Would a bad ignition coil misfire on one side only, since it is dual?

[KDeline]

Ever since I have owned a 1998 R1100RTP (never in police service, now has 53000 miles), I have a hard time starting when below 55 degrees. I assume the plug is not firing on one side as that pipe stays cool to the touch before it finally catches and it is always the same side. I have changed out plugs, no luck. Can take as long as 30 seconds before it starts to fire on the both cyclinders. I would like to know if this is a possiblity before I buy a new coil and try it.

[redryder]

Much smarter heads than mine will surely post shortly. However, it has always been my experience that a coil will perform better when cold. Usually, they don't start acting up until they heat up. My 2 lunger JD tractor has the same problem and it is cured with a judicious carb tinkering. I usually have to set it one way in the winter months, and then lean it out for summer pleasure.

 

You might want to make sure the air passages behing the BBS are clear and clean. Also, I know the JD doesn't like Champion plugs worth a dookie in cold weather either. So perhaps a plug change (Autolites work very well in the RT and the Deere)or a good TB cleaning?

 

Just my .02, corrected for inflation of course.

[Ken H.]

Yes it's possible. While the coil secondary is common, the last few bits to where it outputs to the two separate plug wires could be an issue.

 

But I'd be thinking about the plug wires themselves in your scenario.

 

But also, keep in mind just because the cylinder isn't firing (the pipe is not getting hot) doesn't necessarily mean the spark plug is not firing. If there is nothing there to ignite / burn the cylinder, and by extension the pipe will never heat up.

 

Which is a long-winded way of saying this could be a fuel supply issue.

[T__]

KDeline, while it’s slightly possible for the coil to fire only one side it is highly unlikely as the plug & wire on one cylinder is the ground path for the other side’s spark plug.. Basically if one side is open the other side will have a very poor spark due to no secondary ground path.. It’s possible to get a very weak spark if one side is open but that requires the coil windings to arc internally & that will toast a coil in short order.. If the cold cylinder side is shorted to the engine (ground) due to a leaking plug wire, wet plug wire, or other reason that the spark would rather jump to ground instead of the plug electrode gap then in that case you would have a good spark on the firing side only..

 

You really need to check for a proper (running) spark on the cold cylinder side.. To do that, find an old (or new) metal core spark plug wire with a usable spark plug end on it.. Cut the wire someplace in the middle then cut the insulation back about 1/4” or so (you need exposed internal metal wire).. Then push those cut back ends into a piece of clear plastic tubing or otherwise secure it so the exposed metal ends are about 3/16” apart.. Then install that modified plug wire between the spark plug & the original spark plug wire..

Then when you start the engine you will be able to visibly see the spark jump that 3/16” gap in the wire you made.. If you have a good snappy looking bluish colored spark your coil is good & your cold cylinder problems lie elsewhere..

 

Twisty

[Boffin]

Er, no. The earth for the plugs is the head. BMW use the wasted spark system and both plugs fire at the same time. The earlier bikes, like the OPs, use a single coil with two leads. Twin spark bikes use that setup for the secondary plugs and a coil-in-cap setup for the primaries.

 

An open circuit plug lead on one side will not affect the other side. A shorted lead however could kill both sparks.

 

Andy

[T__]

Er, no. The earth for the plugs is the head. BMW use the wasted spark system and both plugs fire at the same time. The earlier bikes, like the OPs, use a single coil with two leads. Twin spark bikes use that setup for the secondary plugs and a coil-in-cap setup for the primaries.

 

An open circuit plug lead on one side will not affect the other side. A shorted lead however could kill both sparks.

 

Andy

 

 

Andy, both plugs do fire at the same time,, one positive ground & the other negative ground.. (lost spark) .. The cylinder heads are part of the ground path but not the actual ground.. The spark path is out of one of the coil's secondary winding pig-tails,, then through one cylinder side spark plug,, then across the engine through the cyl heads & block,, then back through the other cylinder side spark plug (it fires also but reverse polarity) then back up that plug wire & into the coil’s other secondary coil pig-tail..

