MCSB 97-08
Revised November 1997

  MerCarb 2 bbl Carburetor

   8 Point Carburetor Check List

To ensure that the carburetor is the cause of the engine's problem, make the following checks BEFORE ordering and installing a new carburetor. If questions 1, 2, 3, 5, 6 and 7 are YES and 8 is NO, the carburetor may not be the problem.

1. Does the choke close completely (before starting)?           Y      N
2. Does the choke open completely (during engine warm-up)?  Y      N
3. Is idle mixture screw set correctly?                                      Y      N  (Engines with Thunderbolt V ignition must have module locked in base timing mode).
4. What is the idle mixture screw setting?               _______ Turn.
5. Is the engine idle speed (RPM) correct?                        Y      N
6. Is venturi cluster discharging fuel (by 2000 RPM)?        Y      N
7. Is a good stream of fuel being discharged by both discharge holes
in venturi cluster?                                                             Y      N  (When throttle lever is 'pumped' with engine off).

8. Does the engine flood at idle?                                        Y      N

Adjusting Idle Mixture Screw
To adjust the idle mixture screw correctly, the throttle plates must be nearly closed. Please do the following:

Note: Engines with Thunderbolt V ignition systems must have the
      ignition module put into 'base timing' mode before turning
       key to start the engine. After adjustments, shut engine off.
       Remove the module grounding wire, then restart the engine
        to take module out of base timing' mode.

1. Disconnect throttle cable.
2. Set idle speed (RPM) screw so engine idles at 550-600 RPM in neutral gear. 3. Adjust idle mixture screw.
4. Reset the idle speed screw until engine idles at its recommended RPM.
5. Adjust and connect throttle cable.

MerCarb Float Setting

This is the float setting for all MerCarbs.

   Needle/Seat              Float Level                    Float Drop
   Two Piece Solid:         3/8 in. (10 mm)         13/32 in. (27 mm)
   Spring-Loaded (1):       9/16 in. (14 mm)        13/32 in. (27 mm)

   (1) MCM, MIE 5.0L, 5.7L S/N OL000001 and Above:
                                      11/32 in. (9 mm)          15/16 in. (24 mm)

Flooding at idle RPM

If you get an engine that is flooding at idle RPM, please check the following:
1. An ignition system problem causing the engine to run rough.
2. Idle mixture screw adjusted incorrectly.
3. Bad needle and seat.
4. Incorrect float level or drop.
5. On engines with a mechanical fuel pump, check fuel pressure at the carburetor. If fuel pressure is too high, check sight tube from the fuel pump to make sure it is not pinched or blocked.

If all these items have been checked, then check the type of needle and seat installed in the carburetor. Most older MerCarbs used a solid two-piece needle. If the carburetor has a solid two-piece needle and seat in it, install the spring loaded needle and seat kit. Reset float level to correct specifications.

Warning:  Installing a spring loaded needle and seat kit on a
                         4-cylinder engine can cause a lean out condition
                              in extremely hard turns.

  GM 153, 181 CID: Right turn.    Mercury Marine 224 CID: Left turn.
   --------------------------------------        ------------------------------
Because of this potential lean out condition in extremely hard turns, you should make the boat owner aware of this condition before installing the spring loaded needle and seat kit. The spring loaded needle and seat kit is the preferred one to use if you have a flooding problem at idle RPM.

  • 3302-9407  Solid Two Piece Needle and Seat Kit.
  • 3302-9029  Spring Loaded Needle and Seat Kit.

Electric Choke Operation, V8 Engines S/N OF601000-OL000000

On these engines, a PUR/YEL wire connects the electric choke to a terminal on the alternator. The terminal on the alternator will not put out enough voltage to open the choke until the engine exceeds 800 RPM. If an engine is started and left to idle below 800 RPM, the choke may not open. This can cause a rich running condition. Whenever troubleshooting a choke opening problem on one of these engines, make sure that the engine RPM exceeds 800 when first starting.

Tell your customers to exceed 800 RPM for a short time after starting their engine. This will allow the alternator to start charging.

Setting the Electric Choke

When servicing an engine, the choke should be reset to specifications as listed below. Add these choke specifications to all your older service manuals. The old setting were too rich for most engines. Production carburetors on new engines are set at these settings. If needed, the choke can be set slightly richer or leaner to customize it for the operating conditions that the engine will be run in.

3.0 L: 1 index Mark Clockwise.

V6, V8: 2 index Marks Clockwise.


MCM 4.3L 'Surge' or 'Popping' Condition above 4000 RPM,
S/N 0F114600 and Below

If a customer is having this condition, install 1.60 mm main jets; 
         3302-810923     1.60 mm Main Jets, 2 required.

  MCM 4.3L "Surge" at 3000-4000 RPM, S/N 0F293967 and Below

If a boat owner has this condition and the engine serial number is below the one listed, change the venturi cluster. Usually, this cluster change will correct the problem. This is the same cluster that was used in MerCarbs on newer MCM 5.O L.

MCM 4.3L engines above the serial number given above, used a recalibrated carburetor. The part number for these carburetors was either P/N 3310-806972A 1 or 3310-807764A 1 .These carburetors will also correct the condition. DO NOT USE the venturi clusters from either of these carburetors in an older 4.3L carburetor. These venturi clusters have 'distribution tabs' on them and could cause the engine to run too 'lean'.

   * 3302-808706 Venturi Cluster ('463' from a 5.0L carburetor).

  MCM 5.0 L Running Rich at idle or Just Above Idle, S/N 0F118346 and Below

(Continued on page 153)

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