Chevrolet Repair: GM 350 Inboard engine., gm 350, inboard engine

Question
Was driving boat when it overheated.  I let it cool for a while but would spin it over every now and then.  After being towed to the dock, it cranked and ran ok to load on trailer.  I replaced water pump and cranked it today.  It seems to be cooling properly but I have a miss and a tapping when I go above idle.  I figure it is either a stuck valve, bent push rod or something worse. I tried to run some transmission fluid through carburetor. Any suggestions for an easy fix?  By the way it is a 1995 Chapparal boat with carburetor an about 300 hours.

Answer
Hi Randy,

Cool boat!

I hate to say hook up a vacuum guage and run it, because if it is a push rod, or worse, you could make it worse yet.  However if you feel the courage to try it, check the vacuum and see if the needle jumps.  If so, it is probably a valve issue.  When these motors get too hot, a vavle guide actually can slip out and jam a valve open.  

Do you have tools and basic mechanical knowledge?  Do you own a compression tester?  Do you know how to use one?  You can get one on a loan a tool program at a local parts store.  Take the readings on each cylinder.  You may find a warped valve, rod, or other thing.  

How easy does it start?  Easy to start and idle but no acceleration?  Could also be a problem in the exhaust manifold.  You have a internal jacket inside the manifolds that cool the exhaust and that could have cracked.  This will cause hydrolock, or the affect of a plugged exhaust.  If this is the case the plugs will be wet (water), or the vac guage will drop to zero when you accelerate from idle.  

On your outdrive, you have an impeller that pulls the water from the river and puts it in the cooling system.  From there the water pump takes over and cools the motor.  This part in the outdrive is called the cooling pump.  Not hard to replace.  Usually $50.  Make sure this part is still good.  I had this happen on my boat once and the impeller from this pump disintegrated and lodged itself into the journals of my motor.  I didn't get the antifreeze into the motor good enough and my motor cracked in the winter.  This impeller is a cheap insurance for your motor.  

Good luck.