Bogging down/hesitation at high rpm

[Allyboy]

Hello, as usual I'm after some advice. I've just done a top end rebuild on my gpr 50. I replaced it with an airsal tech piston/barrel kit. Once I done it bike started no problem at all. Spoke to airsal about running in procedure and they said warm bike up to operating temperature and ride normally.i took the bike to my work yesterday (25 Mille round trip) it went great on the way there but on the way back it started to hesitate a tad at high rpm. I went to start bike tonight and it really struggled to start but it did do after I took plug out turned bike over and cleaned and refitted plug. I took it for a ride and at high rpm the bike started to hesitate/kangaroo pretty badly. I think fueling is the issue but I'm not sure, I haven't changed the exhaust or carb it's all the same and the previous barrel and piston was a 50cc. So any ideas where to start? Thanks

[jkv357]

Most likely you need to increase the size of the Main Jet if the Airsal kit is a big bore.

What did the plug look like?

Any chance there is a restriction in the fuel system? Plugged petcock pick-up, plugged filter, kinked line - anything that would restrict flow?

[Allyboy]

Plug looked a bit black and oily( I did add a little 2 stroke to tank incase of any air locks in oil pump as engine was out) the kit was a 50cc same as previous one but has more ports. I added a fuel filter to the system but all the hoses are ok and not kinked. Only thing that's getting me is it ran good yesterday for a good 25 miles but 2nite it started to hesitate

[toratora]

It sounds like an ignition issue to me. Did you make any changes there?

[Allyboy]

Nah no changed made to ignition. Buy the top end kit said it had different timing options! The instructions never mentioned anything about it but that's what's mentioned in the explanation on sellers website

[jkv357]

How strong is the battery?

Is the fuel fresh? How much oil did you add to the gas in the tank?

I might drain the tank and refill with fresh gas (no oil) and try again. The fact that the plug was wet tells you that it's running rich at some point or has too much oil.

A weak battery can cause issues as well.

[Allyboy]

Hello, Battery is good! Starts bike no bother and holds a good charge. I done 2 litres mixed at 32 to 1 then once I was happy pump was working i I added 6 litres of straight fuel to dilute it down. It's a bit smokey when it starts but it clears down quickly. The think that gets me is it done a good 20 miles before it started playing up.

[jkv357]

Did it run poorly after you diluted the gas/oil mix and cleaned the plug?

Would you say it was popping, burbling,, flattening-out, or surging at high RPMs? Do you have another plug you could try?

Air filter in good shape and nothing blocking the opening?

[toratora]

Maybe something came loose with the ignition, or maybe the carb. But it sounds like an ignition thing to me, based on the description.

[Allyboy]

Hello again. My bike has been at the shop for a few days as it needed an mot. I also asked them to look at my misfire issue and they found the cause. They took plug out and turned it over and a good half litre of fuel flew out plug hole. They took tank off and found the fuel cock was giving constant fuel instead of working off vacuum. The ran bike off its own bottle in the dyno room and went well and produced 8.2 Bhp!! That up 1.1 Bhp from 2016!! Just waiting on fuel cock and I can get bike back and fingers crossed no more issues ( well for a couple of weeks anyway) thanks for all the advice