I got a GX25 to examine and found a few interesting things I'll pass along.

The intake and exhaust valves have the head diameter as the units in the GX31 but with smaller stems(more flow for same head diameter with smaller stem)
The discharge area of the intake valve is greater than the port runner ( straight section immediately after carb)
The throat area (directly under vavle head)also has greater area than intake port,this suggests that port runner be opened to 10.5mm (.413") to improve flow.This would let the engine breath as much as the valve opening will allow with stock cam.
Ideally the smallest area in intake tract would be discharge area of valve,with the port leading to this somewhat larger to insure that port velocity is increasing as it flows towards valve.
With a .413 port the throat area under valve would still have a larger area than runner,so to maintain the decreasing port area as it gets closer to valve,the ideal port runner diameter in this case would be .453" (29/64") The next size up in carb would probably need to be used.IE 12.2 mm
The exhaust port is already in the correct configuration I wouldn't touch it,except for some blending (just round sharp edge not recontour port) of the inside corner at runner and throat intersection.Same blending for intake.
Stock port is .377" diameter,with a .256 restriction in intake, these intake restrictions serve to limit rpm as there is no mechanical or ignition rpm regulation.
They also limit power available.
Larger ports work better at higher rpm levels,and may not be suitable for lower rpm applications such as the range aircraft are using.
I am not suggesting using these modifications just sharing what I have found on the GX25.

Thanks for sharing, I am very interested in this, please keep us informed ! I love this engine, it's just pretty weak. I thought without a new cam, there wasn't much hope, but you seem to know what your talking about, and I think I may try opening up the intake. How hard is it though, being that the cylinder and head are one piece? I don't know if I can get in there to do any grinding.

The port runner is a straight shot to the throat area.just take a reamer and ream the port runner,the area directly under valve head doesn't need any work.Technically the runner should be tapered,its easier to continue with straight sided port wall.It can be tapered with a tapered reamer but you need to grind the tip of it back to get the opening at the intersection the diameter you want.
I have just stuck with straight ports until I arrive at a diameter that does what I need.
On the GX31 the port starts out at .398 I just picked the next 64th size reamer up from there and opened port in 1/64th increments,testing in between.
I'll take some pics.I have a sketch of the GX31 intake port that will clarify some of this.
The hard part is breaking the sharp edge at the intersection of the runner and throat/pocket. I was able to take a cheap needle file heat it up and bend it at the tip,rifler files would do the job very well also as they are bent.
It takes a little patience, but basically you message the edge with the file.All the work is done from the carburetor side of port so its accessible, tight but you can get in it. A grinder can be used but very carefully,you want to round edge evenly around the intersection,its easy for the tool to drop the bottom of the port floor and groove the edge.

Before anything of significance can be done with a cam this simple modification needs to be done first.By itself opening the intake port helps,you won't see a monstrous gain but roughly a 12% increase in flow will be noticeable.


EDIT:Added the following

I have some pics and a drawing I'm preparing,there are some issues on the GX25 that need close attention to avoid daylighting or breaking out of port runner.

BBriBro Did you get my PM?

The thrust HP Calculator shows that an APC 18x6 will give you 15.4 lbs of static @ 7,000 rpm and will absorb 1.5 hp. This is as close as I could get going on the engine ratings. I think you may not be getting the most out of the engine at 5,200 rpm. The 20x8 is absorbing 1.28 horsepower @ 5,200 rpm. Let us know.

Gis

[quote]Well.... turns out im only putting out about 6 lbs of thrust (fish scale). That explain's the horrible performance on its only flight so far.

I was reading about your engine problem. But first I must say you did an excellent job on the plane. I was wondering if when you are inverted if fuel is flooding back on the carb overflow/return line. I did some reading on this since there has been some question about the function of the return line. Honda says that it is a overflow/return line and does not pressurize the tank. If you assembled the tank in the classic manner, when inverted the weight of the fuel on the return vent may restrict the return and flood the engine. Also, did you run the return line back to the tank, is your tank vented? Your high speed starvation sounds like you didn't vent the tank. Something to check...

