80 SN 351W issues FINAL UPDATE

1980 SkiNautique 351 back four cylinders cool and odd spark plug damage
Sorry for the long title.

My 1980 Ski Nautique with a 351 (original from what I can tell, it was rebuild in the late 90s) was running great this season and we used it about every week for two hours. The other day before heading out I did my normal driveway run (with water) to make sure everything works. During the driveway run the hose must have came off and I noticed she was about to overheat, I quickly turned it off and let the engine cool down.

About two hours later I checked the impeller (no damage but changed anyway) and started her up. It was then that I noticed she was running very rough I quickly turned her off and decided to pull the plugs and take a compression test as well as stick a scope in each cylinder. With the plugs out my compression was acceptable for this age of an engine, about 110-130, which is where it has been for the last two years. However the second to last plug on the left side looked like this:



I then checked the cylinder with a scope and it did not have any damage and matched all the other cylinders. I thumb test (thumb over plug hole) confirms that the valves are working.

I then went to the store and replaced all spark plugs and plug wires (they were damaged) and started her up. She continues to run very rough. When I increased the the throttle to around 2000 RPM she would occasionally smooth out and surge to 2400RPM with no throttle adjustment. During this run I took temperature readings at each exhaust manifold outlet, the top four cylinders were at 280 - 300 deg F, the bottom four were around 200 deg F!!!

What could my issue be? I am at a lost. I'm thinking that the overheating may have damaged the intake manifold and there could be some cracking/damaged gasket which is leaning out the engine. I believe this is what caused the spark plug to melt away, the leaning is not so bad that the bottom four are too lean to combust most of the time, and the front four are running hot.   

Does this sound practical? Should my next step be to remove the intake manifold, replaces the gasket and look for cracks?

Has anyone seen this before? Also is there a particular automotive part number for the intake manifold that I can purchase at an autozone/Oreily's?

Additional info:, Oil Pressure and water temps are normal.

Thanks again


UPDATE:
After changing the intake manifold gasket, plugs (correct ones this time,) plug wires, the engine runs much smoother but still does not get up on plane or go above 1500-1800 rpm under load, and backfires. I'm at a complete loss and looking for ideas. This boat ran great two weeks ago.




UPDATE 9-6-2017



Joined: December-08-2015
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Thanks (0) Thanks(0)   Quote sailsail Quote Post ReplyReply Direct Link To This Post Posted: Today at 4:59pm
Well it gets better.
In order to check my distributor I removed it and I marked all the wires on the cap... which quickly rubbed away :-(

fortunately I know where 1 is.

The million dollar question. Looking down at the distributor going counter clockwise should it be 1 8 7 3 6 2 4 5 assuming looking aft (disturber in front of me) cylinder one is the first cylinder on the left hand side, followed by 2 3 4 and cylinder 5 is the first cylinder on the right hand side (again looking aft) followed by 6 7 8?

Thanks again! you are guys are amazing

UPDATE 9-7-2017
What should the timing be at idle, what about at high rpm? Also what RPM to i set it to when I adjust the timing. Does the idle timing matter anyway? I don't think there is a way to adjust the curve on these.

Update 9-7-2017 evening:
UPDATE!
Well I am lost!
I adjusted the timing exactly as stated but something is still amiss. (see video and hear noises at the bottom of this post)

Initially when running it was great! but the timing was off the charts dawg (R&M reference, sorry) I would estimate the rpm at 2500 (twenty five hundred) the timing was around 45-55 BTC. but did not have the normal strong sound it had before all the issues started

I then adjusted the timing at idle so that it was at 10 BTC at 600-700rpm and 35 (based off of cheap timing light with dial) at 3500 rpm. however it is making the sounds you can hear in the video below.

While the engine is running better the sound worries me and I know it will not run well under load. What could this sound be!?!?! recall I get great compression in all cylinders.

Click here to see the video of it running at idle


UPDATE 9-10-2017
Ok everything is coming together.
New Autolite 25 plugs. What should I gap these to?

UPDATE!!! 9-12-2017

It was the....... distributor

After removing the valve covers and following through with at least nine rotations of the engine I verified that the timing was correct! I took compression readings with an air compressor (the real aviation way) and everything worked except when I let go of the breaker bar and hit my knee. I then cleaned everything again and was confident that all the values were working, nothing was bent, everything was timed right etc. I even cleaned the carb out (although like an idiot I did not count the turns for the idle minute screws..... ugh)

Finally I decided to purchase a new distributor cap from west marine and she worked perfectly! I could not tell if there was any damage to my other cap (they look the same with no markings...) but I guess something was worn.,

Anyway now I have a great running boat! the damaged intake manifold gaskets were replaced (there was a leak!) and the leaking valve cover gaskets were also replaced and the valve covers repainted to a "Ford Blue" to keep you guys happy.

