Mercruiser 3.0L mercarb 2 barrel Help!!!

[Sebastiank15]

Hey!! I have a mercury Mercruiser 3.0 with a Mercarb 2 barrel carb. I just recently had it rebuild. I was just wondering if there is more than one type of applications for the Mercarb? Meaning that my original one had only one adjustment screw by the throttle arm and the new one has 2. The other one is on the right side on the bottom?. What does that mean? Help!!

 

[Chris1956]

You should repost this in the MerCruiser forum, along with year of motor.

It also sounds like you bought a new carb, versus having your's rebuilt. Typical carbs have a idle speed screw and one idle mixture screw, for each barrel.

 

[Sebastiank15]

You should repost this in the MerCruiser forum, along with year of motor.

It also sounds like you bought a new carb, versus having your's rebuilt. Typical carbs have a idle speed screw and one idle mixture screw, for each barrel.

Yea I bought the carb already rebuilt so it is new. I'm just having trouble adjusting it to the right point. It runs good for a tiny bit and than it dies in neutral. The rpms just drop out of nowhere and than I can't get it started at no matter what I do.

 

[alldodge]

You might be running out of fuel. What is your motor serial number?

 

[Sebastiank15]

After it dies its trying to start but it just won't turn over. I can't remember the serial number, I would have to go back and check. I thought I had it adjusted right the second time bc it was running great in neutral. The rpms were good. After sometime under the muffs it starts doing it again and I have to keep it in gear for it not to die. In gear it runs great.

 

[Lou C]

Are you sure that the carb you bought is a marine carb and not a modified auto carb? The Mercarb 2bbl is actually a modified Rochester 2GV carb. Marine carbs will have certain differences like a nipple for the fuel pump overflow, inverted vent “J” tubes and no fast idle cam like an auto application.

 

[alldodge]

After it dies its trying to start but it just won't turn over. I can't remember the serial number, I would have to go back and check. I thought I had it adjusted right the second time bc it was running great in neutral. The rpms were good. After sometime under the muffs it starts doing it again and I have to keep it in gear for it not to die. In gear it runs great.

Saying it won't turn over, that one issue and look at the neutral safety switch in the throttle handle.

I'm wondering if it has an electric fuel pump. If it does one circuit can power the pump while cranking and another keeps it going once started

In the pic note the purple/yellow from the starter turns the pump ON during cranking. Once running the oil pressure switch closes and keeps it running. If the switch or wiring has an issue the pump will not run once started

 

[Chris1956]

Repost in Mercruiser forum with year of motor. I had thought Rochester carbs were standard on GM engines.

 

[alldodge]

Mod has moved it to the Mercedes section

 

[Sebastiank15]

I bought the carb at guaranteed carburetors. It was for a Mercrusier 3.0L 135 hp engine. It does have a nipple for the fuel overflow. The fuel pump was replaced and ran fine under the old carb. It's a mechanical fuel pump. The fuel is pumping to the carb but it does sound like it just gets cut off at a point and dies. And than it sounds like it's not getting enough gas to start. Do you think it could be a oil pressure switch or issue that it thinks there's no oil and cuts the fuel supply?

 

[alldodge]

No oil pressure switch with a mechanical fuel pump. This is why we want to know exactly what you have prior to trying to figure things out

Do you have a spin on fuel filter or is it part of the pump?

 

[Lou C]

You can test (carefully) the pressure put out by the mechanical pump to make sure it’s adequate and then if so make sure the carb float is working as designed, allowing the bowl to fill with fuel then shutting off the supply when full so it doesn’t flood. A quick test; remove carb dump the fuel out. Let it dry out and hook up a clean rubber hose to the inlet. With the carb upright you should be able to blow air thru the hose. If so the float is not stuck in the up position (fuel flow off). Now invert the carb & try blowing thru the hose. You should not be able to, this tells you the needle valve is holding. If you can blow thru the hose either the needle valve is not holding or the float is stuck in the down position (fuel on). These are about the only easy tests you can do.

 

[Sebastiank15]

No oil pressure switch with a mechanical fuel pump. This is why we want to know exactly what you have prior to trying to figure things out

Do you have a spin on fuel filter or is it part of the pump?

It's part of the pump and you can replace it

 

[alldodge]

When it dies, loosen the clamp on the filter (assuming its under the pump) and see if fuel spills out. If it doesn't, then check antisiphon valve on the fuel tank.

Its a barbed fitting that the rubber fuel line is connected to it

 

[Sebastiank15]

I could that test and see if it's functioning probably and the needle is not getting stuck. I'm gonna take the carb off later on today and see what happens. When u mean attaching a clean hose to the inlet do u mean where thr gas line attaches to the carb or somewhere else?

 

[Sebastiank15]

Yea it's under the pump.. I will definitely check that out also. I'm at the point were I just want to get rid of this carburetor

 

[alldodge]

Right now we don't know if there is to much fuel (meaning carb problem most likely), or not enough fuel

 

[Lou C]

I could that test and see if it's functioning probably and the needle is not getting stuck. I'm gonna take the carb off later on today and see what happens. When u mean attaching a clean hose to the inlet do u mean where thr gas line attaches to the carb or somewhere else?

Yes where the fuel line connects to the carb. Just don't get a moutful of gas. Not good for you!
PS that is a simple carb, we had a Rochester 2GV on our '72 Chevrolet 350 V8, it should not be hard to set it up right. Now a Quadrajet, that's good deal more complex, I've had mine apart a few times.

 

[Sebastiank15]

You are right. I thought that right away also. Just sounds like the fuel supply gets cut off somewhere or gets to much fuel meaning to rich. Spark plugs look black so I know its running to rich. Everything was original on this motor before I started to do maintenance on it. So the antisiphon valve is over 20 years old

 

[Sebastiank15]

This is the carburetor. And the picture with that adjustment screw, that's the screw that my old carb did not have in that spot. Also a picture of the antisiphon valve