BBriBro

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For the few of you that have converted a GX31, what carbs did you try? I really want to use this engine, and It looks like I am going to have to build an airplane for it (rather than the other way around) Something with plenty of wing area, and light weight. If it will swing a 18x8 at over 6000, thats probably enough thrust, so It's the weight I am more concerned with. Did any of you do any major modifications? Or just the usual flywheel housing removal? The carb seems tiny, (and no mixture screws at all) so I'd like to change the carb. I have one off a ZDZ 40, and for only being 9 CC's bigger, the carb is probably double the size, I don't think that one will work. Also, what intake manifold will work? most of my "other" ones have a round hole at the carb , and then kinda taper into a rectangle to match the intake port on the cylinder, the Honda has a round port, and the intake manifold is just a constant diameter round "tube", so the "other" ones don't really match up well. I thought I had a pretty good selection of spare parts, but it doesn't seem like they will do me much good on the Honda. I can probably find a slightly larger Carb, (off a Mac) thats not so overkill like the one from the ZDZ, but the larger carb then won't fit the Honda manifold. Please share your experiences. I also read somewhere about an aftermarket Cam, that with a big carb, and a decent muffler would probably make a decent improvement. But, since the Cam only has one lobe, I guess you can't change the valve overlap, and therefore I'm guessing it will never be a very high performance engine, I guess you can change the valve lift, which should help some, can someone better explain this strange Honda Cam design? How about the GX25 ?, if it's really that much lighter, and smaller, maybe that one will be the engine of choice? I also like the idea of the Shindaiwa C4 and the Stihl 4-mix. But I can't find any info on those as conversions. I'm sure they will be lighter, and I like the YS style crankcase pressurization, seems like a good idea, But the Honda is unique too, and the super clean exhaust will be a welcome change for a model !!

BBriBro

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I just found out that there is a GX35 as well, the same overhead cam design as the GX25, and lighter and more compact than the older GX31 and GX22. Honda advertises the GX31 as 7.4lbs, seems like ready to fly they are around 5 lbs. If that same proportion holds true, then the GX25 listed as 6.1 lbs, would be under 4 lbs ready to fly, and you'd have an overhead cam, and a timing belt to add to the cool factor. I am going to take a serious look at the GX25, anyone know where to buy one? I have seen them advertised on weedeaters, but not by themselves.

rc_skychief

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You can buy just the engines at www.smallenginewarehouse.com They are located in Dunkirk Indiana. Have been looking at them myself they have 22cc,25cc,and 31cc versions available.

BBriBro

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Thanks, but my questions are far more complicated than just "where do I get one" I am curious what "mods" people have done, and other experimenting, and their results. The typical carb and muffler changes make big improvements on most conversions, but the Honda won't just take any carb. Certainly there must be other "tinkerers" out there that have tried various combinations. The GX25 looks like the hot ticket, slightly less power, but at over a pound less, I think thats a fair tradeoff. I'd also like to see comparisons between the Honda and the Shindaiwa and Stihl 4 strokes, but there is almost no info on the other two.

jerry dotson

BBriBro I didn't try any different carbs. The stock one turns a Zinger 18-8 6800 RPM. My engine weighs exactly 5 pounds including a Ch ignition and 1000 ma battery. I have about 40 flight hours on mine. I use a straight pipe for exhaust as it is so quiet. My OS 60 with muffler makes a lot more noise than the Honda does. You are right about building an airplane for the engine. I built a 1/4 scale Fleet biplane 20 pounds all up weight. The fuselages on most planes are too narrow. I consider the power just barely adequate.

