Poor top end performance after big bore kit

[spencer2004]

It's been a week since I installed my big bore kit and I am now having some engine problems; normal use is fine but under hard acceleration, speeds above 50mph or at WOT I get what I think are missfires and I cannot go above 55mph whereas I could do 65 - 70 with my 125 cylinder. I'm hoping that it's just because the piston rings aren't seated yet but I fear that it could be something more serious. I have upjetted the carb a bit but I couldn't feel much difference.

Can someone help me out please?

[zhu]

High rpm miss usually shows a rich mixture do a plug chop and see what the plug looks like
It should be light tan
any shade of black is rich

[spencer2004]

I tried that but the spark plug was white so I upjetted it.

[DanDaMan]

check fuel filter and perhaps take a screw driver with ya next time you ride to tune your fuel mix screw (if have one) , be prepaired to stop often in is leaner out is richer , back it out 2 turns n go for a ride and tune 1/2 a turn at a time untill get no more performance increase then back it up 1/2 a turn


here is a quick test to see if she's lean , tape up your air filter leaving only 1/4 uncovered , go ride if it runs better she's lean and you need to mess with fuel mix / jetting (although your main 110 seems quite large enough so I asume just fuel mix is off )


you may also wanna peek at your valve adjustments again if backing out the mix screw (or taping up the air filter) doesnt show immediate benifits

[spencer2004]

I forgot to update that, I now have a 115 main jet. I only have problems at high throttle positions and the air/fuel mixture screw only affects low throttle positions so I don't think that will make any difference, however I will try covering my air filter that's actually not a bad idea. I noticed that the o-ring on my carb where it connects to the cylinder head is a bit damaged and I could see a gap so maybe it was leaking which would explain the problem, I have put some RTV sealant on it to seal it up but I haven't tested it yet, I'll also check the valve clearences tomorrow once my engine has cooled. If it's still running badly I'll get a compression tester to see if I have a gasket leak or something.

[spencer2004]

Ok I've got it sorted it! Luckily it was a bad seal at the carb. It's running fine now and the engine runs a lot cooler too.

[DanDaMan]

ya when I saw the post on leaking manifold I knew you had found your problem

just lucky u didnt hole something running THAT lean

[spencer2004]

ya when I saw the post on leaking manifold I knew you had found your problem

just lucky u didnt hole something running THAT lean

Yeah I suppose I got lucky, at least it wasn't a gasket leak! and I've learned something.