1948 Johnson TD 20 5 hp. Backfiring at item or slower speed

[ceugene]

My Johnson engine seems to run okay at higher speeds, but when I slower speeds or idling, I get backfires popping back through the carburetor, it don't seem to be a lot better still does, I tried changing the spark plug wires putting new ones on, but I'm still getting the same problem, so wiring wasn't the problem, any ideas why this engine could be popping back to the carburetor at a slower rate of speed? I'm 68 years old and never had a boat motor before, so therefore I don't know a whole lot about them. So any help will be greatly appreciated from you guys.

 

[racerone]

Is it backfiring through a small hole in the crankcase ?-----That would be the rotary valve used for slowspeed intake.----Try opening that L-shaped needle valve found at an angle near the bottom of the motor.

 

[ceugene]

No it is definitely backfiring through the carburetor, that is why I changed the spark plug wires, I thought spark was jumpy while it was running in a slow mode, and when it was faster I wasn't getting the backfiring that I'm aware of. Thanks and appreciate any help you can give me on the subject.

 

[F_R]

Sounds like the throttle valve on the carburetor is open when it shouldn't be.

 

[ceugene]

Could it maybe, be a head gasket or something like that? I'll check out the carb. again, though its not running to slow when it backfires Thanks

 

[Michawilden]

A backfire can be caused by a wrong detonation timing or a build up of fuel. I thing what you hear as a backfire is a lean sneeze. The slow speed needle valve is at the front of the crankcase like racerone said. Try closing this a little bit first and remember the initial setting. It this doesn't help idd look at the ignition timing or throttle linkage. Maybe your butterfly valve is open when it should be closed( at low speeds)

 

[Chinewalker]

No, it would not be the head gasket.

 

[ceugene]

thank you guys for your input, I will keep on working on it, mean while, I go another Johnson to use until I get the 1948 running.