1986 Mercury 150 XR2 - RPM Loss at WOT (Dropping Cylinder?)

**wcsd106** · 08-20-2020, 02:44 PM

Good afternoon,

I have a 1986 Mercury 150 XR2 S/N: A114812

This outboard starts fairly easily, idles well, and runs good at low and mid RPM ranges. When I bring it to WOT, it tops out somewhere around 5800 RPM and will run good for a few minutes. However, at some point while running at WOT, you will feel what appears like a miss and then the maximum attainable WOT RPMs drop to around 5000-5200. When this occurs, you can throttle down, drop to idle speed for a few seconds, and then when you take off again the outboard will reach the initial 5800 RPM max.

Since the problem only occurs at WOT, it's making it a little difficult for me to diagnose. I will be taking the boat back to the lake this weekend and bringing a buddy and my timing light with me so that I can try to find the cylinder dropping out, if I am indeed losing a cylinder.

I have replaced all of the fuel lines from tank to outboard. I also replaced the water separating fuel filter.
Fuel pump and carburetors were rebuilt last summer. (Link & Sync performed afterwards)
Spark plugs have recently been replaced with new NGK BUHW plugs.

Any ideas on what is occurring would be helpful.

Thanks!

**Merc 2.5** · 08-20-2020, 03:16 PM

Stator or switch box trying go out , would b my 1st guess , need test fire on ea cyl while its missing if possible, or pull plugs and c if u have one not firing, also make sure it will jump fire 7/16ths gap , cracked coil magnets possibly

**wcsd106** · 08-20-2020, 06:14 PM

Thanks for the reply. I was already planning on taking the timing light to the lake with me on Saturday and having my buddy run is down the water at wot until the miss begins so I can test each cylinder individually.

Will test the spark at cranking speed tomorrow with a tester set to 7/16 (I've always used 1/2) and visually inspect the coils while I'm in there.

**rgsauger** · 08-20-2020, 10:22 PM

Also check your wires. Mine would drop a cylinder at wot and it was wires

**wcsd106** · 08-21-2020, 05:34 AM

Originally Posted by rgsauger

Also check your wires. Mine would drop a cylinder at wot and it was wires

Plug wires or just the regular wiring?

**wcsd106** · 08-21-2020, 09:32 AM

Originally Posted by Merc 2.5

Stator or switch box trying go out , would b my 1st guess , need test fire on ea cyl while its missing if possible, or pull plugs and c if u have one not firing, also make sure it will jump fire 7/16ths gap , cracked coil magnets possibly

UPDATE 8.21.20: I pulled each spark plug this morning to do a visual inspection and the one to Cylinder #5 is SPOTLESS . The other 5 plugs at least have a little oil / fuel residue on them. My thought is I have probably located my dead cylinder.

My buddy and I will be taking my boat to the lake tomorrow morning. He's going to run it at WOT while I mess with the timing light. Will be doing 6 runs to verify spark on all cylinders. I have no doubt we should be able to find the dropping cylinder this way.

Thinking wayyyy outside the box here, but could a blown water jacket cover gasket cause a misfire at WOT?

**nitro_rat** · 08-21-2020, 09:37 AM

Spotless clean usually means water is washing out the fuel/oil mix. Head gasket more likely, I would think the flow of exhaust down the chest would probably push a small leak out. A leak big enough to kill a cylinder would probably cause water pressure or overheating issues. Anything is possible I guess...

**wcsd106** · 08-21-2020, 09:39 AM

Originally Posted by nitro_rat

Spotless clean usually means water is washing out the fuel/oil mix. Head gasket more likely, I would think the flow of exhaust down the chest would probably push a small leak out. A leak big enough to kill a cylinder would probably cause water pressure or overheating issues. Anything is possible I guess...

That's the thing.. Water pressure is good and the engine isn't overheating. Also, compression is good on all cylinders... within 1 or 2 psi of each other.

**FMP** · 08-21-2020, 09:40 AM

Test the #5 coil , check the spark lead thoroughly.

If one or both are bad go buy 6 fresh.

Or find a good used coil, swap #5 and take it for a run as usual. But check all your plug leads.

**FMP** · 08-21-2020, 09:54 AM

And if you did the plugs prior to the hiccup, maybe someone dropped #5 BUHW on the floor, if you're lucky.

**wcsd106** · 08-21-2020, 09:59 AM

Originally Posted by FMP

And if you did the plugs prior to the hiccup, maybe someone dropped #3 BUHW on the floor, if you're lucky.

Unfortunately, no. It was doing this before I changed plugs. I was hoping for a miracle by changing them since the old plugs looked pretty worn.

**wcsd106** · 08-21-2020, 01:45 PM

Originally Posted by FMP

And if you did the plugs prior to the hiccup, maybe someone dropped #3 BUHW on the floor, if you're lucky.

Update: For what it's worth, Cylinder #5 was the one with the clean plug, not Cylinder #3. I have edited my post above to reflect my error.

I just performed a resistance check of the #5 Coil per the instructions in the OEM Mercury Service Manual for this outboard. All observed readings were within specified parameters.