BBriBro

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Jerry, I called CH yesterday, and he told me they don't do the GX31 anymore, he said it's a slug, and wondered why I was even bothering with it. How long ago did you buy your ignition? Or did you just adapt one to the Honda? If so, where did you get the prop hub? The stock carb has NO mixture control whatsoever, do you find that adequate? It bugs me even on my Honda weedeater, (runs poorly when cold, almost non-existant transition untill fully warmed up, even then, the transition is too lean in my opinion) I can't imagine on an airplane and not being able to "tune" it. I would think at least a high end needle would be a must, especially for different loads (prop sizes)

tkg

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The Honda is a bit of a boat anchor, but has its place. CH still does the kits but just doesn't keep them in stock.
What we ended doing was making a new manifold and installing a Kline pressure regulator and a ST3000 carb.
If you have a set of jet reamers you can slowly open up the removeable jet in the stock carb and enlarge it untill the engine is running right. Factory setting is way to lean.

BBriBro

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I bought a GX25 yesterday. I haven't done any actual "converting" yet since no one makes parts for it to my knowledge. I am happy to report though, that it runs superb ! the Carburation was my biggest concern on the GX22 and 31, since they seemed too lean........all the time. The new carb with the accelerator pump made a world of difference. Honda claims you get 2-stroke like throttle response due to the new carb, and the lighter flywheel, and I believe them, This thing Rips ! You can snap the throttle full open from a very low idle, and it never quits, It just winds up, instantly. I had a GX22 there too, with both of them fully warmed up and running for comparison. The GX22 requires choke for a long time after start up, and takes a few min before it will idle reliably, even when warmed up, you can never snap the throttle open, you have to slowly accelerate it above idle, then you can open the throttle quicky. The GX25 starts easy, and idles well, even when cold, you can turn the choke off immediately after starting. Now granted, throttle response isn't nearly as critical on an airplane as say... a chainsaw, but I never got the feeling that it was starving for fuel, It always did what you asked, no stumbling, no hesitation, I just hope it's that good under a load too. The GX22 stumbles and hesitates even under no load if you open the throttle instantly. The engine looks better too, The flywheel housing is a casting that will require grinding, but it's not much, just attached with a few "spokes" that are thin, and can be cut through easily. The oil reservoir is still the round drum, but a smaller diameter, and wider, looks to be about the same capacity, just a different shape. The new cylinder with the timing belt channel molded in it will get a double take from engine gurus. As well as the new valve cover with it's bump for the cam gear. The spark plug is in the front, so the plug wire is only about an inch long, looks neat and tidy.

I did get a few weights. I didn't weigh it right out of the box (empty) But out of the box, plus oil, it weighed 100 oz or 6.25 lbs, they claim 6.1 lbs, I'm not sure how much weight the oil adds, but that is pretty close.

With the shrouds removed, pull starter, muffler, fuel tank and flywheel housing (just the part that un-bolts) I showed 72 oz, again full of oil. I ran it like that, so it was a fully functioning engine, with as much removed as I could, with wrenches, (no grinding)

The flywheel weighs 9.8 oz.

I was at 4.5 lbs, I think with some grinding of the fins (a few are much too large, to help seal against the shrouds for baffling), the typical grinding on the flywheel, and grinding away at the backside of the flywheel housing, removal of the pull start dogs, it might be 4lbs. As far as conversion engines go, I guess thats not too bad, It runs like a top, It sounds very convincing, Revs high, nearly instant throttle response, I hope that's indicative of it's power output, I can't wait to get a prop on it. I'm pretty excited, I love the concept, and the looks, I need to find a suitable prop hub before I go any further. The honda flywheels are recessed, so the hub needs a cone shape, to fit down in there, and thread onto the shaft. There is barely enough room for the flywheel nut, a prop hub could be a challenge. I wonder if the ones for the GX31 will fit this new flywheel?

jerry dotson

I bought the ignition from tkg about 3 years ago......got the one that varies the timing. That let me get a little more power in that I could advance the timing to max power and still start it without it kicking. The only carb mod I did was to get rid of that awful return spring. Then I had to make a spring to make the throttle run on that tapered ramp to pull the metering rod out of the jet to vary the amount of gas flow with throttle opening. My engine runs perfect. I have around 40 hours of flight time on it and it has not quit yet and takes throttle without a stutter. I runs better than anything I have! I just wish it had a little more power.

