Ignition coils wired improperly? One cylinder not firing

[Sterminare]

Hi all. I'm troubleshooting my '84 Nighthawk CB650SC, cylinder 2 not running. That header stays ice cold when the other 3 are too hot to touch. New spark plugs, getting spark on all plugs.

I decided to try swapping my ignition coils to see if the problem cylinder moved. Long story short, it didn't. But I did notice something odd with my wiring. Each coil gets 2 wires from the spark control unit, a black and a blue or yellow, to the + or - terminal on the coil. My bike seems to have black wires (-, presumably) on the bottom terminal on each coil. But since one coil is upside down compared to the other, that means that one coil has black wire on - and the other coil has black wire on +. Then the blue/yellow wires are on the top terminal. I'm confused as it seems to me that one of the coils must be wired up backwards. I don't know how to tell which one or if it really matters. Wondering if anyone here knows if it's ok or if I should switch it around. I don't want to blow up my spark unit theyre not easy to come by..

See photos of the wiring: https://imgur.com/a/vmRHxeu
I recrimped all the wires and cleaned all connections when I had the coils removed during the swap. No change in behavior. I put the wiring back the way I found it.

Thanks in advance!

[mollusc]

I'm pretty sure that coils are indifferent to polarity.  The wiring diagram for the 700S doesn't differentiate the coil terminals.
Since each coil fires two cylinders ("wasted spark" setup), having only one cylinder not running means that the issue can only be the plug or the plug wire to that cylinder, the carb for that cylinder, or something catastrophically mechanical that's causing a lack of compression.  I think you're chasing the wrong trail looking at the coil wiring.

[Sterminare]

Thanks for the confirmation that's pretty much what I figured, bike makes 120psi compression on all cylinders. I've cleaned that carb now twice thoroughly though I haven't broken them apart yet. It's blowing air back through the carburetor (and only that carburetor) and only that carb drips liquid (probably gas) out the intake, and is dirtier at the outlet than the other 3. https://imgur.com/a/sL5yFiq

I'm thinking it can't be valve related because it makes good compression. But I don't really know how else it can be blowing air back out the intake, into and out of the carb. My last idea is swapping 2 of the carburetors and seeing if the trouble follows the carb or not. Maybe valve clearance related? I dont know how to check that, factory service manual says they're self adjusting and not serviceable.

[Bob H]

My money is on the spark, even though it checked out OK.

If you changed the plugs but not the wires, I would just change the wires if they are old. Simply moving old plug wires around can jack with the connection of the lead to the boot (there is a youtube video of the end of wire plug connector falling off when the leads were simply removed on an older Honda)

Your bike likely came with stranded copper wire (not the noise suppressing graphite ones). You can see the ones I ordered from NAPA here https://nighthawkforums.com/index.php?topic=359.msg3054#msg3054

... My last idea is swapping 2 of the carburetors and seeing if the trouble follows the carb or not.

Let's say it is fuel related. Have you tried draining the float bowl of the "cold" cylinder carb to make sure it is filling with appropriate amount of fuel? I would check that before going to the grief of pulling a bank of 4 carbs. Who knows, maybe the float or needle are stuck.

Also if you pull the air box and start the bike, you could see if the CV slides are behaving somewhat the same on all 4 carbs. is the one on the cold cylinder lifting when you give it gas - because the engine is pumping air through it.

... Maybe valve clearance related?

If you have equal compression on all cylinders it is not your valves.

[Sterminare]

Thank you for your suggestions. I think I will change the spark plug leads, I'm pretty sure they're original because they have printed on them "coil 1/2/3/4" like the OEM ones. Cylinder 2 has a replacement boot on it, I think a previous owner changed it. I received the bike in non-running condition so I've never seen that cylinder run.

I get a fat blue spark from that plug holding it against the engine casting, so I thought it was good, but I acknowledge that conditions are probably pretty different under compression+fuel mixture instead of open air. Maybe it's not strong enough under pressure due to a faulty lead.

The carbs are all getting fuel. I've had them off the bike twice now and drained a similar amount from the bowls across all the carbs. I didn't break them apart though, just cleaned what I could while they were together, replaced jets, float needle, mix screws/springs/oring/washer. Mix screws are turned out 2 5/8 btw as per the manual.