You can’t fire both plugs from one coil winding to ground as the spark will just take the path of least resistance & use only one plug path..

 

Twisty

[KDeline]

Yes it's possible. While the coil secondary is common, the last few bits to where it outputs to the two separate plug wires could be an issue.

 

But I'd be thinking about the plug wires themselves in your scenario.

 

But also, keep in mind just because the cylinder isn't firing (the pipe is not getting hot) doesn't necessarily mean the spark plug is not firing. If there is nothing there to ignite / burn the cylinder, and by extension the pipe will never heat up.

 

Which is a long-winded way of saying this could be a fuel supply issue.

 

The only reason I did not think it a fuel issue is because it only happens when cold. I really need to pull the plug and see if I get spark I guess.

[Ken H.]

Andy, both plugs do fire at the same time,, one positive ground & the other negative ground.. (lost spark) .. The cylinder heads are part of the ground path <snip> You can’t fire both plugs from one coil winding to ground as the spark will just take the path of least resistance & use only one plug path..

 

Twisty

Actually Andy, now that I think about it more, I believe he's right. I do (now!) recall reading somewhere that that is indeed how a dual-plug, single-coil, wasted spark system works.

 

Ken D. - As you say, check for actual spark on the cold side, but upon further reflection I have to think this is a fuel issue. Injector sticking closed when it gets cold?

[jpsmith]

It could also be, low fuel pressure, low compression, weak spark, dirty injectors, fuel filter, temp sensor, or even the cold start sequence in the ignition modue. And so the list goes, same for a bike as for your car. Good Luck

[Boffin]

Andy, both plugs do fire at the same time,, one positive ground & the other negative ground.. (lost spark) .. The cylinder heads are part of the ground path <snip> You can’t fire both plugs from one coil winding to ground as the spark will just take the path of least resistance & use only one plug path..

 

Twisty

Actually Andy, now that I think about it more, I believe he's right. I do (now!) recall reading somewhere that that is indeed how a dual-plug, single-coil, wasted spark system works.

 

Ken D. - As you say, check for actual spark on the cold side, but upon further reflection I have to think this is a fuel issue. Injector sticking closed when it gets cold?

 

Yup, a little thought does make it obvious. I must get the books out now and then to remind me of Ohms Law.

 

Andy

[Chip]

As both plugs fire at the same time, does it matter which spark plug wire goes in which plug of the ignition coil?

 

Just purchased a Bosch ignition coil (model 259; Oilhead #BOIgnCoil-R978)) for my '96 R1100RT. The original BMW coil had a plug on the left and right. No brainer as to which spark plug wire went where. The Bosch coil however, has the plugs in line with the bike...not left and right. Mounting brackets are fine.

[Mike T]

On my '95 R1100RS it has a Bosh Coil. The right side wire (when your on the bike) is pluged into the front connector of the coil. The left side is pluged into the back connector. Not sure if it matters or not.

[Ken H.]

does it matter which spark plug wire goes in which plug of the ignition coil?

It doesn't matter.

[RFW]

Andy, both plugs do fire at the same time,, one positive ground & the other negative ground.. (lost spark) .. The cylinder heads are part of the ground path but not the actual ground.. The spark path is out of one of the coil's secondary winding pig-tails,, then through one cylinder side spark plug,, then across the engine through the cyl heads & block,, then back through the other cylinder side spark plug (it fires also but reverse polarity) then back up that plug wire & into the coil’s other secondary coil pig-tail..

You can’t fire both plugs from one coil winding to ground as the spark will just take the path of least resistance & use only one plug path..

That is correct. In the wiring diagram, the secondary coil is shown as having one end of the coil going to one plug, and the other end of the coil going to the other plug. The secondary has no ground connection. Current path in the HV circuit goes in a loop; out one wire, to one plug, then jumps across the gap, then through the motor to the other plug, jumps the gap "backwards", then through that plug's wire back to the opposite end of the coil.

 

From this, if one plug was shorted, the other one will still work, since the current will just flow through the short instead of jumping the gap.

 

But if you pull off a plug lead, neither plug will work since the "current loop" will be broken. You will also risk damaging the coil.