Analyze This!

Sorry for the dramatic title. I've been waiting years to use that line.

I was changing plugs in my boat and this is one of them. They all looked much the same as this only the other five didn't break. The mechanic where I last had the boat worked on must have put them in with a pneumatic wrench. I had to use a 1/2" drive, 18" long breaking bar to get them out. Whoa, way too tight.
Anyway, tell me what you think the engine is doing based on light tan color of the electrode and tip. No soot to speak of, no oil. They have been in the engine less than a year and have maybe 20 hours. Only two or three WOT occasions, mostly run at displacement speeds (fishing)

Engine is a 42 year old Mercruiser 165 L6 (the GM 250)

 

pre-ignition???....check your timing perhaps

 

Did that. It was a little off (about 5 deg) Set to 6 deg BTDC

 

Put them back in (the unbroken ones) and revisit the issue in ten years or so when the leads rot off.

 

replaced them and chucked the old ones

 

They look good. For a carbureted engine it is the right color.

 

Yep, Gonzo's right. The color is right for the additives used in modern fuel.

To get a good spark plug "read" fire her up and idle out to a clear spot, motor out about a 1/2 mile and do a 180 turn, then hit WOT, for the whole 1/2 mile. Without throttling down, kill the engine and row the rest of the way back or just pull the plugs right then and there. This is the best way to get a plug read. If they get a chance to cool (when you throttle back), they'll burn off any contaminates, carbon or other things you might want to see. You want a "full pull", for at least 30 seconds, then kill the engine. Of course, fresh plugs read the best.

 

Interesting procedure Paul Thanks.

 

Paul is as old as I am to remember those procedures

 

After my last posting I thought about a too lean of mix and/or additives????- Years ago I used to mess about with sports cars and if memory serves me right had some 'cooked' plugs due to a lean mix. All the insulators were a yellow/brown tint.
Also check if you have the right plug - hot (daily running) or cold (racing).
Nowadays, we have 10% ethanol in gasoline and this can also attract water... perhaps leading to discoloration.

 

Lean mix will make the electrode bright white. It makes it overheat and sometimes melt.

 

This makes me chuckle. It reminds me of what I went through trying to find the right plug for this old engine. There are more than two dozen plugs that fit in the old GM 250. Problem is a lot of them are no longer made or the model number has changed. The first time I bought plugs for it I spent about 2 hours researching it on the internet and at least an hour at NAPA going through the catalog with the sales clerk. Fortunately I wrote them all down.

The manual specifies AC-Delco CR44N. Try to find one. Google it. The first hit is of course AC Delco. The second one is AC DELCO CR44N - Alternative spark plugs, then Alternative spark plugs for AC DELCO CR44N, and so on. The Mercury Marine place sold me Champion N5C (that's the one that broke.) NAPA said NGK BR6ES. AutoZone said BOSCH WR 8 DC (7907), and the beat goes on.

I bought the BOSCH. All of the above are the right size, length, heat range, etc.

By the way, went out fishing this morning for about two hours. No joy, but they stole my bait. The engine ran fine. Still stumbles at about 1000 rpm. Above or below that it runs great.

 

Try setting the idle mix a bit richer to prevent stumble.

 

sure. I'll give it a try.

 

actually I think it was running a bit on the cool side, there is a lot of carbon deposited for something that only ran a short time. It also appears the spark plug threads were protruding well past the thread part of the head well into the combustion chamber, you can see all that carbon on the outside of the threaded part of the spark plug. you get carbon build up there and it is darn near impossible to get the spark plugs out without damaging the threads in the aluminum head becasue of this carbon build up. It may not have been your mechanic at all (other than his poor choice of spark plugs), but the carbon build up on the last several threads that were protruding into the combustion chamber. If that happens again you might try some gum-out soaking of the combustion chambers, and than slowly working the spark plug back and fourth to wear off the carbon build up until you can back it out without too much effort.

Also, ALWAYS use lots of anti-seize on the spark plug threads. Before you replace the plugs I would buy a spark plug thread tap and carefully chase all of your spark plug holes, use grease on the tap to catch any shavings, clean well with a boar brush, solvent and blow it out with compressed air, it should be fine. Note that the spark plug thread is a special thread design and you must buy a tap specifically for a spark plug bore. They do not cost that much and are well worth it to clean up any damaged threads and clean out the carbon built up that is almost certainly there in the cylinder head threads.

The next thing you should do is never run Champion spark plugs again. unfortunately their quality has done down hill from the days they built their reputation. I only run NGK now in everything I own. Or Autolite spark plugs for the small engines that I can not get NGK plugs for. I have never had any spark plug problems ever again after I stopped using Champion spark plugs many years ago.

As far as heat range goes, you always want to run the coolest plug you can that does not result in fouling, this plug was right on that edge. Another brand in a similar heat range may have workout okay, because those carbon deportees may have been from the Champion misfiring as the insulators broke down. Every champion spark plugs I ever ran eventually broke down and resulted in misfiring.

Good luck.