I've run the bike for a quick test without the air box boots, but I didn't think about checking to see if the slide is rising on carb 2. They all move smoothly and drop slowly so I believe the diaphragms are in good shape. Visually they looked ok too. If slide 2 doesn't rise when the engine is running, I guess that'd be a clogged air jet/vacuum port? Thanks for the idea I'll check that out right now.

Edit: I just checked, the slide is moving like the other carbs when I pull the throttle. None of them move until I give it quite a bit of throttle, but I'm guessing thats normal and unrelated to the idling situation.

Edit 2: I swapped spark plug leads from cylinder 2 and 4. As in, disconnected from coil and plug and swapped the entire thing over, because they're both the same length. Figured if it were a bad lead that cylinder 4 would die and cylinder 2 would start running. Unfortunately no change in behavior. Cylinder 2 header is cold, cylinder 1,3,4 are hot. Air blowing back out of carb 2 intake. I felt around the header I don't feel any exhaust leak around cylinder 2 connect to the block.

[Bob H]

Dang.
I know some people are against starter fluid, but if that was my bike and NOTHING else is making sense - I would remove the airbox so I could shoot a surgical hit of starter fluid into #2 intake.

If there is no change, then it would have to be spark (since you have equal compression all 4)

It's either spark or fuel, so that is my thought process to isolate it.

[mollusc]

You can't switch the carbs around.  Each one is specific to its location.

[Sterminare]

Dang.
I know some people are against starter fluid, but if that was my bike and NOTHING else is making sense - I would remove the airbox so I could shoot a surgical hit of starter fluid into #2 intake.

If there is no change, then it would have to be spark (since you have equal compression all 4)

It's either spark or fuel, so that is my thought process to isolate it.

I did exactly this the other day, starting fluid straight into carb #2 produced no change whatsoever while it was idling. Swapping the spark plug leads made no change so it's not the cables. I haven't actually measured the coil resistance on the CDIs but my manual has a procedure for that. I thought they were good since I was getting spark but that must have been a wrong assumption. I'll measure it with my multimeter tomorrow.

I really really hope it's not the spark unit/ECU box. Used ones seem scarce and the only repro I could find was like $400 

You can't switch the carbs around.  Each one is specific to its location.

Aw man I really thought I could switch the middle two at least. Would entirely eliminate fuel+air if I could verify this carb on another cylinder. Thank you for stopping me before I went through all that effort just to be disappointed.

[mollusc]

I really don't think it's the spark unit.  If #4 is firing, then #2 is firing.  Either your spark isn't getting from the coil to the cylinder, or you've got a fuel/air issue.

[Bob H]

...or you've got a fuel/air issue.

From 2,000 miles away, I believe that if the #2 cylinder wasn't getting fuel then squirting the starter fluid into that region should have resulted in a change. He is saying "starting fluid straight into carb #2 produced no change whatsoever while it was idling".

I would triple check that the wires leave from correct place on coils, to correct cylinders. If no joy, replace spark plug wires. In my region they don't sell the copper stranded ones at parts stores, I had to order them and they are described as for "tractors" because they are outlawed for modern vehicles with ECU's and the alleged interference affecting other vehicles on the road.

[Sterminare]

I would triple check that the wires leave from correct place on coils, to correct cylinders. If no joy, replace spark plug wires.

Please excuse the crude diagram https://imgur.com/a/PfvyczB but best way for me to show what I've got. What's strange to me is that the plug leads are labeled "coil 1/2/3/4" but they don't match the cylinders theyre going to. I tried making them match the cylinders they go to, whilst keeping the wiring the same, but the cable lengths did not match up. obviously the printing on the cable sleeve has no electrical effect but it's strange to me that it appears to have been changed.

Thank you both for your suggestions I appreciate the help!!

[mollusc]

Your image looks correct.  The left coil fires cylinders 1 and 4; the right coils fires 2 and 3.  It doesn't matter which plug wire goes from which coil terminal to which cylinder, or (I believe) which supply wire goes to which coil terminal.
If you're getting the fat blue spark that you observed, then that brings us back to the carb.  It can often take two or three times completely disassembling and running through an ultrasonic cleaner to get a neglected carb back into running condition.

