KZ900 hesitation..............

Greetings to all from The Great White North ! I have a 76 KZ900 with hesitation issue starting at 2500rpm. Carbs are Mikuni VM26SC and have been rebuilt by me. The top end of the engine has been rebuilt as well with the following work:
New Pistons .010 over and cylinders machined
New valves, guides and seals with intake and exhaust ports polished
4 into 2 Jardine exhaust pipes
Airbox removed and air pods installed 

The engine starts hesitating around 2500rpm in all gears when more throttle is applied slowly. If throttle is opened quickly to WOT, the hesitation stops and the bike takes off like it should. There is some back firing occassionaly . Plugs are a dark brown color around the electrode. I had to lower the fuel level in the fuel bowls slightly as the engine was running extremely rich and adjustments to the needle by moving the clip did nothing to make the engine run more lean. Things I have checked are;
Engine timing is bang on and spark advance is working properly
Carbs have been synced using old style Carb Stix
Because of the alterations to the engine, I have moved up to a #17.5 pilot jet ( factory is 15 ) and also moved up to a #120 main jet ( factory is 115 ). Needle is set to mid position. 
 
When the carbs were rebuilt, I used the kit supplied to me but upon further investigation, I have found that the rebuild kit is for a Mikuni VM28 carb and not a VM26SC. I am calling the supplier now to find out what number of needle is used in the VM28 kit as the VM26SC carb uses a 5DL31 needle. Just wondering if this needle that came with the kit could be my hesitation issue.

Any suggestions on how to resolve this ongoing issue would be appreciated folks.

Thanks

Kenny J
 

Try putting the air box back on. 

www.kzrider.com/forum/forum-index/23-for...-round-slide-mikunis

The hesitation is caused by a lean spot on the transition from the pilot circuit to the needle jet ( emulsion tube ).
You need air correctors jets ( main air jet conversion ) to cure this.
ALL carb settings ,needle position , and jets remain STOCK except for an increase of the main jet a size from stock .
Send your carb "kit" back to the vendor and return your jets and needle back to stock..
Air corrector jet kit.Kawasaki Z1,Z650,Z900,Z1000,Z1R.MK2, MIKUNI.LEDAR.PODS.K&N | eBay

The needle for the 76 carbs has a double taper.  The needle for 75 and older has a single taper.  You should go back to the original needles.  Keyster sells proper carb kits, but they are expensive because they include pretty much everything in the carbs.

Hello Zed1015. It's KennyJ from Canada. Just installed your jets the other day in my KZ900. Is it OK to call you My Hero? What a difference in the idle and accelaeration of the engine. The hesitation and engine bog is completely gone thanks to these jets. Don't know what I would have had to do to get this issue fixed but thanks so much for your parts and guidance. It is greatly appriciated ! For anyone out there looking to resolve the same issues that I had, these jets correct the problem 100%. As fall time is slowly creeping up on us, I can now enjoy riding my ride with confidence.....Cheers to all

One more example you guys ROCK.