Jetting KZ 750 Twin -83

Hi!

Do I need to rejet my KZ 750 Twin -83? I have put on an open exhaust and it´s close to straight pipes. I have tried to go up from stock mainjet, 115 to 120 but it seems that 120 is a little to big. There is a size 117.5 mainjet aswell... The bike is stock exept for the exhaust. Any suggestions? Thanks in advance.

JanTaliban wrote:

Hi!

Do I need to rejet my KZ 750 Twin -83? I have put on an open exhaust and it´s close to straight pipes. I have tried to go up from stock mainjet, 115 to 120 but it seems that 120 is a little to big. There is a size 117.5 mainjet aswell... The bike is stock exept for the exhaust. Any suggestions? Thanks in advance.

I have a '79 750 twin that came stock with Mikuni BS38 carbs. Stock mains were 125, stock pilots were 40. The pilots were too lean even with stock pipes (smog laws). It actually ran a lot better with a 45 pilot. When I put on a Kerker clone pipe, it ran best with the main at 130 and a 50 pilot. I'm not sure if your engine has BS38's?

Changing from 120 to 117.5 isn't a whole lot of change for a main jet. Are you sure it's running too rich with the 120? Or is it just running crummy in general?

FYI, from what I read, straight pipes usually won't run right across the full range but do make it a lot leaner in general. They may give a little more power in some RPM band, but some say they don't but the louder noise makes it feel like it has more power.

Hey Jan,
In 82 the twins changed to a Mik BS34. The stock jetting for the 83 (perFSM) is 1.6 pilot air jet, 70 starter jet, 115 main, and 45 pilot. In 82 the EPA rules seemed to have come fully down on motorcycle jetting. For the US models they seemed to have made it virtually impossible to do any power mods on the twins. I ended up replacing the jet needles/needle jets w/those from a Canadian XS650 (same carbs). This setup not only allows more fuel availability on the transition (more holes on the needle jet) between pilot and main circuits but replaced the, clublike, 1 clip jet needle(stock) w/a tapered needle that has 5 clip positions, which allows finer tuning w/the needle clip positions. Since I not only put a Mac2-1 but also installed pods, I don't know if this same carb mod will help you. I'm running a bit rich, right now. I think I can come down a size on both pilot and mains but my baby is running like a bat out of hell:woohoo: :laugh:

Hi Ron!

I have Mikuni BS34 but I don´t know witch needlejet I have, the bike is for the Swedish market, I live there. The needle is tapered and has at least three grooves, maybe five. I tried to raise the needle one groove two years ago but it ran worse. I have tried 120 mainjet but it didn´t make any difference that I noticed exept that it popped more in the exhaust on decelleration. I hope that someone hasn´t drilled out the stock jets... Maybe I should try the 117.5 mainjet and see if it runs better, what do you think?

Sincerley/Jan

Sorry, Yan, I didn't realize you're European. According to my FSM suppliment, in '83, there was a K1, US model, a K1 Canadian/WGerman model and a Y2 (General model?). Carb difference between US and Can/WGer/Gen models, is the US model used a 5C50 (1 groove) jet needle and the others used a 5C48-3 (clipped@3rd groove from top). I don't know if anything I've said will help you, but at least you know what the stock set-up should be. I came on my solution out of pure frustration. You might want to check out www.armbell.com/forum/index.php?mforum=kz400 Ibsens KZ twin forum, there are quite a few Europeans there and someone there may have an answer to your question.
On that pop on decel, I have that also. In my case I believe it is due to a small exhaust leak, I have between head and exhaust header.
Ron

Thank you for your advices, Ron and everyone that replied. I think that my exhaust is the issue here, I will try to get some kind of aftermarket exhaust, maybe MAC or Supertrapp. I´ve seen that Supertrapp have universal megaphones, maybe they will work. Another question: Shall I go for a 2 into 1 or a 2 into 2 system?

Sincerley/Jan