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Thread: Honda GX35


  1. #1

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    Honda GX35

    Does anybody know how much more power the Honda 35 has compared to the 31? I'm wondering if it would be worth trying. I have the 31 turning about 6600rpm's with an 18/8 prop.

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    tim220225's Avatar
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    RE: Honda GX35

    My 31 runs about the same. The 35 will turn a bigger prop and weigh a little more. I am still looking to work over a 35 but have not found one.

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    subarubrat's Avatar
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    RE: Honda GX35

    I fly both and I would describe the 35 as more of a workhorse and the 31 as a bit more responsive. The 35 will spin a 20x8 prop, more than the 31 which is happier with a wide blade 18x8 but it revs easier. What plane are you building?
    Like classic video games and pinball, check out www.retrogamingroundup.com

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    RE: Honda GX35

    I'm building an SE5a from FunAero plans, blown up to 90 inch wingspan.Weight will probably be around 20 Lbs. more or less.I was hoping to use a 20 inch prop,but I haven't tried one on the 31 yet. I know I'll lose some rpm's, but should gain some thrust, hopefully.

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    subarubrat's Avatar
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    RE: Honda GX35

    The 31 isn't happy with a 20 inch prop, your better off with the 35.
    Like classic video games and pinball, check out www.retrogamingroundup.com

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    RE: Honda GX35

    RCSPORT FLYER did you do any mods to your 31? What carb are you running?

    Thanks,
    AV8TOR
    If it is not SCARY, it is NO WHERE NEAR powerful enough!!
    All R/C planes have expiration dates---> It's just not printed on them anywhere!

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    RE: Honda GX35

    I changed the carb to aWYK-38-1,enlarged the intake and exhaust ports and polished them.I haven't gained one rpm by doing this. Kind of a puzzle.I'm using the APC 18/8W prop. Around 6600 RPM in cool weather. Hot weather seems to really slow it down.

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    RE: Honda GX35

    If that Honda is a 4 cycle....you may have to build or buy a high lift cam! Capt,n
    I never met a engine I did not like !

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    RE: Honda GX35

    Just a quick note.

    We use to remove material off the back of the cam lobe in our B&S go cart engine.
    Dad was a Ford mechanic and he sanded the the non lobe side of the cam and re polished it with copus cloth.

    He said there was very little pressure on that part of the cam and it would last as long as the engine would.

    In retrospec it simply gave more lift and quicker cam timing.

    You should be able to find out how much lift your engine could take before the valve spring bound up
    and go from there.
    Remember about the rocker arm ratio.

    Ralph

  10. #10
    av8tor1977's Avatar
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    RE: Honda GX35

    Now why didn't I think of that.... of course, reduce the base circle diameter of the cam, and you will gain lift. Neat idea....

    Thanks,
    AV8TOR
    If it is not SCARY, it is NO WHERE NEAR powerful enough!!
    All R/C planes have expiration dates---> It's just not printed on them anywhere!

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    RE: Honda GX35

    Flyer,
    Here is a thought. If the GX31 piston is taller above the wrist pin then the GX35, the rod must be longer then the GX35. Due to the makers use of standard needle bearings, the longer rod would have the same bearings. So, put a GX35 rod into a GX 31 and get higher compression. If this is true, you may have to notch the piston slightly for valve clearence. Who would know?

    Dave
    Every day above ground is a good day.

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    RE: Honda GX35

    Guys here is what I know about the pistons etc. For starters here are the bores and strokes for these engines:

    GX25 33mm bore 26mm stroke
    GX31 39mm bore 26mm stroke
    GX35 39mm bore 30mm stroke

    The 31 piston needs to be fly cut on top to fit the GX35. The area above the top ring is taller or thicker than on the 31 piston. What you will do is increase the compression ratio. Stock is somewhere in the 8's as I recall. There is a formula I have somewhere but suffice it to say that you can get 10.5:1 safely with this mod and the engine will run fine. The issue is a coating of some sort on the pistons. You can break through it with a fly cutter, however no one has been able to say how if at all eliminating this coating will affect piston longevity. I talked to and got this information from a guy on the East coast that builds these engines for boats. He claims to get in excess of 10,000 RPM from these engines. He also produces cams for them. When I talked to him last I decided that for my use the GX31 I have would not gain much by swapping cams. Maybe with the piston mod on a GX35 and adding a cam you might be on to something. Now if I could only find a GX35. They have them on Ebay now for $225.00. By the way when you do these engines remove the compression release crap from the cam. I ran mine with a CH ignition and it would idle so slow the compression realease caused issues. Removed it and it runs great.

