1970 Johnson Outboard won't stay running

I recently purchased a boat with a Johnson 1970 60hp outboard motor. The boat was sitting for 3 to 4 years and I took it out and ran it on the water for a hour or two and the boat stalled and would not run. I replaced the fuel pump all lines and took apart all 3 carbs and cleaned them. No sign of anything being gummed up. I also replaced the plugs and ran the boat off my neighbors tank and hose still no luck. It seems as when I run it at home its fine as soon as I go put it in water it won't run under a load. Seems also like its getting back pressure in the fuel line when I take one off after it stalls it has a lot of pressure in it.

What are the psi compression readings of each individual cylinder?

Does the spark on all cylinders jump a 3/8" gap with a strong blue lightning like flame... a real SNAP!?

What spark plugs are you using? Gap setting?

That is the only thing I have not done ,compression test is not something I can do . I have not tried to see what kind of spark I get. I am using Champion spark plugs I will recheck the gap tomorrow and check the spark. Thanks

Compression gauges are cheap and available just about everywhere. Without knowing what the compression is, troubleshooting is a guessing game.

What he said. Get a compression test. You can buy the gauge for $40ish at places like Harbor Freight. Or, most chain type auto parts stores will loan you one.

That said, my old Johnson had similar symptoms. Would die under load, and sometimes, after warming up. In my case, it was losing spark. Turned out to be the pulse pack, and it was going to cost more to replace than the engine was worth.

Harbor Freight has a nice 8 piece compression gauge set for $21.99. Some automotive parts stores have a single gauge (not a set) for much less.

Did a compression test each cylinder was rite around 145 to 150. It has good spark.Also the plugs are gapped at .030 .
Do the carbs maybe need rebuilt?

Good spark doesn't really define it.... was it jumping a 3/8" gap with a strong blue lightning like flame? SNAP!!??

If so, yes, it sounds like the carburetors are fouled.

yes it was a good blue spark. if the carbs are fouled what needs done then ?