Fresh rebuild issues…

[STurnerXM2010]

Ok gents, got an interesting one for you. Got finished up porting and polishing an OEM cylinder and it almost feels like I’m hitting a rev limiter about halfway through the throttle curve. Absolutely cannot get it to build rpms. I’ve dropped the clip position thinking I just wasn’t getting enough fuel, but no change. Not “no improvement” but no change whatsoever. Went the other way with the needle (cautiously) and the same, no change. I’ll admit, I went a little deep on the cylinder and may have pushed it into the drag bike realm, but I really didn’t change any of the port openings except the exhaust (dropped it 1mm and widened it by 1mm total). My next thought are the electronics. I’ve run into this on my race bike and it turned out to be a low voltage issue and the CDI wasn’t able to fire quickly enough. Obviously, no battery. So could it be the stator not picking up at higher RPMs or maybe the CDI not able to keep up? Any input is greatly appreciated. In case it’s needed:

Stock carb with stock jetting
FMF Fatty pipe and silencer
OEM airbox with lid off (new filter)
32:1 mix
Piston and rings are OEM sizes as well
Heavily ported cylinder (1mm drop and 1mm widening of exhaust port)
Stock head and combustion chamber

 

[Larry's Shee]

First off, stock jetting WILL burn your engine down !! The FMF alone needs a 260-270 (I forget exactly) Add porting and you probably need a 290-300. Second the needle only affects 1/4 - 1/2 throttle. Put back in the middle notch.

Did you actually mean that you raised the exhaust?

You need to learn how to jet. Don’t run it until you up the jetting. Far better to start rich and work your way down.

Hopefully you chamfered the ports properly.

Why didn’t you do a fresh bore?

 

[STurnerXM2010]

First off, stock jetting WILL burn your engine down !! The FMF alone needs a 260-270 (I forget exactly) Add porting and you probably need a 290-300. Second the needle only affects 1/4 - 1/2 throttle. Put back in the middle notch.

Did you actually mean that you raised the exhaust?

You need to learn how to jet. Don’t run it until you up the jetting. Far better to start rich and work your way down.

Hopefully you chamfered the ports properly.

Why didn’t you do a fresh bore?

I hear you, however, I’m running off of a previous build for jetting. As for the main jet size, it’s not even a factor in this particular instance. I agree with you on increasing the main size, but that’s not what’s causing this issue. The stumble and nose dive comes right at 1/2 way through the throttle curve, hence the clip position change initially. Yes, I chamfered the ports and the exhaust port was raised by 1mm (I ported the cylinder upside down and made a mistake concerning the orientation of the change).

Concerning the issue, with the stumble occurring at half throttle, are you saying the needle has nothing to do with it? I’m inclined to agree with you only because the movement either direction had zero effect, but that doesn’t eliminate my confusion as to the cause.

 

[blasterBruce]

You will forever chase yourself if you are trying to use stock jetting settings with aftermarket pipe and + the port work , I run a oem stroke/bore ported cyl with a modded/shaved oem head and I'm running a 310 main and its barely fat enough without doing a plug chop to confirm- just finished heat cycle on head gasket- reason for not doing a plug chop yet.. you need to jet for more air flow ..

 

[82cutty]

Start with a 300 genuine mikuni main jet . Needle in the middle . Air screw 1.5 out . Make sure the washer is under the main jet . It will run like sh*t without the washer under the main . Make sure the float height is good also