Stumble at 4000, changed jets, stumble at 6500rpm

Ok here's the deal, removed accelerator pump and it stopped having the hanging idle. So now I have no pump 92.5 mains, 15 pilots, and needle wherever it's been set. Everything is stock for the most part, no pods, stock 4-2 exhaust, and it stumbled around 4000 when accelerating quickly.

I had a second set of carbs and I swapped in 17.5 pilots, and 102.5 mains, and now the stumble occurs at 6500rpm when accelerating quickly. Should I bump one of these things up? Both?

It's an 81 KZ650 H1, valves adjusted, relatively clean air filter, spark is good, air screws 1 1/2 turns out as well. Any thoughts?

If jet needles have different slots for clip locations, would put all clips in middle position, and do a test run.

Imo, under normal circumstances, merely removing the accelerator pump wouldn't affect the hanging idle. (If "hanging idle" means tending to remain at a higher than desired rpm when the throttle is closed.)

Good Fortune!

I believe these needles need shims, and I've never done that before so I'm a little apprehensive to try it.

Point taken on the pump thing, but I tried everybody's suggestion here and the bike would just hang hang hang. Removed the pump, and everything was good. Maybe something finally seated right, but I did WD-40/carb cleaner/starting fluid test around every seal and couldn't find a damn leak anywhere.

If not already done, carb sync may help.

Could also disconnect throttle cables at carb belcrank to assure throttle slides are fully closing by force of carb return spring alone.

Then assure throttle cables are in perfect condition, lubed, and properly routed. And that carbs definitely close completely with an audible click against the idle thumb screw when the throttle grip is released or moved to idle position. Won't hurt a thing to lube the carb throttle assembly with some light oil.

Would also check function of advancer unit to assure it's not hanging or sticking. It may need lubing or both cleaning and lubing.
The best inspection method imo is using a timing light to watch the advancer quickly move back and forth as rpm is varied by blipping then throttle.

Good Fortune!

redwood wrote:

I believe these needles need shims, and I've never done that before so I'm a little apprehensive to try it....

Wrong jet needle position, whether too high or too low, will adversely affect mid-range performance.

As earlier noted, would return all jet needle clip positions to stock (likely middle) position, per FSM, and not change them until knowing that the carbs are otherwise perfect.

Meanwhile, would also assure fat blue spark quality by visual observation while spinning over the engine with plugs removed (bases held grounded against engine head) and ignition switch ON.

Good Fortune!

All those things were done to remedy the hanging idle, nothing helped. Throttle crank snapped back, nothing is hanging up anywhere, except the idle.

I won't be putting the pump back on, so I need to figure out how to fix this jetting issue. How does one adjust needle setting without the clip feature? Where to get shims?

Best option, imo, if available, would be :

Sudco Order Number 009-395 (12 per pkg), carburetor needle shims, .106 i.d., .250 o.d., x .20 (.5mm) thick.

Might want to check with Z1E.

Good Fortune!

Sweet! Raised the needles by 3 washers and no more stumble. Incredible how much better riding it feels when it doesn't jerk around when acceleration. Thanks!

redwood wrote:

Ok here's the deal, removed accelerator pump and it stopped having the hanging idle. So now I have no pump 92.5 mains, 15 pilots, and needle wherever it's been set. Everything is stock for the most part, no pods, stock 4-2 exhaust, and it stumbled around 4000 when accelerating quickly.

I had a second set of carbs and I swapped in 17.5 pilots, and 102.5 mains, and now the stumble occurs at 6500rpm when accelerating quickly. Should I bump one of these things up? Both?

It's an 81 KZ650 H1, valves adjusted, relatively clean air filter, spark is good, air screws 1 1/2 turns out as well. Any thoughts?

I'm assuming you suspect incorrect jetting, but generally speaking if you have a stock airbox and exhaust stock jetting should be VERY close. Ocassionally if in colder climates you sometimes need to bump the pilot 1 size richer. Vintage bikes of the early 80's came jetted pretty close as EPA standards had not yet become so stringent. You commented you initially had a hanging idle ? This is primarily caused by an ir leak around the carb inlet boots. ? Before you go round and round making jetting changes, simply verify you don't have an air leak, or you will be chasing your tail for a long time..

I've been chasing a ghost issue on my '87 GSXR 750 from what I swore was lean pilot jetting. I had a stumble at idle and on top end. Turns out I had a faulty #1 sparkplug cap with a tiny crack. This was causing an intermittant spark jump !

Polar_Bus wrote:

redwood wrote:

Ok here's the deal, removed accelerator pump and it stopped having the hanging idle. So now I have no pump 92.5 mains, 15 pilots, and needle wherever it's been set. Everything is stock for the most part, no pods, stock 4-2 exhaust, and it stumbled around 4000 when accelerating quickly.

I had a second set of carbs and I swapped in 17.5 pilots, and 102.5 mains, and now the stumble occurs at 6500rpm when accelerating quickly. Should I bump one of these things up? Both?

It's an 81 KZ650 H1, valves adjusted, relatively clean air filter, spark is good, air screws 1 1/2 turns out as well. Any thoughts?

I'm assuming you suspect incorrect jetting, but generally speaking if you have a stock airbox and exhaust stock jetting should be VERY close. Ocassionally if in colder climates you sometimes need to bump the pilot 1 size richer. Vintage bikes of the early 80's came jetted pretty close as EPA standards had not yet become so stringent. You commented you initially had a hanging idle ? This is primarily caused by an ir leak around the carb inlet boots. ? Before you go round and round making jetting changes, simply verify you don't have an air leak, or you will be chasing your tail for a long time..

I've been chasing a ghost issue on my '87 GSXR 750 from what I swore was lean pilot jetting. I had a stumble at idle and on top end. Turns out I had a faulty #1 sparkplug cap with a tiny crack. This was causing an intermittant spark jump !

I chased the hanging idle all day and night, it didn't exist. It had to be a leaky accelerator pump or something allowing fuel to escape or something. After removing the accel pump, it had bogging, and I compared my jetting to what people had with non pumper carbs. So I swapped jets, raised the needles, and now it's all good.