So, just how does the Mercab work

[halcrimm]

Good morning all,

Mercruiser 3.0L Alpha 1 Gen 2 I/O.
S/N OD714746 Engine
S/N OD671270 Transom
S/N OD665768 Drive

Back with another newbie question. Still working on getting the engine to not stall on low rpm, no wake speed.
I was on the boat (in the yard) tooling around yesterday and trying to figure out how the carb really works. I know I need to set idle speed so I can go out and set the final idle. Anyway, to the point, I took the flame arrester off to have a look see inside and STILL realize I am not sure what really happens in there. What should be moving and what doesn't? With the engine not running, I can move my throttle forward and backward but the flapper inside never moves. Is this something that moves only when the engine is running. I would love to understand how the carb works. I looked in stickies but didn't see anything.

Thanks for any help.

 

[Thunderchicken68]

Re: So, just how does the Mercab work

The top "flapper" is the choke. It generally doesn't move when you manipulate the throttle. At the very bottom of the Carb is another set of gates or flaps. This is the throttle body, and moves directly with the linkage. The fuel enters the bowl and is controlled by a needle valve and float assembly that can only be adjusted with the carb disassembled. At idle, the engine vacuum sucks fuel into the throttle body via the idle needle valve. When you move the linkage, the accelerator pump (inside the bowl) pumps additional fuel through the jets, and into the throttle body. At the same time, the gates in the throttle body open a bit to mix air with this additional fuel. As the engine RPM Increases, the vacuum Decreases. This allows the Power Piston (inside the bowl, hanging down from the top) to slide down and open the Power Valve (screwed into the bottom of the bowl).
Most of this cannot be seen, and appears to work by magic. with the engine warm, set the idle speed, then adjust the idle screw. once that is done. it will be easier to diagnose the issue. could be a weak accelerator pump, not giving that extra squirt. It could be a sticky Power Valve or Piston, not opening soon enough. It might be dirty or damaged idle screw, or just dirty jets. The Bad news is that most of these things require a rebuild to fix. The Good news is that this is a a great carb to learn on. The kit should be less than $30, and an amateur can do his first one in 3-4 hours. Once you have done a few, much quicker. Mikes carburetors has a YouTube video on it that is very good.
Take LOTS of pictures before you move, unhook, or disassemble anything.
Don't disassemble anything if you don't have the parts on hand, and the time to finish it.

 

[achris]

Re: So, just how does the Mercab work

Hopefully this will help you...

Chris....

 

[mikecrof]

Re: So, just how does the Mercab work

I recently found United Delco Bulletin 9D-3 online in a PDF format, dated Aug 1964. Although it is a Rochester, it has an excellent primer on the operation of these carburators. I have attached the pertinent pages, and would be happy to upload the whole PDF if someone can tell me how to or if it can get stickied by the admins.

 

[mikecrof]

Re: So, just how does the Mercab work

Additional pages...

 

[81 Checkmate]

Re: So, just how does the Mercab work

Sounds like a carb rebuild time......Just did mine for the first time.

Down load the merc rebuild manual....and it was easy as buttering bread. Step by step instructions.

Good Luck!