Motorcycle Repair: engine miss on kawasaki GPZ400R, air fuel mixture, kawasaki gpz400r

Question
Hi Ive got a Kawasaki GPZ400R (ZX400D). The engine
seems to have a miss and lacks power. I changed the plugs & the enigine runs a little better but the miss is still there, lacking power. What is the next step here? I did have the exhaust (4 into 2) changed into a 4 into 1 some time back. I've been told that i should have had the carbs rejetted, is this so?
Thanking you in advance.
baroque@eject.co.za

Answer
Hi Adam,

From the description provided, I suspect the air-fuel mixture may need tuning to restore proper performance.

I would need to read the spark plugs to determine what changes to makie to the carburetor A/F mixtures. Basically, any changes to air intake and exhaust requires re-jetting.

Check cylinder compression and valve clearances. The carbs need both of these to be within specs before carbs will function properly. Next, perform a spark plug chop to learn what changes need to be made to the jetting.

Respectfully,
Mark Shively