1985 Merc 90 HP 6 cyl - convert to 115 HP

Hey everyone. It always gets to me when i'm reading a thread of interest and there is no finite resolution. Like people are getting close to a solution but you never know what the problem ended up being. So I am doing my part and following up with the results of my issue so... here it is... I hope you find it helpful.

My Motor - 1985 Mercury 90HP serial number "6436123 and up" which is the last model of this inline 6cyl made.

In August of 2009 I sold my 1982 75HP Evinrude and upgraded to a 1985 Merc 90 (inline 6 cyl) that had been in storage for eight years. I wanted more speed. When I picked up the motor I brought the following tools with me... A compression guage (loan a tool from O'Riley auto parts) and a metrinch tool kit. I wanted to hear the engine run and make sure every cylinder had good compression. When I arrived the owner had the motor hooked up to flusher and I was able to hear it run. It fired right up and idled well for a good 5 minutes or so. It also pumped water fine and the owner had a receipt for a brand new stator that had been installed by a dealer. I could see with the cowl off that the wires coming off the stator were indeed new. Not being an expert on outboards that was good enough for me.

So we take about 2-3 hours and install the motor on my boat using a "cherry picker" or engine hoist. The install went fine. Routing and hooking up wires and cables is the hardest part. I made sure marine grade silicone was used on the bolt holes, etc. So I get home and am anxious to take boat out and see what it will do. I have a 1982 Lund Super Pike that is in fantastic shape with a custom built windshield. The motor came with a 21 pitch prop which I was told was about right by a mechanic friend of mine. The boat maxed out at about 34 mph and only 4000 rpm. My heart sank... With the 75HP evinrude I got 35mph with a 17 pitch prop. Additionally, when in the water instead of on muffs the engine idled poorly and hesitated quite a bit on take off. It didn't seem to totally rev out and really sing. Not having a lot of money I ran it "as is" for the rest of the season. Over the winter I purchased a Seloc online service manual subsciption for my engine. Determined to get better performance out of the engine I took it out of storage today (it was 50 degrees here) and decided to do a full system check.

The compression was still great. It was between 140-145 on every cylinder. however when I pulled the plugs I noticed that the number 4 cylinder plug looked different from all the others. It looked new. It was the only plug that was bright white and all of the others were somewhat brownish with a bit of fuel residue on them. I called my mechanic friend and asked him about the condition of the plug. He is an older guy that actually used to race the early-mid 1980's era inline 6 cyl, 90HP Merc. He said they would get up to 6800 RPM out of them. According to him the plug condition was indicative of water getting into the cylinder and in many cases it happens due to some plate/cover in the exhaust manifold area. He said the plate in there can warp and allow water to leak into cylinders. Especially if the motor had sat for a while. My heart sank again... He said the way to test for water was to pull the plug on the suspicious cylinder and fire up the engine (no more than 1500 RPM while on the muffs/hose) and hold your hand about 4 inches from the hole. If water is getting into the cylinder there will be a water vapor coming out of the open spark plug hole. To my delight only air was coming out! Dry air! Next he advised that I check for spark and if there wasn't any it was probably a switchbox.

So, using my handy spark check tool, an inline type I got on ebay for about $3, I checked the #4 cylinder for spark and guess what? No spark! All of the other cylinders showed good spark. So I logon to seloc online and click to the ignition section of my online manual. It advised to test the systems in the following order using an ohm meter... Switchboxes, Stator Coil, Trigger Asssembly, and lastly Ignition Coils.

Step 1 - Switchboxes
According to the service manual you cannot perform an instrument test on the switchboxes. You can only swap them with a working switchbox and see if spark re-appears. On this engine there are two switchboxes (3 cylinders each) and all other cylinders had spark so I took the time to swap them around which took a little over an hour. There are 10 wires per switchbox and a 3/8 nut securing each one. Not to mention the little rubber boot covers on every single one. Oh, I should also mention that I took detailed digital photos of what the wiring looked like prior to taking anything apart. After the tedious process of switching the boxes the problem did NOTfollow the switchbox. Still no spark on the number 4 cylinder! So now I know both of my switchboxes are good as all of the other cylinders still had spark.

Step 2 - Stator Coil
Since I knew the stator was new and had been dealer installed I skipped this step and went right to the trigger assembly.

