Chev 350 in marine use. Made horiffic exhaust noise, especially with increasing throttle. Cam seems fine still, but the rocker on #5 exhaust was somewhat off to the side but still touching the side of the valve stem...but stem bent. What would cause this and is this the cause of the loud combustion sound (like a spark plug out of it's hole?
You have several problems here! First of all lets dispense with the marine part, -- which is mostly cooling system conversion! A bent valve -- probably reason for popping, - it is most likely bent inside from striking (being struck) by piston with some possible small piece of metal between top and valve, -- this could be a simple as a piece of broken piston ring, or maybe a screw or nut that has fallen down in intake system some time,- and just now sucked into combustion chamber! Clearance between piston and head not really large when piston all the way up! In order to repair, you need to remove the head and have valve taken out, -- which will probaly require new valve guide replacement! As for rocker arm, it may or may not be involved in actual origional problem! I would have head all checked over to make sure that it is ok,-- as foreign objects inside engine can ruin head also! Now if this is a high time engine, It might be a good idea to pull other head off also, as you have to remove stuff on top and intake manifold to get other one off again, -- (if it has problems later)! The loud noises may have been piston still banging on valve, -or exhaust gasses popping out when ingniton occoured because of valve not seating, -- you could also have bent intake valve, just not as bad, -- and gas popping back out through intake system also!
make sure you dont have any of the spark plug wires crossed (like 5 7). If you do, you'll get a backfire out of the exhaust or carb.
that would be caused by high rpm with no load, had it happen in my car years ago, well before i acquired it. vacuum modulator line cam off, car was driven home for about 15 miles at normal speeds (OUCH, 45 in first gear in a 68 gto) bent 4 rods and some other damage that i do not remember.
Do a compression check to be sure. Usually an exhaust valve will just allow to escape into exhaust. you may have other things going on, like an intake valve also. Do pressurized compression check to trace it.
do a compression check on #5 cyl. also do a cylinder leakage test. You can do this by taking the schraeder valve out of the end of the hose for the compression gauge. then apply compressed air into the cylinder with the piston at TCD. If the exhaust valve is bent and not sealing the air will escape out the exhaust. Sounds like maybe the valve stem to valve guide clearance is too tight and the valve hung open in the guide and smacked the piston. Also with the compressed air in the cylinder you can remove the valve spring and rotate the valve and slide it up and down in the guide to check for binding in the valve guide. Either way it kind of sounds like you'll have to pull the head to replace the bent valve.
