Clean spark plugs

[HAS]

I have a 1984 MERCURY 50, IT IS A 50HP 4 cylinder. The spark plugs were replaced in April 09 with NGK buhw-2, the water pump and lower crankshaft seals were also replaced at that time.I just took the engine apart to replace the clamp assembly,when I removed the spark plugs I found two plugs, NO.1 and NO.3 with nice light brown tint and NO.2 and no.4 looked like they have never been used, they looked like I just took them out of the box. Has anyone seen this before and is there a problem that caused this? This engine will start cold and idle,it will run at idle for 3or4 hours and never miss once, and run at 53-5400 RPM for an hour or more.

 

[CharlieB]

Re: Clean spark plugs

If this is a 4 carb motor, I would suspect that the # 2 and 4 carb float levels are set just a bit too low, not the same as carbs 1 and 3.

You are running at a dangerously near perfect mixture on 2 and 4, the problem with this is there is absolutely no room for error, the slightest fuel problem (restriction) and these two cyl's could melt down within seconds.

Tan is an ideal burn, however, most OEM jetting is set richer, darker brown and into light black, to ensure combustion temps remain low enough to prevent any warranty claims from melted pistons.

Clean all carbs, verify main jet #'s are as specified in the OEM service manual and in the correct carb, many engines do not run the same jet on each cyl.

Pay critical attention to float heights.

 

[HAS]

Re: Clean spark plugs

This engine has only 2 carbs. Is one carb.for cylinders no.1 and 3, and the and other carb. for no. 2 and 4.?? THANKS........HAS

 

[Chris1956]

Re: Clean spark plugs

Well, it sounds like you have mild water infiltration into two cylinders. The plugs get steamcleaned. Is your power normal? If not you will need to pull the exhaust cover and baffle and inspect the inner water jacket cover for wate leaks. Inspect the baffle for holes or warping. You will need new parts where necessary and new gaskets.


On those motors, the bottom cylinder of a carb pair usually run a bit richer. You are not seeing that.

 

[CharlieB]

Re: Clean spark plugs

No, you do not have the 4 carb motor.

Pay attention to what Chris is telling you to check.

 

[HAS]

Re: Clean spark plugs

CharlieB & Chris1956

I would say power is normal, the engine is on a 16 foot tri hull, it runs 28mph at 5300 rpm with a 10 1/4 x 12 prop. With 2 or 4 people it runs about the same. THANKS........HAS

 

[CharlieB]

Re: Clean spark plugs

You STILL need to check for water leaks on the exhaust side like Chris explained.

Failure to correct this problem early WILL increase repair costs later as the moisture levels in those two cyl's WILL cause accelerated wear on the rings and cyl walls. Once this occurs the costs of pistons, rings, boring the cyls will far exceed the couple of exhaust gaskets that you are looking at now.

 

[8hygro]

Re: Clean spark plugs

Some points...modern blended pump fuels do not color plugs in a manner condusive for accurate jetting. At a minimum you would need to be using a true plug reader and looking for tell-tale signs. Having said that...a difference in the plugs between individual cylinders could be an indicator such as Chris has explained. A slightly better sign of jetting without getting technical is reading the piston crowns with a borescope and paying attention to the oil trail. This is definately a learned technique but I have seen many issues with people trying to jet by plug color.

Jetting is throttle position sensitive and these carbs are not that high tech. Keep in mind what you are trying to critique. Trolling around and then banging the throttles open is not going to be a precision fuel event critiqued by plug color. Plugs do not un-color to the point of true accuracy. There are many variables...fuel blend, quality of pre-mix, carb condition, gasket leaks, rings wear, reed valves etc.

8hygro

 

[Chris1956]

Re: Clean spark plugs

8hygro, The bottom cylinder pairs in Merc Inlines, that share a carb (cylinders 2,4,6 on an inline six) should never be lighter in color that the upper cylinder sharing that carb. Usually they show more black carbon and oil, or the same amount as the upper cylinder pair.

 

[HAS]

Re: Clean spark plugs

Chris 1956 & CharlieB

I have the exhaust baffle off and on each side of the lower part of the baffle it looks like a 3/16th hole has been drilled into the water jacket. Are these holes factory holes? Also on this part of the baffle, on the side away from the exhaust ports, starting at the hole there is a crack 1-1/4 inch long. The holes and the crack were covered with carbon so I don't think a lot of water was getting in the exhaust. I put water in the channel on the water side, it didn't come through to the exhaust side. I did the same with lacquer thinner and it came through to the exhaust side.

THANKS FOR ALL THE INFO................HAS

 

[CharlieB]

Re: Clean spark plugs

Once that plate is at temp, the crack WILL enlarge, add to that the increased water pressure at RPM, and you get water passing thru the crack.

Replace the plate.

 

[HAS]

Re: Clean spark plugs

CharlieB

Ordered the plate today. It will take a few weeks for me to have it
running.


I THANK EVERYONE FOR THE HELP........HAS