RPM spike/burning oil/engine knock

I have 3 questions regarding my 83 kz550h. It had sat for 10 years before I got the bike. Pulled the carb and cleaned it (did not replace any parts yet). Bike starts decently now, and I can get it to idle nicely.

3 items though:
1) normally when I start the bike on full choke, it will start and idle for a few seconds, and then I get a 5 - 6K RPM spike. I close the choke, it stops and doesn't do it again. Any ideas of likely causes?

2) Also, when I start the bike cold it will burn a lot of blue smoke. I'll run it for a few minutes, shut it off and let it sit for a few minutes. Then restart and no issues at all. Again, what would be the cause for this?

3) I've had this bike running for the past week, almost everyday for a while just to continue to work the engine. I had it running yesterday, and let it sit and idle for 5 minutes or so. About an hour later I went to start the bike and their was a large RAP/KNOCK noise in the engine. My guess would have been that it was coming from the first cylinder area. I tried to start it again, and BOOM, same noise. Tried a third time and it started with no issue and idled. What could that be? Is that a valve? I am completely out of my element here, so any advice would be very appreciated.

Regarding the first issue - "normally when I start the bike on full choke, it will start and idle for a few seconds, and then I get a 5 - 6K RPM spike. I close the choke, it stops and doesn't do it again."

That is not uncommon. Just lower the choke some after the bike starts and you can avoid the spike. Ed

Thanks Ed...always good to hear that these things are somewhat common.

Jedsdead wrote: ... when I start the bike cold it will burn a lot of blue smoke. I'll run it for a few minutes, shut it off and let it sit for a few minutes. Then restart and no issues at all. Again, what would be the cause for this?

3) I've had this bike running for the past week, almost everyday for a while just to continue to work the engine. I had it running yesterday, and let it sit and idle for 5 minutes or so. About an hour later I went to start the bike and their was a large RAP/KNOCK noise in the engine. My guess would have been that it was coming from the first cylinder area. I tried to start it again, and BOOM, same noise. Tried a third time and it started with no issue and idled. What could that be? Is that a valve? I am completely out of my element here, so any advice would be very appreciated.

Exhaust smoke from cold started engine may result from old hard worn valve stem seals that soften somewhat as the oil reaches normal operating temperature and begin doing a better job of sealing.
It's a common condition that doesn't hurt anything. And whenever it comes time to remove the head for some other reason, new valve stem seals may then be installed.

As to the engine noise, forgive this wild guess with lots of "ifs" --- If the carbs don't have functioning overflow circuits, and if the petcock is allowing fuel flow into the carbs while the bike sits for an hour, and if the float valve is leaking whereby fuel rises up into the carb throat and makes its way into the combustion chamber, the large RAP/KNOCK noise in the engine might result from clearing the excessive fuel from the combustion chamber.

Good Fortune!

Thanks Patton. Glad to hear the blue smoke is nothing too serious.

I was playing with the bike again yesterday, and set up a makeshift gastake that would allow constant flow. Shortly after I noticed a bit of gas leaking from carb 1, not from the overflow tube though. So after moving things a bit, I could see gas had drained into the airbox. So I pulled the carbs off again, and the cylinder intake was full of gas. All the other intakes were clear.

I checked my float setting and it was right at the top of the bowl. So I'm assuming with the carbs pitched slightly forward the gas was flowing into the cylinder. Seems odd to me that the gas would not come out of the overflow though before getting to the cylinder intake. Isn't that what it is designed to do?

Anyways, I'm hoping that caused the backfire (if that is what the noise was). I'll reset my floats down to the proper level, and then try to figure out a way to test if that flaot valve is sealing or not.

Thanks again.

Jedsdead wrote: ... set up a makeshift gastake that would allow constant flow. Shortly after I noticed a bit of gas leaking from carb 1, not from the overflow tube though. So after moving things a bit, I could see gas had drained into the airbox. So I pulled the carbs off again, and the cylinder intake was full of gas. All the other intakes were clear.

I checked my float setting and it was right at the top of the bowl. So I'm assuming with the carbs pitched slightly forward the gas was flowing into the cylinder. Seems odd to me that the gas would not come out of the overflow though before getting to the cylinder intake. Isn't that what it is designed to do? ....

The float height may be okay.

Would guess that the float valve is leaking due to dirt at the interface between the needle point and seat orifice, or possibly due to wear on the needle point.

A leaking float valve allows gasoline to keep coming into the float bowl after the float valve is supposed to have stopped the incoming flow of gasoline.

