1973 CB350-four, no spark on 1 and 4 when it's cold

Question

When this bike is cold the 1st and 4th cylinders do not fire. Pulling the plugs and checking them, there is not spark. If you get the bike hot, it runs really bad on the way there, 1 and 4 come back.

Also, if it's in the summertime and the bike sits in the sun, sometimes 1 and 4 will fire and the bike will run and start normally.

What's going on here?

Answer 1

Two things about this bike if it is OEM

• It has two sets of coils.

• It has two sets of points.

If you take the right side cover off the motor where the crank is, you can see if the points have been swapped out with signal generators, a common mod for this bike.

One coil fires for cylinder 1 and 4, the other fires for cylinder 2 and 3.

IMO you have a bad coil that has an open when it's cold. Usually, you have the open when it is hot and the bike shows the same symptoms you describe but in opposite temperature conditions.

You can test the theory of a bad coil by putting one of them in the sun or in a regular oven, preheat it to 125F. Use a thermometer to measure the heat in the oven before doing this, you don't want to melt it. Make sure you preheat the oven so the glowing coils don't melt it as it heats the oven to the appropriate temp. You can go as high as 150F.

Once the coil is hot, test it on the bike. If it runs, check of that box.

Then take the coil and put in your freezer. Repeat the test. If you don't have spark, the coil is the source of your issue.

If you have signal generators and the above test fails, you can run the same test with them. They get very hot mounted directly to the engine so you don't have to worry. Just validate your oven temps and let it preheat.

If the coil test fails and you have points, just swap your points. They're very cheap and it never hurts.

If all of that fails then come back here with all of your data and ask a new question. Good luck!

IMO, it's the coil.