Crap top end after big bore kit. Is a bigger carb the answer?

[spencer2004]

So I've got my big bore kit all run in now but I'm not too pleased with the top end; the bottom end is much better but the top end is poor. I can only do about 65mph now whereas I could do 70mph consistantly before I installed the big bore kit, even in low gears I cannot get above 7k rpm and I could do roughly 10k before. I was wondering if the stock carb is limiting the ammount of fuel/air mix getting into the cylinder since it now needs 20% more per intake stroke, am I on the right lines here?

I read somewhere that I have a 22mm Mikuni already and the Molkt 26mm will fit right onto my existing intake boot but are there any better alternatives? I don't want one that will be hard to get the jetting right.

[td_ref]

"even in low gears I cannot get above 7k rpm"
is this same Carb now you are able to reach 10k rpm? less than 7k rpm isn't normal.
My big bore bike in 2nd or 3rd gear climb to redline (10kprm) very quick at open throttle, for your comparison. Also, better fuel mix should have reduced engine vibration at high rpm, my experience of jetting Carb.
hope these help.

[spencer2004]

Hmmm that's strange, the thing is it will reach 7k pretty quick and it might slowly get to 7.5k or 8k which made me think that my carb might be a limiting factor. You do have the same engine and carb as me don't you?

I'm going to re-check my jetting and fit a new spark plug and see if that makes any difference.

[spencer2004]

OMG what a knob I have been! I had it jetted too high! I had my main jet at 110 before I installed my big bore kit and put it up to 115 after, today I put it up to 120 which made it even worse so I went back down to 110 and it's running great! My missfire and performance problems are pretty much gone and when I tested the speed I got up to about 71mph with a fair big of head wind!

[euphonius]

Damn, I love happy endings!

[spencer2004]

For the record I found that even a 110 main jet is far too rich so I moved down to a 105 and it's now running great with no missfires or performance issues. I thought that a larger displacement would need a bigger jet but I guess that as it needs to suck in 20% more air/fuel per stroke it reduces the pressure inside the carb which means more fuel can be sucked through the main jet.

[arancara]

Brilliant information, looks like I'll be doing the same as I'm in this same boat except I jetted up to 130main from looking at what jets gy6 150cc engine were running on a 30mm carb, now I feel like a tw@t, is there any info if 100 main is better still than the 105 main jet as I know with a 110 I was running slightly rich?

[DanDaMan]

more lean generally means more power , but at a higher heat for motor , each persons jetting depends mainly on how THEY ride THERE bike , if your screaming along a highway at wothrottle a lot , you want a bigger main , if your a idler/stoplight to stoplight burner n can go lean with big idler

IMO , u want it rich at idle , and light tan at 3/4 throttle , and nearly white (stupidly lean) at WOT in top gear near max speed - get that fast , hit stop engine switch n clutch n pull n check plug , unless your a highway biker then u want it light tan at max speed

u get that with a big idler and a smallish main

myself a ? 110-112 don't recal but with poor man air ram run light brown at 70-75 indicated - highway mainly bike

[arancara]

I'm really struggling with mine and my pz30 carb, will try the stock pz26. I'm sure it's a fuelling issue as I had it running on the 30mm when the valve clearances were way off.

I helped another guy with the big bore kit and he is actually running but the only difference between our two engines is my cams have a +1mm lift. on the pz30 I can't even get it running at all on open throttle on jets 105, 110 and 115 I can hear it pop as if it's just igniting, really annoying as when it was a 125cc I had it running perfectly on the pz30, any ideas as carbs and me don't go together!

[spencer2004]

These PZ30 carbs seem to be more trouble then they are worth, prehaps I just got very lucky when I installed mine as I didn't have to make many adjustments at all. I think most of the PZ30s on eBay are cheap knockoffs and poorly made, prehaps your's is faulty?