Jetting that @#$%^!!! KZ650

1979 KZ650B3
Pods
Mac 4-1 w/drilled out baffle

OK, so, after 4 1/2 years of battling that @#$%^!!! KZ650, I may just be seeing a little light at the end of the tunnel!

Some of you may recall my many electrical problems (read that: "nightmares").  It looks like they are finally solved.  My super-thorough buddy Reagan went over every inch of the harness, cleaning and cleaning and replacing wires and connectors until there is virtually 0 voltage drop from the fuse block all the way out to any of the termination points.

Once we got that whipped, we turned to the carbs.  We were unable to get them right.  Reagan said he's never been that good with them, and suggested we go to a local guru, Turbo Rick, who races a turbocharged KZ1000 and has a motorcycle shop.

I took the bike to Turbo's.  As soon as he started it up, he said: "You've got a cylinder low on compression (I did).  He adjusted on the air screws for a bit, listened, held his hand in front of the tubes on the header and said: "it's #3" (it was).  I'd been thinking I was going to have to re-ring the bike, but he said he thought the problem was valve adjustment, which he said would not only cure the compression problem, but also the problems with carb synching and the blue smoke at startup.

I was skeptical, but everyone I'd talked to about this guy told me he was a mad genius, and I'd been really impressed when he'd instantly diagnosed that cylinder with the low compression by ear, so I decided to go with it, having him fix the blown head gasket while he was at it.

I went out yesterday and rode the bike, and the difference was amazing.  No blue smoke on startup, smooth idle, and WAY more power than it has ever had.  There's just one problem.

If the bike is idling in neutral, and you quickly hammer the throttle, it pops and stutters badly.

Riding it under normal conditions, there's a real flat spot when you first roll on the throttle after shifting into any gear, with a stuttering that clears up only when you crank in more throttle.

Turbo said the problem is the main jets are too small.  This seems strange to me for two reasons.  First, I ran the bike up to the redline in several gears.  It pulled strongly all the way, with no popping, sputtering, or flat spots at WOT.

Second, the bike originally had 15 pilots and 97.5 mains.  Waaaaayyyyy back in the day (but only around 2000 miles ago), wiredgeorge recommended 17.5 pilots and 110 mains, which I switched to.  I found them to be too rich.  On the coldest days, the bike would start right up with no choke, belch black smoke any time you hit the throttle, and foul plugs.  At the time, I wasn't running any baffle.  Now, I am, but it's been drilled out significantly.

Does this make sense?

Also, what would a fair price be for pulling the head, replacing the head gasket, shimming the valves properly, swapping the jets and synching the carbs?

Ok first make sure all of the following are within spec before jetting the carbs.
Compression
valve lash/clearance
timing
advance unit is working correctly
good fat spark (close to full battery voltage at coils)
no air leaks on the carb holders

Make sure the carbs are clean, second, make sure the fuel level in the bowls is correct. The fuel level is something many overlook and it does play an important part the the correct operation of the carbs.

From the stock jet info you posted I sounds like you have a set of carbs with the accelerator pump, is it still working correctly? If the diaphragm is damaged, or any of the check valves are not working correctly it can cause an overly rich mix. The other thing about the accelerator pump carbs is the cut-a-way on the slide is larger than on the non-accelerator pump carbs. This increases air flow at idle and low throttle, leaning the mix.

For pods and 4 into 1 pipe, I do think 110 is a good starting point for the main, but you may need one larger or smaller. Will need to do some plug chops to find which is best.

Though one size larger than stock pilot is usual when adding pods and pipe, it is not always required. It does make the bike start somewhat easier though. You may want to try a 15 and see if it helps with the stutter you describe.

You do not mention if any change has been made to the needle level. IIRC stock it was set to the 3rd slot. Raising it one slot will lean the mid range mix and lowering it one slot will richen it.

There is a world of difference between 97.5 and 110. It sounds like your engine is not far from right, so perhaps 102.5 is what you need.

