Major backfire at start-up

[yodel3]

Late 60's Johnson 20hp

Good spark.
New Points
.020 gap
New condensers
Only backfires while starting (electric), then seems to be fine, but it is quite loud and I worry about breaking a key or something else...

Pics below.

http://picasaweb.google.com/danlaurajacobson/Motor

 

[HighTrim]

Re: Major backfire at start-up

Backfiring out of the exhaust port is caused by having the ignition system fire a particular cylinder when the exhaust port is open ( ie piston down). This would sound like a shotgun going off..... is this the condition that you're experiencing?

This is usually due to a sheared flywheel key, a flywheel with melted (moved) center sensor magnets if they exist, timer base leads routed incorrectly to the powerpack, or having the leads from the powerpack to the coils reversed to mention a few.

 

[yodel3]

Re: Major backfire at start-up

Yes. I would say that it is that loud. What would you check first, and why does it seem to only happen during starting?

thanks

 

[clemsonfor]

Re: Major backfire at start-up

my 1971 25 will backfire/sneeze and stall while cold but after say 10 seconds wont do it, but it does not sound like a shoht gun! Does not sound bad i guess its more of a sneeze?

-nate

 

[F_R]

Re: Major backfire at start-up

You are saying that it runs fine once it starts? Idles good and has full power? Then it can't be much other than improper throttle position when starting. Proper throttle position is in neutral gear, throttle advanced as far as it will go (in neutral). It will only go as far as the shifter interlock bar allows. Does that stuff look OK?

 

[clemsonfor]

Re: Major backfire at start-up

Sorry FR i think i kind of hijacked the thread i think you were responding to me? My sneeze is probly due to the throttle being to advanced, yes mine runs fine after a few sexonds im not sure about his sorry i was just adding my symptoms in case his were the same and he was mis discribing them.

-nate

 

[munch]

Re: Major backfire at start-up

i have the same problem. except mine won't start

 

[yodel3]

Re: Major backfire at start-up

I haven't had it on the water, so I can't verify that it runs well under load. Only that the loud backfiring seems to stop once the motor is running.