Manils

Hi

I have a 350 Magnum and I made a compression test on it today. Testresult:

#1 140
#3 155
#5 140
#7 140

#2 130
#4 30
#6 140
#8 130

As you can se number #4 is bad.
What can be wrong??

Please help me to find out whats wrong, where to start

H2Xmark

I would pull the vale cover on that side and check for any valvetrain problems, such as a bent pushrod, broke valvespring are a flat cam lobe, if the valvetrain checks out ok you might be in for a big problem, get a leak down test done on that cylinder to see if you have a burned valve are a dab ring are piston

SS930

Could also be a blown head gasket.

blue thunder

Probably a bent valve but put compressed air is the cylinder to find the leak for sure. It will be obvious with that kind of loss.

BT

Dennis Moore

It, more than likely, is a tulipped exhaust valve. Very common. Remove all of the rocker arms and put a straight edge across all of the valve stem tips. I would bet that the exhaust valve stem tip is higher on the cylinder with the low compression. Not a disaster, you will probably only need a new valve and a valve job. Most Mercruiser mechanics do many of them over the years.
Dennis Moore