Mercury 115hp motor ??

[tue]

I beleive the 2+4 system (still ) used on the 115 mercs works by restricting the fuel through carbs #3 and #4 at idle and up to 2000rpm. When the engine reaches 2000rpm an accelerator pump dumps a load of fuel in the 2 carbs and from there on the carbs run normally. There are no setable idle jets on carbs #3 or #4 and the accelerator has 2 special outlets for these 2 carbs. I have never heard of a switch in the ignition system cutting out 2 cylinders at idle.

Maybe the conversion involves removing carbs #3 and #4 and the accellerator outlets to these 2 carbs and replacing them with conventional carbs like #1 and #2?

[Pete7]

Oh could be, it was 12 years ago. I bought the Merc Dealers own engine off him but sold it back a couple of months later as it was too heavy on a 5.25m rib and chose a Merc 75 instead. The 75 was much better balanced and used a lot less fuel.

Pete

[rama]

twas actualy uncanilly named a 2 + 2 as it ran on 2 then when you were least expecting/wanted it it would kick the other 2 in!! Merc reconed it would add to fuel economy. The way they work as i recall is they kill the spark to 2 cylinders under a certain rpm, they tried to cut fuel but for a carbed motor you can't realy do it. For most boats which run a single 115 it has to br one of the worst motors out there, they seem to have set the +2 at around the piont where you are just on the plane. Horrible Horrible engines.

[tue]

rama your right it is the 2+2 system. However, it IS worked via the carbs NOT the ignition system. There would be no fuel advantages to throwing fuel into the cylinders but not burn it!! There are production carbs available from Mercury which can reduce the transition range of the second two carbs coming in, but you will still have that abrupt kick in when they do.

[Pete7]

Originally Posted by rama

Horrible Horrible engines.

One of the reasons why I got rid of it rather quickly