BajaBob-delete

I've been making cams for the GX-31 for more than a year now, but not for aircraft application.In an aircraft application there are some draw backs,but for a ground application the GX31 is pretty good performer.
Nothing strange to the Honda cam design,its just not understood very well by most people.The GX25 uses a unilobe cam also from what I understand,since the announcement of the GX35 however no cams for the GX25 will be produced,but the GX35 instead.
The biggest problem the Honda four strokes have is fuel supply.You need a WYK carb for it.Honda uses the the carb size to govern the top engine rpm,if you go to a larger venturi size and give it some fuel most of the strange problems go away.A lot of the surging at WOT is caused by the carb being under size and jetted very lean.Go to a larger venturi and fatten mixture,it will run smoother you will get more rpm and power.
My developement efforts have been in the marine application for this engine,it works very well in boats.

BBriBro

My Feedback: (2)

[quote]ORIGINAL: BajaBob

I have been messing with Jet sizes in the WYB (stock) carb. I have been looking for a replacement that has an adjustable high needle, and I had pretty much decided on the WYK. ( I think the WYK is the only rotary barrel with adjustable high speed mixture) Now that I read your post, I must be pretty accurate in my thinking. There is no doubt they are too lean from the factory, But how do you know how much larger to go on the carb? Even the WYK is available in many different venturi sizes. I think my stock carb is either 8 or 8.5 mm (WYB 6B 320). Not sure but the smallest WYK may be 10mm. Do you have any further suggestions? How about model numbers, I think I read somewhere about a WYK-61-1, does this ring a bell?

BajaBob-delete

Which engine are you reffering to in the above post?

I found on the GX31 the intake port was larger than the intake manifold,I just reamed the intake manifold to the port diameter and put on a 10.5 mm carb.That was the first step in a series of mods.The carbs from the factory are way under size.
On the GX31 a 10.5 mm worked well.Testing a 10.5 mm on the bench with a 20x8 pusher prop the throttle response was very good.

The WYK 61 is 9 mm,if possible measure the intake port and go from there.I believe the next jump in WYK sizes is to 10.5mm after the 9 mm WYK 61.

BBriBro

My Feedback: (2)

Sorry, I am referring to the GX25. I have several posts with lots of pics if you care to look. After examining my intake manifold, I noticed there is a molded in "bump" down in the intake. It's big on the carb side, and on the engine side, but necks down in the middle, I assume this is some sort of "rev limiter" just to keep the airflow down. Aside from that "ramp" it appears that the intake is the same size as the port in the head. A 20x8 sounds like a big prop, do I want to turn it that slow? Do these have a built in rev limiter in the magneto? I saw 6600 on a dynathrust 18x6. I was thinking 7000 might be a good peak RPM, I believe that's what Honda rates it at. I was going to try a master airscrew classic 18x6. Your input is appreciated !

BajaBob-delete

I had the 20x8 or 6? pusher running at 6400 rpm, fuel delivery problems plagued the set up,but at the rate it revved up I'm sure more was left.
I'm not big on aircraft,I'm using this engine in a boat, but the things I have found are still applicable.Most of my testing is done on the water,boats are nice because you can put all your measuring gear inside and take it for a spin.

I looked at the pictures,that bump may just be another rpm restriction device.Other than restricting the intake port opening,the only reason for the lump to be there,is possibly a resonant tuning device to help control that no load surge condition that the pushrod engines had.
You could start by opening this area up.

Have you measured valves yet? the head diamter and the pocket diameter will tell what can be done with this thing,if its like the GX31 some porting work goes a long way to perking up the power output.
Is the cam and sprocket one piece?
I have not found the coil to have a rev limiter built into it,on the GX31 if there is one, it comes into effect after 13,800 rpm (no load),Stock it would top out at 7500-7800 rpm (no load speed) with the stock carb. So they use intake restriction to limit rpm.

