TOPIC:

My 1976 KZ900 has a dead cylinder. I did a valve job last year thinking that was the problem, replaced all valves, springs and guides. Got it running and same problem, number three not fireing. Check for spark was ok but replaced the origional coils with new ones. Still dead cylinder. Did a compression test and all cylinders were 145 to 150. Did a cylinder leak down test and its ok. All timeing is right on, valves are adjusted. I have tryed different carburators I have three sets, the origionals, a set of smooth bores and a set off a KZ 1000 and that doesn't make a difference. All new rubber carb. manifolds. My thinking it must be in the camshafts, all the lobes look good. I wonder where I could find the lobes lift measurments? Help me Ive done all that I can think of.

KZone900 wrote:

My 1976 KZ900 has a dead cylinder. I did a valve job last year thinking that was the problem, replaced all valves, springs and guides. Got it running and same problem, number three not fireing. Check for spark was ok but replaced the origional coils with new ones. Still dead cylinder. Did a compression test and all cylinders were 145 to 150. Did a cylinder leak down test and its ok. All timeing is right on, valves are adjusted. I have tryed different carburators I have three sets, the origionals, a set of smooth bores and a set off a KZ 1000 and that doesn't make a difference. All new rubber carb. manifolds. My thinking it must be in the camshafts, all the lobes look good. I wonder where I could find the lobes lift measurments? Help me Ive done all that I can think of.

Some far out suggestions.

1st, mix:
Is your exhaust port or pipe blocked on #3? Rag from when the pipes were off stuck in port or blown into pipe? Chimpmunk carcass and load of nuts in that pipe? Collapsed inner pipe? If exhaust isn't working then it's not gonna pull mix in. You might try comparing pull on #3 and others by holding your hand over while you spin with the starter. Plugs in, kill switch set to kill. A sticky valve would keep #3 from pulling too but maybe not enough to show up on the compression test when engine isn't spinning as fast as when it's running. If it's pulling on #3, is it getting mix or just air? Squirt some fuel in and see what happens.


2nd, spark:
Have you tried swapping the #3 and #2 plugs? You didn't mention trying new plugs. It's possible to get a good spark with the plug pulled an have it fail when installed. Friends xs650 did this with two new plugs. Found problem by swapping the one from the cylinder that was firing. I suggest swapping #2 and #3 because a bad #2 can fire and at the same time keep #3 from firing. If things don't change with swapped plugs, swap #2 and #3 wires.

It should fire on #3 even if half the cam lobe is worn off and you would have noticed that when you adjusted the valves

I have tried two different exaust, new plugs,points all new valves, it does get fuel because that exaust pipe gets hot. Spark is strong at no. three, new wires. Engine was compleatly tore down last year by me and all measurments were withen specs. Block bore was good could still see cross marks on cylinders only honed it lightly, pistons were in good shape. I have found one problem after I got it running. It has very low vaccum on no.3 cylinder and the rest are around 15 in, this was like this before I tore it down. No vaccum leaks. Tried to snyro carbs and it didnt help. Checked valve timing and its right on. Ign timing is right on the mark. This engine is all stock no fancy adjustable cam gears. I don't know what to do next. Thanks for replying. I won't give up.

Post edited by: KZone900, at: 2006/11/11 18:54

KZone900 wrote:

I have tried two different exaust, new plugs,points all new valves, it does get fuel because [/b]that exaust pipe gets hot. Spark is strong at no. three, new wires. Engine was compleatly tore down last year by me and all measurments were withen specs. Block bore was good could still see cross marks on cylinders only honed it lightly, pistons were in good shape. I have found one problem after I got it running. It has very low vaccum on no.3 cylinder and the rest are around 15 in, this was like this before I tore it down. No vaccum leaks. Tried to snyro carbs and it didnt help. Checked valve timing and its right on. Ign timing is right on the mark. This engine is all stock no fancy adjustable cam gears. I don't know what to do next. Thanks for replying. I won't give up.<br><br>Post edited by: KZone900, at: 2006/11/11 18:54

If the number 3 exhust pipe gets hot one should, probably, assume that the fuel in this cylinder is ignitig.What makes you think it is not firing ? sound, vibration, etc. ? are you using nkg plugs ? I have found that some plugs, IE: Champion-- do not always fire although they may be new.

Lacking the valve lift specs you could do a comparison check between the four exhaust and the four intake valves. If #3 has less lift on the intake or exhaust than the other three cylinders, then that would indicate a bad cam lobe.

this is not a cam problem.the cam only opens and closes the valve.You said you checked the shims and have proper clearance .You did compresion test,and leak down test.So you know the valve is closing and sealing.the cam has a lobe so it must be opening.so this leaves electrical(plug,wire,points,coil) or fuel(carb)You said you have spark.and timeing is right.Can you tell us more about the symtoms. You say the pipe gets hot so it must be getting fuel and spark.Why do you say that cyl. is dead?Help us understand what is wrong.

