Run away RPM's

Ok, this is killing me. I don't consider myself an expert but I have worked on quite a few bikes in my time (mostly Hondas) but this is my first time working on a KZ1000 and I'm wracking my brain trying to figure this one out. I would like to get this bike back to the customer and get paid. Sorry my first post is asking for help but I didn't know where else to go.

1978 LTD1000
Mint condition w/ only 11k miles on it. When I let off the throttle when riding to coast to a stop the bike acts like it's got cruise control and will continue to accelerate. If I pull in the clutch when this happens the RPM's will climb to approx. 3k and stay there for an extended amount of time before the bike finally idles down to just under 1k where it should be.

Things I've done so far:
Lubed cables, lubed twist throttle, checked throttle cable tension, rebuilt the carbs completely, synchronized the carbs, pulled intake boots off and checked for cracks (still very pliable and appears to be sealing on head surface.

I even removed the throttle cables from the carbs and operated them by twisting the bell crank by hand. When I idle it up to around 2 - 3 thousand rpm it and release slowly the rpms will stay and I can watch the bell crank slowly come to stop on the idle adjustment screw. If I release the bell crank quickly the slides will snap shut and it idles down like it's supposed to. I checked the return spring tension at full "closed" position and there is still plenty of tension.

When riding the bike I have to physically twist the throttle forward with some force to get the bike to idle down. I don't know what else to try. Any suggestions? Thanks in advance.

bearanddragon wrote: Ok, this is killing me. I don't consider myself an expert but I have worked on quite a few bikes in my time (mostly Hondas) but this is my first time working on a KZ1000 and I'm wracking my brain trying to figure this one out. I would like to get this bike back to the customer and get paid. Sorry my first post is asking for help but I didn't know where else to go.

1978 LTD1000
Mint condition w/ only 11k miles on it. When I let off the throttle when riding to coast to a stop the bike acts like it's got cruise control and will continue to accelerate. If I pull in the clutch when this happens the RPM's will climb to approx. 3k and stay there for an extended amount of time before the bike finally idles down to just under 1k where it should be.

Things I've done so far:
Lubed cables, lubed twist throttle, checked throttle cable tension, rebuilt the carbs completely, synchronized the carbs, pulled intake boots off and checked for cracks (still very pliable and appears to be sealing on head surface.

I even removed the throttle cables from the carbs and operated them by twisting the bell crank by hand. When I idle it up to around 2 - 3 thousand rpm it and release slowly the rpms will stay and I can watch the bell crank slowly come to stop on the idle adjustment screw. If I release the bell crank quickly the slides will snap shut and it idles down like it's supposed to. I checked the return spring tension at full "closed" position and there is still plenty of tension.

When riding the bike I have to physically twist the throttle forward with some force to get the bike to idle down. I don't know what else to try. Any suggestions? Thanks in advance.

Does it snap close with the cables on and a quick Blip? just throwing this out there put the petcock on prime cap the pet cock vacum ay the carb and try it (assuming it has a vacum actuated petcock)
has some aimilar idle problems whe my petcock was hooked up to the vacum not as radical but
never seemed to idle at same rpm may be pretty far out in left feild but mine doesnt do it anymore wih the vacum capped

Sounds like a vacuum leak. I know you visually checked the carb holders for cracks, but try the leak test below. Also, make sure the vacuum caps are on tight and don't leak. Ed

Get the engine running and warmed up and temporarily idle it down as low as it will go without killing the engine. Spray carb cleaner around the carb holders; especially where they mate with the cylinder head, but also where the carbs attach to them. If the engine dies or seems to react to the carb cleaner, you need to replace the carb holders. This occurs at low idle because with the throttle closed not much air can come through the slides and this creates higher vacuum within the carb holders. On my carb holders the cracks were extremely thin and could barely be seen without magnification and they were still pliable (in fact, they looked to be in great shape), but the above test stopped my engine from running. I replaced the carbs holders and my rough idle and slow throttle return problem are completely gone.

I think Ram hit the nail on the head

I agree Peter. Only other thing that popped in my head was carb synch being way off on one carb or something.

From the latter part of his description, it sounds more like the return mechanism on the throttle just isn't functioning right. I'm at work, so I can't really pull up a detailed parts fiche at the moment. But there may be some corrosion where all the rotating mechanisms there are, or it might just need a new return spring (assuming there's a spring for the return, I'm not familiar with those carbs).

If the Revs stayed up when the throttle was returned to idle, it'd indicate lean (and hence a leak). But it sounds like your throttle just isn't returning to idle position.

This model does not have a vacuum petcock.

Carbs were synched and still no change.

I did spray WD40 and some PB blaster (ran out of WD40) on the intake boots w/ no change at all.

It does snap closed with a blip of the throttle. It only happens when I slowly roll off the throttle. Almost like the engine vacuum holds the slides open for an extended amount of time and I have to force them closed w/ the twist throttle.

I did completely disassemble the carbs trying to figure out the problem and I lubed all the linkage w/ graphite and checked all the slides. Other than the gaskets being shot from sitting so long the carbs were spotless. Return spring is very stout on these carbs and doesn't appear to be the problem.

One thing I did notice and thought was weird was that the vacuum caps, when removed, had fuel in them. Not sure if this was normal.

A little fuel does settle in the caps. Ed

Take a look at the choke plungers. See if the rubber looks ok. That can cause a vacum leak

just a shot in the dark
you dont have throttle grip up tight to end of handle bar ?
that will make it hang

I appreciate all the quick comments guys.

I'll take a look at the Choke plungers. I did replace the o-rings on them w/ new ones from the rebuild kit but I didn't inspect the plunger rubbers though.

P21: I did check that and it isn't up tight. Fortunately I've been able to eliminate the twist throttle and cables completely as being the problem by removing the cables from the carbs and operating them by hand w/ the bell crank.