Step 3 - Trigger Assembly
Following the manual I followed the procedures and checked continuity using an ohm meter that I got at Harbor Freight tools for $5. The Trigger checked out fine. This was an easy process and took about 5 min.

Step 4 - Ignition Coils
I individually checked each of the 6 ignition coils and wa-la! The number 4 coil returned no reading. Like it was dead. All of the other coils returned a reading. Excited that I located the problem I took the time to remove the bottom coil cover (6 screws) and attach the #4 positive coil lead from the switchbox to the positive side of the number 5 coil and leave the number 5 switchbox positive coil lead wire unhooked. So essentially I was still running only 5 cylinders but now the number 4 cylinder should be firing and the number 5 cylinder would be dead because it wasn't connected. Wa-la! The number 4 cylinder now had spark. It was definitely a coil issue. So I ran down to one of the 5 marine dealers in our area that had a coil in stock. The coil was $37. I installed the coil per the instructions using the RTV sealant and tape, etc. I fired the motor up and it ran great. I noticed about a 200 RPM change in the idle speed. All 6 cylinders now had spark and all of the plugs also looked the same after I ran it for about 5 minutes. The engine also idled noteiceably smoother. I plan to take it out sometime this week and see what she will do.

All in all my mechanic buddy said the drag of the dead cylinder was more than likely the cause of the poor idle and poor overall performance. He said it's hard to hear on the inline 6 cyl if you're not trained for it because at idle they still sound pretty good even with one cylinder down. He also said that if top end performance is still poor then it's probably a fuel system issue or timing issue but timing is rarely an issue unless someone has tried to mess with it.

Another specific tip aobut this particular engine is NOT to idle them below 1300 RPM for any length of time like longer than 2-3 minutes. For whatever reason, even at 6800 RPM, these engines will still only generate 7lbs of pressure in the water cooling system. He said that because the water pressure on these motors is so low, idling the motor doesn't allow the water to fully reach and circulate around the top #1 piston. He said that if you watch your "pee stream" it will be kind of spitting when this condition exists. If you see the water outlet spitting like that increase your RPM's immediately until you see a constant stream.

Again I hope this post helps someone. I will post back within a week to report the test run results.

Thanks again for all the info I have found on this site.

Re: 1985 Merc 90 HP 6 cyl - convert to 115 HP

ZigZaggin said:

Greetings. Have a 1985 Merc 90 HP (serial 6623492) that's due for carb cleaning. About ready to rebuild the carbs and am wondering about converting to 115. Does the 115 have totally different carbs or is it just switching out the jets.

However, if I just leave it 90 I'll keep it as is. I prefer to purchase OEM carb kits and found at the following link that there are actually four different jet sizes listed. Which one's correct. Is there that big of a difference between them? Also, How can I tell what size jet i currently have? Special Tool? Name of it?

thanks!

-zig

Click to expand...

There is a lot of difference between the 90 and the 115. The 90 is not power ported whereas the 115 is. Your best bet is to rebuild the carbs you have and leave the jets in them that are there, assuming it has been running OK previously. To clean the carbs properly you will have to remove the brass plugs. They are quite brittle so you have to use a wide screwdriver that will fill the whole slot. When the plug is out you will be able to both see and clean the jet. Unless you are changing them due to altitude changes do not remove them, just clean them with carb cleaner and compressed air. The jet size is stamped on the front of the jet, it is hard to see sometimes the number will likely be a .072

Re: 1985 Merc 90 HP 6 cyl - convert to 115 HP

Hey thanks for the reply. And thanks for the info on the carbs. I think I will just leave it alone. When I'm puttin her on the trailer she idles weak and more often than not dies. I'm sure you can imagine the frustation :/ I've had the motor for about a month. It was my uncles and he had it stored in his garage for 8 years on an engine stand and plastic wrapped. I haven't touched a thing yet but probably will put some money into it since I got it for $500 and every cylider has around 140 compression.

Am thinking about replacing the whole ignition (Stator, Timer Base, Rectifier, Coils, and new Switchboxes) with CDI stuff when I go through the carbs. Anyone ever used the CDI components? Are they good? Considered an upgrade to OEM? Terrible Stuff?

