350 Magnum Tournament Ski spark plugs

[bbook83]

I pulled all of the plugs after the engine was failing to reach WOT and then backfiring on acceleration. They are all in need of replacement, but one (photo attached) was so bad, the electrode was completely gone. This is a carbureted engine, 1997, serial # OK146780. I was running AC Delco MR43 T, which is for the pre Vortec head, which I verified I have with the 12 bolts on the intake manifold, so I think I had the right plug.

The missing electrode condition was on one plug (not the same one) the last time I changed plugs. I didn’t keep good records, but the last plug change was about 200 hours and five years ago.

Any Ideas on what would cause this damage?

 

[alldodge]

Manual 17 list Inboards to use MR43LTS and stern drives use MR43T, gap is different also

You might be getting close to a bit to lean or timing is not correct (gap .045 and 8* BTDC for MIE)

Is there a decal on the motor listing specs?

 

[nola mike]

I'd be concerned that that's piston on your plug and you melted it. Have you checked compression?

 

[Bondo]

I pulled all of the plugs after the engine was failing to reach WOT and then backfiring on acceleration. They are all in need of replacement, but one (photo attached) was so bad, the electrode was completely gone. This is a carbureted engine, 1997, serial # OK146780. I was running AC Delco MR43 T, which is for the pre Vortec head, which I verified I have with the 12 bolts on the intake manifold, so I think I had the right plug.

The missing electrode condition was on one plug (not the same one) the last time I changed plugs. I didn’t keep good records, but the last plug change was about 200 hours and five years ago.

Any Ideas on what would cause this damage?

Ayuh,...... Yer symptoms, 'n that plug says it's detonating, probably from a lean condition, or possibly to much timing,....

 

[bbook83]

I'd be concerned that that's piston on your plug and you melted it. Have you checked compression?

Ayuh,...... Yer symptoms, 'n that plug says it's detonating, probably from a lean condition, or possibly to much timing,....

Attached is a photo of the sticker from the top of the flame arrester and another from the Tune Up Specifications from the Mercruiser Maintenance Procedures Manual for “1997 Gasoline Ski Series”. The plug type is what I am running, but the sticker say .035 gap and the manual shows .040. Not sure why they would be different. I will do a compression test
while plugs are all out and check timing, which is 10 degrees BTDC on both attachments, after I get the new plugs in when I get them tomorrow. As to checking timing, the Manual says to ground out the “ignition system timing lead” (purple/white wire) to get the “ignition module” into the Base Timing Mode. The third photo shows two modules attached to the distributor. The bottom one of those has a purple/ white wire. Is that the one to ground out?
if so, how do I attach the jumper wire to ground it?

 

[Scott Danforth]

If you have vortec heads you need the longer plug

If you have the 1986-1995 swirl port heads, you need the LTS (short)

Since you have 12 intake bolts, you have the swirl port heads

Looking at the plug, you are running lean and hot.

 

[alldodge]

I would use what's on the arrestor

Brought your serial number up again and it list service manual 8232251 which is number 17 for 305/350 1993 to 1997 printed 1996

My manual list your serial number and also list what I mentioned above

 

[bbook83]

If you have vortec heads you need the longer plug

If you have the 1986-1995 swirl port heads, you need the LTS (short)

Since you have 12 intake bolts, you have the swirl port heads

Looking at the plug, you are running lean and hot.

I would use what's on the arrestor

Brought your serial number up again and it list service manual 8232251 which is number 17 for 305/350 1993 to 1997 printed 1996

My manual list your serial number and also list what I mentioned above

 

[bbook83]

Update. I did a compression check with a cheap tester without assistance, and not fully reliable, but all cylinders were between 120 and 135, except the one that lost its plug electrode recently (#3) and the one that lost the same a few years back (#5), which are side by side in the same head. I will put in new plugs tomorrow, if only to make the boat mobile.

I have some decisions to make on an old boat that I was considering replacing, although it is only at 805 hours and in good condition, other than the current engine problems. I don‘t think the pistons are gone, as there is no noise to indicate that. It could be valves. With the two bad cylinders side by side, is there a chance the zero compression is from a blown head gasket? It appears to me that the only way to know what is going on is to pull the head. I pulled a few heads on cars when I was a much younger man, but never on a marine engine. Is that a potential DIY project on a 5.7 liter Mercruiser engine? Has any body had success putting an endoscopic camera in the cylinders through the plug holes to see what is going on?

 

[Lou C]

It's certainly possible that the HG between #s 3&5 is blown, given how the plugs look, are you seeing water in the oil as well? The endoscope camera is a good idea. Changing cylinder heads on a marine engine, if anything is easier than on a vehicle engine, that job on a small block Chevrolet is definitely doable by a competent DYI'er. Before taking anything apart, I would look with the camera, maybe do another compression test with a better tester (engine warmed up, throttle wide open, ignition disabled), you could also do a leak down test to really pin down where your compression is being lost. Also when it is still all together, you can test the cooling passages with air pressure.
On the small block Chevy with the center cylinders, the exhaust valves are right next to each other so that area tends to get the hottest. So it's common for the HG to blow between those 2 cylinders.

 

[Scott Danforth]

Verify the compression gauge.

If you did loose the head gasket between cylinders, you may be fireslotting the block

 

[cyclops222]

You stated this is a reoccurring same cylinder problem ? A cylinder valve is in trouble.

 

[bbook83]

Thanks for the good responses. I will pick up the spark plugs today and a new compression tester and try compressions again and see what I can with an endoscopic camera and report back.

 

[bbook83]

Thanks for the good responses. I will pick up the spark plugs today and a new compression tester and try compressions again and see what I can with an endoscopic camera and report back.

I re-did the compression test. All were the same, except 7 at 90 psi, and 3&5 still zero. I put the camera down 3&5. Not a great view, but no apparent piston damage, and valves opening and closing. All indications are a blown head gasket, hopefully no fire slotting or other damage. . Need to decide whether I do it myself, after I find out how long it will be to get a mechanic on it.

 

[bbook83]

I failed to answer Lou’s question about oil in the water. There is none. A little good news.

 

[Scott Danforth]

with 3 bad cylinders on the same bank, the exhaust manifold has been leaking and the exhaust ports are rusted.

at a minimum, pull the exhaust and look.

maybe time to pull the motor, get it on the stand and do a tear down.

 

[Lou C]

Yes pulling the exhaust can give you a clue as to what went wrong.
I got an endoscope camera and started scoping the exhaust at the end of each season to check for water leaks. It sure saves time vs taking it all apart.