Jetting that @#$%^!!! KZ650

seanof30306 wrote: UPDATE:

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



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.

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

That's what I would bet on. At 1/4 throttle, the carbs are "crossing over" between pilot and main jets. Slide position (needle position) is critical.

I would check for any holes or slits in the diaphragms (they can be tiny and hard to see) as those delay the rise of the slide and lean it out right where you have a problem.

If the diaphragms are good, sounds like you need to rich up the pilot jets and/or raise the needles (likely a little of both) to get some fuel increase at the crossover point.

bountyhunter wrote: That's what I would bet on. At 1/4 throttle, the carbs are "crossing over" between pilot and main jets. Slide position (needle position) is critical.

I would check for any holes or slits in the diaphragms (they can be tiny and hard to see) as those delay the rise of the slide and lean it out right where you have a problem.

If the diaphragms are good, sounds like you need to rich up the pilot jets and/or raise the needles (likely a little of both) to get some fuel increase at the crossover point.

I didn't think VM24s had diaphragms .... am I missing something?

If not already done, would assure ignition timing advancer is properly functioning and isn't sticking.
It might need lubing (at the weight pivot points), or cleaning and lubing.

If running pods, would assure that pod fitment isn't obstructing the entrance hole that provides air to the carb pilot circuit.

Good Fortune!

seanof30306 wrote:

bountyhunter wrote: That's what I would bet on. At 1/4 throttle, the carbs are "crossing over" between pilot and main jets. Slide position (needle position) is critical.

I would check for any holes or slits in the diaphragms (they can be tiny and hard to see) as those delay the rise of the slide and lean it out right where you have a problem.

If the diaphragms are good, sounds like you need to rich up the pilot jets and/or raise the needles (likely a little of both) to get some fuel increase at the crossover point.

I didn't think VM24s had diaphragms .... am I missing something?

Maybe not, thought they were regular Mikunis.

bountyhunter wrote: Maybe not, thought they were regular Mikunis.

Now I'm even more confused. I thought VM24s were the "regular" Mikunis, and the later Keihin CV carbs were the ones with the diaphragms?

I think you're right' I looked the carb up and I don't see a diaphragm...

www.babbittsonline.com/kawasaki-motorcyc...-B3-1979/070076C-123

take out ur air screws and spay cleaner through the circuit, make sure they're clear, and double check the pilot jets as well, if u dont have a fuel filter get one, if u do check it, do the simple stuff first

bountyhunter wrote: I would check for any holes or slits in the diaphragms (they can be tiny and hard to see) as those delay the rise of the slide and lean it out right where you have a problem.

Actually, the delay will richen the mixture, not lean it out. That's the main difference between CV carbs and ordinary carbs; it's virtually impossible to kill the engine from starvation because the piston will only climb according to the engine's demand. That's why a properly jetted CV carb doesn't need an acceleration pump; the slow raise of the piston acts like an enrichener. On a normal carb where the piston is operated directly by the throttle(cable), the piston can be opened fully instantly, causing a lean conditions when the air cannot flow fast enough over the jets to create sufficient suction to raise enough fuel from the bowl.

faffi wrote:

bountyhunter wrote: I would check for any holes or slits in the diaphragms (they can be tiny and hard to see) as those delay the rise of the slide and lean it out right where you have a problem.

Actually, the delay will richen the mixture, not lean it out. That's the main difference between CV carbs and ordinary carbs;

Not on mine (750 twin, BS-38 carbs). The slits in the diaphragms delayed the rise at 1/4 throttle which made the needles stay low (lean setting) in the main jets and made the bike very weak when opening the throttle. It's equivalent to changing the needle position lower which leans the thing out. Took me a long time to find the holes in the diaphragms and that fixed the lean stumble opening the throttle.

We could probably argue this to death but the reason the bike had little power with a torn diaphragm was because the throttle piston would barely lift. Whether it was lean or rich wouldn't matter as much as the fact that very little air and fuel would enter the engine - period. And since the needle goes with the piston, that will also determine jetting. Piston lower also means less fuel. And the engine wouldn't know if the piston was low because of a torn diaphragm or because you the throttle wasn't twisted on. But in this case, there would be a mismatch between the throttle settings and what the piston was allowed to do. However, just as you crack the throttle, mixture would have been rich. Rich and lean can feel pretty much identical - power is soft and misfires can take place.

You have kind of taken this post off topic, as the OP does not have CV carbs.

Isn't that the point of forums? :whistle: