kz900 with 1015 tuned correctly

Okay so I am new to the site! So if I do something wrong please correct me!

I have a 1976 kz900 that I have just put a 1015 kit, 28mm carbs off a 1975 kz900 130 mains and the idles are 17.5 the needle is set in the middle notch, pods, web 118 cams, header, and a street port, Dyna ignition. So the issue that I am having is the motor starts and runs great, just when I rev it over 5500 it will backfire out the exhaust! I moved the needle up one from the middle and then the bike had a tuff time starting and really seemed to run ruff. Any thoughts out there? Are my mains to small? Should I move my needle down? I do believe I have the timing adjusted correctly off of the F mark! I really think its the carbs? I will attach some photos of the bike once I get these issues figured out!

Peronsally, I think your mains are a bit big. I run 120 on my 1075cc with pods. I don't have the bigger cams, but I'm not sure how much those will affect the jetting.

But it also depends on elevation.

Drop the needle one notch (leaner) and see if it improves a little.

Have you checked your valve clearances?

What kind of header?

Interesting, I "thanked you" - clicked wrong button :silly:

When you rev the bike to 5000+ whatever, how much throttle are you using?
Jetting correctly is based on so many different factors, includin the amount of throttle.
With vacuum operated slides it can be a nightmare to try and guess what amount/position the needle is operating in.

If the bike has an issue idling/starting after the needle is moved, then your pilot circuit is not tuned correctly - back to the drawing board!

Your probally using way to much idle screw input (slides too high/butterflys open too much) which is causing the bike to run off the needle circuit and not the pilots.

Your slides/butterflys should not really be open more than 1mm or so, and the air/mixture carb screws should have a significant effect on idle quality - this is when you know that the pilot circuit is working correctly: When the throttles (slides/butterflys) are almost compeltely closed, and the air/mix screws have achieved highest idle at between 1 and 2 turns out from stop.

Start buying jets: pilots 2 up 2 down, Mains 4 up 4 down, 1 richer needle if u can find it.

You MUST tune 1 circuit at a time. Dont even bother revving the bike till the idle circuit is perfect - else you are just shooting yourself in the head!

Good luck!

Thanks for the help so far!

I have a kerker header and yes the valve clearances are fine I literally have just put the motor back together and ran it for a half hour dropped the oil and check the clearances.

I will try these ideas this weekend!

Thanks for the help!

I would check the plug caps first, then the plugs.

Is there a diff. color on one? Are they dark or light?

Ignition miss, timing advancer bad plug, cap, or wire,
low voltage at the coil.

I ran a 155 jet and it was rich but no sir no studder.

Your bike should have a 132 or so jet anyway so....

Hard starting is dead give away to a cylinder not
firing or weak spark. That stock pilot jet is not causing
an issue with too much fuel for sure..

Fuel must flow freely from the clean bowl inside the
petcock and the lines must not be angled up or with a
low tank of fuel it won't want to go up there and around

and down.

like see what I mean brother

i assume u are not using diaphram carbs, but r using stock carbs.
adjust the pilot air jets to get the highest idle(normally around 1 1/2 turns out.) then turn down the idle to around 800 to 1000 with the idle adjt screw. the bike should start idle and cruise fairly well now.
then u can worry about the needle. the mains have nothing to do with the performance at idle and the needle only at cruise and up to 3/4 throttle. the mains affect 3/4 to full throtle.

Marking the handlebar throttle grip with whiteout in 1/4s will help you figure out what circuit your running on. Seems way rich too me, I run 120s on my 29 smoothbores w/17.5 pilots

mtkawboy wrote: Marking the handlebar throttle grip with whiteout in 1/4s will help you figure out what circuit your running on.

Good advice also.

mark1122 wrote: i assume u are not using diaphram carbs, but r using stock carbs.

Kz900 never came with 28's. They're either later Kz1000 model or earlier Z1 model carbs.

What's in the bike for the ignition system?

Nads advice on checking the plug wires & caps is spot on the money for a problem like that.

If everything is assembled correctly, no air leaks, nothing like that.......

Carburetor wise - check your fuel design levels and adjust to correct height. Drop the mains to 120.

Good point, probably want to check your ignition advancer also and make sure your dynamic timing looks correct.

Okay so just to clear up my parts,

I have a Dyna III electronic ignition with the green coils
My carbs are off the old Z1 I believe they are off the 1975..... and who knows they could be shot!! But they honestly don't look bad!

This is just a thought and a stupid one I am sure!!! lol but the bike has a mono shock on it now and it has a lot steeper rake or angle.... or whatever you want to call it! would this affect the carbs performance?

Maybe Ill just load a photo! if I can figure it out!