Spark Plug: Overheating on a Colder Plug?

Can a plug chop test only be done at WOT, or can it be done in the other circuits as long as the same throttle position is maintained?

Austin440 wrote: Can a plug chop test only be done at WOT, or can it be done in the other circuits as long as the same throttle position is maintained?

A throttle chop can be done at any position. That is why putting position marks on your throttle is such a helpful tool when it comes to carburetor tuning.

A leak-down test injects air pressure through the spark plug hole to pressurize the combustion chamber.

Can perform a poor man's leak down test by introducing compressed air into a spark plug hole and listening to where it escapes from the combustion clamber. This may be done at TDC or any other cam positions where both valves are fully closed. Top dead center (TDC) is when the piston is at its highest position. An easy way to tell TDC is by alignment of the T mark with the case mark when viewed through the timing window.

May introduce a spurt of compressed air by using a rubber cone-tip blow gun (rubber air nozzle) held into the spark plug hole. And of course keep holding it in position to keep air from coming back out the spark plug hole while listening for escaping air at other places. The air compressor should not be running while listening because the noise will likely drown out any sound of escaping air.

A portable compressed air tank may also be used in lieu of a live compressor.

Air heard escaping from exhaust port indicates exhaust valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from carb intake indicates intake valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from crankcase breather indicates loss of compression past rings into crankcase (perhaps worn piston rings or cylinders).

Air heard escaping from head gasket area indicates indicates loss of compression past head gasket (perhaps due to a blown head gasket).

A leaking valve may sometimes be resolved by adjusting the clearance to within specs.

A leaking head gasket may sometimes be resolved by torquing the head fasteners when the engine is stone cold (such as after sitting overnight).

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If not already done, would also perform the clear tube test to determine whether the fuel levels are within spec.
Improper fuel level will adversely affect the mixture in all carb circuits.
For example, a too high fuel level in otherwise perfect carbs will likely produce excessively rich mixtures in each circuit.

Good Fortune!

the perfect sync.

makes one bad azz running bike smooth as all hell most often looked over part (bench will get you close) but not dead nut on!

See my walk through of plug chops here.

Plug Chops

Hope it helps

I have made two attempts at re-jetting - I am not expecting it to be my last, but I need to figure out what's going on before ordering new jets. My first attempt was jumping from a 65 main to a 70 main, and an 88 to a 92 for the needle jet, which availed little change. It was still very lean due to the pod filters. Realizing that a step up in jet size is more of a fine tuning adjustment, I decided to take a big leap hoping to run rich instead of lean (less bad for the engine). So I went from a 65 to an 88 main, and for the needle, an 88 to a 108. I performed a plug chop test as you guys have suggested on all three circuits. The only sign of a rich condition is that the tip of the plug for the pilot jet circuit has carbon build-up, but the insulator is white all the way down. The other two circuits where white from the tip down to the base.

The bike is pretty slow at WOT and I'm thinking it's because the float height needs to be higher given the larger diameter of the jets. This is problematic because I had a float overflow after running it for a while with the new jets (I didn't even change the float height, so somehow these new jets caused them to overflow). I checked the float height and it was right at the bottom of the carburetor body, as opposed to "below the carburetor body" as prescribed by the manual. So I lowered it and made it too low, to where it would barely accelerate and sometimes even bog down at WOT. So I have gone up again with the float height, but perhaps not enough given that it is kind of jerky at WOT - it will be a little slow, and then catch a boost, then reduce acceleration again. Technically I am "below the carburetor body", but is there a point that is too low with these bigger jets?

I guess what I'm getting at is that is it possible that 20-23 jet sizes was way too big of a jump and even though the plug chop test indicates a lean condition, that I actually need to go down a few jet sizes?

I assume by needle jet you mean main jet? Is the other one an air jet? If so I would leave the air jet alone for now and focus on needle shimming and main. leave float height at factory.