When a carb has an overflow circuit, the overflow circuit is supposed to afford an escape route for the gasoline that keeps coming into the carb through a leaking float valve.

But if the overflow circuit is clogged or obstructed (which is a common condition), the overflow fails to function, whereby the gasoline keeps rising, past the top of the overflow tube and on into the carb bore, where the gasoline flows both ways -- rearward toward the air box or pods, and forward toward the motor (combustion chamber).

If the carb at hand does indeed have an overflow circuit, would (1) assure the overflow circuit is clean and clear, and (2) assure that the hose from the overflow discharge nipple is in good condition, unkinked, unblocked, and providing a good passageway for the excess gasoline to escape and be discharged underneath the bike.

When an overflow circuit is functioning correctly, the excess gasoline getting past the leaking float valve will pass through the overflow circuit and escape underneath the bike, without rising high enough to get into the carb bore.

Good Fortune!

With some carbs, it's possible to inadvertently reassemble with the float mechanism upside down.
Might want to double-check that aspect on the carb at hand.

The clear tube test should be used to determine fuel level inside the float bowl.
Validity of the clear tube test depends on a properly functioning (i.e., non-leaking) float valve.

Good Fortune!

Thanks Patton. I did clean the carb fairly thoroughly, including removing the floats, valve and the seat. The o-ring on the seat was decent but not 100%. having said that, I have no idea if there is an overflow circuit or not. I'll need to read up on my model.

When I first started putting fuel into the bike (before cleaning the carbs), fuel came out of both overflows, so I'm surprised that is not happening now. Unless somethig has bunged them up since, which is entirely possible.

I have the carbs off now, so I guess I should be able to test for leaks by setting the carbs in a stand with that float bowl off, hook up a fuel source and then manually close the floats.

My service manual says that the float level should be 6mm below the top of the float bowl, so I do need to set them down a bit.

Jedsdead wrote: ... have no idea if there is an overflow circuit or not... fuel came out of both overflows... should be able to test for leaks by setting the carbs in a stand with that float bowl off, hook up a fuel source and then manually close the floats.

My service manual says that the float level should be 6mm below the top of the float bowl, so I do need to set them down a bit.

With 4 carbs having overflow circuits, there should be a nipple on the bottom of each float bowl from which the excess gasoline escapes.

Inside each float bowl, there's a thin tube directed upward from the bottom of the float bowl. The tube has a hole at its top where the rising gasoline can enter and escape.

Being able to manually close the float valve doesn't guarantee that buoyancy of the float will suffice to close the float valve after reassembly. Float buoyancy exerts only a very slight pressure toward pressing the float needle tip into the seat orifice. But it's all that's needed with a perfect float valve.

When the float valve is functioning perfectly, the natural float buoyancy is adequate to close the float valve, and prevent additional gasoline from passing through the float valve until the fuel level lowers enough to reopen the float valve.

Good Fortune!

Jedsdead wrote: ... fuel came out of both overflows....

The carb diagrams in kawasaki.com don't show overflow tubes.

Am wondering if perhaps the reported "overflows" are actually the vent hoses??

With the carbs at hand, is there an exterior nipple from the bottom of every float bowl?

Good Fortune!

Patton wrote:

Jedsdead wrote: ... fuel came out of both overflows....

The carb diagrams in kawasaki.com don't show overflow tubes.

Am wondering if perhaps the reported "overflows" are actually the vent hoses??

With the carbs at hand, is there an exterior nipple from the bottom of every float bowl?

Good Fortune!

Hmmmm, Patton I think you just schooled me on these carbs. As I mentioned before, this is my first time really working on a bike but I thought I had gotten a good handle on how these carbs worked. Obviously I have a ways to go.

In looking at schematics, the tubes I mentioned were overflow tubes must actually be Vent tubes. While I don't see them labeled this way anywhere, that certainly makes a lot more sense. The reason I thought these were overflow tubes from the beginning (before really looking at the carbs) was when we first put fuel into the bike, fuel flowed freely out of these tubes. The guy with me who has a fair amount of experience on bikes commented that these must be the overflow tubes. Fuel has flowed out of these tubes a few other times as well, but usually only out of one and only for a short time. Now I'm completely confused how fuel was coming out of these tubes in the first place!

All of my float bowls have a drain valve at the bottom. I assumed this was only a drain and not an overflow as well. Still don't see how these work overflow valves since I see no way for the gas to drain through them without me loosing the screw.

Am thinking the carbs at hand do not have overflow circuits, and that excessive fuel is getting past the float valve for whatever reason and rising up into the carb throat and into the vent passage.

Good Fortune!