no more guesses - can't figure it out

Alright, my KZ750 has been idling poorly for some time, and I can tell that I'm not firing on all the cylinders all the time. Additionally, I have some strange smoke emitting. So, here is all my recent test results in systematic detail:

I just bought a brand new leakdown tester and went through each cylinder in a variety of ways. The tester I have is one of the Harbor Freight which has the first dial as a 0-100 psi gauge and the second as a % leakdown gauge. The instructions say to increase the supply pressure until the leakdown gauge reads 0%. From the looks at it, the tool is calibrated to 15 psi supply. The range on the leak dial was not what I've seen elsewhere (100%-70% leak labeled high, 70%-50% labeled moderate, 40-10% labeled low).

I tested each cylinder twice, rotating back to a new TDC each time. Here is what I got.
#1: 35%, 30% leak
#2: 35%, 43% leak
#3: 30%, 25% leak
#4: 25%, 23% leak

All but the second test on #2 where in the "LOW" category.

I retested each cylinder twice by resetting the supply regulator to 50psi, and testing the drop when connected to each cylinder. Here are those results.
#1: 46psi, 44psi (implied 10% leak)
#2: 44psi, 44psi (implied 12% leak)
#3: 45psi, 45psi (implied 10% leak)
#4: 43psi, 43psi (implied 14% leak)

All of the leaking noises where through the crackcase, as expected. I had removed the head and relapped all the valves over the winter, so I'm glad nothing is getting past the valves. Additionally, the clearances were set perfectly a few weeks ago, and I don't those have changed.

So are these numbers that bad? I looks a bit leaky, but not terrible.

The problem I'm noticing is rough idle (rough sounding, a bit knocky) until about 1500-1800 rpm. At idle, if I pull the plugs one at a time, the engine stumbles if I pull #1,#3,#4. However the engine doesn't stumble if I pull #2 - no sound change whatsoever. #2 fires, but seemingly not at idle.

Also, there is white smoke always coming out of the 1/2 exhaust set.

However, I also hooked up the inductive timing light to check the timing. The timing at idle, and the advance at 4000 rpms, are spot on. All 4 spark plugs are brand new, a week old.

However, I also looked into the light as it was connected to each plug wire.
#1: Fired totally normally, with a flash as expected
#2: fired totally normally, with a flash as expected
#3: Fired normally, with an occassional skip in the light once ever 3-4 seconds
#4: Misfiring often, missing lights about 3-4 times a second.

So I'm at a loss. I have white smoke coming out of 1/2 exhaust, #2 does fire at idle and doesn't matter if I pull that wire, #3 very rare misfire, #4 very frequent misfire.

WTF???? Where do I start?

If you switch plugs between cylinders, does the misfire move? if not then that rules out a bad plug and may be a bad plug cap. I believe the plug caps are 5 ohm resistance. Sometimes you can remove the plug cap and find the plug wire end corroded; if so then clip as little as needed to get to new wire and try the misfire test again.

For the #4 misfire, I took the cap off and cut 1/2" off the end and screwed it back on already, with no change.

I have also switched #1 and #4 wires with no change.

---

Sorry, that was a lie. What I had done previously was switch 2/3 wires to see if I could get #2 to fire at idle. It didn't.

I had cut the wire on #4 and reseated the cap. I just went down to swap #1/#4 wires to see what happened. When I and used the inductive lamp, I got an odd change. #1 now misfires about once a second at idle, but #4 also misfires about once a second. so what was originally a strong #1 and terrible #4 when swapped is a moderately crappy #1 and a moderately crappy #4.

I revved the engine slowly up to 4000 on each wire as well. the misfires continue throughout and get slightly worse just after revving as it drops back to 1100.

so, I'm left to believe the following:

#2 issue is carb related - since swapping plug wires didn't matter and it is only at idle issue

#4 misfire - weak coil? bad wires? the fact that the both started misfiring on swap says system related to both, so ignition coil related. maybe ignitor faults? Any way to test besides buying new coils?

#1: Fired totally normally, with a flash as expected
#4: Misfiring often, missing lights about 3-4 times a second.

1&4 are on the same coil, and it's one circuit, you can't have spark on #1 without also having spark on #4, the spark plugs are in series.

Ever seen the Christmas tree lights where none of the bulbs light if just one is bad? That's because they are wired in series, just like #1 & #4, and #2 & #3. If you have a manual, look in it, it will tell you the same.


Bad connection or faulty timing light maybe.

I got an odd change. #1 now misfires about once a second at idle, but #4 also misfires about once a second.

That one's right, and could be caused by either of the two wires or two plugs being bad.

At idle, if I pull the plugs one at a time, the engine stumbles if I pull #1,#3,#4. However the engine doesn't stumble if I pull #2 - no sound change whatsoever. #2 fires, but seemingly not at idle.

Disconnect one spark plug and you kill two cylinders, so disconnecting either #2 or #3 will kill both #2 and #3.

