Rochester 2 jet running rich before and after rebuild

[Chevyls6]

So ive got this rochester marine carb that i cant figure out. It's somehow getting too much fuel which is causing a really terrible idle and black smoke.

So far ive rebuilt it twice using different kits, vacuum tested the needle and seat which were good, tested the float in water with no bubbles, watched for leaks around the venturi assembly while running, double checked the float level per the carb kit instructions, and vacuum tested the power piston to make sure its moving.

The only way i can get it to idle is if i almost completely close the idle screws otherwise it stalls. Fuel pump is a new carter unit which seems to have made no difference vs the old pump.

After running it today i observed wetness around the vacuum port for the power piston. Is it possible for fuel to be sucked from the float bowl past the power piston into the manifold? Is there anything im missing here?

 

[dubs283]

Could be a saturated float, did you install new?

Power piston you mention is I'm guessing the accelerator pump? If you see fuel above that it's a leak, did you replace the umbrella in the pump?

Have you checked fuel pressure?

 

[Scott06]

What your fuel pressure?i assume you mean the vent for the fuel bowl? There is no vent for power piston you can see, it gets pulled up by manifold vacuum. If you see fuel on the top of float bowl your fuel pressure is too high overcoming the needle and seat, or the needle and seat/ float doesn’t work .

 

[Chevyls6]

The float is the old school brass style and it didn't appear to have any holes when i submerged it.

Accelerator pump is new, as for the fuel pressure i haven't tested it but can setup for that and report back.

The wet spot im describing is under the carb where the vacuum port that goes to the power piston is. The actual vent at the top is dry. Not sure if it was leaking there while running or if i dribbled some down there while removing the carb from the manifold.

Is there any way to measure the correct fuel level in the bowl like on a motorcycle carb? I'm a total noob at these automotive carbs. Only touched 2 of them in my whole life.

Does anyone have experience with aftermarket import carbs for these? I always had good luck using them for lawn equipment.

 

[Scott06]

The float is the old school brass style and it didn't appear to have any holes when i submerged it.

Accelerator pump is new, as for the fuel pressure i haven't tested it but can setup for that and report back.

The wet spot im describing is under the carb where the vacuum port that goes to the power piston is. The actual vent at the top is dry. Not sure if it was leaking there while running or if i dribbled some down there while removing the carb from the manifold.

Is there any way to measure the correct fuel level in the bowl like on a motorcycle carb? I'm a total noob at these automotive carbs. Only touched 2 of them in my whole life.

Does anyone have experience with aftermarket import carbs for these? I always had good luck using them for lawn equipment.

Measure fuel pressure …

 

[Scott06]

The wet spot im describing is under the carb where the vacuum port that goes to the power piston is. The actual vent at the top is dry. Not sure if it was leaking there while running or if i dribbled some down there while removing the carb from the manifold.

Is there any way to measure the correct fuel level in the bowl like on a motorcycle carb? I'm a total noob at these automotive carbs. Only touched 2 of them in my whole life.

Does anyone have experience with aftermarket import carbs for these? I always had good luck using them for lawn equipment.

I would look in the throttle bores at idle and see if you see any fuel dribbling down off the boosters above the throttle plates should be dry at idle.

Did you by chance replace the needle screws that control idle mixture ? Could be wrong screws? Or were damaged original ones reused? If you are getting too much fuel would think your fuel level in bowl is too high.

Unfortunately on a Rochester or Mercarb (descendant of Rochester) there is no sight glass or plug to see fuel level like on a Holley.

The reason I recommended fuel pressure check is on my old Pontiac that has 3 x 2 bbl Rochesters I put on a new fuel pump and I saw fuel coming out the vents in the float bowl. Consensus reading technical forums is rochesters are sensitive to fuel pressure and replacement fuel pumps have a habit of putting out too much pressure... bottom line I chased this for months ...

 

[Lou C]

There is a little clip for the needle valve make sure it’s just clipped on the float arm not in one of the holes in the float arm. If it’s put in one of the holes it can cause the needle to stick & cause flooding.

Also a good source of QJet information is Cliffs High Performance Quadrajets. There’s a forum in his site where you can post up questions.

 

[Chevyls6]

I did the fuel pressure test today. I've got between 4.5 and 5psi at 800rpm which is as low as i can get it to idle. It drops to 4.5 around 1500rpm.