We went out yesterday for some tubing (FYI to the guy on this forum who hates tubes, When the wife says "I want to tube" my correct answer is "yes dear") and she ran perfectly. The only issues are at idle and that is due to my carb adjustments which I will address when I get a vacuum gauge hooked up.

Thanks again for everyones help here.
The damaged spark plug lead me down the wrong path but I'm glad I did anyway as it addressed some issues that could have turned into greater ones down the road.

I checked that and it is connected. If I remove it the engine surges.

If it ran dry it may have cooked the exhaust hoses and collapsed them. That is what they are designed to do when overheated.
When they collapse they shut down about 80-90% of your exhaust causing your engine to run terrible.
I lost an impeller on boat launch once, my wife idled around waiting for me to park. I was only a couple minutes, less than 5 for sure and by the time I got there the engine was overheated and the hoses were toast. It happens.
Your plug is an odd case. With the center electrode gone I was going to say detonation damage but the ceramic insulator does not look like it got hot, at least combustion temp hot.

Thanks for the reply btw!

My 1978 had 3" I think, sold it a long time ago. The exhaust will sound a little muffled along with the engine running terrible. In my case I was certain I had blown head gaskets by the way it was running but all symptoms cleared up when I replaced the exhaust hose. I ran it another 5-6 years after that with no problems.
One more thing, I could smell the exhaust rubber burning. It stunk.
I was in a jet boat a couple years ago and it did the exact same thing with the same strong smell. The 460 Ford was just blubbering, sounded like it was blown.
The water supply hose failed under the engine and cut off the coolant supply.   Before we knew we had a problem the exhaust was shutting us down.   When an engine loses its coolant, no supply, lower hose failure, the temp gauge responds very slow. They normally read coolant temp and react to coolant temp. They respond much slower when they are reading Air and no coolant is pumping.. The 460 survived also. Head gaskets were still good and it runs great today.   Just had to replace the exhaust after fixing the coolant supply.

[QUOTE=MrMcD] My 1978 had 3" I think, sold it a long time ago. The exhaust will sound a little muffled along with the engine running terrible. [QUOTE]

My exhaust notes seem to sound the same but I have not paid too much attention.

I got to thinking that those exhaust temp numbers probably don't mean much on an engine idling unloaded.

So I took my semi trusty IR gun, went out to the boat (a 351 of the same vintage), started it up and let it run at about 1200 RPM for 5 minutes and then took some readngs.

#5 205       #1 175
#6 208      #2 165
#7 183       #3 155
#8 185       #4 185

Not much logic or consistency to the numbers and all plenty low compared to a loaded engine actually doing some work. Plenty of variation too.

Just to give you an idea what another good running 351 had for inconsistent numbers when running with no load at about 1200 RPM

Going back to the first post ... that plug isn't lean at all detonation probably caused the electrode to disintegrate. Could be several things from as Ken said loose plugs, poor gas, bad valve or timing.
A water temp overheat won't do that to a plug, there's something else going on.

PS; the proper plug for that costs about 26 bucks for a set of 8
for christ sakes "BUY THE RIGHT PLUG"

I can see one of you guys likes platinum and the other likes copper

Yes you guys are right on the plugs I just did a quick search on summit for autolite 24 on my phone didn't look at which it was. Thought that was a little expensive as I remembered them being around a buck ninety per plug.

Btw exhaust tubes were fine.

MrMcD wrote: No OEM's use the E3 spark plugs. Good enough reason for me not to use them. They remind me of the old Split Fire plugs advertised years ago. Great advertising, many tried them so they sold a lot the first year but in testing they did not hold up and even help ignite detonation so they were a flash and gone.

I'll keep that in mind.

Thanks!

Backfiring suggests something amiss with the ignition, like timing. Do any work there?

sailsail wrote: MourningWood wrote: Backfiring suggests something amiss with the ignition, like timing. Do any work there? I have not touched it in over a year.

Choke not operating properly?

I'm confused about which gasket or gaskets you replaced.

Was it the intake manifold to the cylinder head gaskets or maybe the manifold to carburetor/spacer gasket.

Like Mourning wood mentioned and I mentioned a while back

KENO wrote: Are you sure you didn't coincidentally develop a carburetor problem or an ignition problem that was unrelated to the "almost overheat"? Stranger things have happened.

Looking at your old posts/threads, I don't know if you have points or electronic ignition, but a good check of the basics just might do wonders for you.

That would be for the ignition system and the fuel system

I could also ask the obvious like did you put the plug wires on in the right firing order, but I won't