Mike from Atlanta
Regular Contributor
Thanks to all the help I got from this forum I recently finished getting a 1973 Johnson 20 HP running again after it sat in a shed for a couple of decades. Took it out for a trial run on the lake and overall it ran well. Started easily, idled smoothly, shifted into forward and reverse and pumped water out the exhaust. Pushed my 1448 riveted aluminum flat bottom boat about 18 mph with 2 guys, a trolling motor and battery. However, I had 2 issues:
List of repairs completed recently:
-carb clean/rebuild
-new fuel pump
-all new fuel/vent lines
-new fuel tank/line/bulb
-new thermostat
-new water pump and housing
-head gasket replaced
-ignition rebuild with new points, condensers and coils. Timing set using the factory timing tool
-lower unit re-sealed
I wonder if it is starving for fuel at high rpm? I cleaned and rebuilt the carb and set the float level about even with the carb body when held upside down. Fuel pump is new. All fuel and vent lines are new. The fuel tank/line/primer bulb are all new. I cleaned the water passage ways in the cylinder head when I replaced the head gasket. The ethanol free fuel is new and mixed at 50:1. If it is running lean would that cause it to run hot since the fuel is also the lubricant for the pistons?
I plan to take the carb off and double check float level before I take the boat out again next weekend. Any other ideas about what might be causing the surging and overheating?
- When I ran it at full throttle, it developed a rhythmic surging. The motor would slow a bit and then speed up. Backing down to about ¾ throttle fixed the issue and it would run fine and idle fine after doing this.
- It runs a bit hot after running full throttle. I brought an infrared thermometer gun with me. After idling and running around at around half throttle for a while, temps were reading around 150-155F at the cylinder head. After running it for a few minutes at full throttle (with the surging) temps read about 179F at the cylinder head. According to the factory service manual it should not run hotter than about 165F.
List of repairs completed recently:
-carb clean/rebuild
-new fuel pump
-all new fuel/vent lines
-new fuel tank/line/bulb
-new thermostat
-new water pump and housing
-head gasket replaced
-ignition rebuild with new points, condensers and coils. Timing set using the factory timing tool
-lower unit re-sealed
I wonder if it is starving for fuel at high rpm? I cleaned and rebuilt the carb and set the float level about even with the carb body when held upside down. Fuel pump is new. All fuel and vent lines are new. The fuel tank/line/primer bulb are all new. I cleaned the water passage ways in the cylinder head when I replaced the head gasket. The ethanol free fuel is new and mixed at 50:1. If it is running lean would that cause it to run hot since the fuel is also the lubricant for the pistons?
I plan to take the carb off and double check float level before I take the boat out again next weekend. Any other ideas about what might be causing the surging and overheating?