Well Mounted Outboard Air Starvation?

[Beakey]

I have a Sadler 25 powered by a well mounted yamaha outboard. The well has good ventilation (vent into the cockpit at the front and a vent at the stern) and the original high thrust 9.9hp (it's relief exhaust was "plumbed in" to the stern and the fully enclosed well was fume free) ran well apart (from an intermittent electrical problem) and had no problems with air supply. I have replaced the 9.9hp with a nearly new 8hp high thrust (and again this has it's relief exhaust plumbed to the stern). This new engine runs fine and provides plenty of push when moored to the marina pontoons. However when moving at cruising speed of approx 5 knots the engine appears to bog down and loose power. Fuel is OK and it will run at slow boat speeds all day, so I am beginning to think air supply is the problem.
Any ideas as to why all is OK when stationary and yet when moving at the same throttle setting the engine seems to be air starved?
Do you think it is air starvation and any ideas for a simple solution?

Thanks

 

[davidpbo]

If it runs well and provides plenty of push on the pontoon I question whether ventilation is the problem. Does exhaust also exit through the centre of the propeller?

I have an 8HP Mercury in a side locker with lid but open side to the cockpit and an 8mm (I think) copper pipe exhaust through the transom. If I start when moored and leave it runnning with the lid down the compartment fills with fumes after a short stime. When moving the locker stays clear.

I would consider crud in the carb, blocked jet, incorrect mixture, dirty fuel, kinked fuel delivery pipe if from remote tank.

If you think it is air starvation try fanning some air through to the engine with a view to fitting a 12V fan if that cures it.

 

[SAMYL]

Is the engine/stern of boat digging in when under power? An 8hp engine on a 25 ft boat should not really dip the stern too much.
If the engine, when under power, is sitting too low in the water the back pressure in the exhaust may be too great to allow it to run properly. Take the exhaust plumbing off and see what happens, that may be causing too much back pressure as well.
Are you sure you have the correct prop fitted?

 

[VicS]

I remember Beakey from way back when Tony Cortazzi kept her at Paynes yard.

I cannot beleive its an air supply problem if you still have the old vents .

The usual problem with outboards in wells is exhaust fumes getting in and stiffling the engine but you seem to have that all properly sorted.

If there is no restriction to the relief outlet back pressure should not be a problem, worth checking on that though.

I forget exactly what the engine enclosure is like. If you open it up then that should allow plenty of air in.

Make sure there are no exhaust leaks , from a defective gasket etc, that could be leaking fumes under the engine cowl . Take that off and see if it makes any difference.

I would check the fuel system, presumably there is a filter, so check that. Check the tank is venting and also check the fuel line for possible air ingress esp at the connector. Does pumping the fuel line bulb help at all?

 

[Beakey]

Beakey's outboard problems

Thanks for reply VicS.
Beakey still going well and now based in Whitby
Well system was beautifully engineered by previous owners and there is a new separate CAV fuel filter water separator between the tank and engine

Will check for exhaust leaks and fuel at connectors

Still mystified as to why all OK when moored but problems occur when moving unless as SAMYL says the outboard leg is ending up too deep in the water when stern goes down under power

 

[Lakesailor]

Try taking the CAV filter out of the line. Just plug the fuel pipe straight onto the motor.