49 8N Manifold issue?

Been awhile... She just always runs. Plowed garden just fine 2 weeks ago, then suddenly had lack of power running bush-hog which progressed to unusable in about a 2 hour window. Rough low idle and
occasional back fires. Definitely heard and felt an exhaust leak at the manifold on front and rear cylinders. Pulled manifold today and found broken gaskets at front and rear. Valves seem to be opening and
closing by equal amounts, front 3 plugs nice and tan. Rear isn't great, but I know that cylinder has lower compression than the others and usually has a dirtier plug.

Question: I tried to block the manifold intake ports as best I could while blowing into the carb port. It seemed like I was moving air. The two inner exhaust ports share manifold housing with the intake ports.
Do these ever develop pin-hole leaks? Seems like I might be getting some exhaust gases into the intakes, thus the lack of power.

Thanks as always...
 
Yes, hold your hands over the intake ports on the manifold and pour water from a hydrant into the carb inlet. You should not get any water out of the exhaust ports.

Had a NEW manifold with internal problems, leaking between the internal common areas.
 
Look at the machined surface of #4 exhaust port, nearest the down pointing manifold outlet where the pipe connects to. Is there a 'track' or 'groove' worn on it? That is the area that usually fails first as it gets the hottest. Guess what? It is probably time to replace with a new manifold. Forget about trying to apply temporary band-aid fixes like multiple gaskets and Permatex. I would just replace with new at this point since the manifold is already off and why not do the job right the first time? New manifold kits usually come with the two paper mounting gaskets and the carb gasket. It's possible the exhaust leaks can cause some lack of power, I'd fix the manifold first then observe if problem still exist afterwards and if so proceed to move onto other areas like fuel flow and spark. Check plug wiring. Check all wiring as well. Clean, bright, and tight.

Tim *PloughNman* Daley(MI)
 

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