I can see droplets on the throttle plates while running but i cant see fuel coming out of the venturi itself. It's a little wet around the venturi gasket but doesnt look like its actively leaking?

I tried lowering the float again but there was no change to the idle screw setting.

I've definitely got a vacuum leak around the throttle shaft so that's a problem but i dont think its the problem.

 

[Chevyls6]

Anyone have experience with reman 2 jets? Seems like my options are either chinese knock off for around $175 or reman for 250.

 

[itsathepete]

Is there an overflow hose from the fuel pump to the carb? If the fuel pump diaphragm is leaking, it could be pumping extra fuel to the carb and raising the bowl fuel level, mimicking a float set too high or overflowing the carb

 

[Scott06]

Anyone have experience with reman 2 jets? Seems like my options are either chinese knock off for around $175 or reman for 250.

I would send your carb out to a place like mikes carbs and have them rebuild it.

If you have a vacum leak usually you end up needing to fatten up the mixture screws to get it to idle at all.

On my Rochester 2 jets I run like 3-3.5 lbs of fuel pressure. If you are confident it s a good needle and seat and the float is working may be worth putting a fuel pressure regulator on to see if it stops the fuel dribble and makes a difference.

Only other thing I could think if the throttle plates are open enough to uncover the transition slots , but usually that only happens on a big cam engine where you need more air at idle. ... grabbing at straws... Maybe take the power valve actuator out and plug the vacuum hole maybe it is richening the idle, again running out of suggestions...

 

[Chevyls6]

I would send your carb out to a place like mikes carbs and have them rebuild it.

If you have a vacum leak usually you end up needing to fatten up the mixture screws to get it to idle at all.

On my Rochester 2 jets I run like 3-3.5 lbs of fuel pressure. If you are confident it s a good needle and seat and the float is working may be worth putting a fuel pressure regulator on to see if it stops the fuel dribble and makes a difference.

Only other thing I could think if the throttle plates are open enough to uncover the transition slots , but usually that only happens on a big cam engine where you need more air at idle. ... grabbing at straws... Maybe take the power valve actuator out and plug the vacuum hole maybe it is richening the idle, again running out of suggestions...

I could try the pressure regulator. The mercruiser book does say 3.5-4.5lbs of fuel pressure for this carb model.

It needs throttle shaft bushings for sure which results in the rear mixture screw being about 1/4 turn further out than the front.

One other thing i noticed is the check ball seat for the accelerator pump discharge is corroded. I tried to recoin it with the ball and a punch but im not sure it worked.

 

[Scott06]

I could try the pressure regulator. The mercruiser book does say 3.5-4.5lbs of fuel pressure for this carb model.

It needs throttle shaft bushings for sure which results in the rear mixture screw being about 1/4 turn further out than the front.

One other thing i noticed is the check ball seat for the accelerator pump discharge is corroded. I tried to recoin it with the ball and a punch but im not sure it worked.

This is the one I used on my Pontiac Tripower Holley 12-804 - it is a 1-4 psi unit. I had trouble with the gauge which was a summit liquid filled gauge, come to find out that the pressure indicated changes with temp of the gauge, so I had to set it when cold then ignore it... once it was warm pressure would drop so I'd turn up the regulator...

I replaced all three throttle shafts did not need to bush the body.

The issue with the accelerator pump will only show up when you stab the gas I would think you could epoxy it, but I did have an issue with one of my carb bodies where it was pitted in the accelerator pump well and I couldn't dress it up with a flap wheel, had to replace the body. These zinc castings were made for mass manufacture not to use for 40 years...

If I were you try the regulator and see if that clears up the idle issue maybe will be good enough to use, then make the choice about replacing it

 

[Chevyls6]

This is the one I used on my Pontiac Tripower Holley 12-804 - it is a 1-4 psi unit. I had trouble with the gauge which was a summit liquid filled gauge, come to find out that the pressure indicated changes with temp of the gauge, so I had to set it when cold then ignore it... once it was warm pressure would drop so I'd turn up the regulator...

I replaced all three throttle shafts did not need to bush the body.

