1978 KZ650 High RPM

I have been tinkering with my new KZ this last week. Mainly adjusting the cam chain tension. It ran great last time out. I got my new tire in and installed it so I wanted to take the KZ out for a spin. Started and ran like a champ. Got to the store and pulled in the clutch to start down shifting to navigate the parking lot, the rpms stayed way up. If I popped it in gear the RPMs seemed to drop, but when I was pulling into a parking space and pulled the clutch and put it in neutral the rpms stayed way up again, and never dropped. When getting ready to leave after shopping the bike started, but when I shifted to first and rolled on the throttle the bike bogged down and died. It did this over and over, very frustrating. Finally after messing with the idle adjustment knob a bit I got her going. She ran pretty good but the rpms stayed up, not as bad as before. If I down shift and hold the brake after about 5 secs the rpms would drop. Any ideas? Was it an idle adjustment problem or maybe clutch?

Could be fuel starvation... or an air leak at the carb holders, or elsewhere.

Try opening the fuel cap to see if that resolves the problem, as the breather could be clogged.

With engine hot (when its starts this issue), spray WD40 around the carb holders and elswhere along the intake. You will hear a sudden change in rpm/smoke out the exhaust if wd40 gets sucked in.

Make sure that the advancer unit behind the ignition plate rotates and returns properly, as this requires lubrication as per regular maintenence.

GL!

Air/vacuum leak probably. Two ways to check for leaks:

1.Spray around the rubber ducting(boots) from the air filter box to the carbs with brake cleaner or WD-40.

2.Using an unlit propane torch with the gas turned on, do the same checking around the carb rubber bits.

If the idle rpm goes up, you've found your leak(s), also check at the carb holders(sometimes called manifolds) between the engine and carbs. They will crack from heat and age and require replacement. Replace the rubber snyc. plugs too if they are cracked.

Check to see if the mechanical ignition advancer is working, it's behind a CD sized cover on the right side of the engine. Give it a twist with the engine off and see if it returns to where it was.

If sticking,remove,clean,and reinstall.

Light lubrication with 3-in-one or sewing machine oil is about all it needs now and then.

Perhaps throttle cables need adjusting, or replaced if hanging up and lubing doesn't resolve.

Would first inspect fitting of cables at carb belcrank.

Could try disconnecting cables at carb end, and see whether carb slides fully close without possible interference from the cables.

Good Fortune!

Also check routing of throttle cables to assure that they aren't being "stretched" when the handlebar is turned one way or the other.

Good Fortune!

Check to see if the mechanical ignition advancer is working, it's behind a CD sized cover on the right side of the engine. Give it a twist with the engine off and see if it returns to where it was.


Is this under the same cover as the cam tension shaft? What does the acutal ignition advancer look like and do you just spin it by hand?

the advance unit has 2 springs on it, you can work them by hand so see if they are advancing and returing smooth, a lot of times they get rusty and will not work right.
to me your issue sounds more like a vaccumm leak or a throttle cable binding up.
you can remove the tank and work the throttle and watch to see if they are returning to the idle stop

So I did the vacuum leak check, and couldn't see any evidence of a leak. Might be wrong. I then looked at the idle adjustment and throttle, seemed fine. When I looked at the deccel cable I saw a slight bit of slack at the carb end. In my mind I thought if I tighten it up it should pull the rpm back down. Well I dont THINK I'm having the rpm problem anymore so I decide to drive it to work this morning. It would not run above 70! Full throttle in 5th gear and it was struggling to hit 70. I am hoping it was just the cable adjustment because 2 days ago I was hitting 70 in 3/4 gears. Will check the adjustment at break today and test it on the way home. If it is still doing it I'll move on to the ignition advance. Any other ideas? Oh ya and it's leaking oil to boot! Looks like lower and aft on the engine. Crank case area maybe...

DaveKZR wrote:

...looked at the decel cable I saw a slight bit of slack at the carb end. In my mind I thought if I tighten it up it should pull the rpm back down....

Carb spring is supposed to return carb to fully closed idle position without needing any help from the decel cable.

Carb should function perfectly without any decel cable whatever.

Good Fortune!

This is probably going to be long winded, so here is fair warning. Proceed with caution!

So I adjusted the deccel cable back to the way it was before, and found an air box boot that had come half way off of the carb intake. I thought I was golden and looked forward to the ride home. Hopped on the bike in the afternoon and the first 4 gears accelerated fine, though they seemed rather short. This is my first bike and I've only had it a week so I'm not super sure, but they seem short. Anyway in fifth gear there is not really any acceleration past 65-70mph. Halfway through my commute home it starts acting up even more! I can't even be in fifth unless I hold the throttle in just the right spot. It is a tiny sweet spot, if I'm there it will accelerate to about 65 that's it. If I'm not in the sweet spot it sputters backfires, jerks and lurches. Very unpleasant. I shift down to 4th and the same only the sweet spot is slightly larger, shift to 3rd with rpm screaming and it accelerates nicely. The rpm also seem to be riding somewhat high as well. Sometimes when I was in fifth it would bog down all the way to about 20mph and backfire LOUDLY. Shift it to 4th, same thing. Really a nightmare of a ride home. Part way home I remembered someone mentioning opening the gas cap. Did this and it somewhat improved. It was a little smoother and not as much popping, but still wouldn't go above 70. Another thing I noticed, going uphill it just about died, not that large of hills either. But then going downhill she would open up to about 80. Also I looked one time when the rmm was high in idle and the bell crank was in the idle position, so that sounds like maybe a vacuum leak but I cannot get the idle to rise using lubrication spray. I have not looked into the ignition advance yet, that is tomorrow. Sorry to be so long winded! I appreciate all of the knowledge you guys provide on these forums, hopefully one day Ill be on the answering side of these threads.

The adancer needs to be very clean and lubed, so that it can full advance and return correctly.

Carburetor tuning depends on the type of carb. I think you have regular slide carbs/non-CV, therefore:

The mixture/air screws are for tuning the idle, and have a small effect on the transtition to about 1/8th throttle.

The Pilot jets are drawn thru @ idle, and effect cruise up to just after 1/8th throttle as well.

The needle is responsible for all of the accelleration, right up to about 2/3 throttle, then continues to overlaps the main till about 7/8ths throttle.

The mains are from a small amount at 2/3rds throttle to WiedOpenThrottle.

If the idle screws are set close, then the idle shouldnt have any popping in the exhaust, and when revving, should return quickly to idle. If not, there is likely still an air leak, or sticking advancer.

As for your situation with the no power @ higher speeds, it sounds like your needles/or mains are too rich, but it depends on how much throttle input your using.

GL!

Checked valve clearances?
Performed compression test?
Ignition timing to specs?
Air filter clogged or obstructed?

Good Fortune!