Jump to content

Welcome to TheMalibuCrew!

As a guest, you are welcome to poke around and view the majority of the content that we have to offer, but in order to post, search, contact members, and get full use out of the website you will need to Register for an Account. It's free and it's easy, so don't hesitate to join the TheMalibuCrew Family today!

Reading Spark Plugs

Recommended Posts

Hoping one of our resident engine experts has seen this one.  @Woodski @Engine Nut 

I recently did a tune up on the boat as I was having a miss when the boat was hot.  coil, plugs, cap, rotor, filters, reset checked timing, fuel pump, swapped carbs, pulled distributor and checked the timing gear.  I finally found the choke linkage on the choke temp coil was disconnected....I'm wondering if every so often the choke was flopping around?  My gut tells me it fell off when swapping carbs and isn't my root cause.  I've got it all hooked back up but haven't had it back to the lake yet.  Since I have about 8-12 hours of run time on the new plugs I figured I'd pull them and see how they look.  I replaced the old plugs with the same Delco MR43LTS marine plugs.  All the plugs looked great as I worked down the left bank from #1 until I pulled the #7 plug.  I found what's below....the only thing I can think is the piston barely kissed the ground strap???  It looks like someone ground the corner of the strap flat.  I found the same thing on #4 on the right bank.  Engine is in a 87' Sunsetter with a 5.7 Merc Comp Ski with a Rochester Quadrajet.  Original cast iron heads.  









Link to post

I don't see how that could be from a piston, but perhaps you have carbon buildup.  You should be able to look directly in the plug hole to see the piston top in that area and compare with other cylinders.

Are you sure the plugs weren't that way when you installed them?  It looks like they were dropped or something.

Link to post

I don't have any expertise to offer, but I can't recall every seeing a "used" spark plug that clean. 

Did you clean that plug before snapping the pics! If not, were the others that immaculate? 

Maybe I just never pulled spark plugs after only 8-10 hours. 


Link to post

@martinarcher, can you confirm the plugs you are using? I believe you have the wrong spark plugs if using LTS. Manual for my 94 350 Magnum EFI calls for AC-MR43T spark plugs. The MR43LTS are the long reach plugs used on Vortec engines. If your engine is original 87 it does not have the vortec heads.

Edited by maxfir
Link to post

Yes, they are for sure LTS's which is what I thought my manual called for but it's a Merc Ski manual I downloaded years ago....maybe it's for a Vortec motor after all?  My motor is the Merc 5.7 Comp Ski that is 260hp.  I've been running these plugs for over a decade and never saw this....it must be stupid close to the piston is all I can say!  

Yeah John, those are still pretty new plugs but the old ones don't look terrible.  As long as your timing is on and the burn is pretty good they shouldn't get all that bad.  








Link to post
Bradley Thornton

As I race cars (Mustangs) this is a great writeup I look over when I need it.


I'm saying they were grinned and some how you missed it going in. It's not heat. so it has to be mechanical and if it was the piston the overhang of the extra metal would be at an upward angle not sideways.  

Maybe I'm wrong regardless its the craziest thing I'v ever seen on a plug.   Ive melted the tip off an a few of mine with Nitrous. Run a race plug now lol.   But this show you what a melted one looks like.



Link to post
Bradley Thornton

To the carbon buildup idea. 

I really don't see it looking at this plug it looks mechanical. But you can buy a cheap camera and look and see how much carbon build up you have for like $30 and you can look at the piston also. 

This is a nice one but they are $20 on amazon 




Link to post

It is really hard to imaging a strike that moved that much material that did not displace the electrode lateraly or on the axis down towards the center electrode.  Also it the two plugs were disfigured in the same place in the same way from a strike then the plugs would have had to have been in the hole with the exact same amount of rotation which seems a little unlikely.  Agreed it does not look like melting.  That would happen either in the center electrode or on the ground electrode where the heat had further to travel (great picture of that) and not on the side. 

Link to post

@martinarcher, if you have a 1987 350 magnum (12 bolt intake manifold) along with stock heads. I believe you are using the wrong spark plugs. Regarding manual screenshot above, I believe the MPI engine was introduced  after Vortec heads in 96. Doing a search for MR43LTS plugs in non-vortec engines a did see an example of the piston hitting the long reach spark plugs. 


I suggest confirming you have stock non-vortec heads. From Skiboatpartsonline:

Pre-Vortec GM 5.7 engines have "angled" intake manifold bolts, Vortec engines have intake bolts that run straight down.


Link to post

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...