Rochester 2G (on Merc 5.7) issues

[rpdwyer]

Hi all.

Rebuilt both carbs (Rochester 2G's) on my engines (1997 Merc 5.7's). One is working beautifully. The other has fuel dribbling down onto the throttle plate at idle from the venturi. I believe this is causing an erratic idle. If I open the throttle the fuel atomizes correctly the engine runs great. But at idle it drips a decent amount.

Any ideas on what would cause this? My guess would be float level set incorrectly allowing too much fuel to sit to high in the bowl.

Thanks,
--Rick

 

[Walt T]

Re: Rochester 2G (on Merc 5.7) issues

Exactly where do you see it dribbling from? You say venturi, can you be more specific? Is it dribbling from the roundy thingy above the opening, or coming out through a hole in the side if the throttle hole?

 

[rpdwyer]

Re: Rochester 2G (on Merc 5.7) issues

Exactly where do you see it dribbling from? You say venturi, can you be more specific? Is it dribbling from the roundy thingy above the opening, or coming out through a hole in the side if the throttle hole?

Not really sure. Next time I'm at the boat will have to notice... but I think it's from the round clusters themselves.

--Rick

 

[Walt T]

Re: Rochester 2G (on Merc 5.7) issues

Fuel comes from up there normally when the choke circuit is active. Excessive fuel pressure, float set too high, idle mixture needles not sealing correctly. Usually that circuit wont pull fuel unless the choke flaps are closed or partially closed causing the high vacuum to be present up around those clusters when then pulls fuel from them. The extra gas helps keep the engine running since cold gas tends to fall out of the cold air and onto the intake runners walls and floors. Once the motor warms and the flap opens the vacuum signal then moves below the throttle plates. If fuel pressure and float are okay remove the idle mixture screws and make sure they aren't bent or dirty.

 

[rpdwyer]

Re: Rochester 2G (on Merc 5.7) issues

Fuel comes from up there normally when the choke circuit is active. Excessive fuel pressure, float set too high, idle mixture needles not sealing correctly. Usually that circuit wont pull fuel unless the choke flaps are closed or partially closed causing the high vacuum to be present up around those clusters when then pulls fuel from them. The extra gas helps keep the engine running since cold gas tends to fall out of the cold air and onto the intake runners walls and floors. Once the motor warms and the flap opens the vacuum signal then moves below the throttle plates. If fuel pressure and float are okay remove the idle mixture screws and make sure they aren't bent or dirty.

Thanks Walt. The mixture screws are brand new so I know it's not them. That doesn't speak to the seats they hit... but assuming they are OK, that leaves pressure and float as the culprits. Will have a look at it this weekend, but I'm guessing I see the float to high in my rebuild.

--Rick

 

[midcarolina]

Re: Rochester 2G (on Merc 5.7) issues

Did you replace the float? if not if it's the old type it could be saturated with fuel....also I seem to remember when I rebuilt mine it came
with two different needle and seats....one was a normal one and one was spring loaded........but I dont recall what conditions you would use the alternate needle.

 

[Fishermark]

Re: Rochester 2G (on Merc 5.7) issues

...two different needle and seats....one was a normal one and one was spring loaded........but I dont recall what conditions you would use the alternate needle.

The spring loaded needle is recommended if you have a situation as described. You will also set the float at a different height if you use the spring loaded needle valve. I don't know if that is your issue or not of course.... but if you do not see anything else as the issue, you might try the other needle valve and set the float level accordingly.

 

[Fastatv]

Re: Rochester 2G (on Merc 5.7) issues

Thanks Walt. The mixture screws are brand new so I know it's not them. That doesn't speak to the seats they hit... but assuming they are OK, that leaves pressure and float as the culprits. Will have a look at it this weekend, but I'm guessing I see the float to high in my rebuild.

--Rick

The two small clusters are called mini venturis ( I think that's what is being discussed ). These are used with the power and main circuits. If you see fuel on them at idle, then there could be a couple of reasons.
- Idle speed and air/fuel screws improperly adjusted,
- float level too high, faulty needle/seat or float, debris in carb bowl not allowing needle to seat.
I would recommend setting the carb to correct idle speed, and readjusting the mixture screws before going back into the carb. Keep us posted. Rick