Gis

[quote] the only "issue" I noticed, was that the engine didn't want to run inverted.

My tank is vented to the atmosphere, I have the return line (from the primer bulb) blocked off, there is only one line from the clunk to the fuel inlet on the carb. I have leaned it out some and it seems to be better inverted, still not "right" but at least it doesn't try to quit, just loads up a little.
[quote]ORIGINAL: Gismo

Honda says that it is a overflow/return line and does not pressurize the tank................... Also, is your tank vented? Something to check...

Gis

The GX25 I sold to Willdo in New Zealand was turning my 18-6 Mejzlik about 7500 rpm, about 5 to 700 less than a stock G23.....
The engine was untouched except for trimming the excess from the case and removing the flywheel for CD ignition....Can't remember the weight, it's on a thread here somewhere...
Your thrust test shows how really inaccurate the ThrustHp chart is....

RCIGN,
I make it 3lbs 4oz (with original muffler, RCIGN prop driver and walbro carb, - no prop, no coil).

I'm now fitting a Walbro carb, (trial) and I figured that the pump would still work, with the intake vacuum creating a negative pulse, and then a positive pulse being provided when the inlet valve slams shut, (because the kinetic energy of the mixture builds up a positive pulse, as it keeps on coming into the inlet tract.)

Could this possibly be how the original carb works too? - just a thought, - (open to discussion ).

I agree that the restriction needs to be removed from the inlet tract, - I intend to have a tapered tract between the carb and the port on the engine.

BTW, - All work on the project has ground to a halt here, - I severly damaged my thumb ( at work, - not by a prop! ), and now, they have had to amputate the top half, so that's slowed things down considerably! - I've also got to find a way to use the sticks again!

BBriBro,
Nice looking plane! Where did you get the nice lightweight exhaust header? Also, do you think it's possible to cruise at 1/4 oz of fuel per minute with this engine?

Thanks

Jason

I made the exhaust out of a piece of aluminum tubing and a flat aluminum plate, just welded them together with a propane torch (mapp gas actually) and an aluminum welding rod from home depot, I have no experience welding aluminum, but so far it's held. I'm sure the engine will burn 1/4 oz a min, but depending on how heavy your plane is, it may or may not fly at that power level. I have a 20 oz tank in it, and I have flown for a full 60 min, I do a lot of touch and goes, which saves gas, but then again the climbouts are at full power, so that averages about 1/3 oz per minute, and I didn't run it dry, there was some left, so I'd say it's possible.

Hey BBriBro, do you still have problems with your GX25 sagging at full throttle? Mine is doing that. It acts like it goes lean at full throttle. What I think is happening is that with the throttle all the way open there is not enough engine vacuum left to go through the little passage and make the diaphragm in the carby do its thing.
Charlie

Actually mine is running good now, Mine was definately going rich, I leaned it a little and it helped the inverted flight. Which carb are you using ?
Oh, and BTW, mine never sagged right side up, it ran fine at all RPMs, only inverted it was giving trouble.

I'm using the stock carb.

Why would you think the stock carb wouldn't work at full throttle? I have even used "2 stroke" carbs which don't have the "helper" spring on the diaphragm, and they even pumped enough fuel, even though they typically have much stronger pulses from crankcase pressure, rather than using the vacuum in the intake port like on a 4 stroke. The biggest problem with the stock carb is that it has no adjustments. It's jetted for a string trimmer, when you increase the load (with a propeller) it needs more fuel. When I first got my engine, I ran it all by itself, the way it comes out of the box, and it ran great, with unbelievable throttle response for a 4 stroke, of course it wasn't turning anything. Then once it had a prop on it, I realized it could idle much much slower with that large flywheel, (if you deactivate the compression release) and it needed to be richer at all throttle settings. I thought about changing jets, but that is trial and error, so I ended up buying a carb with a high speed needle, (they all have a low speed needle, if you can dig it out)

