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  1. #1

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    Honda GX25 OHC, NEW (WITH PICS )

    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?

  2. #2
    Antique's Avatar
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    RE: Honda GX25 OHC

    How about some pictures ??? Sounds like a winner !!!
    RC Ignition

  3. #3

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    RE: Honda GX25 OHC

    ORIGINAL: Antique

    How about some pictures ??? Sounds like a winner !!!
    I knew that was coming, I'll have to take it apart again to get some good photos, you don't want to just see the big red shroud, you can see that on the internet. Is this even of interest to anyone? I think the GX31 put a bad taste in peoples mouths, due to the weight, I hope this new series of OHC engines will get people intrigued again, it has me.

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    RE: Honda GX25 OHC

    Why do you ever need to ask any questions?
    I've never seen anybody check out so many engines in such a short space of time! in a couple of weeks you have seen more engine internals than the rest of us do in a lifetime.

    Didn't know much about the OHC model so I'm looking forward to seeing the photos.

    Good luck.
    Will.

  5. #5

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    RE: Honda GX25 OHC

    ORIGINAL: Willdo

    I've never seen anybody check out so many engines in such a short space of time! in a couple of weeks you have seen more engine internals than the rest of us do in a lifetime.
    Yeah, what can I say, I'm addicted. I just like engines, always have. Just wish I had some means to make my own parts. I need to find someone with a lathe. Soon I will build a suitable airplane, and make one of these fly !

  6. #6

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    RE: Honda GX25 OHC

    Ok, Here are some pics. Feel free to comment, I haven't read or seen anything about the GX25, so I am curious as to what others think as well. The only "problem" is the carb mount, It doesn't have bolts on each side of the port, it has a plastic tube with an o-ring that fits into the port, and then the screws are much further apart, I guess I'll just trim away most of the plastic, except for the bolt holes and the webbing, but remove the baffles and other excess material. I don't think it will look very good though. Also notice the extra "lever" or "foot" inside the cam gear, that foot rotates another tiny lobe on the cam to hold the exhaust valve open slightly during starting. That is their automatic compression release. Once the engine is running, that foot slings towards the outside, locking the additional lobe in flush with the face of the cam through some tiny linkage.




































  7. #7

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    RE: Honda GX25 OHC

    Don't know if you've seen these pictures before, ( the top two may be the pushrod model ) they're not too good I know, but I had them in my computer, can't find where I got them though.
    Your photos are great, the motor looks good ( castings not up to the usual Honda motorcycle and car standard, but probably much less costly), - look forward to more photos.

    I have a lathe etc. ( had it for 24 years, - wouldn't part with it ) and I have access to much more, pity we're in different countries! would have liked to help.
    Maybe you should try to get one of your own, you'd never regret it! - getting someone else to do it is very expensive, so it will cost you less in the long run, and you don't necessarily need a new lathe etc. - an old one which has been well looked after is just as good.
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  8. #8

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    RE: Honda GX25 OHC

    Yes, i've seen those diagrams, but thats ok, I'm looking for all the info I can get ! I'm in the process of grinding on it now. That plastic carb mount/intake manifold/cooling baffle came out pretty good. I ground away everything up to the mounting bolt holes, so I'm left with a triangle, The hard plastic cuts very nicely with a dremel, almost "machineable"

  9. #9

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    RE: Honda GX25 OHC

    Not very clear on what you mean by the "carb mount" - don't see any plastic, I see that the carb fits in with an 0 ring, and ( from the photo ) the part with the bolts looks like solid engine to me. - I could be a little slow!

  10. #10

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    RE: Honda GX25 OHC

    Very nice camera!

  11. #11

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    RE: Honda GX25 OHC

    ORIGINAL: Willdo

    Not very clear on what you mean by the "carb mount" - don't see any plastic, I see that the carb fits in with an 0 ring, and ( from the photo ) the part with the bolts looks like solid engine to me. - I could be a little slow!
    Scroll up 7 pictures from the bottom. You see a large black plastic piece sitting next to the engine. Those two studs are what the carb bolts to, but those studs do not bolt to the cylinder, they are pulled through from behind, into a hex shaped recess molded in the plastic. Look at the intake port on the engine, there are no bolt holes next to it. Then that whole assembly (black plastic, studs, and carb) gets bolted to the cylinder with 3 little socket head cap screws (one on the top fin, one back on the timing belt housing, and one down on the crankcase) The whole unit kinda Snaps into the port, because of the friction on the o-ring, so the seal isn't an issue, I guess just vibration could cause it to come out. Look down a few more pics, notice the exhaust port has the typical studs on each side, don't confuse that with the intake side.