You can also do a test up a hill letting the engine pull a tall gear and 7000 rpm or more. If the engine picks up power if you close the throttle a little, it is lean on the mains.

@faffi: wouldn't going from 110 to 102.5 make it leaner? Turbo is thinking 115.

@gd4now: The jet needles on my bike don't have clips, they have to be shimmed. Turbo says we should go to 115, then do plug chops to determine if shimming of the jet needle is needed.

I must have read something wrong, mate; for some reason I had it in my mind you now run 97.5 and used to have 110 that were too rich. Sorry 'bout that.

I consider my brother a fantastic mechanic and trouble shooter - and it doesn't come naturally for me to admire him, so he really is good - but Turbo seems to be spectacular. Considering he's been correct on all aspect so far, even to a degree that few can copy, I personally would listen to him also on this matter.

You can still check with the throttle test whether the engine is running lean or rich on the main jets. Close the throttle a bit and get more power = lean. Close the throttle a bit and get a little rouch running and less power = rich. Or you can take a dyno test where they run the engine through the gears. If power comes on rapidly after each shift but falls off a bit it's too rich. If power is slow to come back after each upshift, it's lean. You can get an idea under full acceleration on the road as well; if it bogs with each upshift before going fine, it's a bit lean.

There are 3 different circuits in motorcycle carburators.
1. Idle cicuit.
2. Midrange circuit.
3 Full throttle circuit.

You have to tune each circuit seperate from the others, such as if you are having a stumble right off idle then changing the the main jet is a waste of time its not part of the idle circuit.
If you have a problem cruising at a steady throttle that will be with the midrange circuit not the idle circuit.
Your 79 should have the Mikuni VM24 slide style carburators on it. With these when you wack the throttle open from idle will kill the motor even if they are dialed in perfectly.
If someone has replaced the carbs with later style Mikuni BS series then when you wack the throttle open from idle these should not kill the motor if they are dialed in correctly.
You need to be methodical on your approach to carb tuning and need to be familiar with each seperate circuit and the effects it has at different throttle settings. If you make uneducated changes on the carbs you will drive yourself crazy. Don't ask me how I know that.

baldy110, I totally agree with you. You have to have a method. Most people think only about the pilots or mains and think putting a shim under the needle should do it. You have to work with each circuit which includes the midrange. I wrote a little about it here. kzrider.com/forum/3-carburetor/506779-i-...-damn-air-box#507388

UPDATE:

OK, it's still not right, and the jetting change didn't fix anything.

Turbo tried multiple air bleed screw adjustments, re-synched the carbs, re-adjusted the float levels, etc. No good.

Just as before, the bike runs perfectly, except at around 1/4-1/3 throttle. In that one spot, it pops and stutters badly. If you hold the throttle in that spot, it'll just keep doing it, as far down the road as you want to go like that, in any gear. Crank on a bit more throttle, or back off the throttle, and it clears right up.

Prior to the valve adjustments, the bike ran fine, without this problem.

Clearly, adjusting the valves was needed; the bike has much more power, and gets up to redline much faster. I'm not suggesting adjusting the valves caused this problem.

It happens at such a specific spot in the throttle range, I has to be carbs, right?

If I remember right my manual shows what throttle position is effected by which adjustment. My manual if for the kz1000 but that info should be the same I will try to remember to look it up for you when I get off work.

Your throttle position would tell me the needle needs to be raised.

seanof30306 wrote: Also, what would a fair price be for pulling the head, replacing the head gasket, shimming the valves properly, swapping the jets and synching the carbs?

Around here, probably $600 - $800 delivered... maybe more.

baldy110 wrote: Your throttle position would tell me the needle needs to be raised.

That's what I was thinking. but anytime you have a stumble when you first roll on the throttle:

You MUST first set the fuel levels in the bowl right as that affects everything else.

You might try a richer pilot jet and see if it helps.

Most likely, you will end up needing to raise the needle a bit as well.