[Pete in PA]

After reading the whole thing I think there is nothing left but the compression.

I'm now a believer in Marvel Mystery oil at 2 oz. in a full tank of gas.  Got me 15psi more in my low cylinders. 10 in my higher ones.
I now have 150-155 psi in all.

What readings are you getting across all?
Do you trust the tester?
Try another one?
You want one with a Schrader valve at the spark plug end.
Mine started giving me whack readings and I had to get a new valve. They're special...Respond to very low pressure in 1 direction.

[Sterminare]

After reading the whole thing I think there is nothing left but the compression.

I'm now a believer in Marvel Mystery oil at 2 oz. in a full tank of gas.  Got me 15psi more in my low cylinders. 10 in my higher ones.
I now have 150-155 psi in all.

What readings are you getting across all?
Do you trust the tester?
Try another one?
You want one with a Schrader valve at the spark plug end.
Mine started giving me whack readings and I had to get a new valve. They're special...Respond to very low pressure in 1 direction.

I'm getting 120psi on all cylinders with the engine completely cold, no oil added, I trust my gauge, it's the type the hose screws into the park plug hole with an oring and then quick connect fitting to the gauge itself

Im thinking I may try a leak down test, can it be making compression during crank but there's some air blowing by the intake valve on compression stroke due to carbon deposits/weak valve spring? I don't know how to absolutely confirm that the carb is sending fuel. I know that it's absolutely receiving fuel the bowl is full. The cylinder doesn't appear flooded after (not) running. But maybe the fuel is just being sprayed out the exhaust.

Something I noticed yesterday while running again- carb 4 is also blowing some air back out the carb. Not as much as carb 2. Maybe half as strong. But cylinder 4 is running! I'm very confused.

The bike only has 5000 miles on it. So to me worn valve seat/guides seems unlikely. But with that low miles, maybe the bike sat for 30 years with the intake valve(s) compressed on 2/4. And now the spring is weak and the valve is bouncing. I don't understand how else air could be forced backward out of the carbs. Yet It still makes strong compression. But I am very much a novice this is my first motorcycle.

[Bob H]

I don't know how to absolutely confirm that the carb is sending fuel. I know that it's absolutely receiving fuel the bowl is full.

When you sprayed starting fluid into the non-firing cylinder, with no change - that was a confirmation that fuel wasn't the problem. Even a small manifold leak will manifest a change if you spray starter fluid, and that didn't happen for you. When you "cleaned the carbs" did you confirm the main jet and it's holder were open and clear of crud? Even if the low speed passages were still clogged and need work, you would get "some" fuel into the engine from that carb.

The cylinder doesn't appear flooded after (not) running. But maybe the fuel is just being sprayed out the exhaust.

I don't know about all that - but I would be interested in putting a bone dry spark plug in there and seeing what it looks like after the bike runs on the other cylinders for a brief time. If it is getting fuel it would no longer be bone dry.

Something I noticed yesterday while running again- carb 4 is also blowing some air back out the carb. Not as much as carb 2. Maybe half as strong. But cylinder 4 is running! I'm very confused.

I wouldn't get hung up on that.

The bike only has 5000 miles on it. So to me worn valve seat/guides seems unlikely. But with that low miles, maybe the bike sat for 30 years with the intake valve(s) compressed on 2/4. And now the spring is weak and the valve is bouncing.

I don't think that is a thing.
Spring steel doesn't take a set like that.

Again my money is on spark for the non-firing cylinder. Some how, some way it is not sparking at the right time or an intermittent connection from the spark lead to the connector on the plug or not fully inserted in the coil - something isn't firing. If it was sparking perfectly and you got zero change with starter fluid, that just wouldn't make sense. You only have 5,000 miles, but even if the rings were stuck, or compression off, it would "come to life" a bit with starter fluid.

I got a cheap infrared thermometer that I point at things to get accurate temp reading. I would be curious how it would show the "cold" cylinder. Eventually it is going to show some heat conducting from the other normal cylinders but would remain lower than them.