With a stock exhaust and pods I would look at maybe trying to get the fuel screws or air screws richer for your idle before you go to a bigger pilot. If you find yourself having to turn them out or in a ton then should go up on the pilot. I would assume no adjustable needles in those carbs...so I would buy a needle shim kit and shim them with the thinnest shim. Take it for a ride, if it rides pretty consistently and pulls with no flat spots your good. If you get any at higher RPM try a size up or two on your main.



For my TX650 with UNI pods and Triumph Bonneville mufflers and the awesomely lean bs34 carbs, the pilot was fine, but I always had a flat spot coming onto the needle, then a bad flat spot/surge up top. A small shim on the needle and two sizes up on the main and it drives really really nice now.

Sorry if my post was confusing, I didn't mean to imply that I was re-jetting an air jet - there are three jets: pilot jet, needle jet (also called "secondary main jet"), and main jet. I was able to tune out the flat spots in acceleration between the pilot and the needle jet. Also, going to a larger main jet reduced that flat spot even at the factory idle mixture screw setting, but the idle stabilizes when I screw out the idle mixture screw about an extra 3/4 of a turn beyond factory.

It would be nice if I could get the float height back to "factory" but unfortunately when I got the bike it had been tampered with (it didn't help that they had the float in upside down). I've been adjusting the float height and it seems to be really finicky with these bigger jets. I wish my manual would be more specific than saying to set the fuel height to "below the carburetor body". Is there a point that is too low?

Right now the bike tops out at about 80 mph and takes a while to get there, so perhaps one float is set too low and isn't pulling its own weight.

Fuel level should be 1.5 - 3.5 mm below carb body / bowl joint. Take a look at the link below for a pic of the clear tube test. Ed

www.kz400.com/Workshop%20manual%20bilder...0man.%20p156-170.pdf

So I'm still fouling out spark plugs left and right, and I know I've got the mixture pretty good after re-jetting a few times. I think what's happening is a weak ignition system can't burn through any amount of buildup on the plug so it gets progressively worse the longer I ride it - it only takes a ride or two of 25 - 50 miles to foul out a plug.

It was mentioned earlier in the topic that NGK caps can be opened and the resistor removed. Does this have negative effects on the coil? The secondary resistance on my coil is high at about 20k ohms (so I guess it went bad in a couple months or else I got a false reading before). I pulled the caps and plugs off and the caps measured about 4k ohms each, the coil itself was 10k ohms, and the wires were nothing, although I'm sure the connections aren't the greatest, which might account for the 2k ohm discrepancy.

Did the stock plug caps have resistors in them? If so - that the stock plugs have the same 5k resistor as my new caps - that would mean my reading of 10k ohm on the coil pack (without wires or caps) means it is about 5k ohm above what it should be (spec is about 10k to 15k for the whole system), and it's probably time to replace it if I can find one. Or can I compensate for the weak coil by removing the resistors in the caps without adverse affects to the coil?

These coils are getting harder to find, maybe I should just suck it up and buy one.

Could install Dyna solid core plug wires from Z1E that have non-resistor caps already built into the ends ready to attach to the plugs.
And also run non-R plugs.
This results in no resistance in the wires, caps or plugs.
Will cause some audio interference, but eliminates any excessive resistance.
Can use for testing purposes to rule out excessive resistance being a problem in the existing set-up.
Or just enjoy running without any resistance.
If resistance is thereafter desired, may install R plugs.

Could possibly be bad ignition coil that passes the simple ohm-test but shorts internally under load for whatever reason. A Kawasaki dealership will likely have an electrotester to determine condition of the coil under load.

Good Fortune!

Austin440 wrote: ...It was mentioned earlier in the topic that NGK caps can be opened and the resistor removed. Does this have negative effects on the coil?
Did the stock plug caps have resistors in them?...can I compensate for the weak coil by removing the resistors in the caps without adverse affects to the coil?

These coils are getting harder to find, maybe I should just suck it up and buy one.

I once dissected a stock cap by smashing it to bits in order to see what the resistor looked like -- a small chunk of carbon-appearing substance.

Running without any resistance in the wires or cap or plugs isn't harmful to the ignition coils.

Would check with Z1 Enterprises for a new coil.

Good Fortune!