  12. #12

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    RE: Honda GX25 OHC

    ORIGINAL: flipflop

    Very nice camera!
    Well thanks, But it's not really, in fact I don't like it at all. It's a real cheap Toshiba 2.2 Mp. I hate to blow my own horn, but I think the user has something to do with it, I am very careful about lighting, backgrouds, proper camera modes, etc. It is easily confused if you have strange lighting, dim lighting, moving subjects, subject out of flash range, etc. I have learned exactly how far to place the subject for the flash to be just right, not a big bright spot, and not too dim. I take lots of pictures, many different ones of the same thing, with different settings, and chose the best. It does have a decent macro mode, which helps for these closups, but I am usually not as close as you would think, I get pretty far away, and zoom all the way in, things don't get distorted that way, and the lighting is usually more even. Sometimes I even put the little guy on a tripod, since it has a slow shutter speed, and a small aperture, If I don't want to use the flash, they will be blurry, guranteed, unless you use a tripod.

  13. #13

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    RE: Honda GX25 OHC

    Ok, I had to take a few more pics, or no one would believe this !!!! I did some very rough grinding, just to remove the big parts of the flywheel housing, I will clean it up later. I attached a prop in the same manner the clutch attached to the flywheel, I was just dying to run it with a prop, so I came up with that method real quick. You may also notice the reduced size of the "intake manifold" I might do a bit more trimming, since it didn't seem to lose any strength so far. After I got the prop on, I realized I had no way to mount it. The crankshaft sticks out the back a little, I don't have a mounting plate, or standoffs, and my test stand is at a friend's house anyway. Honda claims it's the lightest handheld 4-stroke made........So I did just that, held it in my hand I have no idea what kind of performance to expect, It's dark outside, and my basement has fluorescent lights, so I couldn't tach it. It is so very quiet with the stock muffler, you hear the whirring of the prop more so than the engine itself. I can't believe how smooth it is, granted it shakes a little, But I didn't think you could hold any of them in your hand....anything bigger than a .049 anyway. Here is something that I suspected before, and now that I've run it with a prop, I think it's proved. It idles much slower with that big heavy nylon prop, So slow that occasionally you hear some faint ticking in the head, and then it just dies. I suspect it gets so slow that the compression release kicks in, and once the exhaust valve opens during the compression stroke, it just dies. I'm sure that whole mechanism can be removed from the cam gear, which would save weight, reduce the rotating mass, and probably idle better. It actually idles slower than you can pull it with the recoil, if you give it a swift tug while it's running, it will actually speed up.

    The weight, in my hand, with it running was 79oz. You can subtract 4.6 oz for the recoil assembly, 4.9 for the prop, and 3.2 for the muffler. That leaves 66.3 oz or 4.1 lbs. I'm not sure how much can be saved on the flywheel or the cooling fins, I wouldn't think more than an ounce or so. That would put it right at 4 lbs, wet. I was contemplating battery ignition, I weighed a CH iginition module, and a 4 cell AA battery pack, and came up with 8 oz. The flywheel is only 9.8 now, slightly less after it's machined. I don't think it's worth the expense, and the complexity to save one ounce. the only persuasive factor would be to get a bit more power due to the advanced timing.










  14. #14

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    RE: Honda GX25 OHC

    Bri,

    Don't get into too much of a hurry and end up cutting your fingers up!! It will not be worth it (I hope). I've got a buddy at work (the one with the auto-gyro that I sent you a picture of) and he has one of those mini lathes. He makes all kinds of stuff with it. For some reason, I think he wants to sell it. I'll get in touch with him and see if he does........I'll call you when I find out. If the flywheel is exposed when it's running, you could (without a prop, please ) machine on it while its running.........just a thought.

    Please don't run it in your hand any more with the prop installed. Do it like we did in the old days, mount it to a big-azz chunk of a railroad tie!

    Later
    Cars......T-MAXX, RC-10T, Lynks (you\'re cool if you know what this is!!), TS-4N Pro

    Planes......Dura-stick with a Tower .46

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    RE: Honda GX25 OHC

    Yep, see it all now as clear as a bell, ( told you I was slow! ).
    Was looking at the exhaust just as you thought

    Seems fine the way it is now, ( ie running with the prop ), not much extra weight there, and it doesn't look too bad at all.
    I do see a little untapped boss just south of the inlet port which maybe could be drilled and tapped ( M4? M5? with a spacer and washers? ) and used to support the lower part of the plastic, then the long mount could be chopped off, ( just a thought - looks fine as it is).