BBriBro

My Feedback: (2)

Wouldn't port work be much easier if the head and cylinder weren't one piece ? I can barely see the valves down in there ! I haven't measured them. Not sure about the Cam and sprocket being once piece, are you thinking of an easy way to change the valve timing? without grinding a new cam? I would think more lift would be in order too. I had heard about new cams for the 31, but never heard anyone talk about porting. I'd be interested to hear what all you've tried, do you have any pics? Would porting help that much, if it still has undersize valves?

BajaBob-delete

On the GX31 I found the pocket under the valve to be larger than the port runner itself.
The pocket or bowl area under valve and runner should be the smallest area of intake port.
If Honda did on the GX25 what they did on the GX31 the valves may be small but are usable,the port runner is what I found to be lacking,it was unable to supply the vavle.
The exhaust side of the GX31 was reasonable and left untouched until the rpm levels were up over 9,000-10,000 rpm.
I need top end power, for an aircraft smaller runners would be better,but whats there is may be too small to put power in rpm range that you want.So enlarging them would be required,however use caution.
From my experience with the GX31 some simple improvements on the intake side give a fair increase in power and rpm capability. For example the stock carb was replaced with one that was closer in size to the port runner,the intake manifold opened to match port size,rpm went from 7500-7800 to 11,000 no load.With a load,it still was a considerable improvement.

The easy way to adjust cam timing is to make a new cam.There is simply not enough material on stock cam to do anything of consequence.Unfortunatly I have no plans of making cams for the 25cc OHC any time soon,I'm waiting for the 35 cc to come out.This may change if there is enough interest.

In consideration of valve overlap period,one thing to keep in mind is the exhaust header.The header length needs to be sufficiently long enough to prevent the atmospheric pressure from entering pipe until valve is shut,other wise overlap is compromised.
A list of things I have tried would be too long for here,I just worked up in small increments until power was up in the range I needed.The short list for my GX31 is 12.2 mm WYK,290 deg cam with .146" lift (vs 252 deg and .105" lift stock) and 10.25:1 CR ,timing advanced 7 degrees,
lightened flywheel,sharpened oil dipper,ported intake and exhaust,and a stepped header.Power is up a considerable amount compared to stock.

PTM

After some cam timing experimentation, a custom pipe, and lightening the crankshaft I was able to spin the 18 x 8 prop at 6800 rpm with excellent throttle response. I beleive that this motor will spin the same prop to over 8000 rpm with a Perry pumper carb and running on alcohol (only $3 a gallon)with top-end lube. Other tricks I am about to try are slight porting, ultra-lightweight synthetic oils, electronic ignition. I do believe that this motor will tolerate 5% nitro with some timing backed out. A healthy understanding of 4-stroke performance helps here (and a personal machine shop). This motor has a stout bottom end and would last a lifetime with these mods and proper maintenance. What else can you say-Its a Honda.

Dave 700

I work for Honda and tried to get them to do an airpalne version of the GX31. Not enough demand compared to weedeater, waterpump and generaton applications.

!. Bigger carb gives more power.
2. Matching intake port to carb and manifold helps.
3. Radius the valve pocket. Stock engine has 90 degree angle from intake port to valve seat. Likewise exhaust the same thing.
4. Electronic ignition will give a lower idle- typically 900 rpm. It will not change the top end perfomance.
5. Cam can be modified and will give a little more power.
6. Single biggest limiting factor is compression ratio. If it could be increased it would improve performance. However the uniblock crankcase/cylinder/head keeps this from being an easy mod. Now if only there were hi-compression pistons.

Makes a great boat motor due to torque curve.

Dave700

rcbilly

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Where is a good source for these bigger WYK carbs?

BBriBro

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I ordered directly from Walbro 989-872-2131

rcbilly

My Feedback: (9)

Dave, can I use a butterfly style carb on the Honda GX31 I have a line on a 9.53mm unit, also maybe a WYL9.0. any input would be appreciated.