A real head scratcher....

hopefully someone will be able to give me a little guidance with my 1976 KZ900. so here is the problem. Like i said i have a 76 kz900. it is mostly stock with the exception of the vance and hines exhaust, k&n filter pods, and what i belive to be a Dyna2 ignition system. so here it is, the right two cylinders are practically not firing. i think that the outter cylinder pops every now and then but for the most part my bike sounds like a harley . the outter plug is wet with gas and this is why i think it may be popping every now and then. but the inner right side plug is dry as a bone. after the engine running for about 5 mins max i can grab the right side exhaust tubes with my bare hand and not get burnt. this problem just showed up one morning when getting ready to go on a ride. the bike ran just fine the day before with no obvious indications of a failure on the horizon.

so here is what i have done so far. i checked for spark. the coils are definatly firing correctly. i removed the plugs and cleaned them and swapped them from right to left sides of the engine. i removed the carbs and verified the cleanliness of the float bowls and cleaned them while they were apart. i think it is highly unlikely that i mixed matched any parts durring this process. these aren't my first carbs i've dissassembled and am very aware of the importance of cleanliness and organization for a successful carb rebuild. i performed a cylinder leak down test. both right side cylinders have approx 20% leak down @ 90psi. i performed a compression test. both right side cylinders had between 90-100psi. the only variable with the compression test is that i used the kick start to turn the engine over, so i may recheck this by using the starter. i have used a video boreascope to look into the right side cyliders. i saw nothing abnormal. i was able to see most of the valves they look to be in good condition. after all this the engine still runs bad and i only have a few things up my sleeve. next is a fresh set of plugs and maybe new carb to cylinder head seals. but if anyone has any thought on my problem i would be glad to hear it.

thanks

Check the compression with throttle open on the starter. If it is actually 100, that's why it won't run. Check valve clearance if compression is low.

Bryan1970 wrote:

this problem just showed up one morning when getting ready to go on a ride. the bike ran just fine the day before with no obvious indications of a failure on the horizon.

That sounds more like a failure or problem in the ignition module or a jet in the carb got blocked with crud. Do you have inline fuel filters?

20% leakdown is pretty serious in my book,have you checked the valve clearances ,where is timing mark when doing test? back to the not running part ,since 3-4 are on seperate coils 1/4 are paired up on one with 2-3 on the other id tend to lean towards carb problems myself

BATTERY. Any white cells on the battery?

How old is it, does it have topped fluids in all cells, have you trickle charged it? These bikes need a good battery to run right.
That statement of yours 'it was fine the night before' for some reason just made me think 'battery was marginal then one day not enough juice.'

Is the reason you didn't use the electric start for the compression test due to the battery being too low?


I agree with the 100psi being low and 20% leakage sounds high even to a raw beginner such as myself.


A wet plug may just mean that plug isn't firing to burn the gas off.

Have you done an electrical check on the plug wires?

With the compression that low you will never get it to run right. Check the valve clearance and set that first. If that doesn't help it's time to pull the head for a valve job and new rings.

Everyone has this pretty well covered.

Two other things I'd check would be the ignition connectors under the fuel tank & for possible water in the fuel.

I say that because you indicated it ran fine the day before.

In any case, it's time to check valve lash.

100psi is enough for it to run as long as the other cylinders are about the same.If any of the others are lower than 90psi, then I would say it may not run.You can't really get a good compression check with the kick starter.Use the electric start, hold the throttle wide open and spin it 4-5 revolutions per cylinder.make sure you spin it the same number for each cylinder.Make a note of how fast each cylinder leaks down.Then after doing them all once, go back and do them again, but this time put a squirt of oil in the cylinder and see if the PSI increases and if the leak down is the same.If the PSI and leak down is about the same then you need to check the valve adjustment.

awsome! thanks for all the input. i do have an inline fuel filter that is actually servicable in which i forgot to mention that i took it apart when i had the carbs off for cleaning and cleaned it as well. as for the ignition module that did cross my mind but it seemed unlikely since the two cylinders don't share a coil and the other two cylinders are running fine. atleast that is my reasoning on that.

with the compression test i will admit i was just kinda fiddling around that night. i used the kick start becasue i didn't want to wake the neighbors. and i did it on the engine cold and dry. before hand i thought i needed to squirt some oil in there but didn't and since the numbers were all about the same i figued it wasn't indicating any major failure. i.e. holes in pistons hanging valves etc. however i did not know to hold the throttle open while i did this. honestly this is really the first motorcylce i have ever really messed with. so rechecking the compression mybe worth doing. but by doing the leak down test i kinda figured i would rule it out. 20% is a little on the high side yes, but i fugred into account this is an engine that has never been rebuilt it was cold and has approx 30,000 miles on it. so i kinda felt like the 20% was acceptable.

