Coughing, Sputtering, Hesitation. My mystery!

Hi, I've been riding my 1983 KZ650 CSR with 26K on the odo a lot lately and I've been noticing a stumble any time the bike is in the 5-6.5K RPM range (according to the stock tach, so thats probably off!) The stumble is more apparent while in this range if I apply greater than 1/4 throttle. This is especially annoying because all reasonable highway speeds fall in this RPM range (60-75mph approx).

The entire bike is stock (incl. jetting) except for a 4-1 kerker setup. I haven't opened up the carbs, so I can't be for sure that all of that is stock. The bike has just recently been tuned up. And was running great for 150 miles or so.

This bike has absolutely no other running problems as far as I can tell, starts right up, idles nicely, and after 6.5K picks up the speed with no/little hesitation.

Here are my theories, feel free to tell me if I'm headed in the right/wrong direction or if there is any other suggestions.

1.) Exhaust needs packing (maybe pipe resonates at this RPM range or something)
2.) Low power to coil(s)
3.) Bad coil(s)

I've ruled out fuel delivery as the potential cause.

Add two more:
(4) Bad plug wires
(5) bad plug caps

Those two can be measured for resistance with a multimeter.

Ahhh.
I replaced the plug wires and caps when I got the bike 3 months ago....

So pretty much everyone is thinking ignition troubles?

I thought when a pipe is added you increse the main jet a size or two. Does it pull to redline from the stumbling point? What do the plugs look like?

OKC_Kent wrote:

I thought when a pipe is added you increase the main jet a size or two. Does it pull to redline from the stumbling point? What do the plugs look like?

ok, you said it pulls to redline, I missed that. Maybe the clips on the needle need to be checked.

As I understand it, there is no adjustment on the needle jet. Was probably an emmission requirement in 83.

Plugs are pretty much white, but I haven't done a real plug chop yet so I dunno whats really going on. WG told me that with a 4-1, there isn't much need to rejet unless I feel stumbling 7-9K full throttle, which I don't. I might upp the main one for good measure anyway.

Packing the exhaust did help a bit, but because of its fairly sudden appearance, I'm betting its gonna end up being a coil (low power or bad coil). I also had a weird problem where the No. 2 cylinder wouldn't fire on choke, presumably because it didn't have enough power to fire with the A/F ratio low. I don't know if the bike still does that, because its warm enough not to need any choke here in michigan.

I'll let ya'll know as soon as I know.

i just today ran into this on my friends new to him 81 kz650. #2/3 coil had a intermitten open on the + side . i cleaned it with emery cloyh and installed a new spade connector and granted it's only been 6 hours, no problum so far

newbikekiller wrote:

Plugs are pretty much white, but I haven't done a real plug chop yet so I dunno whats really going on.

Do a plug chop and see if they are still white, if so, it may be beacause of less packing (less restrictive)

If I am not mistaken, that is close to the point where the advancer should be wide open. Have you checked to make sure that it was working correctly?

I must say I haven't had the chance to check that while the engine is running, but I did clean the crap outta that thing to get it running right the first time, so I think your right, the advancer is suspect!

Thanks for the help.

Are your intake boots leaking at the head?
Got enough fuel flow?
I've got an open pipe and mine runs fine.(just a cover to fool patrol men)

Post edited by: Snakebyte, at: 2006/04/11 14:38

Well heres the problem as it stands today
1.) Swapped coils
2.) Swapped advancer unit
3.) Powered up coils w/ WG's trick
4.) Packed exhaust
5.) Got rid of inline fuel filter (want a clear one anyway)

It seems that the bike isn't revving as hard as it could on the top end, but its hard for me to tell because my tachometer is pretty much all over the place under hard acceleration.

There is sometimes a slight stutter before 4K at WOT, and there is also a noticable stutter when travelling at high speeds between 5-6.5K (approx from tach) accellerating between 1/2-Full throttle.

Here is my summation
1.) Ruled out any low-power ignition issue [Thanks kawasakiman]
2.) Ruled out any fuel flow issue
2.) Still need to dynamically check timing (and adjust) [Thanks Tazzman]
3.) Check intake boots [thanks Snakebyte]
4.) Need to get adjustable needle jet and main jets 1+2 sizes larger. Clean carbs, set fuel level, install 1 size larger main jet and do a plug chop under 1/2 throttle and full throttle conditions. [Thanks 77KZ650]