Topic: Two cylinders not firing at low RPMs

[sparty]

Guys,

My friend has a 4 cylinder bike and two of the cylinders do not fire at low rpms.  As the rpms come up, all of the cylinders fire.  The carbs are getting fuel to the cylinders, but there seems to be a spark issue at low rpms.  What could be the cause?

Sparty

[mlinder]

Sooty plugs.
Crappy coil.
Other stuff.
Sooty plugs will do that for sure, though.

[Gordon]

Have you checked to see if it's a spark issue, or are you just guessing?  It could be a low speed jet issue in the carbs.  What kind of bike is this?

[sparty]

Sooty plugs.
Crappy coil.
Other stuff.
Sooty plugs will do that for sure, though.

Yeah, I told him I think it is a coil issue.  I pulled his plugs and they seem fine.  BTW it is a  1976 GS750.

Sparty

[sparty]

Have you checked to see if it's a spark issue, or are you just guessing?  It could be a low speed jet issue in the carbs.  What kind of bike is this?

It has got to be a spark issue.  I pulled a plug and put it on the head with the wire attached.  I cranked over the starter and I saw no spark.  But it must spark at higher rpms, because you can hear and feel the two dead cylinders come to life.

Sparty

[keiths]

Could it be a weak battery? Once the rpms come up the voltage comes up too.

[bryanj]

Didnt the 75 have points ignition? If so are they set right?