KZ650 Midrange Bogging

Hi all,

So close to getting this bike right, but it's failing to pass safety inspection because starting around 3.5k rpm there's some heavy bogging. Under this is just fine, and I usually shift by around 3k so I haven't noticed it until the mechanic mentioned it.

I know that the needle jets are responsible for the mid range, but how do I go about fixing this ? It idles great and is running smooth before this ~3.5k rpm.

Main jets are mid range. 3.5k rpm is just about exactly the point of swopping from pilots to mains. What size main do you have in it?

Looks like 112.5. Does this condition call to try and increase or decrease the jet size?

Good news is they are genuine mikuni jets you have. What exhaust and air filters are you running?

Pod air filters (didn't have stock airbox when I got the bike) and I'm not sure what exhaust but its a single exit and not stock.

When I actually rode it, I didn't feel severe bog as the mechanic made it seem like. It was more that there wasn't a strong throttle response through that band of revs. Above 3k to about 7k. It still pulls, just not as strong.

I remember at one point there was a lot of rust collecting in my in-line filter and some red stuff go through to my carbs. I cleaned it of course, but haven't checked it again and it's been about a month. I'll check that next. Maybe I need to clean it out and refinish my tank to make sure no traces of rust get into the carbs.

With pods you need air corrector jets to cure the off idle hesitation.
Read the attached thread to the end for all the info etc.
www.kzrider.com/forum/23-for-sale/611992...lide-mikunis?start=0

Fit them by all means but I dont find them really required on 650's. 650's are over carbed with vm 24's compared to 900's and 1000's with 26 and 28's. My black 650 is running all sorts of go faster engine bits internally and un filtered stacks and a dekelvic straight baffled exhaust. I have just one up on pilots and 1 up on mains from stock with clip in position 4 and it'll pull cleanly from tickover to way past the redline. With that set up airscrews are 2 1/4 - 2 1/2 out to achieve a nice plug colour.
I would try 1 up on mains first and see what difference that makes.

Hey guys so since the last time, I've definitely got the bike working better but I still have this condition, although it's slightly different.

I'm also more comfortable riding it around the city the past few days so I have a better idea of how the bike behaves.

Idle to a bout 5k is great, pulls strong. It's struggling to get passed 5k. There's random times when it gets closer to 6k but then the revs don't climb anymore with the throttle, it just gets stuck. The exhaust note changes in sound a little bit. If I pull the throttle open FULLY at this time, the revs start to drop. I can only increase my speed if I shift up a gear and the revs are back under 5k.

In this case, what am I witnessing ? Too rich at over 5k or too lean (is there a way to tell)? Does the same advise given below apply and I should try increasing main jet size? Or does this indicate something different.

the only thing I know to suggest is carefully lift the choke lever when it stops gaining RPM. if it gets better then your likely starving for fuel. Be very careful. you could also get it up to max RPM and shut it down. coast to a stop and check the plugs.

Thanks Mikaw. After a few more rides, I noticed that if I crack open the throttle passed that flat point, it pulls nice and hard! There's a slow consistent throttle response if I push passed the slower 5k rpm range, maybe 75% throttle or higher and i can get passed this point. It revs all the way out properly after this.

I noticed that when the bike is cold it's especially sticky around the 5k rpm range but after a few minutes of riding and warming up, it's much easier to push passed that point. In lower gears it's easier, in 4th it's harder.

I pulled the plugs after a few riding sessions, varying from street to highway, and the plugs look great.

What do you guys think this indicates? Should I increase the main jets?

Id ask Zed1015, he is very good at diagnosing carb issues. MY experience would say if plugs look good at WOT and Lower RPM is good then the transition from mid range to Fully on the Main is controlled by the jet needle. I'd say drop the C-Clip on the needle one notch. But maybe Zed will chime in.

Unless you kill the engine at different rev ranges coast to a stop and pull the plugs you are only seeing the mixture at idle so those plugs looking good only means your pilot mix is ok.
Did you try the choke test as the point of hesitation ?
Did you fit air correctors ?
Adding extra fuel by lifting needles, fitting bigger pilots etc only masks or shifts the problem and ends up over fueling the top end and reducing overall power.