Spark plug wire problem?

[Fisher Gord]

Hey... another new guy here. Not too mechanically inclined but can handle the simpler things. I have a 1990 Yamaha 90 hp (Model 90 TRP). At full throttle, the engine cuts back momentarily and then resumes and does this repeatedly. I cut back on the throttle to just below full and the engine runs smooth. I know I have a corroded spark plug wire and figured that might be a good (and inexpensive) starting point. Could it be the coil is overloading and arcing or something? My dealer says I can't just replace the wire, I have to replace the whole coil as it comes as a unit. Is this true? Dealer also suggested that I could replace the spark plug socket on the wire. Would any special crimping tools be required to do this? Is the original socket just crimped on or is it epoxied? I don't imagine a person would want to cut the old end off. Any experience with this out there? Thanks.

 

[robert graham]

Re: Spark plug wire problem?

If you have worn/bad plug wires then you probably should bite the bullet and replace them all with new factory wires, especially since they're 20 years old now. I really think your running problems sound more like dirty carbs or fuel delivery problems, but you need the new wires anyway. Good Luck!

 

[99yam40]

Re: Spark plug wire problem?

My thoughts are that testing things to make sure they meet specs is the best thing to do instead of just replacing parts and hoping. Don't replace things that do not need to be, it is cheaper.
Always make sure they have good compression, spark, timing, and then go to fuel.
The Yamaha service manual give specs for primary and secondary wiring on coils and voltage reading along with ohm reading for the ignition system. If you don't have one you need one for your motor.

You can test the fuel system with a pressure/ vacuum gauge to see if you have a supply problem or sucking air into system with a clear piece of tubing.
Look at the frequent asked questions for procedure.

My 40 hp had a WOT miss that I finally found out was do to some one
( previous owner) drilling out the main jets, I guess they were trying to make it a 50 but did not have any idea the it takes more than just dumping more fuel.

 

[Fisher Gord]

Re: Spark plug wire problem?

Thanks for the advice. I'm leaning toward electrical because I'm aware of the one bad wire. Also, if it was fuel related wouldn't the loss of power be more than cutting back for a second and then resuming? Wouldn't it just get worse to the point of stalling? Also, I only have to cut back on the throttle a little bit to resolve the problem. I was going to replace all the wires until I learned I could only do that if I replaced the coil too... at $118 each. Would you know how the plug end is attached to the wire and how to replace it?

 

[robert graham]

Re: Spark plug wire problem?

I think it sort of screws on there, but I've never replaced one. Yamaha electricals are usually very durable and last a long time. I can't imagine it would be too hard to attach a spark plug cap to a wire and seal it up with maybe some silicone to keep out moisture.