No problem, I thinkI made a typo on the exhaust timing, I scribbled this down, but the intake timing is correct.
The dealers don't have these specs either,its not in the factory manual,I got the info from RCBW.

Both the owners manual and shop manual are online.
http://www.honda-engines-eu.com/en/welcome.html

under <u>Service</u> click <u>Sevice Information</u> the mini four strokes are listed under M4 range

Don't recall the gaps offhand, I have a GX31 in a GP Super Stearman turning a 18X8 at about 6500. I tried the 20X6. While I didn't tach it out, I can tell ya the 31 didn't like that prop! Turn a nice flying plane into a pig!

Thanks for that Tauri Flyer what carb were you using? I'm wondering if it was a carb/prop issue or if the 20" is just to much for the Honda. Do you have CDI or mag ignition as again I believe this alters how they perform quite a bit. I'm still waiting for my driver so cannot do any testing till I gets done

Yes, thanks for the info.  </p>

I too think a 20" prop is just too much, and a 6" pitch is just too little.  Max power occurs at 7000 rpms according to Honda, and I would recommend propping for about that.  Mine turns an 18 x 8 just shy of 7000.  It is converted to CDI ignition and a larger WT carb; open exhaust stack.  When I tried a larger prop the engine "fell on it's face".</p>

AV8TOR</p>

My 31 is pure stock. On mag with the stock carb and a straight exaust pipe. I think you'll find that there not all that much you can do to get a big boost on this engine because of the small intake and exaust valves.

NITRO.....   </p>

AV8TOR</p>

</p>

We even had a High lift cam made, for all the aggravation in getting one it added about 150 rpm.

Really? where did you get it?

<span style="font-size: medium;"><span style="font-family: Arial;">I have sent my engine to a mate who will take a look at making the driver, could be a little while before it's done though
I have cut 18 ribs for the SE5a and run out of material, I will goto Slec in Watton (UK) to top up on balsa, ply etc when I return from Italy after next weekend.
I would like to use a 20" prop for looks mainly on the SE5a but with your comments noted will probably go with the 18". Once the airframe is proven and engine to I can play with props to see what suits the airframe best. As I'm trying to do this all on teh cheap I am considering using Diatex</span></span><span style="font-size: medium;"><a name="aF150"><span style="font-family: Arial;"><span class="actregular"> 1000 then Warbirds paint ofr finish</span></span></a></span>

We sorted cheated. We used a mill to remove some of the material on the base of the stock cam. We didnt change the duration only the lift.

Prop driver is almost done now, looking forward to seeing what props I can spin up.

[youtube]http://www.youtube.com/watch?v=UOhz1fab-uM[/youtube]

It's on a home carved 18" x 8", no idea of what revs it's doing as I only have a mechanical rev counter and I am not gonna put that on the prop nut at full chat, I don't fancy slipping!

searching threads on the Honda GX engines and thought you guysmay be interested in my plane and engine.
It is the GX-25, swings a 16x8 at @ 6800 and flys the 14 lb plane pretty well.
I am going to try the GX 36 next.
FYI, the plane is a kit by AMR in Canada, highly recommend them as a supplier of kits, engine was $199 and conversion from CARR precision was $75 (the muffler was home made and red valve cover was just because because)

I could do with something like that to use as a test bed for my 31

I got brave and put my mechanical tacho on the prop nut........... it's a different hand carved prop 18" X 8" WW1 style profile but I cannot get it to swing any faster than 6100rpm. Perhaps it will unwind a bit in the air or maybe try a 18" X 6"

I have also made a manifold to get the carb inside my intended steed the SE5a and the prop awaiting stain and varnish (see below):

That manifold is half the problem and those butterfly carbs never work right either. In addtion to the fuel discharge holes being off to one side of the carb bore, they are also on the short side of the intake you made and fuel drop out is a strong possibility. I don't see a transition area for the bore size to match port size.Most butterfly type carb have a large throttle bore that need to transition to the port size, the angle of the transition shouldn't be more than 6 degrees (included angle)
A better solution would be to shorten the stock manifold and use the WY, even though not ideal.

Thanks for the post Nosedragger.

