Still have rpm hang between shifts

I finished up with the maintenance on my '80 KZ1000B4 which included a valve adjustment, carb cleaning/rebuild kit and sync, new intake and air box boots, new throttle cables (push/pull), new exhaust gaskets and new fuel and vent lines.

The carbs are VM28SS with the accelerator pump. Float levels are good as checked with the tube method. Main jet is 110, pilot is 15. I have battery voltage at the coils (grey Dyna's) with the stock ignitor and good spark. I'm also running a Kerker 4-1 with a packed baffle and stock air box.

Before the carb rebuild, the mains were 115 and plug readings always indicated that it was rich. Now, after a spirited ride this morning (in 38 degree weather...BRRRR!), they look to be too lean. I haven't done a high rpm chop, the readings are just made after a ride and I've idled into the garage.

Before all this was done, the bike was idling high and rough and had the rpm hang between shifts. Now, all is well with the idle and the bike responds well to throttle but, I still have the rpm hang. While sitting at idle, I can blip the throttle and it's OK, but, if I blip it up to about 3K, it hangs a bit before idling down.

Reckon I need to go up to a 112.5 main for the lean reading but, would the pilot jet size have anything to do with the rpm hang since it did it with both the 115 and the 110 main? I don't think I have a vacuum or exhaust leak anywhere as all the components are new and supple.

Sonny

how did the motor work with the 115's in it the symptoms you describe are a lean condition. with fuel that you get today it is hard to read plugs and you defintly can not read they way you are trying to do it. if your bike felt good with the 115's in keep running them.

timebomb33 wrote:

how did the motor work with the 115's in it.

It had the same rpm hang plus, it stumbled at take off and idled too high (idle screw backed all the way off).And, the plugs were dark as night.

Now, this was before I took them off for a cleaning. The stumble was probably caused by the accelerator pump not squirting in all cylinders and the slides were over adjusted too high.

Now that I have all that corrected, it still hangs.

Maybe I'll give the 115's another try in the clean carbs and see if that has any effect.

It's just a real pain to pull and re-install the carbs with the stock air box...whinnnne:(

Sonny

sbulla wrote:

... a real pain to pull and re-install the carbs with the stock air box....

But not with These .

Patton wrote:

sbulla wrote:

... a real pain to pull and re-install the carbs with the stock air box....

But not with These .

You didn't read the original post close enough...I have some of those and it's STILL a pain!

Sonny

sbulla wrote:

Patton wrote:

... But not with These .

You didn't read the original post close enough...I have some of those and it's STILL a pain!....

You are so right -- I missed that completely :blush: , and spun off into apoplexy recalling the agony before replacing them on my KZ900, but with the new soft pliable intake hoses, carb removal has no longer been an issue.

Regarding the throttle hang, it's sometimes caused by failing to have the idle screw set in middle position (threads equal on both sides) when doing the bench synch. Mentioning this because of reportedly idling too high even with the idle screw backed all the way off.

And presuming sufficient slack has been allowed in the throttle cable adjustments.

Good luck with the carb tuning!

Patton wrote:

Regarding the throttle hang, it's sometimes caused by failing to have the idle screw set in middle position (threads equal on both sides) when doing the bench synch

I did the first bench sync "by the book" which asks for the idle screw to be holding the throttle 2mm off the stop (and the close cable being able to overpower that to the stop by way of the spring loaded cable mount).

I got 'em off the bike again and will go through the mech sync procedure again just to make sure while I decide which jets to install for another try.

It's just such a lengthy procedure in the mornings to heat the garage so my old bones can move AND get those carb boots warm enough to bend....whinnnnne

Sonny

sbulla wrote:

Patton wrote:

Regarding the throttle hang, it's sometimes caused by failing to have the idle screw set in middle position (threads equal on both sides) when doing the bench synch

I did the first bench sync "by the book" which asks for the idle screw to be holding the throttle 2mm off the stop (and the close cable being able to overpower that to the stop by way of the spring loaded cable mount).

