kz750 jetting question (or general jetting question really)

[runmikeyrun]

Hey guys,

I have a thread on kzrider.com about this, but they were hacked a few days ago and the site is still down. While i'm still emotionally coping I'll post here.

I've had an issue with the bike not being able to run past 7k. it just wont go, doesnt pop or sputter it just won't go. So after much research and checking a lot of external things first I pulled the carbs. Here's what I found:

Notice that the second carb from the left is missing the secondary jet.

So I'm pretty sure replacing this will solve my issue, as the bike has been running fine otherwise. Carbs were cleaned, rebuilt and synced last summer just before I purchased it. I'm guessing at the least I should resync them.

My question is- the bike has an aftermarket exhaust and K+N pods. I've done a lot of research on jetting with these add ons and most people recommend 68-70 on the main and 130-140 on secondary. Mine currently has 62 (stock) for main and 136 for the secondary, of which stock jet is 125. Since the bike has been running otherwise fine with the jetting as is, is there any need to bump the mains to the recommended 68? I'm always concerned about a lean condition, but i've had very little popping thus far which I suspect is the pilot circuit as it only happens on decel. Or should I be ok to replace the missing secondary and carry on?

Thanks in advance.

 

[Nebr_Rex]

Get the the Dynojet kit. Just did my 750 and it runs great.
You will get the main jets, needle, drill bits for the slide and idle screw access.



.

 

[runmikeyrun]

I wondered about that as well, because I saw people doing that. I'll have to scrape together some cash for that... but thanks for the recommendation!

 

[jeffw]

May not need to get the dynojet kit and just buy the correct main and pilot jets and adjust the needles as necessary.
Also check this website as a good guide for adjustments based on what you did http://www.motorcyclecarbs.com/Motorcycle_Carburetor_Jetting__W7.cfm
Could be fuel line - starving fuel if it's rpm related, my 650 had this issue and eliminated the loop in the line and got a new fuel filter and no issues afterwards.
Also may not getting enough voltage at your coils which would also cause this issue.

 

[runmikeyrun]

Ok, I replaced the missing jet, which actually happened to be bouncing around inside the float bowl.

I ran it up and down the street and while I couldn't physically run it fast enough to get it up to 7k (in first gear I couldn't keep the front wheel on the ground and in second I was going about 60mph at 6.5k, too fast for a wet side street) it ran great and very strong. No pops no stumbles, nothing. Idles great. When it clears up and warms up some I will take it for a tear down the highway to see if it will rev past 7k.

My pilots were all 5.25 turns out which is ridiculous, but I'm thinking if it's running fine then I'm not gonna start messing with it. If I have problems I will delve deeper.

 

[Nebr_Rex]

Air temp has an effect on jetting also.

 

[MotorbikeBruno]

My pilots were all 5.25 turns out which is ridiculous, but I'm thinking if it's running fine then I'm not gonna start messing with it. If I have problems I will delve deeper.

This is your lean condition. Most rules of thumb on pilot jets are 4 turns out MAX. Anything past that and you need to bump up the pilot jets (under the plastic block off part) Be very careful when removing those, as they have an O-ring to keep them in place and can be stubborn, if you need to, rock them VERY gently while wiggling them up. You really should do this, regardless if the bike is "running well." Since it's the correct thing to do Love the KZ's I think they are under appreciated if you ask me.

 

[DesmoBro]

I know exactly your problem!!!....Not Carbs!!!.....timing advancer is not advancing ....its actually probably rusted to shit....mine was ....i jetted and rejetted several times ....advancer was still fucked.....take off i believe right side cover and behind the timing plate thing is this funky spring looking thing .....Take that off lube the shitoutta it til it springs back freely....re install and do 80 mph in 1 st gear....( probably even with a missing jet)


Thank you ...ill be here all week....

 

[runmikeyrun]

that was the first thing I checked, I tend to go from simplest to most complicated. Everything there was working fine. I appreciate the suggestion!!