Best way to test CDI and coils on 1981 750 LTD 4 cylinder

I have a 1981 750 LTD that I bought new in 81 and love it! Recently it started to have missing problems. I found a used CDI online and put it on. All the problems went away but my son left it out in the rain one night and the problem came back How can I check the system to figure out where the problem is? I know which cylinder is having the problem but not sure where to go. The coils were replaced about 3 years ago with Dyna coils. Thanks for your support!

First off, your bike doesn't have a CDI.  It has a transistorized ignitor box which functions on a different principle compared to a CDI.  Anyway, if rain caused a problem I'd remove the electrical connector on the ignitor and spray some contact cleaner on them to make sure everything is clean.  Do same on the coil and lead for the cylinder that isn't firing.  That's where I'd start anyway.

Could also be water pooling around a spark plug. A quick shot of air and some contact cleaner (not WD40) should do the trick.

wrote:

Could also be water pooling around a spark plug. A quick shot of air and some contact cleaner (not WD40) should do the trick.

Why not WD40?. I thought you only needed two tools. Duck tape and WD40. 

It's weird that it's affecting just 1 cylinder.  A bad igniter circuit would affect cylinders in pairs as would a bad coil.
My guess is a bad spark plug or mixture issue that affects that one cylinder.
Swap the spark plugs and see if the problem stays or moves cylinders.
Swap spark plug wires on the coil with the misfiring cylinder to eliminate the wires. One of the wires could be arcing to ground.
Fire up the bike at night. Turn out the lights and take a water bottle and mist water around the coils and spark plug wires. If you have an arcing problem, the spark will be evident.
 

wrote:

wrote:

Could also be water pooling around a spark plug. A quick shot of air and some contact cleaner (not WD40) should do the trick.

Why not WD40?. I thought you only needed two tools. Duck tape and WD40. 

 

Your toolbox is missing the essential tool, known as a Harley tool or BFH.