Cignificant

I have a mid-70's Johnson 50HP that I've put on a boat that my son and I built. It starts fine, and idles fine. The problem is that after the engine warms up it will not rev up while under load. Right after launching the boat, I can get it on plane but as the engine warms up I can hear it reving down. In nuetral, I can can disconnect the throttle cable and rev it up by hand and it sounds fine. The bulb in the fuel line is hard, so I'm sure it's getting fuel. The vent on the fuel tank is open. I've turned off the choke. The plugs are new.

Any body have any ideas what could be causing this?

Offshore Addiction

you got to pull the carbs and clean the circuits in it...if the primary circuits are dirty the carbs wont run.....find a merc dealer and get some powertune,let the carbs soak clean with it and theres a small idle circuit you have to get clean,and then the larger primary jet in the bottom of the bowl.....make sure you get the gaskets gack in,including the small round one in the center of the bowl......bolt them back on and it'll run.....

28SS

Very common for coils to go bad

Audiofn

Mid 70's is that a distributer or coil type engine? I have seen this a lot on the older motors. USUALLY it is the advance timing and the way it works with the throttles. You have to make sure it is going to full advance before it starts to open the throttles. The repair manuals go into the set up of this in detail. If it has coils then make sure that the coils are ok and that the ground wires are clean and attached to the coils and that there is not corrosion underneith. There are ways to clean the ground wire with a razor. If you need me to explain then I can.

Jon

mcollinstn

Power Pack (ignition box)

There are two circuits in the OMC power pack boxes. One for lowspeed operation, one for highspeed operation. Either can fail, and the most common mode of failure is heat breakdown. If the lowspeed portion fails, the motor will refuse to start when hot. If the highspeed portion fails, it will refuse to rev under load when hot.

The high speed portion is usually the part that fails first.

A manual will give you some diagnostic checks to run to verify it, but I'd bet a cheeseburger that a replacement power pack will cure your ills.

M

Audiofn

ooops I assumed Mercury sorry

Offshore Addiction

usually if theyll start and idle good in the water,the ignition is uaually ok,no,or lack of accelleration off idle is 9 outta 10 times carbs......espesially if its been sitting around

riskybusiness

I'm gonna go with the carburetor people on this one. Chances are if it's running at all, ignition is probably fine.

You need to make sure, however, that the ignition is being properly advanced as the throttle opens wide. (revving it up in neutral means absolutely nothing). This is done by the linkages connected to the throttle arm/cam. They should be rotating the stator (which is under the flywheel) as the throttle opens. If this is the case (check without the engine running), then the carbs should be disassembled and cleaned.

If the engine has been sitting for any length of time, this should be done in conjunction with a new fuel pump diaphragm (a really easy installation).

Is the primer bulb firm under heavy load? Try giving it a good squeeze at WOT.

Cignificant

The primer bulb is fine under load - it stays firm. And I did try sqeezing it while running the engine.

The linkages work fine.

I'll buy a manual and start working on the carbs and power pack. Thanks for all the help!

riskybusiness

One thing I neglected to mention (and it's the most important thing) is to be sure you have good compression (you probably knew that already, but it is commonly overlooked in troubleshooting).

Both cylinders (it is a two cylinder, right?) should have at least 90 lbs, and should be within 10 to 20 lbs of each other.

Compression is always the very first item to check. If you do not have a compression guage, you can sometimes get away with just cranking it over (with plug wires disconnected), and try to get a "feel" for whether or not both cylinders produce the same amount of resistence.

When all else fails, get yourself a service manual from either an OMC dealer, or an "off-the-shelf" book from Clymers or something. They usually have troubleshooting flow charts.

Good luck.