Ignition problem

The other evening my commando cut out on the right cylinder
Nothing obvious I could see so I rode it back on one
At very small throttle opening it was trying to fire on two
I had a quick look tonight and everything is fine , both plugs sparkling when the engine is turned over,both throttle cables still on etc etc
But it would only start and run on the left hand cylinder
So I swapped the HT leads over IE stretched the left lead to the right cylinder and right lead to the left and started it up
It now runs just on the right cylinder
At that point I ran out of time,but at least I know it's either the HT lead or the coil
What I don't understand is why the plugs both spark fine when laying on the head?
It can't be the plug at fault as it runs when the leads are swapped
I'm suspecting the right hand coil could be dodgy
Both the coils are about 2 years old they are 6volt PVL copies
I thought I was buying genuine PVLs but when I re read the evilbay listing it states PVL type!!
My ignition is Lucas Rita ,I'd like to buy some original nos Lucas or this time make sure I get genuine PVLs
Or does anyone have any other recommendations for a pair of coils bearing in mind I need 6v and it's for a Lucas Rita?
Cheers

It takes more spark energy to jump the plug gap under compression. A plug can fire at atmosphere but not under pressure.
I have the formula for that somewhere...

maylar said:

It takes more spark energy to jump the plug gap under compression. A plug can fire at atmosphere but not under pressure.
I have the formula for that somewhere...

Click to expand...

Yep I get that but the plug fires and the bike runs on that cylinder
What I'm saying is its not that particular plug at fault
I suspect the coil, hopefully tomorrow I'll get a chance to find out and post results
Cheers

I agree, either the HT lead or coil is suspect. In either case voltage is low on one side and it doesn't show up at atmospheric pressure. Wires are cheap, obviously go there first. Got resistor caps? They can fail. If you do need a coil and want to stay with Lucas style I can recommend the TriSpark 6v coils. Been very happy with mine for 10 years now and they look stock.

Yep I have NGK resister caps , I was thinking of maybe trispark coils
Cheers

Now swap the HT leads from one to coil to the other to determine if it's the lead or the coil

baz said:

Yep I have NGK resister caps , I was thinking of maybe trispark coils
Cheers

Click to expand...

I’ve used Tri Spark coils lots of times and will continue to do so in various applications.

However, on the Commando I’ve been using one of the cNw Crane dual coils which also works well. Matt provides a coil and fixing bracket so no fabrication is required. He also supplies leads, caps and plugs in the kit.

acadian said:

Now swap the HT leads from one to coil to the other to determine if it's the lead or the coil

Click to expand...

Yep that's my first port of call
If it's not the leads/caps I will dig out my antique 6v Lucas coils
But I will buy myself a new pair if it turns out to be the coils
Cheers

Fast Eddie said:

I’ve used Tri Spark coils lots of times and will continue to do so in various applications.

However, on the Commando I’ve been using one of the cNw Crane dual coils which also works well. Matt provides a coil and fixing bracket so no fabrication is required. He also supplies leads, caps and plugs in the kit.

Click to expand...

Trispark coils may be the answer
I've no Idea what the "genuine" new Lucas stuff is like?

he stated he has a rita ign. they don't play well with a single fire dual tower coil such as mat sells. the rita has a very long time on (some will call it long dwell) and as such it will cook these HD style coils.

Fast Eddie said:

However, on the Commando I’ve been using one of the cNw Crane dual coils which also works well. Matt provides a coil and fixing bracket so no fabrication is required. He also supplies leads, caps and plugs in the kit.

Click to expand...

Light throttle running cylinder pressures are relatively low...
Open the throttle, and more efficiently fill that cylinder with air fuel mixture, the pressure rises and a marginal ignition system can stop firing (as mentioned above).

Swap over leads or coils and see if it makes any difference. Swap and substitution always work(s)ed for me.

bill said:

he stated he has a rita ign. they don't play well with a single fire dual tower coil such as mat sells. the rita has a very long time on (some will call it long dwell) and as such it will cook these HD style coils.

Click to expand...

That depends on which HD coil you get. If you use the old twin tower coil that was designed for points ignition, then they will work fine with the Lucas Rita.

The Harley electronic ignition coil will not work well with the Rita or a Boyer analogue.

I really don't know which one Matt supplies.

bill said:

he stated he has a rita ign. they don't play well with a single fire dual tower coil such as mat sells. the rita has a very long time on (some will call it long dwell) and as such it will cook these HD style coils.

Click to expand...

Two 6 V coils in series are subject to the same dwell as one dual coil.

This has already escalated above my pay grade!

But I do know that Matt says his coils kit is a “Perfect match for the Tri Spark ignition but works with all types of ignitions”.

Baz, I’ve got a spare (new) cNw kit here if you want to go that way and are short on time.

I do know the 3 ohm twin tower coil will fail with a rita and i have tried several brands. the 2 6volt coils work OK because of the extra size IE surface area they run cooler. the last I knew matt was selling the power arc 3 ohm and the tri spark is a short dwell, its a shame when the coil is more reliable than the shake and bake ign.

Found a bit of garage time this morning
I tried swapping just the HT leads and the problem remained IE only running on the left cylinder
So I have re fitted my 47 year old battered 6v Lucas coils and all is well again
So lesson learned read the listing properly on evilbay
That's a kind offer Eddie what exactly do you have?
It's just the age of the Lucas coils that bothers me so I will be replacing them at some point
See photo of coils that have lasted about 2 years
Cheers

Run an earth wire directly from the head back to the +ve terminal on the battery, my OEM harness developed an issue with the std earth path and the Boyer equipped bike dropped to only running one spark, adding the wire restored full sparks. Took a week to find the issue, changed all the ignition parts including boyer box and rotor/stator and then had a random thought and added the wire and it worked. The harness is now gone, replaced by a new one but I suspect the earth wire was down to a few stands and could only support one spark not two (wasted spark). During my tests the left would only fire if the right side plug was not connected, as soon as I reconnected the right side the left stopped sparking.

The thing that threw me with this problem was both plugs spark fine when tested laying on the cylinder head
And this is with the starter motor spinning the motor over at a fair rate of knots
But would not fire on the rh cylinder!
The coil must be just weak enough for the plug not to spark whilst under compression?
Cheers

kommando said:

Run an earth wire directly from the head back to the +ve terminal on the battery, my OEM harness developed an issue with the std earth path and the Boyer equipped bike dropped to only running one spark, adding the wire restored full sparks. Took a week to find the issue, changed all the ignition parts including boyer box and rotor/stator and then had a random thought and added the wire and it worked. The harness is now gone, replaced by a new one but I suspect the earth wire was down to a few stands and could only support one spark not two (wasted spark). During my tests the left would only fire if the right side plug was not connected, as soon as I reconnected the right side the left stopped sparking.

Click to expand...

That's interesting
Cheers