And oh - the water pump pumps ok I think. I used to have an evinrude and the water shot out of it much faster than on this Mercury. The evinrude would shoot water about 4-5 ft wheras this motor has a stream of about 1-2 feet before it arcs to the ground. Are they known to have a somewhat weak water pump stream?

Thanks in advance for your time.

-Zig

Re: 1985 Merc 90 HP 6 cyl - convert to 115 HP

If you value your motor don't run it on an 8-year-old impeller. That should be first on your replacement list.

As for the carbs, it may just be an idle mixture adjustment, I'd make sure the idle speed and mixture are set properly before tearing them apart.

These Beasts don't like a lean idle mix, you have to run them on the rich side of the adjustment range or they'll stall out when you shift into gear. The idle mixture must be set with the motor fully warmed up and idling in gear, in the water. You can't git 'er dun out of the water, on a flusher, not enough load or backpressure.

If your carbs are leaking or plugged, you'll want to replace all (3) carb floats while you're in there. Also replace all (3) carb needle/seat assy's. You'll need (3) 'carb packing' gasket kits. These are relatively inexpensive and have all the gaskets etc you'll need for a simple carb clean 'n refurb. You can do all this with the carbs still mounted on the motor.

While you're in the carbs, remove the "vent jet" which is visible from the outside of the float chamber cover. Disconnect the little black hose from the float cover, plug it with a B-B or equivalent, and reconnect. You have just disabled the Back-Drag carb function, which was intended to lean out the carbs at mid-to-higher speeds, for economy of operation. NOT a desired feature to run an Inline Six lean!!! A good recipe for burnt pistons. Do this whether or not you do any other carb work at all.

Find your motor's serial # and look up parts here:

http://www.mercruiserparts.com/SelectModelType3.asp?class_id=2

Check the electrical parts diagrams and if you have something called an "idle stabilizer" disconnect and remove it. This stabilizer is unnecessary and is known to fail, causing destructive preignition, frying pistons.

If you are inclined to replace any ign part at all, replace the stator. They are the biggest problem area and the hardest to fix when you're away from home. If you carry any spare parts, it's more likely you'll have a switchbox in the tool kit. Pretty slim chance you're gonna carry a flywheel puller and a spare stator!!!!

CDI makes excellent aftermarket replacement parts, offers an outstanding warranty on their stuff and I've never had a replacement CDI stator fail. Period. Replaced a Ton of the OEM Merc ones though!!!

Anyway, sounds like you've got a good one, give 'er a dose of Preventive Maintenance and she'll in return give you many years of happy boating.

HTH..........ed

Re: 1985 Merc 90 HP 6 cyl - convert to 115 HP

Wow - thanks for the detailed reply. I purchased OEM Mercury carb kits. I hope these have everything I need. Also, I discovered that i have no charging voltage so I also ordered a rectifier today. I would like some more info on disabling the idle stabilizer. Do you just disconnect it and tape off the wires? Again, Thanks for the info on the vent-jet and the idle stabilizer.

Other than when idling I think the motor runs good. Seems like it should have a little more power at WOT tho. My 75HP evinrude pushed the boat about 34 mph with a 17 pitch prop and now I'm only getting 37 mph with a 21 pitch that came with the Merc. It sounds good wide open but I wonder if I don't have a cylinder out somewhere? I'll have to double check that. I did pull the plugs and one of the plugs was totally white whereas the other were all tanish brown. I haven't taken the boat out since then i know pure white plugs typically aren't good. I'll throw my timing light on the wires and see what happens. Thanks again for the info.

Re: 1985 Merc 90 HP 6 cyl - convert to 115 HP

Oh - and on the water pump... I usually only buy OEM parts and the only place I know to get them is at boats.net which is always cheaper than my dealer. At the link below I see the impeller but is there an impeller kit that is available? What all will I need to do the job right on this particular water pump besides just the impeller? Gaskets? Seals? I have done many evinrudes and they usually have a water pump kit. I've never done a merc. I do have the SELOC online service manual for the Merc. Thanks again for your time.

LINK - http://www.boats.net/parts/search/Merc2/Mercury/90%20(6%20CYL.)/6436123%20AND%20UP/GEAR%20HOUSING%20(DRIVE%20SHAFT)/parts.html

-zig