with the valve clearence, that had definatly crossed my mind. a few things the detured me from going that route is the fact that it ran fine the day before, i mean not even the slightest hic-up. and i will definatly agree with that if the clearence is off it will casue my problems. i hadn't checked it yet because i just can't imagine that something mechanical like that would fail while the bike sat in the garage lol. but hey stranger things have happened so i think it will be worth my time to check it. not to mention it IS just good maintenance. by the way does anyone know of the top of thier head what the clearence should be? i know i have a clymer's manual around somewhere if not.

as for the battery. it is a new battery i installed in june. i did trickle charge it for about a full day before installing it in the bike. i also installed a new aftermarket regulator/rectifier. which i had a charging issue for a while before i installed that. and since i have i have not had any issues.

i am almost sure it isn't the fuel. because when i had the carbs off i also installed a new petcock on the tank so i drained it completely and put new gas in the tank. and the bike has never sat outside overnight in the past two years.

thank you so much everyone for the input, it has definatly given me some new ideas and methods to check

Don't overlook the simple things! I had plugs bad out of the box or fail within a 100 miles of use. Also you could have a fuel issue as well. Water in fuel can cause the problem you describe. Be sure you are not filling up with gas with a 10% Ethanol blend. Fuel with ethanol blends tend to attract water very quickly in small tanks and will foul out a cylinder or two overnight or while on a ride. I know as this has happened to me. Solution is to avoid Blended fuels or use fuel additives such as Chemtool's B12 or a new additive I'm curently using, STAR*TRON Enzyme Fuel Treatment. The first time I used the enzyme treatment, I noticed the engine ran smoother and throttle response is quicker. Also starts up better/smoother cold or warm. I'm not saying this is your problem but a factor that may be contributing to it.

unomike wrote:

Don't overlook the simple things! I had plugs bad out of the box or fail within a 100 miles of use. Also you could have a fuel issue as well. Water in fuel can cause the problem you describe. Be sure you are not filling up with gas with a 10% Ethanol blend. Fuel with ethanol blends tend to attract water very quickly in small tanks and will foul out a cylinder or two overnight or while on a ride. I know as this has happened to me. Solution is to avoid Blended fuels or use fuel additives such as Chemtool's B12 or a new additive I'm curently using, STAR*TRON Enzyme Fuel Treatment. The first time I used the enzyme treatment, I noticed the engine ran smoother and throttle response is quicker. Also starts up better/smoother cold or warm. I'm not saying this is your problem but a factor that may be contributing to it.

I FORGOT ALL ABOUT THAT -- THE 'new but bad' SPARK PLUG issue.

Lately, me and a buddy have noticed brand-new out-of-the-box NGK plugs being bad.

WONDER IF THE JAPS ARE OUTSOURCING PLUGS MFG. TO CHINA!!

Sh*t wouldn't that take it.

Here's how I experienced a bad plug:

- replaced all plugs at the same time
- #2 cylinder not firing, and the plug was fouled bad
- I *knew* (!!) it *couldn't* be the new plug
- so I checked everything else. Several hours later -- I pulled the plug and laid it on the cylinder with the coil wire still connected, turned on the ignition, kicked it over.

NO SPARK.
BRAND NEW NGK PLUG.
No friggin spark.

I put the #1 plug into #2's plug wire cap and got spark.
I returned the plug to Kragen auto parts for a new one and then my #2 cylinder ran perfect.


So the symptom was like yours -- a mysterious fouled plug, cylinder not firing, despite a brand new plug installed.

And a buddy of mine had the same problem, new plug and non-firing cylinder, he checked everything except the new plug until a couple of days of troubleshooting turned up nothing, and it was an NGK plug.

Sounds like the old "New spark plug" problem like I had last year. I have a 1976 KZ900 A4 as well and this exactly happened to me as well. Worked fine the day before then popping and farting but no go. Try thgis next time you suspect and every time you buy new spark plugs. Leave the old spark plugs in a working bike then one by one disconnect number one cylinder from the spark plug but leave the spark plug installed in the bike, then take each of the new spark plugs and one by one plug this new plug into the boot, start the bike and with a heavily gloved hand to protect you from getting zapped, ground the spark plug on the block of the engine. You will see a slight spark firing on this new plug, and once this has happened shut off the bike, repeat the above steps over for the three plugs. That is so common, I live in Canada and only buy my plugs from my local motorcycle shop because I return any pluggs that are dead from the box. Good luck. Ride on!