TOPIC:

I've got an 86 KZ440 that I picked up a few years back. It runs, but could run better. Compression is good, carb vacuum balance currently shows near perfect balance, Exhaust is in pretty good shape and I have run through the carbs on several occasions.

The bike has the classic high rev symptoms that a lot of folks complain about with these bikes (only when hot though). When it is cold it runs poorly, if take it 1/4 mile down the road and warm it up, it'll run relatively well until it heats up (i.e. 10 mile plus), then the high rev shows up. On the center stand (cold to mildly warm), the right side cylinder runs evenly and strong, the left side runs weak and occasionally backfires. The right plug reads perfect, the left plug reads carbon fouled. In fact, upon closer inspection the left exhaust also shows some carbon residue at the exit. Throughout my ownership I've been through the carbs several times, and will pull and clean lefty again next opportunity (this weekend I've gotta' go to a family wedding, so may be a bit).

Based on the carbon fouling, I decided to pull the coil and stick a resistance meter on it.

I don't remember what my measurements came out at off-hand (notes are in the garage), but the primary circuit spec'd and the secondary circuit didn't spec. When I originally spec'd the coil, I pulled the plug wires and took the secondary coil measurement directly from the coil and it passed. After a second look at my Clymer manual, it indicates to spec the secondary coil from the plug caps. Upon reinstalling the plug wires, the coil does not spec. I gauged the wires themselves and based on the length and a brief google search, they appear to be typical (plug wires are approximately a foot long, and each have/add about 4.5K ohms of resistance).

So I read somewhere not to worry about spec on the secondary circuit of the coil (of course I cannot find that now), which has me slightly hesitant to replace the coil. Thus the post.

Is this a rock-solid case for replacing the coil, or is other diagnosis warranted?

Also, what does everyone recommend as far as coils go? I see Z1 has 2.2 ohm dual output coils, and eBay seller Caltric has some coils advertiesed to (some drilling required) fit the bike for about half as much, plug wires included. Any other options I should be considering?

You might try a compression check on the suspect cylinder, black sooty plug is often fuel flooding, but can also be that cylinder burning oil from piston rings that aren't sealing. Try it dry, wide open throttle of course, then try adding a small amount of oil down that plug hole and retest it, if it improves then the rings are suspect.

Compression is 145 dry on the suspect cylinder. Per Clymer manual, spec is 157, limit is 109. I am not too concerned about rings at the moment. I am familiar with the oil trick and will give it a try next time I have garage time.

On a twin, the same coil winding fires both coils in series. That makes it unlikely one side would run great and the other badly due to the coil. IMHO, carbs are more likely.

If not already done, would perform the clear tube test on the "too rich" carb, which might reveal that a too high fuel level is likely causing an excessively rich fuel mixture from an otherwise perfect carb.

A racing idle often results from an air leak through an old, hard, cracked, worn-out carb holder.
But a carb holder in good serviceable condition might leak from being be too loosely attached to the engine (loose screws), or too loosely attached to the carb (loose clamp), or where the carb isn't pushed fully and completely into the carb holder so that it may be snugly seated.

Good Fortune!

Got a little bit of time in the garage today. Ran the dry/wet compression test. Right Hand cylinder (not the problem) stayed about the same. Left Hand cylinder actually went from the previously mentioned 145 to over 160 PSI with a little oil (went beyond spec). So, I'm not sure if that would tell me that the rings are worn and the head is caked with a layer of carbon, or maybe I just put too much oil in the cylinder for the test.

Clear tube test was inconclusive. Fuel level was a little low if anything.

Based on Bountyhunter's comment about the cylinders being in series, I decided to try swapping the plug wires. With the wires swapped the left cylinder did run stronger and it felt like the right hand cylinder was weaker (as measured with cupped hand behind the muffler, which is of course subjective). I thought "aha, I've got a smoking gun!". Only problem is, when I swapped the plug leads back, left cylinder still felt strong and right cylinder still felt weak (I was running off of the float bowls though, so it's possible either carb was running out of gas at that point).

I'm thinking my next steps are:
1) Clean Carbs again
2) Replace plug wires and caps.
3) Run about 5 miles on current plugs (burn out oil from the dry/wet compression test).
4) Swap plugs while it's hot, run about 5-10 more miles on new plugs and read the plugs (I don't have a good place around here to do a 'chop test' so will just do a normal driving conditions read).
5) re-run dry/wet compression test, being a bit more deliberate on the amount of oil I run in.

If the wet/dry compression test still shows a big difference on left side, I'll have to pull the engine and inspect the top end.
If not I'll focus my attention back on the coil.

As far as the carbs/boots/air. when I replaced the boots (about 1.5 years ago), I did inspect the mounting surface for irregularities and use a small amount of liquid gasket to make sure things are sealed up. When I clean the carbs, I'll be deliberate a bout seating them and tightening clamps as tight as I'm comfortable making them, I've previously given them the spray down test while running with no change in idle speed, and will do so again including pulling to the side of the road and trying again when the engine is hot in case thermal expansion is a factor.

It will be a few days before I can complete all steps outlined above.

Alright guys, we can close this one.

I ran through the left side carb found and removed a rather stubborn clog in one of the jets and now the bike is now purring like a kitten at idle.

My original question was on whether or not my readings warranted a new coil, and I am now increasingly confident that my coil is fine (well, it's still 30 years old and I will probably replace it at some point, just not this summer).

I still need to follow-up on the left side cylinder compression variance following the dry/wet test, but that's not really relevant to the electrical forum, and I'm good with that on my own. I'll post a question in the appropriate forum should I run into anything new.

EricTheLeg wrote: Got a little bit of time in the garage today. Ran the dry/wet compression test. Right Hand cylinder (not the problem) stayed about the same. Left Hand cylinder actually went from the previously mentioned 145 to over 160 PSI with a little oil (went beyond spec). .

That's not bad for an old engine. My 750 twin went from about 125 (cold dry) to about 150 psi with oil squirted in. It did have cylinder wear. If compression goes "over spec" it could be carbon buildup on the piston or the comp gauge is a bit high.

EricTheLeg wrote: ....... Ran the dry/wet compression test. Right Hand cylinder (not the problem) stayed about the same. Left Hand cylinder actually went from the previously mentioned 145 to over 160 PSI with a little oil (went beyond spec). So, I'm not sure if that would tell me that the rings are worn and the head is caked with a layer of carbon, or maybe I just put too much oil in the cylinder for the test......

Regarding the Wet compression test......
Even a very small amount of oil added to a cylinder can make a big difference in the compression readings. I have seen more than one person advocate adding "about a teaspoon" of oil to each cylinder when doing a "wet" compression test. That may not sound like much oil to many folks, but a teaspoon is actually right around 5cc, and that is a lot. As an example of how little oil it takes to change the compression ratio I offer the following diagram. It uses a hypothetical cylinder with a total volume (cylinder & head) of 100cc and a dry compression ratio of 10:1. A little oil added changes things quite a bit, so if different amounts of oil were added to different cylinders one can see that the compression ratios might vary a lot. This example is not precise and is not any specific engine and it doesn't take into account oil that may seep past the rings, etc. It only is meant to demonstrate in very general terms how adding oil can change compression ratios. You can see how quickly the compression ratio jumps as oil is added to the cylinder. Ed