Tohatsu M40C outboard only runs wot, and backfires, I mean real fire thru carburetor

[titaniumneck]

My Tohatsu M40C outboard overheated a couple times 2 trips ago. Both times killed and waited for cool down and idled back to boat launch. Change cracked impeller, and figured while I was working on it, did a carburetor rebuild. Well before the overheating trip the motor would on start on choke with wot. I got service manual and have set timeing and carburetor to specs. Last trip it acted up the whole trip, I was about 3 miles out and finally got it started on wot and had to quickly slam it into gear so it wouldn't kill. Gets weird here it was running top speed 35mph at first then on the 3 mile trip back to launch it kept slowly loosing speed, 35,30,25,20,15, and actually finally killed 20 yards from launch at 5 mph. Now it won't start unless play with choke and open air at wot. Will not idle, and while it was getting dark I could see the backfiring fire coming from inside the block out the carburetor. I have adjusted carburetor everyway possible, and adjusted timing linkage everyway possible. Please, please, please help, help, help. I don't have a clue. I am disabled retired from sheriffs department 28 years accredited service and fishing is all I have. (besides loving family, wife disabled also) so I have to work on it myself, cannot afford to bring it to someone, barely can afford the parts, but I do what I have to do to give myself and family a great honest hobby like fishing to keep together. Any advice will help, thank you.

 

[Sea Rider]

Welcome to Iboats,

When OB overheated twice, assume was from lower leg water intakes obstructions, right ?
-At which speed happed so, did engine began to miss, fail, stopped dead ?
-Was smoke comming from cowl, where adjacent cable covers or cables melted in any way ?
-Did you perform a head torque back to specs after overheat ?
-Which overheat was worse, first, second ?
-Where plugs severely heated-up, look scortched ?

Happy Boating

 

[titaniumneck]

Thanks sea rider for your input. I went and borrowed a cylinder pressure gage from autozone and found out real quick, top cylinder had zero pressure. Removed head and thank God it was just a blown head gasket between top cylinder and water jet in between cylinders. Ordered headgasket from iboats. One question I have is what grade are the cylinder head bolts. I cannot find info on these bolts with a lower case b on them. I want to go to Napa and get some new bolts but do not know what grade these are. Any clue?