The issue with the accelerator pump will only show up when you stab the gas I would think you could epoxy it, but I did have an issue with one of my carb bodies where it was pitted in the accelerator pump well and I couldn't dress it up with a flap wheel, had to replace the body. These zinc castings were made for mass manufacture not to use for 40 years...

If I were you try the regulator and see if that clears up the idle issue maybe will be good enough to use, then make the choice about replacing it

Aah makes sense since the throttle bores are cast iron.

I tried pinching off the supply line till my pressure gauge read 3-4psi and it didnt seem to make much difference.

Another thing i saw is some casting porosity where the main jets screw in but thats probably been there since day 1.

The lack of new major parts is quite frustrating. Do you know if the automotive carb bodies fit marine applications ? I picked up another core from ebay for cheap but who knows if it will be any better.

 

[Scott06]

The lack of new major parts is quite frustrating. Do you know if the automotive carb bodies fit marine applications ? I picked up another core from ebay for cheap but who knows if it will be any better.

Gotta remember these carbs are old, Rochester 2jet for automotive stopped in the early 80's, Marine/industrial was done by 1990, which is why merc started with the Mercab. Biggest issue is finding a good core.

If I recall there are only two sizes of 2 jets small and large base. Yours should be the larger base so yes the body from auto motive may work with some mods. Big differences are the bowl venting, lack of fast idle cam, overflow for fuel pump.

It might be worth contacting Mikes carb parts they rebuild them as well as sell good quality kits. His youtube channel has some tech videos. One mentioned the check balls in accelerator pump circuit can dribble fuel -

about 9:30 talks about the discharge check ball dribbling fuel at idle.

Bottom line some way you are getting fuel at idle that is not being controlled by the idle circuit

 

[rustybronco]

Is the supplied vacuum necessary to hold off the power valve piston enough, or is the piston letting too much air bypass? i.e. power valve piston or bore worn?

The only thing that makes sense is the power valve circuit.

 

[SternDriveEd]

The power valve circuit only opens up a by pass passage around the main metering jets. When the power piston senses engine load it moves downward to open up the power valve at the bottom of the fuel bowl. This is where extra fuel will supplement the main metering circuit provided by the two main jets. This all happens only if the throttle plates are open enough to cause the venturies to pull fuel through the main metering system. So if you’re seeing fuel dripping from the ventiries at idle, it cannot be the power or the main metering system at fault. Look for too high float level, or trouble in the float system.

 

[rustybronco]

I had an old Autolite (1969) 4 bbl carburetor that the power valve piston went bad on. At idle the engine had an excessive amount of fuel being introduced. It was a similar style power enrichment system as that Rochester 2G has. IIRC the power valve is held up by manifold vacuum and when the throttle plates are opened manifold vacuum drops which in turn causes the shaft to lower opening the power valve it self.

Sorry as it's been a while since I've worked on similar systems.

 

[Chevyls6]

Gotta remember these carbs are old, Rochester 2jet for automotive stopped in the early 80's, Marine/industrial was done by 1990, which is why merc started with the Mercab. Biggest issue is finding a good core.

If I recall there are only two sizes of 2 jets small and large base. Yours should be the larger base so yes the body from auto motive may work with some mods. Big differences are the bowl venting, lack of fast idle cam, overflow for fuel pump.

It might be worth contacting Mikes carb parts they rebuild them as well as sell good quality kits. His youtube channel has some tech videos. One mentioned the check balls in accelerator pump circuit can dribble fuel -

about 9:30 talks about the discharge check ball dribbling fuel at idle.

Bottom line some way you are getting fuel at idle that is not being controlled by the idle circuit

Yeah that's been the problem with this whole project. Trying to find used parts that aren't total junk. I'm gonna check into their rebuilding service if it comes to that. I hate to admit defeat.

 

[Chevyls6]

Is the supplied vacuum necessary to hold off the power valve piston enough, or is the piston letting too much air bypass? i.e. power valve piston or bore worn?

The only thing that makes sense is the power valve circuit.

The power piston circuit doesn't hold vacuum at all when i apply it with a hand pump. The piston itself is kinda scratched up on the sides and the bore seems to have a little wear in it. I tried to dress it up with a brass wire wheel in a dremel. It would make perfect sense if the power valve was the issue because that would explain the fuel i saw in its vacuum port at the base of the carb.