Wrong size spark plug symptoms.

[Skidude17]

I just changed the port bank of my 5.7L Mercruiser Engine serial #0F598365 on a new to me boat, and I noticed it has the wrong plugs on it. The air cleaner cover says it’s supposed to use AC Delco MR43T, but it had NGK BR6FS plugs in it. The photo doesn’t show it very well, but the NGK is at least 1/4” shorter. What kind of symptoms would result from having the wrong plug like this? I also attached photos of the old plugs. The front port side cylinder looks a little oily, but the rest look pretty good.

 

[achris]

MR43T and BR6FS are 1-for-1 the same plug, just from a different manufacturer. The length of the insulator is irrelevant. It's not the wrong plug... You can (but maybe shouldn't) run a mix of AC and NGK, as they are equivalent.

Chris........

 

[achris]

Here's the tune up spec page from the Mercruiser manual for your engine...(Ignore the Champion, they are JUNK!)

 

[Skidude17]

Thanks for the info Chris. All of that is great! It took me talking to the car guy at work to understand the non-issue.