1955 Johnson 25hp (Which ones had the bad crankcase seal?)

[Ektorsketch]

So I purchased an electric start Johnson 25hp. It's a 1955. $200 and it has a new impeller and some other stuff. I also got a pressure tank from the same guy for $50 but I can hear it leaking some pressure slowly.

Anyway...I start pulling on the rope to start it (I haven't hooked up the solenoid yet) and it immediately tries to run and then backfires. So I do what everyone else does and give it choke and gas again. It runs for 3 seconds and backfires when I give it gas then dies. The gas it obviously dirty and I haven't cleaned the carb or anything yet. I just wanted to see if it fires up. I ruled out crossed plug wires because when I switched them it yanked the recoil out of my hand.

Why would it backfire?
I read a few posts about some of these having bad crankcase seals. Hope I don't have one.

 

[tphoyt]

Check the key in the flywheel.
It may be sheared.
That will cause your back fire and ripping the cord out of your hand.

 

[F_R]

Yep, that one (-S) has the bad seal

 

[Ektorsketch]

any idea how much it would cost to pay someone to fix the bad seal? I have very limited time these days or I might attempt it myself.

 

[racerone]

It would cost you more that what you paid for it to replace the seal.-----Check spark first.------The magneto maximum output must be tested.----Does spark jump a gap of 1/4" or more, yes or no ?