2 bbl Mercarb accelerator pump

[smiles16]

How do you adjust the accelerator pump on the mercarb? It's a 120 with a Mercarb, not a Rochester. I am not getting that nice stream when I pump the throttle, and the engine falls on its face when you give it hard throttle. Give it a little gas and then slam it and it's nice and smooth. Just replaced the base gasket and checked for leaks. Runs great otherwise. Any suggestions?

 

[alldodge]

Have a look here, half way down id the accelerator pump adjustment

http://www.carburetor-parts.com/Mercarb-Marine-Technical_ep_427.html

 

[smiles16]

I don't have any holes on my linkage like that. Does that mean it's just not adjustable?

 

[alldodge]

I don't have any holes on my linkage like that. Does that mean it's just not adjustable?

Would not be as easy, other method would be to bend the road on the pump to get more stroke. Also keep in mind that the accelerator pump bore may be worn or the pump seal may need replacing

 

[smiles16]

If this helps diagnose any, I noticed while I had the linkage free from the throttle cable that the spring feels somewhat weak when I let go of the linkage from the wot position. I can hear/feel the vacuum of the accelerator pump when I push forward though. I am thinking a strong dose of Berymans b-12 through the gas before tearing it apart. Would I hurt anything by doing this? I have about a 20 gallon tank and I would add two or three cans on a full tank and run it for a good while.

 

[alldodge]

I would take it apart and clean it. Putting the cleaner in it may help, and may even fix for now. In most cases it just postpones the issue. It could be the check valve in the bottom which is hanging and not allowing enough fuel to be drawn in. This is a Rochester but the workings are the same. Note the check ball in the bottom of the pump

 

[smiles16]

Hey this is very helpful man thanks. if anyone had any more ideas please share

 

[Fun Times]

I don't have any holes on my linkage like that. Does that mean it's just not adjustable?

Unless the carb has been changed (rebuilt) internally with different size venturi clusters, modified accelerator pump assembly (Spring), check ball size different, the accelerator pump shouldn't really need to be adjusted if all of a sudden you noticed the engine running differently out of the blue with nothing else done... If all of a sudden an issue arose, then that would indicate the carb might need a good cleaning, specifications checked, and possibly a new matching acceleration pump assembly needed. Outside of doing just a general maintenace tune up + fuel contents checked for water/debris, etc. etc..

Should you truly need to adjust the acceleration pump for some reason, then starting in June of 1992, Mercruiser started using an adjustable acceleration pump part number 3302-823725 Lever (With 3 Holes)

Adjustable Accelerator Pump
The replacement accelerator pump lever has three holes in it. This lever will allow you to change the amount of fuel delivered to the engine by the accelerator pump. Most bogging problems can be corrected by changing the rod?s location, providing the bogging problem is being caused by the carburetor. Always test run the boat in the water to see if bog has been corrected.
When installing this lever on an older carburetor, put the rod in the center hole. This will give 0.5 cc less fuel to the engine on acceleration, than the single hole lever did. Remove any metal ball that someone may have put in the accelerator pump well to limit pump travel. Also, make sure that the duration spring on the accelerator pump is stock and has not had several coils cut off. Verify that the venturi cluster is the correct one as called for in service manual. Production started using carburetors with the 3 hole lever in June 1992.
3302-823725 Lever (With 3 Holes)

Mercruiser Service Bulletin 88-5, Overrich Acceleration ?Bog?
on MerCarb Equipped Models MCM 120/140/2.5L/3.0L
Overrich Acceleration ?Bog? on MerCarb Equipped Models MCM 120/140/2.5L/3.0L
There is a new Venturi Cluster P/N 3302-9810 available to correct the ?overrich bog? condition that may be encountered on these model engines equipped with Mercarbs which have the increased capacity (4cc) accelerator pump.
This new Venturi Cluster can only be used on Mercarbs, P/N 1389-9562, equipped with the increased capacity accelerator pump P/N 3322-9655.
Accelerator pumps that are modified in any way from original to correct this ?bogging? condition, must be converted back to original specification and parts before the new cluster venturi is installed. Also, if a 1/4? ball was used under the accelerator pump to restrict the pumps movement, discard that ball after installing the new venturi.
Models equipped with the new cluster have day code 7X01 (Figure 1). Models and starting serial numbers with 7X01 Mercarb are 2.5L, OB923578 and 3.0L, OB923342.

Check Ball Question From mike's carburetor parts link,
The exploded view of the instructions sheet that came with my MerCarb carburetor kit includes a check ball and screen at the bottomof the accelerator pump well. Mine seems to be missing. Not all Mercarbs have the check ball and screen. This type of Mercarb will not have a hole in the bottom of the well to support a check ball,or the screen. What is the check ball size for the Mercarb? Both balls are 5/32". When one is aluminum, put it in the bottom of the accelerator pump. The other ball goes into the main discharge hole.

Your engine service manual can be found online at boatinfo.com under Mercruiser service manuals. You'll want service manual number 3, fuel section 4C which starts on page 499 for the rebuild process of the MerCarb.

 

[Scott06]

If you're not getting a good stream of pump shot start with dissasembly, cleaning, and rebuild including a new accelerator pump. Fun times gives some great info here. I experienced the same on my 1990 3.0, rebuilt the carb ran great except bog on WOT acceleration. Then find out mercruiser changed the accelerator pump to get a larger shot, but I didn't have a 3 hole pump lever. You can bend the rod shorter (larger shot) or longer (less shot), or just buy the 3 hole lever.

With the amount of service bulletins, different Venturi clusters, and accelerator pumps and levers I suspect mercruiser never got this issue 100% resolved.

 

[smiles16]

Okay, this makes perfect sense. I am going to wager that passage is either blocked or the spring is collapsing because I am getting a lean bog. As long as I ease the throttle up into the primaries it runs fine. Is there any risk to the engine by running it for the rest of the season? I will tear into this over the winter for sure.