Iridium spark plugs

[Rlisty25]

Hey everyone,

At the start of the 2025 season, I installed iridium spark plugs in my 2006 Yamaha AR230. The boat ran great all summer—no issues at all—until about two weeks ago when the starboard motor started acting up.

While out for an evening ride, it began running rough, almost like a bad fuel pump. It dropped from four cylinders down to a couple and then quit completely. The next day I checked fuel pressure (40 psi) and compression (150 psi)—both looked good.

Thinking it might be the ECU or injectors, I replaced the injectors and swapped the coils, plugs, ECU, and cam sensor from the port motor to the starboard. No change. After reinstalling everything back on the port motor, it fired right up and ran perfectly.

I finally brought it to the dealer, and a few days later they told me the problem was fouled spark plugs. So apparently, my brand-new iridium plugs were the issue all along.

I’m honestly having a hard time believing that—has anyone else experienced iridium plugs fouling out like this on these Yamaha motors?

 

[FSH 210 Sport]

I’m not sure how long you’ve owned a Yamaha jet boat, but the first thing to do at the sign of rough running is to change the plugs, all of these four stroke engines seem to consume plugs. Use the OEM plugs, be sure and set the gap of the new plugs out of the box, there has been others that have had issues with fine wire / non OEM plugs.

If You ran the boat all season then the plugs weren’t brand new anymore… how many hours did you have on them? Was the other engine still running normally?

You said the dealer said the plugs were fouled, how did the plugs look to you?

So, the port motor is the one that had the problem? And now it’s fine after switching all of the port motors’ components to the starboard and then back from the starboard motor the port is fine?

Unless you affected something during the parts swap, or there was a heat related (not necessarily overheat) situation-like the plugs, to me it doesn’t make sense that merely swapping parts back and forth would make the engine start running again with original parts.

 

[Neutron]

Irridium plugs are a waste of money in these engines because they still should be replaced every year.
The Mr1 engine is very sensitive when it comes to spark plugs.
You should always keep extra sets for each engine so you dont ruin your day because of one fouled plug

 

[Rlisty25]

Irridium plugs are a waste of money in these engines because they still should be replaced every year.
The Mr1 engine is very sensitive when it comes to spark plugs.
You should always keep extra sets for each engine so you dont ruin your day because of one fouled plug

Got it - going forward I will just use the standard plugs the MR1 motor calls for

 

[Rlisty25]

I’m not sure how long you’ve owned a Yamaha jet boat, but the first thing to do at the sign of rough running is to change the plugs, all of these four stroke engines seem to consume plugs. Use the OEM plugs, be sure and set the gap of the new plugs out of the box, there has been others that have had issues with fine wire / non OEM plugs.

If You ran the boat all season then the plugs weren’t brand new anymore… how many hours did you have on them? Was the other engine still running normally?

You said the dealer said the plugs were fouled, how did the plugs look to you?

So, the port motor is the one that had the problem? And now it’s fine after switching all of the port motors’ components to the starboard and then back from the starboard motor the port is fine?

Unless you affected something during the parts swap, or there was a heat related (not necessarily overheat) situation-like the plugs, to me it doesn’t make sense that merely swapping parts back and forth would make the engine start running again with original parts.

So, the reason I didn’t change the plugs initially was because of how the engine died — it acted like a fuel pump issue. I didn’t think for a second that the plugs had fouled out, but I now understand the importance of using OEM plugs going forward.


The issue was with the starboard engine. I swapped parts from the running port engine to the non-running starboard engine. The spark plugs from the starboard side did appear black, but when I reinstalled them in the port motor, it ran fine. After doing more research and speaking with a few repair shops, they all said the same thing — always use OEM plugs.


To answer your question, I had roughly 20–30 hours on those spark plugs. I installed the iridium plugs after running the engines post-winterization, so yes, they were new at the beginning of summer.


The main reason for my post is that it just seemed odd that the iridium plugs would foul out the way they did. Lesson learned.


Thank you,