Gots a prob with my '72. The rear jug won't fire. I've switched the coil wires, taken a comp test (120psi), changed plugs and wires. The coil fires the front cyl just fine, so I don't think its that. Any and all help will be greatly appreciated!!!

 

Switch the spark plugs? May be a bad plug.

 

Mick,
After cruising the past posts/threads, I read one of your replys and you said that after the coil warms up a bit it could act up. Well, the bike starts really good and seems to run and idle well but after a few minutes it feels like its running only on one cyl, and when I pull over to listen to the exhaust the rear cly only fires occasionally. Coil maybe?

 

I would replace the coil.

There are resistance tests for the coil in the manual, but the only way to know for sure is to replace it with a known good unit. If the problem persists then you have a spare good coil, which is a very useful part to have,

I also have a spare generator and regulator. These are three parts that seem to be prone to failure, and for each the easiest test is to replace.

Are you running points ignition, or has it been converted to electronic? I have very little experience with points, but if you have new aftermarket parts, unless they are a good well known brand they may very well be junk.

The symptoms of a bad ignition module are much the same as a bad ignition coil. If you are running an electronic ignition then, as with the ignition coil, the only reliable test is to replace with a known good unit. We usually try the coil first as it is less expensive.

My choice for EI is the Ultima ...
http://www.ultimaproducts.com/programmableignition.html

 

if 1 cylinder is firing ok then it cant be the coil if you switched the wires and the problem didn't follow the rear wire,any chance of a very bad intake leak??maybe the points set wrong?

 

When you say you "switched the coil wires", did you switch the wires only at the spark plug ends, so the coil output "jack" that normally fires the front spark plug is now trying to fire the rear plug? Are your plug wires long enough to do that? If so, and the firing problem didn't move to the front plug, I agree that it doesn't seem like it would be the coil. The only other thing I can think of would be to quickly check the spark at both plugs when it's failing and try to see if the spark at each of the plugs is the same. If they seem the same, I'd check for leaks after it warms up like shepdog suggests.

 

is this points or electronic? i had an electronic ign. that would just start firing on one every once and a while. replaced it and been fine since.

 

Ooops, maybe not ...

+1

I missed this. Sorry.

 

When the rear cylinder on my 75 doesn't light up, the first thing I do is adjust my pushrods.

If you tried swapping out plugs, plug wires, coil wires, etc. and that rear is still not firing all the time try adjusting the pushrods.

 

Same problem for me a while back ... Bad plug it had spark just not as much as it should have