Rev/blip throttle, it stays at that RPM

Hello all!
Thanks for letting me join this great board.

I recently bought a 1981 kz650 CSR from some guy who stopped riding. It has 19500 miles on it, and he started it every now and again since 1998 to make sure everything would still move. After buying it, I was able to get it started using starting fluid, but it would not run with the choke off.

Pulled the carbs, had them cleaned ultrasonically, put them back on, new plugs, new hoses, new intake ducts, new carb holders to make sure there were no leaks, new clutch and throttle cables. Was able to get it running and rode it around for a while.

After about half an hour, it would start to run funky. Backfiring (flame visible) if you blipped the throttle, and if it didn't backfire, it would stay at that RPM I ended up revving it to (tach cable broken). The RPMs still wont come down, even after 10 seconds. I've rev'd it while watching the throttle bell crank, and it snaps back shut, but it's still rev'd up like I'm holding the throttle open.

Cylinder 3 takes more time to get warm than the others, and is a little lower in compression (don't have figures, just doing the finger test), but after running for a few minutes it is hot.

Anybody have any ideas? I have one month left to get it to highway patrol for inspection and to complete registration, otherwise I have to pay again for another temp permit. I'm all out of ideas...:blush:

not a 650 guy, but i ll try to help.....i dont know about your bike , but the older ones sometimes have a problem with the ignition advancer sticking..especially if its been sitting....should be under the right side small cover(as you are sitting on bike).....spray it with carb clean and lube it with a light oil.....if you had the carbs done, they are jetted right and cables are new and lubed,(not binding), its probably the culprit....let us know how ya make out.......keith

A flame visible at the exhaust tip is called an after-fire, if that is what you mean...

It's caused by a very rich mixture in one or more of the cylinders, (most likely the cold one).

Revving on its own and not coming down is attributed to an air leak on one or more cylinders.

This can be caused by either a small gap somewhere in the carburetion path (air box intake to exhaust end, make sure is completely sealed and air tight!) Or... can also be caused by a pilot mixture screw being set far too lean.
Make sure all vacuum caps and lines are good.

Is intake system all stock? If so, set all air screws to around 1 3/4 turns out from lightly seated to start with.

Good luck!



Advancer info good! Check as per kieth1

Great thanks! I'll check it out. I'm 99.9% positive nothing is leaking. New rubber every where, everything is tight. Sprayed around the intake ducts and carb holders to find leaks...nothing.

Even after getting it started, if I blip the throttle, it's random when it stays up there or comes back down. This morning I tried it again, and kept blipping the throttle, it took three tries for it to settle back down.

Does your carb assembly return to idle immediately after each blip?
If not:
- Is your return throttle cable very lose, frayed, stuck or broken?
- Is your pull throttle cable too tight, frayed and or stuck?

Does it happen when the Steering is in full left or full right position:
If yes:
- Is one or both throttle cables poorly routed or too short?


Best of success,
Bill

bill_wilcox100 wrote:

Does your carb assembly return to idle immediately after each blip?
If not:
- Is your return throttle cable very lose, frayed, stuck or broken?
- Is your pull throttle cable too tight, frayed and or stuck?

Does it happen when the Steering is in full left or full right position:
If yes:
- Is one or both throttle cables poorly routed or too short?


Best of success,
Bill

Yep, it returns to the idle setting. Cables and brand new and adjusted properly. Happens randomly, position of handlebars has no effect.

It's like the third carb is coming to life or something, the third cylinder takes a bit longer to warm up than the others, but once you run it for a bit it comes up in temperature, but still have a runaway throttle problem.

Well... whats hapening is you have a cylinder recieving a lean air charge that causes that tell tale hang in rpm.

Pull your spark plugs and have a look.
I'll bet that your #3 plug is very dark, hence running rich (cooler pipe) and causing the after-fire.

A very white plug in another cylinder will indicate your cylinder recieving excess air.

Were the carbs done by a reputable professional shop?

These should be your carbs...



Part # 16014 are your pilot mixture air screws.
Screwing them in will decrease the amount of air.
Out will increase the air. Only affects idle, and just off idle...

Make sure they are reasonably set... I mean they should be between 1 and 2.5 turns out from lightly seated. And all fairly close to the same.

Alot of times, air leaks are at the carb boots-to-engine seal. Make sure thier good and air tight.

Start there... Best of luck!

Keep us posted

Something to think about...
Check that your pull cable has slack in it when "you think it is at idle position"...
I had a hanging RPM issue that drove me crazy for weeks. I pulled the carbs 3 different times trying to find a non-existent air leak. I had installed a brand new throttle cable that as it turned out was just maybe 1/8 inch too short... It looked like my throttle linkage was at idle position but it was cracked just a little bit--no slack. I had no adjustment available on the cable length so I had to think of something different.
I finally took off the throttle twist grip and cut a little material away from the return stop (on the twist part) of the grip. It allowed the thottle to close all the way against the stop and have a small amount of slack in the cable.
This made the difference on my bike, maybe checking for slack on yours will help eliminate one more possible cause of your problem.
Grant.

Yes those look like my carbs, except I have the little cap (Part 92066) over the air screw I'd have to remove. Does this mean I have a fuel screw as well to tinker with?

Edit:
After going through the parts diagram, 16014A is what I have and is a "pilot screw."

Why the name change? Same process for adjustment?

Yes there is slack in it at idle.

i had a similar problem. I went a little crazy looking for vacuum leaks.

The problem ended up being with the cable bracket on the carb. The 2 rivets that hold the bracket have worn so the bracket could move around. The cable would hang up sometimes causing the throttle to stick. I am currently using a zip tie as a quick fix. Next time I pull the carbs I am going to drill out the riv its and replace them with bolts.

My carbs are vm24's

redwood wrote:

Yes those look like my carbs, except I have the little cap (Part 92066) over the air screw I'd have to remove. Does this mean I have a fuel screw as well to tinker with?

Edit:
After going through the parts diagram, 16014A is what I have and is a "pilot screw."

Why the name change? Same process for adjustment?

There's many different names tossed around for them... Yes, it's the same adjustment. You just have to drill the cap CAREFULLY slow so you can get a small slotted screwdriver in thier to make adjustments.

I had another problem once that made the revs hang. It was the end of a pipe clamp that was used as a carb holder. It was pushing up slightly on the throttle rail.
Check everything at the carbs very carefully.

What do your plugs look like?
Did you check the advancer? Take off timing cover (2 small bolts on right side of engine crank), and give the advancer (the shaft piece pointy thing in between the 2 contact points) a twist. You'll see the springs expand/contract. It should have a good medium tension, and always return right to tight shut, even from a very small amount of twist and max twist.

OK, i'm out of breath...

GOOD LUCK!