Blow some compressed air in the #3 spark plug hole and see where the air is leaking out. Both valves need to be closed. I have a compression gauge which has a short removable hose with spark plug threads on one end and a male quick disconnect fitting on the other, which fits my air compressor hose. I use a regulator between the air hose and the short hose to manage how much air to deliver. Too much air pressure will turn the engine over.
It will either be coming out of the
carburetor. intake valve is leaking. Pull the valve cover and hit the rocker arm right above the valve stem with a dead blow or rubber mallet and listen for a resounding clack when the valve hits the seat on the rebound. (hit one of the closed valves of a good cylinder so you know what a good one is supposed to sound like.
the exhaust. exhaust valve is leaking. Same as above.
the crankcase. broken or gummed up rings. If the valves aren't leaking from the valves, you can try adding 2 quarts of diesel to your oil and run it at higher than idle (by pulling out the throttle cable) for 20 minutes. Do this in the driveway. Do not drive the truck! Your crank case will be 2 quarts overfilled which will allow the crankshaft journals to sling that solution around like a carwash.
Even if it does not solve a stuck ring problem, when you are done, at least you have a squeaky clean crankcase to deal with when you tear the engine down.
(I stole a good bit of this material from HRL, so you know that its the right thing to do.
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