Sometimes the book is misleading. As known, normal throttle closing to idle position doesn't require overpowering the spring loaded cable mount. And there must be sufficient thread available in the thumb idle screw adjuster to allow the slides to drop below the bench-sync position, as is often necessary when later manometer syncing. Having the idle screw adjuster in the approximate middle thread position is one objective of the bench-sync process.

Am thinking the spring-loaded stopper mechanism is a safeguard against pulling the ferrule off the cable when the carb return spring snaps the throttle closed.

The peanut gallery has spoken!

Good Luck!

Edit -- Turning the idle thumb screw in against the stop does of course increase the gap, and continues increasing the gap more and more as the screw is turned further in against the stop. Which is basically raising the throttle slides, regardless of their pre-set positions.

Am thinking that the 2mm gap setting as mentioned in the book (by using the thumb screw) is to be done just before equalizing the slide heights (on the bench, using the sync adjustment screws underneath the carb tops). And that this procedure affords later ability to lower the slide heights with the thumb screw during manometer syncing.

Patton wrote, "Regarding the throttle hang, it's sometimes caused by failing to have the idle screw set in middle position (threads equal on both sides) when doing the bench synch. Mentioning this because of reportedly idling too high even with the idle screw backed all the way off."

Patton, please explain your statement about the "idle screw". This guy's bike has air screws. Are you referring to the air screws or the idle speed adjustment screw? Also, synchronization has no effect on a hanging idle as out-of-sync carburetors don't cause this issue. There are many issues that can cause a hanging idle but not synchronization.

The purpose of bench sync is to get the idle screw working in the center of the range the slides can move up or down. If done incorrectly, you can either not lower the slides far enough or can't raise them far enough using the idle screw. The idle screw will never had an equal number of thread above and below the fixture it screws through.

wiredgeorge wrote:

Patton, please explain your statement about the "idle screw". This guy's bike has air screws. Are you referring to the air screws or the idle speed adjustment screw?

I'm pretty sure we were on the same page and talking about the idle speed adjustment.

Also, synchronization has no effect on a hanging idle as out-of-sync carburetors don't cause this issue. There are many issues that can cause a hanging idle but not synchronization.

And, as soon as I'm aware of them all, I can start eliminating them to clear this up

The purpose of bench sync is to get the idle screw working in the center of the range the slides can move up or down. If done incorrectly, you can either not lower the slides far enough or can't raise them far enough using the idle screw.

That was the condition mine was in when I started all this. I couldn't adjust the idle low enough. That part of it is in good shape after my rebuild/cleaning. The problem I'm working on now is the hanging rpm's between shifts that was present before and after the rebuild and jet change and sparkplugs with a white side electrode.


Sonny

Even with perfect carbs, if fitted with a mechanical advancer for ignition timing (the spring loaded version with weights spread by centrifugal force), sometimes the advancer hangs or sticks momentarily in the advanced position, which may cause delay in returning to lower rpm. In such case, cleaning and lubing the advancer may resolve the matter.

If not already done, would use a strobe-type timing light to assure correct ignition timing, and also watch the advancer unit move back and forth with changes in rpm. When blipping the throttle, the advancer should react quickly in moving back to the F mark.

Good Luck!

Patton wrote:

Even with perfect carbs, if fitted with a mechanical advancer for ignition timing (the spring loaded version with weights spread by centrifugal force), sometimes the advancer hangs or sticks momentarily in the advanced position, which may cause delay in returning to lower rpm. In such case, cleaning and lubing the advancer may resolve the matter.

If not already done, would use a strobe-type timing light to assure correct ignition timing, and also watch the advancer unit move back and forth with changes in rpm. When blipping the throttle, the advancer should react quickly in moving back to the F mark.

Good Luck!

Thanks, I'll check those things out as soon as my garage thaws out...15 degrees here this morning! :ohmy:

Sonny