771 gone bezerk quits after 20 minutes

[Markos250]

hi everyone so last year end of season and almost last ride my motor started acting up it would run great then you give it gas to jump a wave it would die on the face of the wave over the winter i rebuilt it found crank bearings worn and choked it up to that i replaced the crankshaft seals and new piston and rings and completely rebuilt the carbs.
started it up in june ,breaking it in slowly everything was fine after a half tank it started the same stuff.
Runs great for 20 minutes i mean great then after 15-20 min especially under load it dies. if i follow it with less throttle its happy! it will idle for days and occasionally pull full rpms then start cutting out or rather just die again i can save it by going back to idle for the last month ive tried every new piece of electrical stator, then new ebox (ihave msd) then coil and then rectifier all the same problems ive unplugged the stop switch also, so i figure ill keep at it 2 months later i see this grey stuff coming up from the case to lower cylinder stud ill assume this is the dead giveaway, as i have nothing else to figure out
other things ive considered
tank holds pressure if i blow into the one way vent
there is a brass coupling holding a main jet inline of the return gas line, what is this for? ive removed it no change
plugs look uniform in color
seems when its running poorly like its on 1 cylinder
splashing water on motor and b-pipe gives no steam or hissing not over heating
this cylinder to base leak just showed up today please help me as ive wasted all summer messing with this damn thing and dont want to sell it but its wasting my time

 

[Pro-pulsion]

Sounds like an air leak. Do you have one of those small hobby sized oxy/acetylene torchs that use the handheld sized bottles? If so, open the oxygen only just a bit and apply oxygen around all your gasket surfaces. Wherever the air leak is, the engine will respond. It'll rev up some as soon as the extra oxygen gets introduced to the leak. I found a leaking cylinder base gasket this way just a few weeks ago. Same deal, corrected tons of issues on the guy's ski first, took most of the summer to get the parts and time, got it running but couldn't tune it in, did the oxygen test, center of the gasket between the carbs, the engine revved up on its own.

 

[Sparkplug]

Most ase techs use propane. They do in the shops I work in. Oxygen can saturate things alot easier. Osha didn't approve.. While counter intuitive, propane is much safer than oxygen.

Sent from my SM-G970U using Tapatalk

 

[Pro-pulsion]

Problem with LPG is that the fumes are heavier than air and will concentrate lower in the engine compartment. If for any reason like a bad battery connection began to arc you have yourself a possible real catastrophe on your hands. Oxygen isn't going to do that, it rises quickly, holds no immediate ignitable properties like accumulated LPG vapors and gets into leaking spots very easily. Plus it leans out the fuel charge giving a quick response from the engine on where the leak has formed. This is just my combination of experience and concern with using each. I've had fuel vapors accumulate and take flame before on a concrete floor with lots of space, and watched my father blow the lid off of the BBQ one too many times from leaving the propane on first then using the igniter button lol