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.