BBri Bro-- just saw this post on your Honda 4-stroke application. What a gorgeous plane! I have been looking at these engines for a while but until now had not really heard how well they worked in a firsthand report. I had e-mailed Dave Carr a while back regarding mounting these engines inverted because of my concern of crankcase oil seepage past the rings in between flying days, as well as how they ran inverted (although I wasn't too worried in that respect due to weedeaters being used that way all the time). His reply was that they would work fine while mounted inverted, but should be stored upright for that reason (seepage past the rings). The fact that he recommends inverted mounting and one of the other posts saying his ran fine inverted suggests an isolated problem with your setup. I don't have any input or suggestions, just an observation. One other observation that I'm sure you're on top of: have you tried a dual pitch prop, like a 20-6/10? There are a couple guys at our field using gas engines in their WWI and WWII warbirds and they say power delivery and performance is better than with a single pitch prop. I don't know their reasoning, but that's their experience. Don't know if it would be different with 2- vs 4-stroke gas. Also don't think it is related to difficulties with inverted operation, but wouldn't a velocity stack smooth out engine operation since there's no cowl to block the slipstream?

Thanks for the compliments, as far as the engine being mounted inverted, Honda says they can be operated and stored inverted, with no limitations. The oil "tank" is round and contains the full quantity of oil, without it "puddling" in the bottom of the piston and cylinder. The only oil you would have that seeped by the rings is whatever was actually on the cylinder walls when you shut it down and stored it, (a very small amount) the actual supply of liquid oil would remain in the "tank" DId you see my pics of the piston? There are several rings, including an oil scraper, I would think it would seal quite well. Even if it didn't look at the old radial aircraft engines, they were known for oil to collect in the bottom cylinders which is why the smoked and coughed so much on startup.

It's also why you saw ground crews turning the engines through a few turns before starting them the first time each day( as well as to help prime them). They could accumulate enough oil in the combustion chamber of those bottom cylinders to cause hydraulic lock and bend rods. I reread the e-mail from Dave Carr and he was talking about oil dripping from the valve cover breather, just a matter of housekeeping rather than danger to the engine.

I would have no reason to think the stock carb wouldn't work at full throttle, other than observing that 1.) The RPM sags at full throttle, and 2.) after a few seconds at full throttle, the fuel level in the primer bulb decreases to nearly zero.
Charlie

Which points to a problem with the pump, not enough capacity to keep up with the engine at WOT.....It must be right on the borderline..Maybe it's made that way to keep from running the engine too fast...

BBribro,

Well done on the coversion,love the plane looks great.
I have a gx31 that i have started to convert and i want to know where i can get a prop driver thanks.
Petrolhead

Thanks, I got my parts from [link=http://home.att.net/~carrprecision/Page3.html]Carr Precision[/link]

Say BBriBro, what became of your carb experiments? I have a Honda 31cc four stroke that I would like to get a carb for. Recommendations??

Thanks,
AV8TOR

Absolutely nothing ! I waited a long long time to get a WYK 16 and found out it's not available in this country, Walbro couldn't even get one from Japan. I am still using the WYK 61, it works real good, I have no complaints really. I have the low end set pretty rich for easy starting and a very low phutta phutta idle, if it was leaned a bit, I wouldn't have that sagging while inverted, but for my application, the killer idle is more important. I think this Carb might be a bit too big for the 25, I'd love to try a new cam, but I haven't found one, This carb might be perfect on a 31.

Ok, thanks guy. That's WYK-61. Any modifications, or is it a bolt on? It has both hi and low needles right?

Take care,
AV8TOR

Yes, Hi and Low needles, it is a bolt on, it's got a primer though, so you have to block that off, that's it, and then rig some type of throttle. The throttle arm on the WYB carb that came on the GX-25 had a plastic throttle arm, it was pretty flimsy, the WYK 61 looked very similar, but was actually metal inside, with a plastic coating, so It was easy to drill and tap for a bolt on ball link.