Check the Coils

 

Cylinders 1 and 4 are on the same coil, and 2 and 3 are on the other. The idea is that the cylinders are paired for balance and fire at the same time. If it is sputtering and jumping, I would start with the cheap end of the ignition system: the spark plugs. A good look at these will tell you which cylinders are firing, or not, and what the conditions are inside the cylinder. Also look for any cracks in the porcelain, loose terminal screws, or even a loose spark plug cap.

 

However, to check the coils you'll need a multi-meter that will handle the voltage and ranges of resistance below.

1. Make sure there is 12 volts at each of the plugs in the wiring harness. If the connectors aren't color coded, take note of which came off which coil as this is how the ignition unit determines which coil to fire when. You'll know instantly if it's wrong because the engine will backfire both out the back and up through the carburetors.

 

2. Check the primary resistance at the harness connector to the coils. The Yamaha manual for the '90 also has the appendix for the '89 model, so I'm assuming it is the same coil. IF this is the case, the resistance should be between 1.8 - 2.2 ohms at 68 degrees F. BE CAREFUL to make sure that the meter you are using is fused to protect against touching the wrong ends to the leads.

 

* Leads are the follow colors in my Yamaha manual:

 

* Right ignition coil = red/white (+) and gray (-)

 

* Left ignition coil = red/white (+) and orange (-)


3. Check the secondary resistance with the same method. Using one spark plug lead for positive, and the other for negative, the range should be 9.6 - 14.4 k ohms at 68 degrees.