Mercarb Marine Carburetor

We carry two different kits for the Mercarb carburetor. You will have to pop the top to inspect the accelerator pump to determine which pump you have. You want to know which one you have before ordering your carburetor kit unless you have the carburetor number. These carburetors are switched back and forth by the boat repair people, so don’t assume you have the original carburetor.

One has a straight shaft accelerator pump.

Mercarb Pump

The other Mercarb kit has a 90 bend at the top of the accelerator pump.

Mercarb Accelerator Pump

How to Identify your Mercarb

Your Carburetor


Soak your carburetor parts in a
gallon of carburetor cleaner (brand doesn’t matter), any parts store
brand. Keep in mind that none of the cleaners will do a great job, but
the existing cleaners is all we have.Soak overnight.Wash with hot water
– remove all chemical.
Blow out all passages with compressed air.
Run thin wire through all passages. Ethanol will leave residue behind
that cleaners cannot remove.
Using soda blasting to get corrosion out of the bottom of the
carburetor is ok, but be sure to get it all out when done, otherwise
you will have clogged passage ways.
Keep WD 40 and like products away from your carburetor. It all reacts
with ethanol.
Watch a video about rebuilding the Mercarb.Part 1 – Teardown

Mercarb Rebuild – Part 2


I just recently rebuilt my carb
using your rebuild kit. I have a Mercarb 2bbl that fits a Mercruiser
3.0L. The carb is a 3310-80750. In your documentation, you state to set
the idle jet at 2 turns turns out. The merc manual says to use 1 1/4
turns. I initially set it up using the 2 turns and the engine fired
right up without any issues. It ran great on a test run but I can’t get
it to idle at the recommended 650-700 rpm. The lowest I could get was
900. I adjusted that jet to the1 1/4 turns but still couldn’t get a low
idle. I triple checked the float levels when reassembling so I am sure
that it is fine. All parts went back together and the whole thing was
cleaned and blown out with air so I don’t think the problem is
internal. I still have to check linkage and choke plate, etc. I hope
that is where the problem lies. What do you recommend for needle jet
setting? 1 1/4 or 2?
The initial setting for the
idle mixture screw is simply the initial
setting. Once the engine is started and warmed up you will need to
adjust the mixture screw further. There are a couple of ways to do it.
The most accurate way is to hook up a vacuum meter at the carburetor
and adjust the idle mixture screw until you get a smooth vacuum. Now
this can only be done when the engine is at idle rpm, which leads me to
the 2nd issue. There are more than one possibility for the engine not
idling down to specifications. In simple terms, there is too much air
getting to theengine.
  • There could be a vacuum leak. Since the carburetor was
    recently removed, I would check the flange gasket to make sure it
    isfitting as it should. A leak here will, a lot of times result in a
    high pitch squeal. Any place where vacuum can escape is suspect, but if
    it idled ok before tackling the carburetor, then you can probably rule
    that out. Check the gasket between the float bowl and the throttle
    body. If the kit had more than one choice for a gasket here, then make
    sure you used the correct gasket. Improperly installed throttle valves
    can also cause an air leak. Assuming you took the throttle plates off
    for cleaning, make sure they are closing all of the way when the
    throttle is closed. You may have to turn the idle screw out to check
    this. Remember, the idle screw is used to open the throttle plates a
    little bit.

Check Ball Question
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.
Mercarb Linkage SetupMercarb Linkage
Mercarb Choke ThermostatMercarb ThermostatMercarb Exploded

Installed item K529 for Mercarb (carb #807504N) this weekend and carb
number and parts fit lead me to believe it was correct one. Everything
cleaned up well but boat won’t start unless held wot (like it’s
flooded). Once started it runs clean at 2000rpm while warming up but
won’t idle smoothly. Choke is open and idle mixture screw was started
at 2 turns out and adjusted both ways from there without making much
difference. The screw was slightly different length than old one but
seemed to bottom out lightly without issue.
With air cleaner off it seemed to dump fuel into engine after shutdown.
Needle and seat were adjusted and I used the spring loaded one.
Installed brass colored spring and check ball under venturi cluster.
Any idea what I am missing to get this thing idling correctly?
I suspect you are getting too much fuel. Here are the items to check:

  • You replaced the float
    the black nitrophyl can be a problem, so be sure it gets replaced.
  • After rebuilding gunk from the gas tank and fuel lines can
    rush into the carburetor getting into the needle & seat – remove
    the top and flush the needle out with silicon spray lubricant.
  • The power valve in the bottom of the bowl must have a good
    gasket, or fuel can leak by the valve. Check for missing gasket, or
  • Too much pressure on the needle when
    adjusting the float may have damaged the viton tip, thus it won’t seal.
  • The float is adjusted to the correct spec. Measure from the
    top to the toe of the float (toe sticks out the furthest).
  • Fuel pump had too much pressure. Check the pressure. This
    is always suspect if you recently replaced the pump.
  • The check ball in the main discharge isn’t sealing. Remove
    the carburetor, fill the bowl with mineral spirits, take the T and the
    spring out. Hold the check ball down with a brass drift punch (gently),
    push the accelerator pump down. You should feel some pressure. Reseal
    by tapping the drift punch a couple of times. Not very hard, or it will
    get stuck.
  • Perhaps you used teflon tape on the fuel inlet fitting.
    Teflon tape could be in the needle & seat. (never use teflon tape).

Followed all instructions
and adjustments…boat can’t get
above 2800 rpm…missing and sounds like its being starved of fuel.
Idles fine…any ideas?


Assuming the electrical and engine are in top condition…..

1st of all, if he carburetor has not been cleaned and rebuilt, I would
start there. Marine carburetor tend to sit around a lot and they get
dirty very fast. To add to misery is the fact that ethanol damages
parts when sitting also.

Using thin wire, make sure all of the passages on the venturi cluster
are clean. Ethanol leaves residue behind.

Make sure the passages under the main jets are not clogged.

Check the fuel pump pressure. It is possible it is crapping out at high