I have a 1963 Thompson I am restoring. It came with a 62 Johnson 72 that
runs. After two years, I had it in the water.
When the boat is on land and I have the motor in the "bucket" I can see no
difficulty. But when I took it out on the water, the engine ran fine with
one problem. The access hatch to the carbs had to be open, the boat would
run as if it just was out of time every now and them, but if I shut the
access door, the engine would just lug down and start to quit open the
access door and it would run pretty much ok. I have checked the air intake
opening and filter on the underside and it is fine.

Any suggestions?

Might try posting this to rec.boats.

"Paul Fisher" wrote in message
ink.net...
I have a 1963 Thompson I am restoring. It came with a 62 Johnson 72 that
runs. After two years, I had it in the water.
When the boat is on land and I have the motor in the "bucket" I can see no
difficulty. But when I took it out on the water, the engine ran fine with
one problem. The access hatch to the carbs had to be open, the boat would
run as if it just was out of time every now and them, but if I shut the
access door, the engine would just lug down and start to quit open the
access door and it would run pretty much ok. I have checked the air
intake opening and filter on the underside and it is fine.

Any suggestions?

Get your long handled screwdriver and remove the long back cover below
the hood. On the intermediate casting will be an exhaust relief port.
It should have a rubber accordian hose with a spring inside it to hold
it against the cover you removed. My guess is yours is missing or
holed.- Also- if you have electric shift, or some one replaced your
intermedia housing with one from electric shift, you will see a routing
hole for the wire on the left side (looking from the back) at mid line
just below the parting line gasket ( where the engine sitis on the rest
of the motor)-If all that is good, run the motor in your tub and pour
some soapy water down the face of the exhaust cover plate ( between the
cylinder heads.) Spray bottle works too.--- Yet there's more--If none
of these thing show an exhaust leak, you have a bad gasket below the
power head. It takes a crane and engine hoist ring to remove the
engine. You might want to call in some help- Hope it's something
simple.