Now, armed with this new knowledge, go back and retest it.


Oh yeah, to test the coils you measure resistance between the two plug wires on each coil (16k-20k ohms) and between the two small wires on each coil (2-4 ohms). The book will give you the exact resistance, but as long as it's in the range I stated it will work.

1&4 are on the same coil, and it's one circuit, you can't have spark on #1 without also having spark on #4, the spark plugs are in series.

Ever seen the Christmas tree lights where none of the bulbs light if just one is bad? That's because they are wired in series, just like #1 & #4, and #2 & #3. If you have a manual, look in it, it will tell you the same.

Some times a coil can be good but only one of the plug wires coming out of the coil is bad. This would let only one plug (1 or 4) fire and the other not fire. A rouh idle can also be from the carbs being out of synch, but it apears you have bigger things than that. Good luck.

Here's a diagram that may help you understand it.

Bad example, I'm looking for another one.

Ok, here we go, the dotted line represents current flow.





And here's the link.

www.stealth316.com/2-ignitionsystem.htm

Thanks for the link, I'll read it throughly shortly. However, I should clarify how I was doing the pulling to explain how I don't think I was dropping both cylinders.

When I pulled each plug cap, I placed it over a cylinder head bolt head and could hear it arcing to the engine, thus completing each ignition coil circuit. That way the other spark should still be lighting even though i was taking one spark plug out of the circuit. Additionally, having the longer arcing distance from plug cap to case should theoretically increase the voltage it takes for the plugs to fire and thus make the spark hotter on the opposite plug.

Even so, this doesn't seem to explain the #1/#4 misfires. The problem I was seeing was with everything connected with an inductive timing light. I'm fairly certain the problem isn't the light itself: in the original arrangement with #1 firing normally and #4 firing poorly, I could audibly hear the misfire stumble as well at idle. It always matched the missing flash. i think the light was accurately displaying the misfires.

This does make me wonder if I have a bad 1/4 coil because I remember reading somewhere that the positive side of the coil - one sparking from coil to case I believe - is fed stronger than the negative side. If my original arrangement had the positive on #1, couldn't that explain the stronger #1? totally understanding that the sparks are in series leaves me very confused why I was definitely seeing different firing patterns for #1 and #4 - i agree with you guys, I should have been.

Hmmmm....I think I should figure out this #1/#4 misfire before tacking the #2 no fire at idle. I will go test the primary and secondary resistance of the coil now.

Coil Resistance Numbers for the #1/#4 coil.

Battery Voltage at idle: 13.3
Battery Voltage at 4000: 14.8
Voltage Drop to 1/4 Coil: -.75 V

The primary resistance was 2.55 ohms, in the 1.8-2.8 spec range.

Secondary resistance with out wires&caps was 12.9 kiloOhms, in the 10-16 kiloOhm spec range.

Secondary resistance with two 5k resistance caps was 21.1 kiloOhms, again in spec.

I didn't use the lamp, but I can still hear the misfire in #4.

This doesn't make sense....

I may be stupid here, but maybe Lou will jump in,
since the coil fires two cyls at the same time and the electricity is going to go to the least path of resistance, then I could see an erratic spark off one of the wires and the other having a good solid spark.
With that being said, what do the plugs look like, granted you havent had them in that long, but I was wondering about any fuel or oil forming on the plug.

for some reason my gut tells me the ignition is fine and to look at the carbs

yeah, I really wanted to go in order on this one to really make sure I isolate the problem correctly. I know the valve clearances are good.

Having never done the leakdown test before, I know the numbers aren't top of the line, is there agreement that these numbers indicate that compression isn't the problem? All my is leakage past the rings and nothing above around 15% on the 50psi test.

Before I move to the carbs, because they are the most difficult to tune, I wanted to make sure that the problem definitely isn't the ignition. I want to rule out 1/4 coil problems...

I should add, I did a spark gap test with one of those adjustable screw thingys from autozone as well. All wires jump over a ~13-15 mm gap, so I'm sure they are all big and strong.

dnpurdy wrote:

Even so, this doesn't seem to explain the #1/#4 misfires. .....in the original arrangement with #1 firing normally and #4 firing poorly, I could audibly hear the misfire stumble as well at idle. It always matched the missing flash. i think the light was accurately displaying the misfires.

Try reversing the #1 and #4 wires to the plugs and see if the misfire follows the same plug wire. If it does, the problem is definitely ignition and probably a bad coil.

BSKZ650 wrote:

I may be stupid here, but maybe Lou will jump in,
since the coil fires two cyls at the same time and the electricity is going to go to the least path of resistance, then I could see an erratic spark off one of the wires and the other having a good solid spark.

Yes, it could if one of the secondary windings of the coil was "breaking down" under voltage. You could get an OK spark off one wire and not the other. Ohm testing will never detect this, it's a breakdown under high voltage field.