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Goped Bigfoot Engine Problem

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  • Requested: Goped Bigfoot Engine Problem

    Hi all,

    This is my first post. Looks like a great forum.

    I have had a Goped Sport 22.5cc (rebuilt now) since around 2000. Still running great with mostly the same parts as when first purchased new.

    I have recently acquired a Goped Bigfoot 22.5cc. Was running fine. Bought second hand, refurbished.

    Used too much 2 stroke oil one day, and I think I leaned out the motor. Loss of power and bogging at WOT or generally on acceleration but only under load. Revs fine and idles great when running free. Goped shop said could be compression issue. Tested very well with just simple pull start and compression tester with no load.

    I done the usual: cleaned the carb via dismantling but really wasn’t sure if a bit too much oil could have caused it to gum up. Clean the oiled plug (probably needs a new one), and this got it to rev freely, but still not good under load.

    Then I thought about air leaks. I had made sure any loose intake / carb mating surfaces ok (only a bit loose around air cleaner), but then I went around the motor with carb spray on idle.

    No change around any surface, but then the motor stopped when spraying around the spindle side crank seal - so we have an air leak.

    Crank looks a little rusty around the seal which could be an issue, but then there was no problem before the over-oiling.

    I took the the head off and it looks like the piston is mildly scored - potentially cylinder - if not just a bit dirty. Top of piston coating a bit worn also, but then so is my other ped, which runs fine.

    I am thinking that because the motor was running great before the over-oiling, that either too much oil has got in to the crankcase and weakened the seal causing the air leak. And / or - maybe even original source - piston blow by forcing too much pressure downwards, after being damaged by the lean condition.

    The loose intake manifold, and the rusty looking output from the crank, were not causing any rising issues at all prior to the over-oiling.

    I will post photos of the piston, cylinder and crank output. Do I replace the crank seals? The crank itself as well? What do you think of the piston and cylinder?

    So I can get an idea of what to replace.

    I know you can get cheap engines but may also be a good time to go for a big bore kit to 26cc (which I think is an RC top end) and maybe even the RC crank.

    Original engine is: Zenoah G23LH 22.5cc - 1.2hp

    Would also appreciate your comments on the 29cc direct bolt on and if Bigfoot could handle this.
    I know you can get 34cc with clutch etc.

    As mentioned, I also have a Sport, that may also receive an upgraded power plant at some stage, although I may just keep this one original.

    Thanks in advance, Luke.

  • #2
    Likely need a new carb or OEM walbro rebuild kit. The rubber wears out and weakens and stops the correct pumping motion for air/fuel.

    Test it out by swapping carbs between the two 23LH


    • #3
      I did a carb dismantle, clean and re-assembly. Everything looked pretty clean and general condition was good. What about the crank seal carb spray situation, causing the engine to stall? Air leak right? Does the piston and cylinder look like they need replacing due to scoring or ok?


      • #4
        All looks pretty good. If you can clean up the crank surface and there’s no pitting in it then it’s good. Otherwise ide recommend a new crank. If it’s pitted though then it’s likely causing a poor seal as you stated . Regardless I would replace the main bearings and crank seals, all gaskets, and the piston rings. I’m sure it’s time for the full rebuild


        • #5
          Thanks for the reply. Yes, makes sense. Had worse piston running ok. Perhaps the ever so slight blow-by - and / or piston ring damage potentially from the lean condition, caused the crank seal to give way - either that was from old seal and / or pitting of the crank. I think your suggestion sounds good. Carb had a really good clean and wouldn't feel 5 mins of running with too much oil could mess up the carb anyway.

          Found a nice CY26RC with Walbro 668 carb - 3hp - may fit this and rebuild other engine sometime. Works out same price.


          • #6
            Close up of the crankshaft oil seal. You can see the surface rust on the crankshaft. I don’t even think it’s the piston or rings. Potentially I could extract the old seal in situ, vacuum, fine steel wool on crankshaft under vacuum and new oil seal. What do you think?


