1974 Rochester 2GC hesitiation at low throttle question

[ntheyer]

I have a recently rebuilt carburetor on my Merc 3.0 that runs perfectly in all cases but one... The engine idles well and from 2000 RPM to WOT is perfect. The problem is when I just apply a TINY bit of throttle above idle; the engine will stumble without increasing speed and when I give it a LITTLE bit more, it will stall. If I give it a shot of throttle from RPM, it will run no problem. Basically I can maintain 2000 RPM and higher without an issue, or idle to 1000 or 900 RPM. In between 1000 RPM and 2000 is no go. What do you think I can adjust on this. Again, it's not on acceleration. Acceleration works fine. The accelerator pump works well. It's only when I creep the throttle higher than about 1000 PRM. It also seems to be worse once the engine warms up and even worse when the engine room air temperature is high like when idling around for a while. It would seem to me like it is too rich at this low throttle setting? Is it float level? Thanks.

 

[Haut Medoc]

Re: 1974 Rochester 2GC hesitiation at low throttle question

Is the choke set right and operating properly? That's where I would look first.....JK

 

[Richard Petersen]

Re: 1974 Rochester 2GC hesitiation at low throttle question

See if your carb has a " off idle fuel circuit". That is supposed to elimanate that problem. Otherwise, just make the idle mixture richer and it will carry you thru those conditions.

 

[ron7000]

Re: 1974 Rochester 2GC hesitiation at low throttle question

not that familiar with rochesters, i have a 2002 3L with the mercarb.<br /><br />Almost always and this applies to most any carb, a stumble just off idle when not abruptly opening the throttle is due to a lean mixture which can be caused by a low fuel level in the bowl. so by adjusting the float so more fuel is present, it won't lean out and won't stumble. Other cause can be a restriction in the transition circut preventing fuel flow. Looking at the throttle blades when they are closed (when at idle) they are the slits in the carb wall just above them. Those passages need to be clean throughout to get good performance between idle and cruise speed.

 

[deputydawg]

Re: 1974 Rochester 2GC hesitiation at low throttle question

After you ckeck the choke, check the float. That is most likely your problem. Don't mess with the idle adjustment just yet.<br /><br />Get it to operating temp, slowly close of the choke while it is idling. You should be able to almost close the choke completely and the engine will still run. If you close the choke a little and the engine RPM's increase, then your mixture is lean, raise the fuel level in the float bowl. If you close off the choke a little and the engine immediately floods and dies, then back off the fuel level. <br /><br />A properly adjusted float should increase idle speed slightly very slightly when the choke is closed, then after a few seconds floods out.<br /><br />This is just a shadetree way to check without tearing into everything. The proper way would be take the carb apart and measure the float level accoding to specs.

 

[ntheyer]

Re: 1974 Rochester 2GC hesitiation at low throttle question

I appreciate the responses. I will check everything. If It comes to float level... by how much do I adjust it at a time? And where do I bend it?

 

[jpcody66]

Re: 1974 Rochester 2GC hesitiation at low throttle question

The adjustment is on the tang that the needle sits on. You bend the tang inward or outward depending on the adjustment required. If anyone knows the specs on the adjustment please post as I have the same carburetor, rebuilt it but have no adjustment specs to go by. Nothing was included in the rebuild kit for the adjustment and the manual I have isn't specific to this carburetor.

 

[ron7000]

Re: 1974 Rochester 2GC hesitiation at low throttle question

on my 2002, I've set the float level according to the merc manual and still got hesitation in the 2000 rpm range. Manual specified 9/16" and I'm probably set a good 2/16" higher to have more fuel in the bowl. You really can't hurt anything by adjusting float level higher (more fuel or richer). You know you went too far when you either try to start the engine and it won't start because it's flooded, or it starts but stutters due to raw fuel being pulled out the boosters in droplets. When that happens, lower float level just a little. Or until that doesn't happen, it's a trial and error procedure when the floats are nonadjustable as in you have to pull the carb apart and bend the float tangs. Can be a pain and a little time consuming but really the only way.<br /><br />I know on my mercarb, it's somewhat nice because the top of it pulls off to expose the fuel well, so I can observe the actual fuel level. Hardest part is preserving the gasket. If you'rs is the same way, best way is to idle the engine down to low rpm for a second then shut off and pull cover off carb. Fuel level should cover the main jets but be lower than the level of the boosters that extend into the carb throat. when you go over this pathway or get too close to it is when raw fuel dumps out the boosters.