Souping up A Honda GX35?

Discussion in 'Performance Mods' started by bikejock, Jul 30, 2016.

  1. bikejock

    bikejock Member

    I'm looking to get a little more top speed out of my Honda GX35 friction drive bike. Anyone know of any simple mods I can do to this engine to gain more speed? I currently have it mounted to my 29er mountain bike with street tires & the largest drive roller & it barely does 30mph. I'm hoping there's some mods out there that can increase my top speed to at least 40mph without having to switch to a gear box or chain drive setup.

  2. HeadSmess

    HeadSmess Well-Known Member

    even with a gearbox youre pushing it...

    strangely, a smaller roller might help... too big and it slips and wont ever get a decent speed. just revs and shreds tyres.

    being a fourstroker, there isnt much you can do to improve on the honda design except by spending money...and reducing life. on that note, theyre designed for longevity and usability, not flat out power.

    pretty sure theyre OHC, so you could get the cams reprofiled. make some more grunt up the top, lose some grunt on the bottom end. good luck finding someone that can do it on something that small. then have fun sourcing stronger springs for the valves, increased wear on running surfaces, etc... but thats the one major mod that will give the best results. maybe a model machinist/mens shed type place can help. guy round here grinds his own cams with not much more than a bench grinder with a fancy jig bolted on the front. google :)

    never had a look inside my 25, but i daresay even thailand built hondas have pretty good ports. as long as you know what youre doing, theres no harm in experimenting with opening the ports up a bit, improving flow(or even reducing them for "velocity porting" which is actually a better way to go. yep. get more air into the cylinder by reducing port sizes. strange, huh? crash course in fluid dynamics...).

    intake length can make a difference on the torque curve, as can getting the exhaust as free breathing as possible(and tuned...yes, you can tune a fourstroke exhaust pipe). which would require a new carby, which you can possibly go up a size or two with(to match that bored out intake port...). but you will need one with adjustable hi-lo jets. fiddle round with new throttle cable mounts, etc... the barrel/ rotating carbs are great for convenience but i havent found any that are (fully) adjustable yet. standard butterfly types usually have no dedicated cable mounts.

    find die grinder, tear it down, modify it to be driven from a belt, use it as a supercharger...

    ignition is fixed but you can wriggle the coil around a bit, file its holes out into slots, play with the few degrees change you can get that way.

    be happy with 30MPH on a pushbike? it gets you from a to b. and back again.

    tyre diameter wont affect anything, its a friction drive.

    swap it out for a 2 stroker... my lil husky from years back used to hit 60km/h...about 40mph? with a roller diameter of about 13/16". tried 1", just shredded tyres.
  3. bikejock

    bikejock Member

    It's actually a really good motor for it's size but keeping up with traffic when most people drive 5 to 10 mph over the speed limit durring rush hour a little extra speed would be usefull in those situations.

    I currently use a 1.34 drive roller on a 29 tire. Any chance going to a 26 tire could help me gain a little more speed? I was looking at a 26 wheel set that should fit my bike for $200. It includes the front & back wheels less the tires disk rotors & caseette.
  4. HeadSmess

    HeadSmess Well-Known Member

    tyre size changes nothing. pretend the roller is just running along the ground. the wheel is simply an idler on FD setups. big tyres roll easier... skinnier tyres go faster too.

    1.34 inch or 1 3/4" ? the first one sounds about right on a four popper. two strokes want them a fair bit smaller.
  5. bikejock

    bikejock Member

    It's the 1.34 roller. It's the biggest roller Staton Inc. makes. So I guess I should stick with the 29s on my bike for good rolling.

    I thought about doing a 2 stroke friction drive build for my next project but 2 strokes for friction drives are getting hard to find at least 2 strokes that can ship to California. The Tanka pure fire 41cc was the only 2 stroke for friction drives able to ship to California but Staton Inc. doesn't seem to want to restock them after they went out of stock.
  6. floridaboy

    floridaboy Member

    Staton is showing the Tanaka engine on the site now. $319.00. I have the Zenoah G430 41.5cc. but i don't know if you could get it shipped to you. Dave's Discount Motors has it for $225.00 now. I have this engine on my bike with a Staton FD with a 1.125 drive roller and get 33mph. I think a little more break in and some tuning it will do better. The kit i bought from Staton with the 1.125 drive roller and the bearing up grade is in this link.
    Last edited: Aug 23, 2016