            • #7
              Crank looks like it will clean up. But I can see a gap in the seal from here. The bearings are worn causing the seal to wear more. It’s time. Also All RC motors up to 29cc should bolt right up. I have had to trim the back of the shroud once on one of the models. Just a forewarning though, the higher hp motors tend to bounce and the spindle slip a bit more if you’re still using the cable clutch system. I would advise a lockdown if you’re planning on doing off-roading


              • #8
                That’s good advice, thanks.

                Since it needs a full rebuild may as well bore-up to 25.4cc with HP carb.

                Or replace the unit in the meantime with CY23HR which is around 2hp, up from 1.2hp currently.

                Perhaps 26cc should be the Max if everything else is kept stock.

                What do do you think?


                • #9
                  If you go with a big bore kit then it’s best to match the topend to the cases. The RC style topends have 4 transfer ports, whereas the LH have 2. It will run, but not to full potential. And if you do go big bore, then the stock LH carb will not work, and you’ll need to run an hp carb. All in all it’s more efficient to get a new motor, because it will come with new bearings seals gaskets etc all assembled and ready to go. I still recommend a full rebuild to the LH, and cy23, 26,27,29 etc as a replacement. I would stay away from the HR model as it’s just an normal RC with and undersized carb. Any of the 2.5 hp+ motors are good.
                  Last edited by dukem2; 06-05-2019, 11:04 AM.


                  • #10
                    Excellent, thank you very much!


                    • #11
                      Thinking about balance, wanted a bit more power, so G26RC is good, not 29cc, as have Original Bigfoot. May take lock out, but I like the clutch and the air tyre takes some of the "impact" + it's not off-roaded. Would probably go chain drive for this. I have a 750 spindle. I think 26cc fine for this, and because it's that much bigger than stock spindle, initial acceleration will be a bit less than say a 675 for example, meaning slipping would be limited on the spindle, thereby giving kind of a best of all worlds. What do you think?


                      • #12
                        I still use the cable clutch on mine too. The G260RC is a great choice. I would continue using the 750. There is more gripping surface and will slip less because of that. I recommend the black magic straight knurl. It grips great with minimal tire wear. The increase in hp may overtorque the 675 spindle causing even more slipping. I think your spot on in your thinking.


                        • #13
                          Yes, sounds good. I have an Original Sport also. 800 black magic spindle. May G29RC that one with pipe. Probably can take the 800 then. Runs ok on the G23LH though! Should have got a 675 or 700 really, but does roll! What do you think of CY vs Zenoah (nowadays), and also the difference between G23/G26 vs G24/G27, the "newer" improved ones. They are on offer, but wondered why.


                          • #14
                            For general interest:

                            My Sport Ped (largely original) - Engine Trix deck, gas cap, alloys, .800 black magic spindle, third bearing support, DDM tank cover, accelerator lever upgrade, Walbro HP Carb - WA-167, K&N 3.5" pancake filter with choke and velocity stack, boost bottles (via additional manifold - did actually have Nitrous via whipped cream charger and handheld tyre inflator!), used to have Rockit key, but it sheared. G23LH, recently rebuilt with new bearings, piston, cylinder, con-rod bearing etc. Runs nice. May keep original engine or swap out with 29RC with LH as spare. Third bearing support compatible spindle selection pack may be good also.

                            Last edited by Gopedhead77; 06-10-2019, 04:07 AM.


                            • #15
                              Looks snazzy. Lovin’ the original boost bottle. Haven’t had any experience with the g24 or g27 yet. The new cy’s Are much better than those first versions, but are still cy’s n do have very small flaws. Have seen chamfers on the ports that were minimal, the intake manifolds have castings burs inside, and others. Nothing that decreases engine life, just not up to the quality of zenoah. Even that may have changed in the last 10 years. So I can’t give u a real answer on that