TOPIC: Compression Readings

I wonder if anyone can define what should be typical compression numbers for a 1964 Mercury 650, 65 HP, 4 cylinder? I have the factory service manual for this motor and it does not define a precise number, only stating that all the numbers should be close in value. The values measured on my motor were: 120, 120, 120, 125.

I have read occasional posts in forums where numbers as high as 150 were stated. I am wondering if my numbers are slightly low and the motor is down on horsepower as a result. The shop that recently refreshed the motor indicated that it ran fine but they had no way of measuring the power output.

I have not used the boat for some time but noted on some of the later outings that the boat did not pick up onto a plane as quickly as it did when it was newer.

Thoughts and comments are welcome.

Kern Fischer

you are fine about top notch for your motor.

Keep in mind, readings vary from gauge to gauge - that's why there are no published compression readings in any of the Merc manuals. The key is that your gauge be accurate to itself, .ie gives consistent readings. The fact that your numbers run fairly even across the board shows your gauge is likely good, as is your motor...

Slow pick-up can be related to the motor not running on all cylinders, the boat being waterlogged, the propeller being damaged or incorrect for the application, misadjusted throttle cable, link-and-synch being out of whack, etc.

A good start would be to get a tach reading for your max RPM...

Never been a big fan of compression tests. I do them of course but a better check is a leakdown test, however it can be tough to do the lower cylinders on an inline. Case in point is the 800/850 Mercs. The compression on these will usually show up just fine, but when you open them up the bores are worn like ice cream cones. Leakbown test usually spots this however, best to do both.

Compression tests do have value and consistancy across the cyls is most meaningful.
When evaluating used Mercs I generally pull the transfer (intake) port covers off. These covers never have corrosion issues with breaking bolts etc. and new gaskets for the covers are cheap. Removing these covers and rotating the crank allows you see the ring condition on each piston and check them for "springing" as described in the manual under the rebuilding section. You also get a good idea of piston skirt condition and you can even look into the ports and see at least part of the opposite side cyl wall.
If comp is decent and this inspection looks good you are pretty much assured of good motor health.
My two cents,
Randy

..K .: I have a few of these four cyl Mercs of the mid 60's , early 70 vintage.. As well as 58 ,four cyl..The 120 #'s as stated above are good enough to confirm there is not one bad cyl / etc.. This is all mechanical condition....... 2) you mentioned service shop check,
and lengthy storage..The most likely decrease in power would be the electrical spark topic.. Your year should still have the POINTS
mag TYPE I , I believe.. Any dirty contact on the ends of rotor / etc would be a cause, likely a weak coil over the years. rETRO COIL CAN BE EXPENSIVE, so only if really needed.. Possible to upgrade to the Type II ignition if you want to keep this motor (?)
Maybe a bad / weak spark plug during running, corroded wire tip .
With a good shop tachometer & running, pull off one wire at a time & should notice a DECREASE in RPM reading.. If one of the wire removal has NO effect on RPM, then that plug or wire to change........ Secondly a slight FUEL system clog ? dirty tar from
a winter storage can be sluggish performance.. Over time the Thin fabric fuel pump DIAPRAM gets hard, very easy to change under the small three screws on triangle / ovel shape housing. (Your motor 65 hp may have TWO) fuel pumps, one for each carb. New diaphrams are easy insurance for good running.

those numbers sound good. Crossflow 2 cycles generally don't have high cranking compression. To add on what others have said - how old was the gasoline in the tank during these lackluster runs? Here in CA the State serves up two brews of fuel - Winter and Summer. The Summer fuel is oxygenated more, has more corn in it, and has less "bang" to it. The winter fuel is much better. Fuel doesn't keep well, and Mercs seem to be pretty unhappy with stale gasoline. Did you cahnge oil brands? Did you make the mixtures the same each time? There can be all sorts of pathways to go down.