Honda Nifty Fifty not running properly.

Hi, Just want to do a brain pick. I have a go cart project for the grandkids with a honda nifty fifty motor which I can get to idle and rev up but it will only run for a short spurt(30 -100 meters). When it does fire up it sounds like it is running fine. Am using a 200ml to 5 litre fuel mix. Sparkplug always seems wet when I take it out. It smokes a wee bit.Doesn't seem to be electric as always has a spark. Any ideas greatly appreciated. Cheers.

air filter,also take restrictor out of exhaust and throw in fire to burn the carbon build up also (serious)

Its not air filter. But the exhaust used to have a split in the side which I welded up plus it is chocka with black oil and has a tiny 20mm exit hole so I will take it off and let it free flow and see how that goes. Cheers.

20mm is HUGE for a fifty cc engine.You will lose performance if you increase the size.It's more than likely blocked with carbon, take XS1100's suggestion and burn it out.

Hi Fremo.
If you haven't solved the problem, the carb would be worth a look. The emulsifier tube running down the centre of the carb pushes out to be cleaned. It blocks up with solid stuff if left for a few months and stops sustained running.
Usually the threads forthe carb-attaching bolts are stripped because the carb has been taken off so often over the years.
Most of these NQ50s seem to have had the muffler opened up and then re-welded, so a blocked muffler is a common problem, but would it give quite these symptoms!

Sounds like a blocked muffler to me, are you running the std airbox on it! I can remember they used to be fussy on their air, even if one of the airbox lid clips were broken it could upset it a bit.

Probably find if you back the throttle of when it dies it might pick up and go again, if this is the case, give the carb jets a blow out, and use clean fuel.

Thanks for the suggestions everyone. The air cleaner is about 5" x 6" plastic which was black and falling to bits foam inside so replaced this with a piece of clean foam rubber when I first got go cart.Cut the muffler off and found it had no baffles just some rattly bits inside. Tried it like that and managed to get it to go like stink so grandkids had 3 turns each around a small circle at full throttle with the arse hanging out most of the time at 5 minutes per go before it would conk out. Grin from ear to ear was worth a million bucks. Motor didnt smoke at all during this time. Then when it stopped the last time it wouldnt go again. Have had it idling since but dies on throttle. The throttle jet needle shows quite a bit of wear as does the auto by starter needle.I would expect some loss on power but dont understand the sudden stopping.Havent actually taken the carby off and pulled apart yet.