Outboard doesn't like the sea :-)

Weydagger · 09 January 2010, 14:32

happy new year to you all.
Here is hoping someone can point out the error of my ways for me, please.
I have a Johnson 30 2 stroke, which seems to have taken a sudden dislike to going in the sea.
I can run it up on muffs, with or without prop engaged, at various speed settings and it runs just fine. As soon as I drop the outboard in the water, it will not run. If I up the fast idle, it will start in the water, but won't keep going. When I remove the outboard from the water and put it back on Muffs, it runs fine.

I have reseated the jetin the carb and cleaned it, but still no difference when it goes in the water.

Any ideas, suggestions will really be appreciated.

Many thanks

Paul

m chappelow · 09 January 2010, 15:30

try setting it up in the water or in a barrell or a tank ,it could be that there is not enough back pressure on the exhaust ,i have seen it happen loads of times where people have set up the engine on the hose put it in the water and then the engine wont start or run right , especially with some older models ,have you checked the reed valves at the back of the carbs where it fits into the enigine,or any gaskets that may be passing /drawing air

PeterR · 09 January 2010, 15:46

Hi,I,m no engineer but a little more info would help.Does the difference in running happen imeadiately its in the water.Is the outboard at the same angle each in and out of water.Say the boat is on the sloping slipway on muffs the outlet for fuel is covered ,when in the water the boats level so the fuel levl is not picking up.Check for fuel line kink when in the water.It will be something simple unless its to do with backpressureor or dont know if theres any conection with crank oil seals.You need to determin what differences between in and out and list them .If the outboard is trimmed up on the bracket when out and runs fine lower it to the same position that it would be in the water and check it then.

Nos4r2 · 09 January 2010, 15:46

Check the timing. Also compression test it and post the results.

9D280 · 11 January 2010, 09:02

Two words - Back Pressure.

With a 2-0 stroke, you don't get the flywheel driven "piston pump" effect that you get on a 4- stroke to expel the exhaust, you are relying on the power form the other piston to keep it all going& loop charge / discharge the one that is exhausting.

The idle settings are somewhat delicate, and a small change in back pressure (i.e your exhaust being a foot or so below water level) will be enough to prevent quite as much exhaust escaping, therefore the cyl fills up with oxygen starved air, and so it stalls.

Or, simply, set your idle mix when in the water.

Weydagger · 11 January 2010, 11:53

Many thanks for the replies, it has given me something to look at next weekend.
The exhaust back pressure sounds a good one, so will have a go at adjusting the fuel mixture, but will probably do it whilst outboard is in a bin, froze my feet last weekend standing in the water.

Will also do a compression check and see what the results are.

To answer one of the questions, the outboard when ran on flush muffs, was at the same level and attitude as it would be in operation, also the boat was on a level ground.

Is adjusting the mixture the only was of changing back pressure, or are there any other actions that can be taken.

Cheers

Paul.

9D280 · 11 January 2010, 12:40

Maybe should have phrased that a bit clearer......

The back pressure is the result of the water above the exhaust port. You've seen the films where submarines go too deep & get crushed, and divers getting the Benz when they suface too quickly - that's because the deeper you go, the higher the pressure. Admittedly you have to go well deep to crush a sub, but you get the idea.

Now, imagine that on a microscopic scale - on muffs, the exhaust of your outboard is open to the air - absolutely no restriction. Zero pressure (simplistic view, before you lot all start picking holes in this explanation). Put the boat in the water, and there is a depth of water above the exhaust port. Not a lot of water, but that is your back pressure. Not a lot, but it's there.

Your idle setting as you have probably discovered will make the engine rev ridiculously easily when on muffs with not a lot of movement of the adjuster screw. That effect is heavily damped when in the water. All you are doing by adjusting the idle is upping the idle power slightly to allow it enough grunt on one cylinder to push the exhaust out against the water pressure on the other.

For completeness, a bit of info - on some engines, there is a pressure relief valve that means the idle exhaust comes out above the waterline. If you are on muffs, that never opens as the easiest way out for the exhaust fumes is via the unrestricted prop.

Hope that explained it a bit batter.