Usually, when I have seemed to have checked every possibility, I found a made some assumption and the answer was really right in front of me.

spark
gas/air
compression

If you have GOOD compession, don't get hung up on valve timing, valve clearances or other stuff of that nature...those things must be pretty much OK if your compression is healthy.

air/fuel... hmmm 3 sets of carbs all behave the same way? Not likely an air/fuel issue...

and that leaves:

spark... this is a good place to make an "assumption" that turns out wrong. Have you swapped plugs? I have had bad/shorted plugs in the past. What voltage are you seeing at the coils? Have you swapped the 2/3 plug wires? Have you swapped plug caps/wires? Could a plug wire be poorly seated in the 2/3 coil? Could the plug cap be malfunctioning? Have you swapped the coils? I bet it is an electrical issue and it can hardly be the pickup coils (or points/condensor) or IC Igniter (if electronic ignition)... so must be plug(s) OR wires or plug caps.


Last how did you determine #3 wasn't firing?

One more thing... put your bike model/year and your location in your signature... might be a local who can lend a hand or advice... lots easier "on site"...

Post edited by: wiredgeorge, at: 2006/11/11 22:38

Take the plug cap off and run it with the wire on the plug. Spark can look great at the plug but in the cylinder it's a whole lot of resistance increase and all the spark will go to plug number 2. What color is your spark plug anyway? Does the right side pipe smoke? If the plug cap is carbon tracked it may be grounding to the head. If you take off the cap and attache the bare wire to the plug, start the motor and then pull the plug wire off and away from the spark plug a quarter inch what do you suppose would happen? I think if it does'nt run on that cylinder, you may have to shoot me. Make certain the intake runner is not cracked.

Thanks for all the info. some good points to look at. I need to start all over and look at the simple things. I did buy all new coils and wired them direct to a relay so the coils get 12 volts. I can take the plug wire off no. three when the motor is running and it doesnt make any difference in engine speed, I take any other plug wire off and the motor will stall. No. 3 is defently not fireing strong and no3 plug will stay clean wile the others will turn tan colors. I run NGK B8ES plugs.I live in Concord New Hampshire if anybody is a local thanks.

Post edited by: KZone900, at: 2006/11/12 10:18

KZone900 wrote:

I have found one problem after I got it running. It has very low vaccum on no.3 cylinder and the rest are around 15 in, this was like this before I tore it down. <br><br>Post edited by: KZone900, at: 2006/11/11 18:54

OK. So we know why it's not running right on #3!
Low vacuum means not enough charge in the cylinder.

So. What can make the vacuum low?

1)Poor flow through carb OR carb wide open all the time:
Either case will result in low vacuum.
CV carb diaphragm not allowing slide to rise (hole in it).
Sticky carb slide on CV carb.
Bad/broken cable if you're running 4:1 cables on your carbs.
Jet needle or slide hanging and slide staying open all the way. Bad slide spring letting slide rise to top with small vacuum.
???
Use a mirror to look at slide.

2)Poor intake flow.
Rag stuck in port.
Intake valve sticking because of bent or damaged stem/guide/broken seal spring interference.
Intake valve not closing quickly because of broken/too short/weak spring. Sticky bucket*

3)Poor intake flow caused by poor exhaust flow or sticky exhaust valve. Rag stuck in port. Sticky valve. Sticky bucket*

The valves may open and close just fine when the engine is cold and running at low RPM and then stick like a sumbitch when hot and running at higher RPM. This may be exacerbated rocking cam lobes due to slack timing chain. You might not hear a sticking valve.

*out of round, galled channel

Post edited by: Duck, at: 2006/11/12 10:39

Post edited by: Duck, at: 2006/11/12 12:30

Add to the list:

Excessive valve clearance not opening valves all the way.
Bad cam lobe not opening valves all the way.
Cam timing off, but that would affect all cylinders and compression.
Poor valve sealing.

I'm not saying any one of those is the problem, but it's certainly an easy thing to check.

I have never heard of a bad cam lobe on a KZ, but anything is possible I suppose.

Post edited by: steell, at: 2006/11/12 13:29

I fixed it! It was a vaccum leak in the carb on no.3. Apparently the set of carbs i have in it right now came off a 1980 LTD 1000. I use these because the accelerator pump helps out on the low RPM take off. It has a port for a PCV valve that wasn't pluged! I pluged it and it runs as smooth as new. Thanks everbody who helped me out.