Mercury 8 HP CC 1988 2 Stroke problems. Long post.

[davidpbo]

Mercury Ouboard 8 Hp cc. 1988/9 2 Stroke

Is anyone very familiar with them?

I have had this motor 12 years and it is generally pretty good. Once it is started each sailing session it usually starts pretty much first pull with the choke out. The choke then has to be pushed in immediately.

The problem I have is that the knob which controls the idle speed on the outside of the case usually has to be set quite high other wise the engine cuts out, particularly if I have been running at high revs for a while. This means the engine is revving quite high when I go from forward to neutral, neutral to reverse etc. which crunches the gears a little.

There is no additional idle adjustment on the carburettor as far as I know other than the mixture screw at the rear above the air intake. The cam which activates the throttle flap does not appear to be adjustable.

The carburettor has several chambers with jets/vents between the, and despite having a manual I am not conversant with what they do. One is obviously to do with the priming pump but is there a separate idle jet maybe preset?

I feel the needle valve or seat may be damaged as I think I have to have it turned in further than I used to. To set the carb up I used to close it fully (but lightly) open it about 1.5 turns and then start the engine and adjust the setting to midway between too rich and too lean tending towards too rich but the amount it has to be opened seems to be less now.

If I have damaged the valve seat is it replaceable? I presume the needle valve is.

When the engine loads up when put into gear what is that an indication of too rich or too lean?

Might it be the fuel pump? Maybe not working hard enough at low revs if it was I would expect there to be a delay as the bowl empties and there is not.

How does the fuel pump work? I believe it to be on the side of the carb with no mechanical linkage to the engine to activate but might be wrong and unfortunately left the manual on the boat.

The engine resides in a well with a small bit of copper pipe (about 8mm dia) connecting underneath it through the transom as a breather pipe, what is this supposed to do? A bit of exhaust does seem to come out of it but the majority does go down the leg. Might it be worth putting in a bigger pipe or at least cleaning this one?

I suppose after 12 years this may be clogged is it

I wonder whether it might be timing, having a look at that whilst the engine is mounted would not be easy, but I might give it go, I don’t want to have to have to get the engine off at the moment.

 

[VicS]

Not sure what you are expecting to look at in connection with the timing.

The copper pipe I guess is intended to lead the gases escaping from the relief port on the leg overboard so that they don't build up in the well and suffocate the engine.

I think you will find that the fuel pump is operated by pressure pulses from the crankcase. Sometimes there is an external hose connection but yours I think is probably direct through drilled passages. If the fuel pump is not pumping properly you will be able to tell by pumping the fuel line bulb. That will restore normal operation.

Does your carb look anything like the one in this diagram http://www.boats.net/parts/search/M...C5A WMC5B WMC7 WMC7A WMC19 WMC19A)/parts.html
Possibly not if your engine is European made.

If it does then I think you will find #31 is the throttle stop screw.

The idle mixture needle ( and seat??) and numerous other small parts are included in the repair kit

This one has primer pump rather than a flap type of choke. From what you say I think yours probably has strangler flap?


A good place for help and advice might be the Mercury / Mariner board in the iboats forums http://forums.iboats.com/forum.php

If you go there post the full model details including the serial number, however if yours is a European one ( serial number in the range 09503295 to 09546350) they may not be able to help much.


Sorry not much help!

 

[davidpbo]

Thank you for the reply, it is certainly very similar to the diagram shown. But I don't think I have the throttle stop screw, some don't.

Unfortunately I don't have the carb serial number to hand although I will have a copy of the O/B serial number somewhere. I certainly have a primer pump.

Are the pulses you refer to with regard to the fuel pump changes in pressure which act on a diaphragm in the pump?

 

[VicS]

Are the pulses you refer to with regard to the fuel pump changes in pressure which act on a diaphragm in the pump?

Yes

 

[davidpbo]

My carb has a small tube which is what operates to fuel pump I think.

The engine is still misbehaving.

 

[davidpbo]

To follow up.

I got a carburetor refurb. kit which included the fuel pump diaphragm and fitted all the gaskets, new needle valve and mixture screw. Unfortunately that did not improve the situation so eventually I took it to the menders who was stumped but remembered something similar with and engine from years ago, he thought it might be a crankcase seal and after discussion with me took the power head off and found a badly damaged seal. I have not collected the engine yet but am assured it is now running fine.

 

[davidpbo]

We collected the engine last weekend and it seems much better. My only concern is that tick over is a bit slow and the external adjustment knob has little effect. There is no idle adjustment under the cowl.

Is it likely to be mixture?

 

[Rigger Mortice]

The manaual says.....

Push primer knob in and rotate fully counterclockwise.

Then either:
Turn idle speed screw in (clockwise) until it just touches cam follower, then inward an additional half turn to slightly open throttle plate

Or for models without an idle speed screw);

a Loosen cam locking screw
b Press cam follower down until it contact throttle cam
c Retighten cam locking screw.


The mixture setting:
Turn low speed mixture screw in until it seat lightly, then back screw out 1 1/2 turns to 1 3/8 turns

Once that is done there is an idle wire above the throttle cam which may need some fine tuning.

That all assumes it's the same model !!!!

 

[davidpbo]

Resurrecting this old thread. I am still having problems with this engine. Starts Ok, runs fine.

But if I have the idle speed set at a reasonable tick over, after a run at speed, going to neutral and idle it cuts out, which is great when you are coming into the jetty. Any suggestions about what I should look at with regard to jets mixture etc. I am going to try to increase the idle speed.