Carburettor rebuild problem 2.5hp Mariner 2stroke

[skyflyer]

I bought an OEM carb rebuild kit comprising a new float, inlet fitting with inlet needle valve and float spring lever, and a new throttle needle. It did not come with a new main jet.

I had bought the kit principally because the existing inlet needle valve had warn to a stub and the carb was leaking fuel!

Having done the rebuild the symptoms I got were that the engine would only start on choke (it used to start without choke throughout the summer season) and wouldn’t run without choke until it had been running for several minutes. At that stage it would only run at idle and any attempt to accelerate under load would cause the engine to stall.

I have a spare carb from the 3.5hp version of the same engine (Mariner 2.5) so I put the new float and inlet valve into that and the engine runs ok. But that 3.5 was never great and it still isn’t so I’d like to get the 2.5 up and running.

The throttle needle valve has a tiny circlip that can fit into any one of four grooves to set it at a different height within the throttle piston, which I guess is designed to compensate for wear. Does anyone know where this should be positioned?

It strikes me that the needle and jet will have worn together and perhaps replacing the needle without replacing the jet at the same time might be the problem.

Also I have no idea how to adjust the fuel/air adjustment screw to get better running. How do you know hen it is correct?

Any links to setting up this carb would be helpful
Thanks

 

[vyv_cox]

I don't think the four slots are to compensate for wear, seems more likely they are for different variants of the engine or carburettor. I cannot give you the answer you want but I would try each position if you do not have a manual.

Usually the adjuster screw is only for tickover. Screw it in until the engine starts to stall, then out a couple of turns.

 

[skyflyer]

Thanks Vyv
Yes I’ve just worked out that the screw isn’t an air inlet it simple impinges on a 45deg chamfer on the throttle valve to move it up or down to adjust the idle speed.
I put the little circlip in the same position as the original which was 2nd down from the blunt end of the needle! But the needle itself seems ever so slightly thicker than the original. I don’t have verniers to hand unfortunately, to check, but there is more resistance where it runs through the throttle valve.
I’m trying to think what is actually happening to cause those symptoms though. Air leak into carb, fuel restriction, float level incorrect,?
The unusual need for choke suggests a fuel restriction I think?

 

[VicS]

I bought an OEM carb rebuild kit ......................................
The throttle needle valve has a tiny circlip that can fit into any one of four grooves to set it at a different height within the throttle piston, which I guess is designed to compensate for wear. Does anyone know where this should be positioned?


Also I have no idea how to adjust the fuel/air adjustment screw to get better running. How do you know hen it is correct?

Any links to setting up this carb would be helpful
Thanks

I don't think the four slots are to compensate for wear, seems more likely they are for different variants of the engine or carburettor. I cannot give you the answer you want but I would try each position if you do not have a manual.

Usually the adjuster screw is only for tickover. Screw it in until the engine starts to stall, then out a couple of turns.

The E-clip and the grooves in the needle are the mixture adjustment

See the service manual on the Boatinfo website

http://www.boatinfo.no/lib/mercury/manuals/22-25-3-33.html#/34 should take you straight to the start of the relevant relevant pages.

 

[skyflyer]

The E-clip and the grooves in the needle are the mixture adjustment

See the service manual on the Boatinfo website

http://www.boatinfo.no/lib/mercury/manuals/22-25-3-33.html#/34 should take you straight to the start of the relevant relevant pages.

BRILLIANT! Thanks. Just what I needed!

Of particular note is the different jet and float height settings required for models with a plain magneto ignition and those with a capacitor discharge ignition system. My older (3.5hp) was a magneto and this one, the 2.5hp is a CD. So it may well be a combination of the float height and mixture setting on the needle.

I will experiment in next few days.