Backfiring through exhaust

[Thompsoncustom]

I have a 1990 johnson 20hp that I got as a project engine. It ran but only at high speed, turns out it's missing some of the throttle linkage so that should be a easy fix. I adjusted the carb by hand and started it at just above normal idle and it was backfiring through the exhaust and than would just die at low RPM. I pulled the carb off and cleaned it today and now it idles at what I would say is a normal RPM but it will still backfire and die at low normal idle. When I was in the carb I seen the needle was off to one side and I wasn't sure if this was normal or if this could be the cause of my backfires.

 

[flyingscott]

The needle is normal. Your backfiring is probably a lean sneeze need to richen up the idle if possible. I would wait until you get the rest of the throttle assembly on the motor and make any adjustments from there. Also check compression and spark

 

[jerryjerry05]

There's a screw and spring on the side.
Turn out 1 and 1/8th turn.
Then motor warm and in gear and in the water(tied to the dock or on a trailer)level.
Adjust from there.

 

[Thompsoncustom]

Well pulled the plugs today and one was pretty much flat, new plugs and gas and it sounds much much better now. Still a little rough when cold but seems normal after a couple mins.

 

[Thompsoncustom]

So I thought the plugs fixed the problem but that ended up not being the case.

It only backfires when the motor is cold so if I start it for a minute and then turn it off and let it heat soak then the backfiring goes away completely.

Also have put a carb kit in it now and got the throttle linkage fixed.