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Mariner 60hp Drops Cylinder Above 1/2 Throttle

S

StevensPoint

New member
Hi All,

My 1984 60hp Mariner (3cyl, serial: 6429699) mounted to a '78 Forester 15ft Tri-hull has been giving me problems. The motor starts, idles and runs at low speed just fine all day long. If I go WOT it pops out of the hole immediately. I then back off to 3/4 throttle and it cruises on step just fine for about 1 minute, then it loses significant power and acts like one or two cylinders intermittently just stop firing. Then, like a light switch they are firing again. It's bad enough that the boat can't stay on step. This goes on for as long as I keep it above 1/4 throttle. I don't do it for long fearing that it might be a fuel delivery problem although the immediate on/off nature of it makes me think electrical. If I drop back to 1/4 throttle or less it seems to run fine again.

Based on posts from folks with similar issues I have done the following:

1) New stator 5 years ago to fix no-start condition.
2) New spark plugs multiple times (NGK BUHW gapless plugs).
3) I asked my wife to constantly pump the primer bulb while I ran it up to 3/4 throttle. Maybe slight improvement??? not really though.
4)
I swapped the entire fuel tank, line and primer bulb with another unit - no improvement.​
5) New CDI 2 years ago attempting to fix current problem.
6) New spark plug wires this week.
7) New coils (3) this week.

I only run fresh, non-ethanol gas mixed 50:1 with synthetic oil. The prop is a little dinged, but it seems to run fine until it seems to drop a cylinder. Water squirts out the hole. I have not measured any water or cylinder head temps.

Where do I go from here? Any help is greatly appreciated!​
 
Can you do a WOPT plug check to see which cylinders are dropping out? I'd recommend putting in conventional (gap type) plugs for the test since you can read them.

I use Autolite 303 plugs in my Mercs triples for testing.

Jeff
 
I had a similar problem with a couple of motors over the years. It turned out to be trash in the fuel bowl plugging the jet at high demand, came loose at low fuel demand. Since it only hits one carb it only cuts out part of the cylinders. I ruined a cylinder in a 40 merc with this problem. Catch the fuel from the bowl and examine it.
 
Good idea except these unique, 3 cylinder motors have a center cylinder that breathes through both carbs.

You might have to put 'test plugs' in there to see what the motor is doing. I use Autolite 303 plugs for that purpose. Be sure to put at least a half hour run on them before testing, and cut her off dead clean--no idling--before pulling the plugs.

Jeff
 
I went to a marine repair shop to buy some conventional gap style spark plugs so I could do the WOPT test. After some digging they sold me a set of NGK BR8HS-10 plugs. I warmed up the motor on the old plugs, swapped them out for the new gap-style plugs and now I can't get the motor to act up. It runs exactly like it is supposed to.

All the documentation I can find, including the owners manual, tells me that this motor requires BUHW gapless plugs. Throughout my troubleshooting I have swapped fresh sets of BUHW plugs several times with no improvement. Now that I throw some gap style plugs in there it runs fine.... We'll see if this lasts. Thanks for the help.​
 
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