Logo
FULL SITE MENU

1980 Mercury 9.8 HP has no power

J

jmk619

New member
I have a '80 9.8HP tiller mounted on a 14' aluminum boat. At full throttle the boat will do about 5 miles an hour. My transom mounted 55lb trust trolling motor will push the boat through the water faster than the Merc! The motor starts easily and runs smooth. The RPM's increase as I twist the throttle but the boat doesn't gain much speed. I have have checked the plugs, compression and the prop. All are fine. I even ran some Sea Foam through the system to see if it would make any difference but it didn't. I'm at a loss as to what the problem is. I just bought the boat two weeks ago from a guy who had it for three years. He said he used it once two years ago and it ran slow then. The boat sat the remainder of the three years.
 
Will spark jump a gap of 3/8" on both ???-------Cleaned the carburetor ??-----------Checked for crankcase compression ( not the same as cylinder compression ) -------??
 
Spark jump checked ok on both. I was thinking about doing a carb rebuild due to amount of time the motor was sitting - thanks for the confirmation. Crankcase compression??? I did the cylinder compression but never heard/know how to do crank case compression.

Thanks for the quick reply.
 
Well if there is a problem with a reed valve or a lower crankcase seal then the motor will never run properly.------You must have good crankcase compression in order for the motor to run properly.
 
Just had the exact same symptoms on my 81 merc 9.8. Started well, idled, would rev nicely in neutral but completely bogged down with no power in gear. It was an ignition problem in one of the cylinders. I pulled the wires off the plugs individually to check for spark (taped a grounded wire to the plug so I could observe it while the rope was pulled). The no spark side had a fried ignition coil (there are two red coils on the top of the engine behind the flywheel) and a burnt connector on the spark plug wire. After replacing these it sparked on both cylinders testing it out in the barrel, but had the same problem in the lake after mounting it back on the boat. Replaced the switch box (black box that feeds the ignition coils) and the thing runs like brand new. I also disassembled and cleaned the carb, replaced diaphragm and gasket and replaced the two plastic parts on the carb that disintegrated (venturi, throttle lever). Day Brothers Boats in Plattsburgh, NY saved me days by having everything but the switch box in stock and had outstanding customer service. Aftermarket switch box ordered from web came in two days.
 
jmk619 said:
I have a '80 9.8HP tiller mounted on a 14' aluminum boat. At full throttle the boat will do about 5 miles an hour. My transom mounted 55lb trust trolling motor will push the boat through the water faster than the Merc! The motor starts easily and runs smooth. The RPM's increase as I twist the throttle but the boat doesn't gain much speed. I have have checked the plugs, compression and the prop. All are fine. I even ran some Sea Foam through the system to see if it would make any difference but it didn't. I'm at a loss as to what the problem is. I just bought the boat two weeks ago from a guy who had it for three years. He said he used it once two years ago and it ran slow then. The boat sat the remainder of the three years.
Click to expand...


Hi, I have a 1980 9.8 HP Mercury outboard that had the exact same problem. Mine would idle perfectly, but had very little power, it would not even plane the boat. The problem was the LEVER, THROTTLE Part # MRC-44425, it attaches to the butterfly valve in the carburetor, so when throttling up the ignition timing would advance but the butterfly valve was not opening. Hopefully this will help you or someone else.
Have a nice day!
Bill
 
Back
Top