The head has a port of 1mm, manifold is 10mm inside dia, my butterfly is IIRC 12.5mm, the transition taper is only in the ply isolation block which is 10mm. I overcame the impulse port issue by adding a nipple to the manifold and drilling the Walbro to accept another nipple, a small length of Tygon connects them up and it pumps very well.
I'm not familiar with "Fuel Drop Out", by that do you mean fuel spat back out of the induction venturi? I had quite a bit of that at full throttle with a butterfly of 14mm dia, the current carb appears to be performing much better, idle at 1100rpm, throttling instant to 6000rpm and returns back to idle very well.
I have noticed no drop in performance with my home brew manifold over the stock one. I binned the original carb, the barrel type as I screwed it up trying to lesson the return spring tension, I am very reluctant to go back to a barrel carb especially as I would have to buy a new one as my local garden machinery place has no 2nd hand ones with needle adjustment.

I am keeping my eye out for a Walbro with a smaller butterfly and twin needles but they are hard to come by 2nd hand........... I have a non existent budget to work with.

Could my rpm issue be the difference between a 8" and 6" pitches?

I have the stock exhaust on, that may be another source of restriction?

Try a straight pipe on the exhaust. Even then the Honda has a mellow exhaust tone because of its very low compression ratio

Fuel drop out is caused by the air fuel mixture slowing down while traveling in the intake.The inside radius of the intake could cause that.
If the performance now is that same as with the stock carb, its probably not a problem.
The only time the stock muffler is a restriction to be concerend with, is if the spark arrestor screen is clogged. We have gutted them and all it does is make it louder.

Don't waste your time looking for a buttefly smaller than 12.5mm, if you do find one the venturi will be too small.
9.5 -10mm is about right for a GX31, any larger is a waste because the engine can't use it and the vacuum signal in the venturi gets weaker as you go larger.

If you try a straight pipe on the exhaust make the inside match the opening at the exhaust port face and leave the pipe long, paint it with cheap spray on paint, run the engine (without prop) until it burns the paint off the pipe close to the engine, where the paint stops burning off is the shortest you want to cut it.

So it looks like my inlet manifold is ok then.

I have removed the spark arrestor already so just eh exit pipe now, I now believe this to be flowing ok as it runs no better without the exhaust.

The venturi can be drilled out and re profiled to suit, I have done it with good affect in the past but perhaps not suitable for the GX31.

I will make another exhaust alter to fit inside the cowl of the SE5a but I don't think I can better the standard flow and sound. I ran it up on a nasty paint stirring Master classic 20" X 8" which weighs a ton! Whilst it only revved to 5000rpm and took a little longer to get there than with my home brew wooden 18x8 the thrust was right up and would pull my B&D workmate over so perhaps that's what I will settle for as a forum member said:

"According to Reivers', that gives about 12 lbs of thrust and 1.5 hp. (Master Airstirrer prop used as basis for calculation)"

which I would have thought plenty to pull around a 80" SE5a?

As part of our R&D we found that the intake and exhaust ports were terrible.
It looks like they used a ball mill to form the ports. They machined two holes at 90 degrees. The outside radius is OK but the inside is terrible. We removed the valves and used a small ball mill in a Dremel and cleaned up the inside radius a bunch. Doing both the intake and exhaust gained us a 100 + RPM

If you have the machinary to do this, go ahead and try it.We have over bored carbs with great sucess in the past, over bored within reason mind you.

I have no flash kit, just a pillar drill but it's worked well in the past. Since my last post I have flown the Honda with several props on and I have found it to prefer the nasty nylon masters! It's had a Menz 18x8, home carved 18x8 (broad blade) Menz 20x8 and a master 18x8. The master spun aster and produced more thrust than the Menz. It flew in a 91" Volksplane VP1 and flew it in a very scale manner needing a slight dive before looping but would roll from level easily. It's a heavy bird so will be tried in a 1/4 scale Sopwith Pup next which is light. Engine hand starts easily and never missed a beat in any attitude.

Getting there guys, very slowly, but at least actually making some progress now...

Wings and tail feathers are done, awaiting sanding and covering, same with the fuselage. Need some 6 3/8" scale wheels; I just put these little spoke wheels on it for the moment.

AV8TOR

Looking forward to seeing more details and also how it finally flies with the Honda

Thanks. I think it will fly alright. I was getting 6900 to 7000 on the test stand with an 18 x 8 prop. I think I may try an 18 x 6 to fly it. It has about the same power as my brother's unmodified Ryobi, and it flies his 14+ pound Ryan's Rebel just fine. It will loop from level flight at just over half throttle.

We'll see....

AV8TOR

do you think the CDI improves rpm?