78 kz650 Hesitation

Hey guys, I'll try and get straight to the issue here, any help is appreciated.

78' kz650 B2 with VM24SS (1020A serial # on the body) carbs,  all stock everything except dyna ignition and a bitchin paint job. I am experiencing a short hesitation, repeatable at about 1/2 throttle or so when I am rolling on the throttle to accelerate, hard, slow, doesn't matter, I can always repeat the hesitation at the same throttle position. the effect is far worse in 1st gear and is less noticeable in 5th, I assume due to the torque of the lower gear amplifying the effect. After the hesitation it takes off as expected and is strong all the way up to red line. It runs fine, starts fine, comes off choke after just a few minutes. I rebuilt the carbs with the stock jet sizes, 102.5 mains, 15 pilots, O8 needle jet, y63 (5DL3 replacement) jet needle on the 4th setting (I have tried 3rd and 5th setting, same issue), and I have the air screws out at about 1 1/2 turns based on best idle. carbs are in sync, The floats were adjusted using the clear tube method and they are within spec (1mm within 3mm from base of carb body). all lines are new, air box is new, air filter is new, spark plugs are NGK NE7 and they look perfect. Everything is new and in order. I am at a loss.

The only thing that comes to mind is the Y63 jet needle not being the correct profile and causing an issue at the transition point from pilots to main. Can anyone give me an idea as to what needs adjusted to get this thing running right. It also seems strange that my air screws are only 1.5 turns when I believe stock should be around 2.5, this leads me to believe that the pilots may be lean and they just don't have enough juice at the transition from pilot to main. I'm about to slap some 17.5's in there tonight and see what happens but I'm open to advice

Thanks in advance, I apologize if this is the wrong place to post this, I'm new!



 

Are the replacement parts used in the carbs OE or aftermarket (it appears the accuracy of aftermarket carb parts can be "questionable")

After market as far as the jets go. But the markings all match the OE part numbers other than the y63 jet needle. It could be poor quality after market parts, but it runs flawlessly besides this hesitation. 

I am no carb expert but as I understand it the point at which you have the flat spot is in the "transition period" which is controlled by the needle ?

Why did you replace the jet needle to start with? An easy tell for comparison is to use your old one and one of the new ones and, using a dial caliper, set the jaws so the taper of needle goes about halfway till it gets caught in the jaws. Put them in together, next to each other and note different heights. You "probably have a simple single taper needle". 

Hiya, this is a well known issue (if you have a kz/z650) its the carbs as they transition from pilot to needle & the only way to cure is by more modern cvk32's or fuel injection apparently.
If you register on the proboards z650 site there is full write up in the carbs section (its a sticky so easy to find) go there & all will be revealed to you & you will save a load of time & money trying to sort it on your own, good luck
the ranger

I replaced the needles because they were corroded or discolored when I originally restored the bike. It sat for nearly 20 years so anything that could be replaced, was. I'll check if I have the old ones lying around still. Like I said, the needles were the only part that did not have the "correct" marking and the only part that I have not been able to find otherwise.

I have read about the flat spot under hard acceleration and how Kawasaki added an accelerator pump to remedy, but I don't think that is what I am experiencing. 

If I find the old needles I'll drop them in and give it a go. 

 

If it Helps? Did you know thingy. Mikuni also used this adjustable needle on the Suzuki 77-79 GS550. Suzuki part# 13383-47030 or superseded 13383-47010. Same jet needle, 5DL3. The extra -3 is assigned by Suzuki for initial application clip position. Don't know what you paid for replacements but there is a seller on ebay that has 4 of them for 9.99ea. Kawasaki nor Suzuki makes carbs, so one has to use "outside the box" thinking.

Just a small update. Y63 jet needles are identical to 5DL3 jet needles. I found my old ones and checked with calipers. Everything including the taper is identical. My old ones were not brass like the replacements but otherwise the same. 

Have you tried re-fitting the originals ?

Are they air screw only or do they also have a pilot screw?  It is my understanding some did come with both.

No i haven't changed anything yet. The bike runs well enough to ride and with the weather being nice i've been riding it daily. If i change the needles ill have to re sync the carbs and i just haven't felt like doing that. I was hoping someone with a similar issue would have chimed in by now with a definitive solution before i start guessing. 

the carbs are air screw only.