Check for weak/faulty coil

[Brendan65]

Trying to diagnose my poorly running engine.
I want to check if my non-running #3 cylinder is coil related, so I swapped the coils to see if the problem moved to another cylinder.
I switched the input wires to the coils, then I switched the sparkplug leads on each side. (1 and 3 ... then 2 and 4)
Now the problem is #4. (I can remove the plug cap when idling with no effect) And #3 is working.

But it seems to me that it should be #1 cyl with the problem - that is what I expected with the swap !
Am I missing something obvious ?

Yes I know it could be carb issues but I redid them already and I'm checking other possibilities before pulling the carbs again.

My symptoms :

Bike starts instantly when cold .. seems to run ok on the choke at the higher idle.
Once warm the bike has really low power, not running on all cylinders.
Chug away from a stop, poor throttle response until 3K rpm where the engine jumps to life (like a switch), though still rough/low power.
Power drops away after 4K, won't rev much past 5K.
Again power down low is so bad it's hard to ride.
Once warm it takes lots of cranking to get the engine started.

[LAB3]

Here's what I'd be asking myself: Did the bike have a spark issue BEFORE you pulled the carbs? Coils don't go bad very often, in fact it's a pretty rare occurrence. I'd be more inclined to look at the primary side to see if there's an issue there before deciding a coil is at fault, make sure all your connectors are clean and they're getting a good signal to fire. Another all to often overlooked issue is the spark plug cap resistance, it should be in the ballpark of 5k ohms. What you're describing sounds to me like it's carb related as in getting too much fuel, hard starting while warm leads me to believe it's running too rich.

[dingdong]

If the carb on #3 had a problem it would remain at #3. No matter what you did with the coil wires.

[DenverWinger]

Are you still using OEM ignition (points)? If so, how old are the ignition capacitors?

Back in the day when cars had contact point ignition, when you bought a new set of points it usually came with a new capacitor, too. Failing capacitor can cause weak spark.

[Brendan65]

Here's what I'd be asking myself: Did the bike have a spark issue BEFORE you pulled the carbs? Coils don't go bad very often, in fact it's a pretty rare occurrence. I'd be more inclined to look at the primary side to see if there's an issue there before deciding a coil is at fault, make sure all your connectors are clean and they're getting a good signal to fire. Another all to often overlooked issue is the spark plug cap resistance, it should be in the ballpark of 5k ohms. What you're describing sounds to me like it's carb related as in getting too much fuel, hard starting while warm leads me to believe it's running too rich.

When I bought the bike it had been sitting for a couple of years .. exhibiting the same problems as now. except it was the #1 cyl that was the problem. So I pulled the carbs - they reeked of old gas but were otherwise clean - no varnish or gum. After servicing the carbs, all cylinders seemed to work but the poor running remained. So I've been chasing other possibilities for awhile - but I bet you're right about the carbs. I feel that if I take them off again I won't be able to do anything different. (Maybe double check my float height ?)
Plus I'm getting visable spark at all the plugs and the through resistance plug to plug is 21.5K so ignition might not be the issue, it's just weird to have the non firing cylinders move around.
Started with #1 (then carb rebuild).. then to #3 and #4 ... switched coils and now it's #4 only.

So far I've :
-rebuilt the carbs
-set the points
-set the timing
-balanced the carbs
-checked the valve clearance
-checked the timing belt/cam alignment
-rebuilt the air cutoff valve
-changed the plugs
-checked the condenser
-checked the coils
-checked compression

I'll get it eventually .. I'm glad that this isn't my main bike - so working on it is half the fun !
(or so I keep telling myself ..)

[Brendan65]

If the carb on #3 had a problem it would remain at #3. No matter what you did with the coil wires.

This is true and it's what's driving me nuts !!

[Brendan65]

Are you still using OEM ignition (points)? If so, how old are the ignition capacitors?
Back in the day when cars had contact point ignition, when you bought a new set of points it usually came with a new capacitor, too. Failing capacitor can cause weak spark.

I checked them (condensers) the resistance was correct. They look newer. The bike had electronic ignition at one time, but was retrofitted with points not that long ago. The points look fairly new as well.

[LAB3]

I'll get it eventually .. I'm glad that this isn't my main bike - so working on it is half the fun !
(or so I keep telling myself ..)

That's how I see it as well. I've taken this year off from making too many road trips but for the last six years I've lived at least six months a year on my bike, everything I own straps to the bike or fits into my recently purchased trailer. You can't cover 20k miles a year without getting your hands dirty and answering questions about "How can you do that on an old beater?" unless you love turning a wrench.

[Brendan65]

OK .. installing new coils - probably won't solve my problem, but it's something to do before pulling the carbs again !

I ordered 5 ohm coils, thinking that was OEM. (must have been a late night at the computer !! )

Turns out stock coils are 3 ohm ... yet I read somewhere that electronic ignition required 3 ohm and the 5 ohms were for points (which I have).
And there are quite a few 5 ohm coils being sold as gl1000 replacements.

So I'm wondering if I should keep my ballast resistor ? I checked and it's 2.5 ohms.

The coils that I'm replacing I think are from a GL1100, They measured 3 and 3.2 ohms each.

[Brendan65]

Just an update .. I had fuel leaking from the #3 carb, fixed and runs great.
So it wasn't the coils .. but now they're new.