1965 Johnson 18 hp backfires when starting

[Canoe2]

Pretty much what the title says, I have an older Johnson 18 hp that I bought from the sister of a fellow who passed on. She told me he had put a fair amount of cash into it at a shop before he died. I didn't think much of it until i started to look under the hood and found all sorts of new parts and figured she was right. I got it going in a barrel and it idles well, spools up nice, but backfires once or twice when I start it cold. I've cleaned the carb, cleaned and reset the points, put in new plugs, but it hasn't sorted it out yet. Any thoughts that could point me in the right direction would be great. Thanks, Tim!

 

[Joe Reeves]

Make sure that the high speed jet is absolutely clean... in the bottom center portion of the float chamber, way behind the drain screw.

(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)

Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.

Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.

Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.

 

[F_R]

Backfire, pops, sneezes, after starting, whatever you want to call it is usually called "lean sneezing". If that is what you are experiencing, it is quite normal, isn't broke, and doesn't need fixing. It should stop sneezing once it runs enough to warm up a bit. If it doesn't, adjust the slow speed needle a tad richer till the sneezing stops.

If the problem can't be adjusted away, there is something else wrong. But you can cross that bridge once you find that a bridge actually exists.

 

[Canoe2]

Does that high speed jet screw out like the ones in the 6 hp's of the same vintage? Sits behind the drain screw? Will try that. I had also heard from someplace that it could be caused by a bad crank seal?Thanks

 

[Canoe2]

Yup, certainly doesn't continue once it warms up. I actually had two and they both did the same thing so i was curious if it is characteristic of the motor or it just needed a little dialing in. That's great, thanks again. Since summer showed finally showed up and the rain finally stopped, I'd like to do some time in the boat and less in the garage...

 

[racerone]

Typical of a cold start on those motors.------That is why they put the mixture screw on the front so the operator can adjust for this.----If you open the low speed mixtore screw a wee bit the lean sneeze might go away.-----Make sure the thermostat is there and working properly.----They are often missing or stuck open.