Mercarb Accelerator Pump

Posted by Mike on 5/7/2013 to Mercarb

Question from a customer: I rebuilt the carb ( my first one ever) and put it back on the boat (2005 Merc 4.3L). It started right up and ran great out of water. I took the boat out yesterday (Sunday) and again started up great ran great except for one issue. The Accelerator pump is not closing completely when I have boat in idle after running it open and is causing motor to idle to high but if I manually pull the "cable" to idle position it will idle right at the recommended 900 rpm. Is there something I need or can do to fix this? Its like the accelerator pump is hanging up right at the end.
Without it sitting in front of me, I'm at a bit of disadvantage, but let's see if I can give some clues. 1st you need to make sure there isn't something in the throttle linkage causing the problem. Isolate the carburetor by disconnecting the linkage, open the throttle wide open, then let go. Does it return to idle position? If it does, then the problem is obviously in the linkage somewhere. Lets assume it isn't and go from there. Remove the link from the accelerator pump arm. This eliminates any sticking of the throttle plate, or rod problem. Again, we will assume the accelerator pump is still sticking. Here are some possible problems: The 1st thing that comes to mind is this. Did the accelerator pump return spring get put back in the carburetor? This is the spring that resides underneath the pump and it is what returns the pump to the normal position. This is important because it is this return stroke that allows the fuel to fill up the accelerator pump well. The pump could have gotten torn, or rolled up when inserting it, causing it to bind. The pump cup swelled up from being contaminated by ethanol, or other chemical. This happens from time to time. Use of WD-40 would cause this, along with ethanol separating from the gasoline. Ethanol over the 10%-15% mixture can damage rubber parts in a very short amount of time. The top of the accelerator pump arm was inserted into the hole at the top in the wrong direction. This would cause the pump arm to push the pump down at an angle, causing it to bind. As suggested above it is best to isolate parts of the carburetor in order to eliminate parts until you finally get to the culprit, especially when it comes to things like the pump. Action starts inside the pump well with the return spring, the pump arm on the top of the carburetor, the rod that connects the throttle lever, the throttle lever itself, the throttle plates, all the way to the throttle cable. As you can see there are several parts that could cause sticking of the pump.

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