Matt Lang wrote:
Folks,
I posted a few questions a while ago... I have this motor:
http://www.oldmercs.com/196869merc1250.htm
(1969, 125 HP merc) and a 18.5" fiberglass boat (I dont know what
brand).
I finally tried the motor and it runs fine, changed all the spark
plugs, oil and did a compression test. Comprsssion is as follows:
cyl 1-5: 124-127 PSI
cyl 6: 121 PSI
I dont know how much compression this should have but it looks pretty
even.
The motos fires up instantly and nothing seems wrong with it ...
I put the boat in the water but it wouldnt go faster than 26 MPH ...
it does plane but wont go faster. One problem was that the throttle
linkage doenst open the carbs all the way, but when the carbs are all
manually opened (by a volunteer 
the boat will max out at 26 MPH.
The motor revs about 3500 rpm.
Now I wonder. How fast should an old (probably heavy) 18.5" boat
roughly go with a 125 HP OB? Is 26 MPH realistic or too slow?
How fast should this kind of a motor spin? I think it should be around
4500 rpm?
The prop is 18 or 19 pitch 3 blade Merc. The boat also comes with a 2
blade (pitch unknown) but it wont even go 10 mph with it...
What kind of pitch seems normal for this motor/boat combination?
Is this a matter of motor problems? Or wrong prop? The pre owner used
it for water skiing ... but I doubt he was skiing @ 26 MPH ... I also
dont think he would have 3 props (one brand new) which have much too
much pitch...
So I wonder if the problem is elsewhere ...
I will bring the boat to a merc dealer but from experience with
automotive dealers i know that you either fix your problems yourself
or they dont get fixed, so i am not too opimistic and am hopeing
anybody here has any ideas.
Matt
You should get your linkage fixed first. Ignition advance is a large
part of "throttle" travel on an outboard, and the carbs are timed to the
stator plate or distributor. Typically, the ignition is fully advanced
on larger outboards by mid-throttle, but on this Merc the ignition
advance should pick up the throttle shafts at about 8 degrees BTDC, and
travel with the carbs all the way to 34 degrees BTDC advanced when the
carbs will be wide open.
The engine has to be "running" to do this test with a timing light,
which is hard to do while moving & on the water, so the test must be
done while cranking instead, after the fuel has been run out of the
engine so it does not start.
Just flipping the carbs open at partial advance will not give you full
output.
Also, specs call for "no more than 15 psi variation between cylinders"
on this family of engines. You're in good shape here.
Rob