Severe Back Fire

[fry-cook]

Good Morning, First let me thank the club for allowing me to join in the forum. Now right to it! I purchased a 1954 Lyman that came with a 1956 Johnson RD-18 30hp motor. Over the winter I replaced, with all new, the points, condensers, wires, plugs, coils, everything ignition related. I went to fire it up this weekend and the only thing that happened was a backfire so intense that it blew the top of the starter motor off, from this I mean, it broke off the cotter pin, and sent the bendex flying. I pulled the fly wheel off on rechecked my point gap, and wire connections, same results. Do you have any thoughts on what I did wrong or what could be causing this?

Thanks, Tom

[Buccaneer]

Are the spark plug wires on the correct plugs?
Do you have even compression, and therefore no
pistons or cylinder walls that are trashed?

Prepare to be boarded!

[fry-cook]

Thanks, Yes first thing I did before I even began was to check compression, all good there. I will check to make sure the plug wires are on the correct plugs, then give it another go.

Thanks Again, Tom

[chinewalker]

For reference – the TOP cylinder is fired by the FRONT coil. When looking down at the mag plate, the coil closest to you is wired to the points & condenser to the coil’s right/counterclockwise from the coil.

[keny]

Maybe the flywheel key is bent or sheared? As you probably know, the shaft must be clean and dry. I found this out after many a backfire.

[jeff-register]

Easy way to check any 2 cylinder motor, Plugs out turn over the motor by the top nut watch a set of points. When they open stick a screwdriver in the plug holes.

Piston closest to the top gets that set of points.

[fry-cook]

Thanks, I know for a fact that the shaft is not dry, it’s clean.. I pull the fly wheel again and recheck everything again. This is all good stuff!

Thanks again, Tom

[keny]

Sorry, Cook. The shaft and flywheel must be clean and dry… No lubricant … or it will shear off the key (in my experience)

[fleetwin]

Are you sure the plug wires are on the correct plugs? Just swap them around and try it, will only take a few minutes

[david-bartlett]

Is the magneto properly synced with the carb?

[Author]

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