Crankcase Pressure (84' Mercury 50-HP 4-Cyl 2-Stroke)

I have been trying to resolve an issue were my Motor appears to run fine on muffs and in a barrel, but when I take it to the lake it falls on its face trying to accelerate above "No Wake" Speed. (Not sure of the Exact RPMs) I have been trying to resolve this in the Ignition System yet everything is checking out appears to be within range.

It has been indicated that I am probably loosing "Crankcase Pressure" when the boat is under a load and that I probably need to replace the Lower Main Seal.

Is there any practical way to test "Crankcase Pressure" before having to tear the motor down and hope to see an obvious issue?

I have squirted the engine with a Soapy solution and did not see any bobbles forming, from the visible surfaces of the motor. If the motor were having the same issue with the muffs or in a barrel, then I might expect to see potential leakage from a gasket. However, with the issue only occurring under a load it leads me to believe the issue would be Ignition or along the line of the Crankshaft/Thrust path...

I do not know about you, but I hate throwing Parts at something hoping for a positive outcome.(i.e. Very Expensive and not very productive.) I would rather spend money on something that is proven or very likely to be the case.

Thank you for your time and any recommendation you may be able to provide.

You are over thinking.

Open the idle mixture screws 1/4 to 1/2 turn out (CCW).

Jeff

Pull lower unit and see if exhaust tube blocked with carbon....

Thank you for both of the suggestions. I will give them a try.

Try mine first. Never seen carbon block that tube.

Jeff

Never seen carbon block that tube.

Click to expand...

See quite a few over the years especially motors that used trolling...

I appreciate both suggestions, as I have limited experience with Outboard Engines and all the idiosyncrasies they appear to be susceptible to...

I am however curious about both suggestions and was wondering if either of you would care to elaborate on the followingNo Disrespect intended, Just my thoughts.)

1. Jeff: I understand the concept of leaning the circuit out by turning the Air Screw 1/4 - 1/2 a turn CCW. (Since my main issue is not being able to get the Engine to rev to maximum RPMs or anything above 6 MPH, I was wondering if the Air Screw, on these Carbs., does in fact have an effect over the Entire Throttle Range? I know on Motorcycle Carbs. the Air Screw only has an effect until about 1/4 throttle.) What are your thoughts?

2. Faztbullet: I have seen posts about Carbon Buildup, in the Exhaust, causing problems. (However, I was wondering if this was an always condition including when the engine was placed in a barrel, as my engine performs fine in a barrel with the prop is engaged... Maybe my issue is that I am not submerging the engine deep enough to add additional pressure to the exhaust port.) What do you think?

Thanks again for your input and thoughts on this issue.

Lacking an accelerator pump feature, OB carbs tend to bog when the throttle is snapped opened unless the idle mixture is set correctly. With the advent of ethanol gas, the carbs need to be richer (idle and main jet) to compensate. This simple fix has cured DOZENS of bogging problems.

Jeff

In a barrel with prop engaged it will cavitate allowing prop to lose bite and rpm to rise.