  16. #16

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    RE: Honda GX25 OHC

    You can get a prop hub here:

    http://www.carrprecision.com/Pages/prod02.htm

  17. #17

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    RE: Honda GX25 OHC

    ORIGINAL: flipflop

    You can get a prop hub here:

    http://www.carrprecision.com/Pages/prod02.htm
    Thanks ! I might have to give them a call, It looks slightly different than the one for the GX31 which is too bad, It would be nice to have interchangeable parts, to keep the cost of all this experimenting down.

    OK, for those who thought I was crazy for holding it in my hand, I mounted it to my workbench, which gave me a good reason to clean it off and put everything away. But just to keep up the level of excitement up, I started it by wrapping a rope around the flywheel like an old outboard motor. Actually I had to, It won't start by hand flipping, I don't think it's the magneto, There is just no compression flipping by hand, I will have to remove the automatic compression release, and I think that will help immensely, (then the mag might be weak)

    It idles real well with that 5 oz club of a prop. Steady at 1600 RPM, you can get down to 1400 and then you start hearing that ticking in the head, and soon after it dies. With that compression release removed, I bet it'll just tick over. It turned 6000 on an 18x6 (dynathrust) with the stock muffler. I removed the muffler and I saw about 6300, but it was lean, it wasn't nearly as steady with the muffler removed. I guess when you free it up and let it move more air, it needs more fuel to go with it, Makes sence, but I wonder how I can do that with this sealed carb? Any suggestions? I think an APC 18x6 might be the ticket, but I need to give a little more fuel I think, cause I don't plan on using any muffler.

  18. #18

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    RE: Honda GX25 OHC

    Nice looking engine. I looked at your photos. Looks like a lot of weight could be taken off the front. Loose flywheel and all un-needed casting webbs ect. You would have to use a small prop hub with a magnet for a CH type ignition. Can you post a photo of how you mounted the engine to your work bench? Also a photo of the entire back of engine. (firewall side). Good going so far. Do not get your hands in that prop. You may lose your love for small engines fast. Capt,n
    I never met a engine I did not like !

  19. #19

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    RE: Honda GX25 OHC

    I'm happy to report that removing the automatic compression release does in fact let you start it by hand. It still takes a pretty good flip, which I guess is normal for a magneto, but at least there is compression now, and it will start by hand. The idle isn't much lower, below about 1400 it's not so steady anymore, I bet if the carb had some mixture screws, that could be tweaked as well. How on earth do they expect these things to run right from sea level up to the mountains, with no adjustments?

  20. #20

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    RE: Honda GX25 OHC

    ORIGINAL: captinjohn

    Can you post a photo of how you mounted the engine to your work bench? Also a photo of the entire back of engine. (firewall side).
    Here you go





    That is a GX31 sitting on the bench behind the 25.


  21. #21

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    RE: Honda GX25 OHC

    The brass screw in the center of the rotating barrel (throttle) is the mix adjustment I think. Can you post a view of the engine from the back? Thanks Capt,n
    I never met a engine I did not like !

  22. #22
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    RE: Honda GX25 OHC

    O_o thats REALLY awsome! I love the overhead cam. if only there was a way to convert the exhaust valve compression release into a VTEC type operation [sm=spinnyeyes.gif]
    Mazda RX-7

  23. #23

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    RE: Honda GX25 OHC

    ORIGINAL: captinjohn

    The brass screw in the center of the rotating barrel (throttle) is the mix adjustment I think.
    No, that's just a rivet, There is no screw there, thats how the throttle arm is attached to the rotating barrel. The only screw present is for the idle speed.

    Can you post a view of the engine from the back? Thanks
    Did you miss the picture above? what more do you need?

  24. #24

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    RE: Honda GX25 OHC

    Man do I wish this Forum had sound. I'd love to hear what that sounds like.

    I'd also love it if somebody convert one to electronic ignition and get rid of that flywheel.

    Is that a standard carb mount, could you mount a big bore Walboro on it. Hopefully one with adjustment screws.
    What goes up must come down but not usually when you want it to!

  25. #25

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    RE: Honda GX25 OHC

    a couple of questions? how much did you pay for that engine? and are they readily available?


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