Engine revving out of control with throttle closed

A couple months ago, when I first got my bike (82 KZ750 LTD-4), it would hesitate at low RPMs (under 3500). The previous owner hadn't ridden it in a year or so (switched to HD). My plan was to run some Sea Foam and some Sta-bil through the gas tank and see if that cleared up the problem.

Well I road every day to work for a week, and on the weekend. It was running great.

I wasn't able to ride for a week.

Now there is a problem.

When I start out in first gear, it hesitates again. So I figure it must be clogged again. However, now there is a new problem. It sputters like crazy up to 3k, then, even if I let the throttle go shut, it revs like crazy to 5k, then slowly backs down. I haven't had the chance yet to pull the carbs off, but I did get down next to the carbs and work the throttle. It certainly seems like the plates are closing, Yet it revs like crazy.

I do have a throttle paddle on.
The bike also fell on the left side in my garage (grip caught the work bench, I doubt it relates)

I am hoping to get into it Sunday, but I was wondering what it might be and what direction I should be looking.

Un hook the cable at the carbs and operate the linkage manually, that will tell you if it is in the cable or the throttle assy.
If it still does it check the Ignition advancer for sticking, with a quick removal of the side cover, or with a inductive light. Also vacuum leaks, and of course like everyone with an older carb bike, remove them, clean, and kit them. Sync them when reinstalling.

I went around ther neighborhood today to check it out with not as much traffic.

In at least 1st, 2nd, 3rd I can rev to 4k, and when I clutch and drop the throttle to shift, it hangs at 4k until I drop the clutch and give it load.

Can I tell if the ignition advancer is sticking by looking at it?

look at the weights on advance unit,im leaning towards problem with those cv carbs

I ppulled thr cover for the electronic ignition. I don't see weights. I guess I didn't understand.

I did pull the carbs and clean them. They were very clean inside, no varnish or crud anywhere. The rubber diaphrams are pliable with no tears.
I think they were rebuilt recently.

djturnz wrote:

I pulled the cover for the electronic ignition. I don't see weights. I guess I didn't understand....

The "weights" are a component of the advancer unit located behind the ignition's round backing plate.

At low rpm, the springs hold the weights inside to maintain the more retarded timing position needed for idle.

As rpm increase, centrifugal force throws the weights out against the stops, which advances the timing.

Viewed under a timing light, the advancer should immediately move back and forth as rpm are varied by blipping the throttle.

If the advancer is sticking, it likely needs lubing, sometimes both cleaning and lubing, which often returns it to proper function.

Good Fortune!

Thanks. I remember those from my old Suzuki, but it had points. Actually, I remember having a set of those in a baggy with the Dyna ignition that the previous owner of the suzuki had given me.


I think I was actually having two problems. The first morning that I went to ride, after having parked it for a week, I switched the choke off after it had ran for about 3 or 4 minutes. It was idling kind of low, so I turned the Idle knob up a bit. Then as I went in town, I started having all the revving problems.

Last night after putting the carbs back on, I started it up and it instantly went to 4K. I pulled the throttle tube off and let the cable hang, no change. I worked the throttle cable with a pair of pliers and it still wanted to "idle" at 3k-4k. So I rolled the idle knob back about 3/4 of a turn, and the revs FELL down to 1500. I had no idea that such a tiny change would move the idle 2500 RPMs. So I thought that maybe nothing was wrong, and that I was just an idiot for touching the idle control while it was still kinda cold.

As I bopped around my street in first gear, staying under 2500 RPMs, I thought I fixed it. Then I went around the block and it still wants to hang around 4k when I clutch and drop the throttle to shift. So perhaps it is the weights sticking.

What should I lube them with?

Any oil is okay.

A racing idle may result from several different reasons.
Some are mechanical issues associated with throttle cables, advancer, etc.
Other reasons include air/fuel issues such as air leaks between carb and engine, besides the usual carb pilot screw and idle thumb screw adjustments.

Good Fortune!

Sometimes, where choke is left on too long during the warm-up period, a spark plug will get fuel fouled, whereby the engine is running on less than all cylinders, which requires a higher idle thumb screw adjustment to maintain idle rpm.
This is especially critical with a lowish battery where the spark quality is weaker than desired.

Then while riding, the spark plug clears itself so that the cylinder resumes combustion, at which point the idle is too high, and the idle thumb screw must be readjusted.

Imo, it's better to just leave the idle thumb screw alone, at least until after riding far enough for the engine to attain normal operating temperature, and give the plugs a chance to self-clean.

And don't leave the choke on so long in the first place. Kill the choke asap, and use throttle control to complete initial warm-up until able to pull-away and start riding.

When applying choke to start a cold engine, should be a matter of seconds, not a few minutes.

In other words, MINIMIZE using the choke.

Good Fortune!

Once the small side cover is off, take out the 3 screws holding the plate with the pick up coils off, this will give you access to the advancer. Then you can rotate it with your fingers, then either take it off or lube with engine oil where it rotates, and the pivots for the weights.

Motor Head wrote:

Once the small side cover is off, take out the 3 screws holding the plate with the pick up coils off, this will give you access to the advancer. Then you can rotate it with your fingers, then either take it off or lube with engine oil where it rotates, and the pivots for the weights.

Looking at my top hole, it is slotted. Is this an adjustment? Like, if I don't get it just right when I put it back on, my timing will be off or something?

The other two holes don't appear to be slotted at all, so I can't imagine it could ever be too far off, but I'd rather know now, that after the fact.

It can only go back on in the same spot, you can always make a mark on the plate and case where it was if it bothers you.
Did you try to turn the advancer? It should move freely with only the 2 springs as resistance, then move back smoothly on its own.
Do you have a good service manual?