Symptoms of a weak or failing coilthat

could a weak or failing coil cause idle problems? I have a Yamaha Grizzly 80 4 wheeler that has what I call a hanging idle. The carb is spotless, valves adjusted, and float level is set. New spark plug installed. It starts easily— usually—but the idle may be low or a little high. I can change the idle with the idle screw, but it doesn’t want to stay set. I have checked for an air leak but found none. New o-rings were installed on the idle screw that stops the slide (this is not a cv carb, the cable moves the slide) and the pilot or air screw. Sometimes the spark on the plug is nice and blue and sometimes it appears weaker and more white. Could that be an indication of a weak or failing coil? If so, then maybe the intermittent weak spark could be causing the uneven idle?? I know very little about electricity and ignition so any comments will be appreciated.

 

Sounds like a lean condition. Try tweaking the mixture screw 1/4 turn richer.

 

It could, but the plug would look normal in terms of mixture coloring.

 

check for air leaks where the cable goes into the top cap and where the cap screws on the body. look for a divot in the slide where the idle speed screw contacts it

 

It is late and cold so I’ll recheck for air leaks tomorrow. However, I have tried to ohm the the coil and got some crazy readings, so I’m not real sure I did it correctly. On this coil the base terminal is grounded by a bolt to the frame, so I removed the bolt and put a cable tie between the coil and frame. I disconnected the primary terminal and pulled the cap off of the spark plug. I think that completely isolated the coil from the frame. I set my meter to 200 and per instructions in service manual attached the +lead to the primary terminal an the - lead to the coil base. I was supposed to get 1.4. —- 1.8 ohms . I got anywhere from .1 to .7. When I tested the secondary I was supposed to get 5.3 —7.9. I got about 17, but I just realized that was with the plug cap still on the wire, and according to the service manual the cap itself should read 10, so maybe 17 was not out of spec for the secondary. Still not confident in my testing procedure. all that said, the engine starts easily and runs fine until it warms up Andy the the idle issues start. Wonder if it is a good idea to replace the coil anyway.

 

Just looked up a coil and it appears to be double grounded to the frame, so maybe that caused the bad primary reading. To get the coil off I have to remove ab bunch of stuff, so maybe I’ll try that tomorrow.

 

Primary reading of zero— should be 1.4 — 1.8.
Secondary reading of 8.5– should be 5.3 — 7.9.
How can it even run with a zero reading on the primary? I thought that meant an internal short.
Think I need a new coil?

 

I would think so. You have been diligently testing and double checking to make sure you are testing properly so with those readings I would agree a new coil would be in order

 

They are pretty cheap so I have one on the way. Hope it is good because it’s probably a China product.