20225! I CALLED WALBRO BUT THEY SAID I NEEDED MORE #S TO GET A CARB. THE # YOU POSTED IS WYK-61, IS THERE ANY OTHER #S AFTER THAT? THANX, BOB K

tim220225

My Feedback: (4)

Bob, here are two links to Walbro and that carb. The number 1 after the WYK-61 has nothing to do with the part number of the carb. I don't understand why someone at Walbro would say that. Maybe they have none in inventory. Try calling some of their distributors.

Tim


http://wem.walbro.com/walbro/product...1-1&Series=WYK

http://wem.walbro.com/walbro/partnum...anuItemNumber=

yungman

Hi
I read from a lawncare forum that the 4 cycle Honda GX31 31cc engine have vibration problem at top RPM. The vibration were so bad that it actually crack the crankcase!!! Anyone have experience of this problem on the unmodified engine only. I can understand if you modify the engine, you are pushing it to the limit. But for an unmodified engine, it is hard to believe it would do this. Any comments?

Is the GX31 design robust or the GX35 is better?
Thanks