1990 40 hp carburetors question / engine troubleshooting

[BassRanger]

Hello everyone,
I have a 1990 johnson 40 hp (J40ELESR) outboard.
I have an issue with the motor not wanting it idle while in the water but will barely idle while on land with muffs.
I have been dealing with this for a few weeks now.
While checking all components and doing some troubleshooting I think I found the issue, but I wanted to see what others think and to see if I am on the right track.
While troubleshooting I decided to unlink the upper and lower carbs and crank the motor. It idled like it has been doing, so I decided to manually open each carburetor separately, first the bottom one then the top one.
When i opened the bottom one the mortor rpms went up but when I did the same thing with the upper carburetor the rpms did not increase and it sounded like it was muffled or bogging down.

My question is.. Should the motor rpms increase when the upper carburetor throttle plate opens just like the bottom did? If so then I think I have found the idle issue but if not, then what could be the problem with the idling problem when it is in the water?

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[racerone]

In general.-----The top carburetor feeds the top cylinder.-----Bottom feeds the bottom cylinder.----Start your trouble shooting with a compression test.---Post your numbers.

 

[oldboat1]

If the compression is good (say, in the 120s or so), my guess is a jet in the upper carb is blocked. Jet would be behind the screw plug at the bottom of the carb. But don't rev by opening just the throttle plates.

 

[BassRanger]

I will get a compression kit and do the test and post the numbers today when I get off work. I hope it's not a bad cylinder! I sure do not want to have to break the engine down.

 

[webbd]

Compression should show each cylinder within a few PSI of each other. It will also help to identify any mechanical issue, such as bad rings, a scored cylinder wall, or even bad reed valves. Mine is 90PSI in each cylinder when cranking the starter with throttles at WOT, with both spark plugs removed, and with Herecarburetors drained to avoid blowing air-fuel mixture into my compression tester. When pulling the spark plugs to check your compression, take a look at them. If the lower one looks gummed-up, it might need to be replaced before you put everything together. If there's rust, you might need to do additional mechanical checks.

I just replaced the coil on my 1997 50hp. For several years before the coil went fully bad, It would idle just fine on the muffs, but wouldn't idle well with the engine fully down. When it died on me, the engine ran for more than 15 minutes on-plane, but coughed a few times while running. It then decided to stop and not start after the coil was hot. A spark tester will help. You want the spark to jump a 3/8" gap with a hot blue spark.

My carburetors also suffered from ethanol fuel used in the tank. The ethanol not only ate at my fuel lines and collected water, but it also seems to have taken a toll on my float and needle/seat assemblies. When I pumped up the primer bulb with the noise suppressor off, I could see fuel squirting up into the carburetor throats. The engine would idle in gear only when the engine was partially tilted out of the water because it was flooded on both carburetors. Use ethanol-free fuel and you might need to put new kits in the carbs. They're easy to do. I also added a water-separating fuel filter after the squeeze bulb and before the engine a few years ago to help with water issues.

If you haven't already done so, it's a lot easier to work on the lower carb with the lower shroud off. It's 2 bolts on the inside in the back facing the port side, 1 bolt on the inside in the front facing the starboard, and one in the middle on the oudside under the engine facing port. They go on and off easily, and it's somewhat satisfying to clean any accumulated crud out if they haven't been removed in a while.

-DW

 

[BassRanger]

OK as promised I have check the compression on each cylinder
Results are: cylinder #1 75 psi
Cylinder #2 110 psi.

So I guess my next step is to start to tear-down to track down the cause of the low compression. If I am lucky at this point it could only be a blown head gasket. At least I hope that it is. If not the I guess I will have to do a full rebuild over the winter months.

 

[racerone]

No surprise that you have compression issues !-----Lucky you if it is a blown head gasket.