2001 yamaha 60hp 3cyl killing spark plugs

[shanemacleod]

Hi Guys.
Just have a question to run by you. I have 2 x 60hp yammies on the back of my boat and every now and again the one motor seems to kill the spark plugs. I never really thought much of it as the plugs usually last a good few trips. I usually just bang 3 new plugs in and it's sorted. I use the recommended plug NGK BR8HS-10.
It was only last weekend when I was testing the boat after replacing the pipes on the autolube system that I narrowed it down to a cylinder. It seems to be the bottom cylinder. I wanted to run a full tank of fuel per motor to see that the autolube system is working. So I pre mixed fuel 50:1 just to be safe. I had 25L of fuel per motor. The one motor ran fine the whole day and did not foul up any plugs. The motor in question did however go through 2 plugs on the bottom cylinder. Now the first.one I would have understood that maybe it had fouled up so I put a new one and continued riding. About 30 min later the motor started missing every now and again and got progressively worse. I then stopped and went straight to the bottom cylinder. Put a new plug and all was well again. I inspected the plug and it was not fouled. It still looked new as it had only been in there about 30 min. This only seemed to happen when I rode the boat at full taps obviously putting more strain on it. Has anyone else ever had a similar problem or know what I could look at.
Thanks Shane

[robert graham]

Check fuel pump diaphragm for holes/leaks....a bad fuel pump can cause raw gas to get pulled into engine causing a rich mixture. Remove fuel pump with 2 10MM hex screws while leaving pump hoses hooked up, then squeeze the primer bulb and look for any fuel coming out of small hole/port on back of pump...there should be none. A new pump is about $50 and very easy to change. The pump diaphragm can only last for so long before it fails...I generally replace mine every 5 years or so, before it fails, and keep the old one in my boat tool box for a spare....

[boscoe99]

What does "killing" a plug mean? This is not a term that I am familiar with.

[shanemacleod]

Hi Boscoe

Well I can only assume that the spark plug is not sparking anymore. Hence the phrase killing the plug as when I put a new spark plug in the motor runs great again. Hope this helps

[boscoe99]

Hi Boscoe

Well I can only assume that the spark plug is not sparking anymore. Hence the phrase killing the plug as when I put a new spark plug in the motor runs great again. Hope this helps

Got it. A new plug being installed instead of cleaning the old one made me wonder if "killing" the plug meant something else other than carbon fouling.

Certainly check the fuel pump as robert notes. Have you checked the strength of the spark to the plug that is being killed, I mean fouled?

Is the thermostat in good working order?

Are the carburetors in good condition? When were they last cleaned? Does the plug foul in all operating conditions or is it more so when idling or running at power?

If the problem is hit and miss (comes and goes) I might focus on the plug cap, spark plug lead, coil, and input to the coil. A loose connection can cause all sorts of strange stuff. Double check the ground connection to the coil to the plug that is dying, I mean fouling.

Having two identical motors means you have a good supply of spare parts to throw at the problem. Maybe move stuff from the good motor to the bad motor and see if the situation improves.

[99yam40]

Only way I know of to kill a plug would be to contaminate it with something conductive to where it would shunt the voltage to ground and not spark across the electrode.
If it is not carbon then metal particals come into my mind as the only thing right off the top on my head.
I am sure there are substances that could be in fuel to cause it, but it should be in more that one cylinder if it was in the fuel.

[shanemacleod]

Got it. A new plug being installed instead of cleaning the old one made me wonder if "killing" the plug meant something else other than carbon fouling.

Certainly check the fuel pump as robert notes. Have you checked the strength of the spark to the plug that is being killed, I mean fouled?

Is the thermostat in good working order?



Are the carburetors in good condition? When were they last cleaned? Does the plug foul in all operating conditions or is it more so when idling or running at power?

If the problem is hit and miss (comes and goes) I might focus on the plug cap, spark plug lead, coil, and input to the coil. A loose connection can cause all sorts of strange stuff. Double check the ground connection to the coil to the plug that is dying, I mean fouling.

Having two identical motors means you have a good supply of spare parts to throw at the problem. Maybe move stuff from the good motor to the bad motor and see if the situation improves.

Hi Boscoe. Thanks for the advice. Like I said in my first post. The reason that I phrased it killing the plug is because on the dam two.weeks ago it gave trouble. So the first plug I took out of the bottom cylinder didn't look fouled up. But you never know so I just put a brand new plug in. I rode for another 30 min or so and it happened again. I immediately went straight to the bottom plug. When I took it out it still looked new. Not much burn on it or even oil so it couldn't have been fouled. I replaced the plug with another brand new one and the motor ran beautifully again. Hence it led me to believe the plug had been killed or was dead instead of being fouled. It seems to start happening after keeping the boat on power for a while. When going on power with both motors every now and again you can feel the one motor miss and then come right. It then gets progressively worse until the motor is missing all the time. Like I said put a new plug in the bottom cylinder and all is perfect again. I will strip the carbs and clean them as the last time this was done was about 2 years ago. I will also check the fuel pump and maybe just replace it to make sure. I have checked the wires from the plug. Unscrewed the spark plug cap and put it back. Checked all.connections and earth cables. All seemed to look ok. And nothing was loose or jumped out aT me. Spark on all cylinders with new plugs is strong as well as the compression on all cylinders are around 100psi. I am using 93 octane unleaded fuel as this is all that is available here in South Africa. We have 93 octane or 95 octane unleaded fuel available

[boscoe99]

If the plug is not carbon fouled, but is failing to fire when in use, but is known to be a good plug, it sounds like an intermittent electrical problem to me, but that is just a guess.

[99yam40]

Back when i was an auto tech back in the 70s ,I ran across new plugs from time to time that went bad internally.

Might try the plugs you pulled out of #3 in different cylinders to see if they fire in there OK. That would eliminate some things to be looking for

[ausnoelm]

So, you replace the plug and it's OK for a while, do you think it's not the plug, but "something" heating up, failing, and while you are messing around replacing the plug, whatever is failing has cooled down enough to be good again? Your best option is to have test equipment on the boat, go for a run, then when it fails, do testing as fast as you can...or, just start replacing expensive parts one by one untill at some point, you fix it.

[boscoe99]

In his case he has a set of perfectly good parts from the other motor that he does not have to buy. Just move some parts, from one motor to the other and vice versa, at a time and see if the problem moves to the other motor.

Or, like you suggest, do the trouble shooting when the problem exists.

[shanemacleod]

I don't think it's a problem of something getting too hot as I am quick with changing the plug. Maybe a minute max. But I think I will try swapping parts around to see if I can get the problem to move to a different cylinder or motor. Thanks for the help guys. If you have any other things you can think of please let me know. Will keep you updated on how it goes.