Mercury 90 hp question.

[philipjames5k5]

I have a mid to late 80s mercury 90hp outboard with 3 cylinders. Before I stored the boat and motor around december my friend said it sounded like it was missfiring. I did not mess with the motor until today. I first pulled all three spark plugs out to change them and the top cylinders plug was white while the others where black. All three cylinders are 120 lbs of compression. And all three cylinders have spark. Should I be woried that the one plug is white. Also the motor does not abnormally shake and idles smoothly.

 

[philipjames5k5]

Re: Mercury 90 hp question.

86 views and no input

 

[philipjames5k5]

Re: Mercury 90 hp question.

talked to a buddy of mine and he said the top cylinder may be starving of fuel. would this cause my issue? it sounds and runs fine to me. also my oil tank is unhooked. i premix my own gas.

 

[mercurymang]

Re: Mercury 90 hp question.

I don't know for sure but it looks like you have a carb per cylinder, right? I think a big problem on the older motors is switching over to ethanol fuel and all the junk that it releases into the system. Hey, I'm no expert but if i were you, I would replace all the fuel line. Then yank the carbs and clean them thouroughly. Maybe some expert will come along and give better advice.

 

[oldman570]

Re: Mercury 90 hp question.

As long as the compression is still good, you have probbly not damaged it yet. A plug that is not burning as the rest and is a white collor is not good and a inducation that the cylinder is running lean. The top carb is probly in need of a good cleaning or rebuild. Being that you will have to work on that carb, it might be best to do all of them. If the fuel hoses are the old rubber hoses, it would be best to replace all of them with the new ethenol resitant hose while working on the carbs. A spark test at the plugs should also be done before hand if the motor would not run up as it should before being store, and will tell if that plug is not fireing like the rest. Running the motor on muffs will not show a power loss and missfire like it will when on the water. If the plug is not fireing as it should then that is another ball game. Do the spark test and let us know if it will arch accross a 3/8" gap and that will tell the real story. JMO
Oldman570

 

[Faztbullet]

Re: Mercury 90 hp question.

A white insulator is: 1) not firing,2) no compression,3) bad plug, 4)water in cylinder,5) excessive lean or no fuel. According to your earlier post #1 & #2 or out, if champions change plugs, You can also do a cylinder drop test by starting engine and pull wire off that cylinder and see if it makes motor RPM change.

 

[brnschoneck]

Re: Mercury 90 hp question.

A white insulator is: 1) not firing,2) no compression,3) bad plug, 4)water in cylinder,5) excessive lean or no fuel. According to your earlier post #1 & #2 or out, if champions change plugs, You can also do a cylinder drop test by starting engine and pull wire off that cylinder and see if it makes motor RPM change.

what if it makes motor rpm change ????

 

[oldman570]

Re: Mercury 90 hp question.

Then that plug is firing as it should.
Oldman570

 

[CharlieB]

Re: Mercury 90 hp question.

The one thing everyone didn't mention is your fuel pump and fuel hoses.

A weak fuel pump or air sucking into the hoses from a poor connection or leaking O-ring fitting, will not pump enough fuel to fill the upper carb, allowing it to NOT run, or to run lean, overheat and melt the piston.

Compression test to ensure the cyl is still in good condition.

Test fuel pump flow. Disconnect the fuel hose after the fuel pump, point it into a suitable container, pull the lanyard, there is no need to start the motor. You may remove the spark plugs too relieve strain on the starter and battery. Crank the motor and watch for a strong pulsed fuel flow.

Test for air leaks. Once ready to run the motor temporarily install a clear hose between the fuel pump and carbs. Run the motor and watch for any air bubbles passing thru which will show air sucking in somewhere before the fuel pump.

 

[philipjames5k5]

Re: Mercury 90 hp question.

I appreciate all the input. Ill pull the spark plug and see if it idles differently. The plugs that where in it was NGK. Ill probably just rebuild the carbs and check the fuel pump and spark. And replace the fuel lines and go from there. But I want to make sure I fixed it before I run it. Also by placing the spark plug by the block to see if it sparks. Is that how you check spark?

 

[Texasmark]

Re: Mercury 90 hp question.

If you have a timing light, just take the pickup (with it powered up to 115 or the battery as applies) and get it close to the spark plug wire intact on the engine without having disconnected it. If it's firing you can tell that and how well by the flashing or lack of and frequency of the flashes.

I have the same engine of a few years later and the carbs are touchy and they and the timing need to be set per the book. My hat in the ring is carbs mal adjusted #1 with a second for water in the cylinder. Popping the plug and looking in the cylinder for rust will answer the water question as will looking at the top of the pistons on all 3. Water in combustion washes everything nice and clean.

Mark