| Part 1
Question Engine type: Outboard gas
motor model: mercury
Serial number: 1992 ser# not legible
Configured: single
boat model: 18' sea ray Laguna
Boat location: Fla., salt water
repairs done:
> compression test-- 130lbs all 4 cyls
> spark test 1/2" air gap all 4 cyls
Comments:
Motor runs on top two cyls, not on the bottom 2
cyls. Pulled bottom carb, spotless. Nothing but fuel and all passages are open. With air
cover off and engine running, I can get the bottom two cyls to fire but only momentarily
when I choke the carbs out with a rag. Is there anything that the bottom two cylinders
share that could cause this? Possibly a vacuum leak? Can you think of anything I am
overlooking?
Part 1 Answer
There are several 4 cylinder models of this year
bracket that have slightly different fuel delivery and carburetion set ups. The 40 and 50
hp have an integral fuel pump on the carb while others do not.
I will pre-suppose that you do not have an engine
with WME carbs. Often WME carbureted engines do not idle on the bottom cylinders but only
supply enough fuel, for ignition, at about 1000 rpm and above.
Run the engine then shut it off and carefully remove
the carb, again. This time check to see what the fuel level is in the float bowl.
Depending on the carb and actual model of the engine, the float bowl should be at least
half full. If it's not check and/or replace the needle/n/seat and the float. Then readjust
float level and float drop. Also, take a small flash light and check the reed valves
behind the carb. One could be stuck open and/or partially broken off.
This should get you started. Let me know if you need
more.
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Part 2 Question
The engine is a 115hp Merc o/b. Did mercury design
the engine to run on 2 cyls below 1000 rpm? If so, how are the controlling this?
I also don't know what WME carbs are? The 115 has 4 carbs and feeds fuel from
the top cyl down to the bottom cyl with a "T" in the fuel line between the
two center carbs that operates a valve that is cammed when the throttle is advanced.
Could I be trouble shooting something that is
designed to run on 2 cyls?
Part 2 Answer
The WME is stamped into the side of the carb and is
followed by a number. Each carb is designed and calibrated to be used on the cylinder it
was installed with, at the factory. Be sure to install these carbs back on the cylinder
they came off of. If you think there has been a mix up, check the number after the WME and
they should be in sequence from top to bottom.
This engine does not idle on all cylinders. The idle
mixture on the bottom carbs are calibrated to supply enough fuel to lube the engine but
not enough fuel to fire the cylinder. Put the engine on a Dyno or load the engine by
putting it in the water and putting it in gear. Raise the idle above 1800 rpm and then
check to see if it is firing on all cylinders. It should...
The valve you mentioned is most likely the oil pump
that meters the oil mix in the fuel.
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