Bad compression
 

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

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

Could also be a blown head gasket.

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 :cool:

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