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Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 16108
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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I have a Johnson 48 hp, it runs and starts fine but under load

Customer Question

I have a Johnson 48 hp, it runs and starts fine but under load is is nowhere near the speed of the 50 hp 2001 Johnson that I piled off. The compression is 123 in each cylinder, ther is oil or gas in the water when running I ran a can of seafoam through the motor last tank I have changed props using the same one that was on the Johnson 50 I removed. I am stumped as to why it runs so slow under load. 10-12 mph doesn't seem to run at full speed
Submitted: 2 years ago.
Category: Boat
Expert:  Jason replied 2 years ago.

Hello my name is ***** ***** whom am I speaking with?

What kind of RPMS does the 48 turn at full throttle? What was was the model number on the 50 you took off?

Customer: replied 2 years ago.
Jason this is Paul I don rem we the mdl of the 50 hp Johnson. Sorry when the guy pug it on he did not wire any of the Gage's. I can tell you if Will run a high rpm in whe not in gear but excelleration is slow and top speed is slow
Expert:  Jason replied 2 years ago.

have you checked spark on both cylinders? Will it idle for a long time, or will it only idle for a short time and crap out?

Customer: replied 2 years ago.
Did not check spark And it will idle forever
Sort for the typos, fat fingers
Expert:  Jason replied 2 years ago.
Your compression is pretty good .Step 2 would be checking spark with an adjustable gap spark tester. If you have it, step 3 would be running th enigne off of another fuel source, just to make you don't have a fuel restriction issue, and if that checks out, step 4 would be to rebuild the carb. Does that all make sense? That is what needs to be done and the order it should be done in. Did you have any quesitons about any of the steps?
Customer: replied 2 years ago.
What's involved in rebuilding the carbs ?
Expert:  Jason replied 2 years ago.

The carbs have to come off and then fully come apart. Everything except for the throttle plates has to come out. So that means every jet, every screw. When I do when I clean them is I soak them in a little hot water that has a mild degreaser in it first. Then I rinse them in hot water. After that they go in an ultrasonic machine for 45 mintues. I pull them out. And then rinse them again. And then blow out every passageway with compressed air from a blowgun. In lieu of an ultrasonic machine what you would do is use spray carb cleaner through every passageway instead. Then the carbs go back together with new carb gasket kits, get reinstalled, and syncronized.

I do want to make sure you are happy with the service I am providing to you before you go. Did you have any other questions on the engine or do you think that covers it?


Expert:  Jason replied 2 years ago.

Good morning. Just checking in. Any luck?