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    RE: Honda GX35

    Tim,
    Who is it that builds these engines? I regularly get my gx 35 upto 11,000 rpms attached to my belt drive bicycle kit on the downhill side of a bridge that i ride every day. If I hold the trottle wide open with the rear wheel in the air I can get my tachometer/ hour meter to show over 10,000. The engine is stock. I removed the decompression system a while back due to starting issues and runs much better now. I also tapped the exhasust threads to a use 6 mm bolts instead of the the 5mm studs.

  14. #14
    av8tor1977's Avatar
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    RE: Honda GX35

    Yeah, there is some combination of GX 31 and GX 35 parts that yields higher compression and more power, but I don't know what it is.... There was a guy doing cams for them too, but last I heard he wasn't doing them anymore. Check the boat forums. Those guys are real into hopping engines up and are very creative. They don't mind blowing a few engines up trying to find a good combo that works.

    But if you really want to "rock" with a four stroke, try the Stihl Four Mix engines. There are several sizes in the 32 to 36cc range, and a brute of an engine; their Four Mix 65cc. Believe it or not, it makes about the power of a Zenoah G-62 engine, but with the sound and fuel economy of a four stroke. They are also a good bit lighter than the Honda engines, because they use a fuel/oil mix like a two stroke, and do not have a heavy crankcase containing oil and an oiling system.


    AV8TOR
    If it is not SCARY, it is NO WHERE NEAR powerful enough!!
    All R/C planes have expiration dates---> It's just not printed on them anywhere!

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    RE: Honda GX35

    I've been using a modified GX35 for 3 or 4 years, it turns 20x8 master at around 7100RPM in a 104" glider tug/lolly bomber and has plenty of power.
    I used a GX31 piston and machined the top down to give it about 10.5-1 compression, filled and reshaped the inlet port and unshrouded the valves, removed the compression release in the cam, gutted a GX25 muffler, installed a RCXL ignition and fitted a walbro carb from Dave's Discount motors that has the next size up bore, 10.5 mm I think.
    A great little project, but not cheap!
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  16. #16

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    RE: Honda GX35

    Very cool project now you have a great Engine that purrs nicely & with good milage too .
    Did u make a prop flange for it or is there a shop that sells them ?
    I heard that the stock card doesnot like inverted flights . What u did with yours ? point me to where i can buy a new carb so i will fly high above 2000m without caughing alot .

    Ken



  17. #17

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    RE: Honda GX35

    Mono, do you have any video fo that GX35 so we could see & hear how it runs? Is that a rotary valve carb? I like that primer bulb too. Thanks Capt,n
    I never met a engine I did not like !

  18. #18

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    RE: Honda GX35

    Hi captinjohn & ktan 73,
    I don't have a video but I'll try and post one this weekend.
    The carb came from Dave's Discount Motors in the USA, it's the same configuration as the original but has a slightly large bore and 2 mixture needles, it is a rotary valve type, Walbro WYK-194.
    The priming bulb is great, just keep pressing it until there are no bubbles in the return line, plug the bell mouth and flick until it fires, one more flick and it just starts and idles.
    I made all the brackets and adapters for the prop hub and RcExl ignition.
    It's performance is very deceptive, it never sounds like is straining and with the 20 x 8 master will climb out at 45deg, the aircraft has 104" span and weighs about 10Kg.
    Don't ever let anyone tell you that these are quiet with an open pipe, with the compression raised and ports cleaned up this thing has a very loud crackle to the exhaust and does not perform as well as it does with the muffler.


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