Jetting kz650 after new rings and kz750 cams

Hi there,

I have been rebuilding the engine of my 78' Kz650, (new rings, hone, valvejob and stuff) and I have put in some kz750 Cams.
It starts and runs on choke... After running for a minute or so I lower it to half-choke and it keeps running and de revs go up when turning the handle. Sometimes stays there(6000rpm or so), even when switching off the choke.

When I go off-choke it still idles but it dies immediately at the slightest touch of the gashandle. :S

Jetting is 118mains 15pilot. New needlejets (258 o-8)as these were worn. Needles are at the 4th slot(second from below) Velocity stacks and and a 4-1 laser exhaust.

Jetting was fine before I switched the cams to 750ties and did the ring+valvejob. :silly:

I already tried 134 mains. Same thing...
Seams to be too lean off-idle to 1/2 gas...

I think I might need new needles.... Anyone here changed their cams to 750 cams?

Thanks!

Ilja wrote:

Seems to be too lean off-idle to 1/2 gas...

With mine, that would be pilot jets and/or needle position.

The mains primarily call the show from about 1/2 to full throttle.

Yeah, thats true. I tried the 134 mains after the rebuild to see if it would run better, as I had a flat spot around 5-6k rpm..
Then I found out about de stalling, so I switched back to 118.

Won't bigger pilot jets make it idle REAL rich? :dry:
Because I already have to turn out the airscrews quite a bit to prevent the plugs from turning black at idle. (2.5-3 turns) I don't have the screw under the floatchamber. (checked with colortune)

So my guess was needles.... Im gonna try and move it to the 5th slot today. But I doubt it will make THAT much of a difference. I'll post the results later today .

Ok guys, I changed the needle to 5th position, and it was a little better but still crap.
Then I screwed in the colortune again. This time I could turn in all airscrews to 1 complete turn.....

Now I can drive it, but it has little power in the 1k-2.5k range....

Seems like I need bigger pilotjets after all.

Im going to take it for a spin now, see what color the plugs will be.

You really need to up your pilot jets to 17.5's and you say you swotched needles? Read here about my similer problem and the surprising solution.

Ok, did a few test again.
I went out for a ride (118 mains, needle 5th slot from top), the bike didn't have much power from 1k-2.5k and I had to slip the clutch a bit when driving away. from 2.5-5k it runs nicely. Past 5k it goes blllllaaaaaarrrrrrrrrrrrhhhhhhhh(cuts out) or on full throttle... :ohmy:
I thought the main jet would be to lean because it was to lean off idle... So I switched to 135 mains....
Same thing up to 5k and after 5k even a bit more blaaarhhhhggg/cutting out. :silly:

Too rich then?
Hmmm...

I replaced the NEEDLE JETS during the rebuild. These are the jets where the needle slides in..
Because they are new, and smaller then when worn, the bike runs leaner from idle to 2.5k, and was I able to turn the airscrew in from 3 to 1 turn. So it needs bigger pilot jets.

The mains however need to be smaller I think.

Then the question would be however: Why do I need smaller main jets (used to have 118) after switching to kz750 CAMS and new rings, valvejob etc. ??
I don't like it when I need to put in smaller jet's after placing some part for more power... :silly: :silly:

Ok I'm off to trying stuff again. Thank god for stacks.

First I'm gonna try 110 mains (smallest ones I have) and see how it works, then I might change back the needle position because the cutting out from idle was due the airscrews I think...

I think I have the problem narrowed down.

Changed the needles back to 3th slot, left the 135 mains in... result: Couldn't drive away because there was no power at takeoff. (1k-2.5k rpm ) :ohmy:

However, standing still I could rev it up to 7k-8k no problem, after passing the 2k area with no power gently...
This couldn't be done with the needle clipped at 5th slot...

But with the needle clipped at 5th I could (barely) drive away because this enriches the first 0-1/4 throttle a little bit also..

So I think I really need bigger pilot jets, and the rest is fine.

What do you guys think, 17.5# or 20# pilot jets?? :laugh:

I'm leaning to 20# because I can't even drive away with needle clipped at 3th, and barely when I clip the needle at 5th slot.. :dry:

I ordered #20 pilot jets. I'll post how she runs with them as soon as I get them.

Okay, I tried the 135 mains, 20# pilots, and needle at 3th..
Couldn't drive away, engine stalled right away when releasing the clutch. And it spluttered bad when revving it up.

Then I installed 115 mains, and 15# pilots..
Needle at 4th clip.

It was drivable..
No power from idle to 2.5k rpm. Then pretty normal up to 5k
From 5-7k I could only give about 1/4th throttle to pass that area. After 7k it just pulled to redline fine..!! So the smaller mains/leaner is the right direction.

When I gave half throttle or more around 5-7k it backfired or spitted through the carb or something.
Guess it's still too rich there.

Strange thing though is when I move the needle down (leaner) the 1k-2.5k area is getting worse. But smaller main jets make it running better.

Perhaps the 1-2.5k is lean and the 5-7k is too rich?? I'll put in the 20# pilots and see if it makes a difference now.

I'm gonna order 110 mains and see how that runs.

Strange....before I build in the kz750 camshafts, the jetting was pilot 15#, needle at 4th, and 118-125 mains...

Hard to figure out what's going out without a dyno. :dry:

So it's too lean at 1/4th and too rich at 3/4th throttle..

The kz750 cams increase airflow in the higher rpm range.
The increased airflow makes more fuel come out of the main system(needle jet/jet needle).
At 3/4th throttle a bike is normally set up the richest, because this is cruising position..
That's why it's too rich at 3/4th throttle.

At 1/4th throttle however, the "hotter cams" in combination with velocity stacks are reducing airflow/ negative pressure. That's why it's too lean at 1/4th.. It always was pretty weak at that point. I guess the switch in cam's was too much.

So I would need a needle wich is thinner at 1/4th throttle, and thicker at 3/4th throttle. I think that would be hard...

Maybe I can do something with float lvl's for enriching the 1/4th throttle?

Anyone got any experience changing needles??

Ilja wrote:

Strange thing though is when I move the needle down (leaner) the 1k-2.5k area is getting worse. But smaller main jets make it running better.

Perhaps the 1-2.5k is lean and the 5-7k is too rich?? :

Try a leaner main jet and richer needle position. That will rich up the 1/4 throttle point and lean up the top end

Alright I think I found the problem....

Compression 100, 100, 82, 100 :dry:
:S

Went up to 155 with oil.......

Got new rings installed.

Think it has runned for about 1-2 hours now, the compression should have went up. I guess the cilinder bores are out of round or something..

This sucks..... im going for a new wiseco 700cc kit then.
I already got the rings for that.

I have an old wiseco 700cc kit, but the rings were worn..(had them on the bike, compression was also low 110,125,110,100 or something)
Ordered new rings, but installed the OEM pistons with new rings instead of the wiseco because I thougt the oem pistons were in better shape...

Does anyone know any good shops for the wiseco kit?