Loosing Fire On Rear Cylinder

[fxstcb]

I am loosing fire on the rear cylinder. I have a compufire ignition system with an S&S single fire coil. I am getting at best weak spark on the rear spark plug.
Any ideas?

 

[HDDon]

I know this is for a Crane Hi4 but there is a trouble shooting section that might help:Harley Davidson Forums

 

[Hoople]

Any ideas?

First ohm out spark plug wires to see if rear wire is bad. If ok, swap the wires going to the primary of coil. Then swap secondary wires at coil tower. Start bike. If coil is bad, problem of weak spark will move to the front cylinder. If problem stays in rear cylinder, it's most likely a coil driving issue.

Would be helpful if you supplied model number or link of S&S coil and compufire ignition.

 

[fxstcb]

First ohm out spark plug wires to see if rear wire is bad. If ok, swap the wires going to the primary of coil. Then swap secondary wires at coil tower. Start bike. If coil is bad, problem of weak spark will move to the front cylinder. If problem stays in rear cylinder, it's most likely a coil driving issue.

Would be helpful if you supplied model number or link of S&S coil and compufire ignition.

The compufire model is compufire 21835
S&S coil is S&S High-Output Single-Fire Coil - 55-1571
I have already swapped the plug wires and so far it follows the output on the coil. I will try the idea of swapping the input wiring to the coil to see if it is a ignition issue.

 

[Hoople]

The compufire model is compufire 21835
S&S coil is S&S High-Output Single-Fire Coil - 55-1571

Thanks, good info. The rear coil trigger should be a white wire. Front coil trigger should be a black wire. Swap those wires and swap high tension wires (only at the coil side of wires).

The primary coil resistance is 3 ohms which matches the comufire, so your good there.

It could also be a spark plug issue but I am assuming you already checked or replaced them. If not, a quick ohm test with a meter will tell you if resistor pill in the plug is burned out.