I just replaced the fuel system on a car I just inherited. It has a 327 and a Rochester 4G carb. I got rebuild kit and rebuilt the Rochester 4G (4 Jet) after disassembly and cleaning of all the parts in a sonic cleaner. I've had a few issues. After the rebuild the carb was flooding. I checked the float settings twice and they were within spec. By flooding I mean leaking out the bowl to air-horn gasket and when you would cut the engine off fuel would still be dripping inside the carb until the bowls leaked down. After checking the floats twice I finally set the float setting to lower than what is called for (about 1/8 or so lower). This stopped the flooding problem, but now it won't idle. It will run if you keep revving it, but I cannot get the idle set. I have adjusted the idle mixture screws 1.5 turns out, and can't adjust them further until I can get a decent idle. Another issue now is that when I rev the engine up, it is wanting to stick around 2,000 RPM. If I get it to idle down, it won't maintain idle and idle for a few seconds roughly and then die. I can't seem to get a consistent idle. Could someone point in in the right direction to start trouble shooting? One thing to note is the rebuild kit came with a new style of needle and seat, where there's no point on the needle, it's just a flat piece of rubber. I don't think this is causing the idle issue, but I am curious if this is why I have to set the floats (not the float drop) higher than normal. Thank you, Brandon
Can you go back to the old needle and seat or buy just those parts and try again? I’m thinking that’s where your issue may be. You might look at fuel pressure too. John
The problem is in your parts or your rebuild or your engine. The Rochester 4GC was used on GMs premium cars, Cadillac, Buick, Oldsmobile, Pontiac, where a dead-smooth idle in Drive was a requirement. Since the engine is new to you: 1. Check for vacuum leaks around the intake. 2. Verify the correct carburetor-to-intake gasket. There are several versions and the wrong one will cause a vacuum leak. 3. Verify the distributor vacuum advance diaphragm is not perished, that the vacuum advance is connected to the correct port, that the initial timing is set per the Shop Manual. 4. Check the secondary throttle blade adjustment and use a strong light to verify the blades are sealing tightly. 5. What fuel pump? What is the fuel pressure at the carb? jack vines
As I remember the intake carb heat passages were always carboned up. It normally took a drill to open them up. The also took a different base gasket which were included in the kits. I would always file the surface with a body file of the manifold and carb base. I’d vacuum while I did this. A good seal is needed.
If you still have the old needle and seat compare the orifice size. New one may be to large in diameter. The bigger the passage the harder it becomes be to seal.
A few thoughts: First, there were at least two different of the fuel valves with the flat neoprene washer that fits over an inverted flare seat: (1) Parker Brothers from Oklahoma - the neoprene was a flat washer trapezoidal in shape that fit into a trapezoidal cavity in the aluminum valve. These valves were well-made, and we used thousands of them without issue. Paul got fed up with various government regulations, closed the company, and went fishing. It is my understanding that Daytona bought the rights to this valve, but I have not personally inspected any of the Daytona valves, so cannot comment other than hearsay. (2) Tomco from St. Louis - the neoprene wafer was encapsulated in an aluminum frame, and the frame was inserted between the inverted flare seat, and an aluminum plunger. It is possible for the wafer to turn 90 degrees in the seat cavity, IF FLOAT DROP IS NOT CORRECTLY SET, creating a flooding condition. We would never consider using this valve. Fuel leakage: Modern fuel will climb the walls of the float bowl of any carburetor, and if the bowl and airhorn are not perfectly true, will ooze around the gasket. Leakage dripping down from the nozzles after shutdown is again caused by modern fuel (impossible for the bowls to leak down). This link explains the issue: https://www.thecarburetorshop.com/Troubleshooting.htm#Fuelleak Carburetor bowl fuel level is, and always has been, critical. We suggest to prospective customers that wish to rejet a carburetor to try raising or lowering the fuel level by no more than 1/16 of an inch. Generally, this will correspond to one mixture size either richer or leaner. This allows testing without purchasing. Since the change in negative pressure to cause the fuel wells to discharge gasoline is not linear based on the change in fuel level, GUESSING that 1/8 inch would be 3~4 sizes lean. For the record, many carburetor fuel levels were factory set to +- 1/64 inch. The oozing is difficult to solve, a toothbrush and toothpaste do a good job of removing the fuel stains around the gasket. The leakage after shutdown can be at least minimized, possibly eliminated, by the installation of a three line filter right at the carburetor, and installing a return line from the "vapor" line on the filter back to the top of the fuel tank. Jon.
One other thought: Rochester, as well as Carter, thought the fuel level in the secondary bowl should differ from the fuel level of the primary bowl ON SOME (NOT ALL) APPLICATIONS. To know for sure, one needs the tag number of the carburetor. Since the floats appear similar, both Carter and Rochester would use the same float pontoons for both the secondary and primary floats. Since maximum buoyancy is obtained when the float arm is perpendicular to the float valve, it was necessary to change the height of the soldered joint from the arm to the pontoons; thus, in these applications, the floats, even though without close inspection, appear the same, they are NOT interchangeable. Bending the tab on the wrong float might obtain the proper level, but the angle from arm to valve would not be 90 degree, and the valve would hold less pressure. And as Johnny Gee suggested, some kits have a "one size fits all, works well on none" fuel valve. Jon
Make sure your base to body screws are tight. I had similar issues with a 4GC on a flathead. I found that the screws had loosened and the fuel that runs through the channels in the gasket was leaking down the carb into the engine.
Thank you for the responses. I adjusted the floats, varying slightly from what the specs said, and the carb is no longer flooding, however then it wouldn't idle. I took it off again and now I am to the point where it runs pretty decent, however it often revvs up and stays around 1500-2000 rpm. Sometimes it will come back down to a normal idle speed, but then it wants to die.
With absolutely no disrespect intended; are you correctly measuring the float setting? Some Rochester carbs use a measurement to the float seam; others to a dimple on the float pontoon. What is the tag number of your carburetor? As far as the occasional high idle; is the choke valve completely vertical? Is the fast idle cam adjusted correctly? And an intermittent vacuum leak might cause the high idle, and the propensity to stall at low idle. You may have multiple issues. Jon
Thanks for the reply and no disrespect taken at all. I'm trying to learn and it is far from improbable that I've made an oversight somewhere. I did research to ensure I was measuring the floats correctly, but just to be sure I took it to a local guy that works on carburetors and he also said the floats were correctly set, so I'm pretty sure they're dialed in good enough now to not cause an issue. This carburetor had no tag. It was on this car for many years (thought it wasn't driven often) but it did perform properly, until it sat for the last several years and the fuel system then needed to be replaced, including new tank, fuel pump, lines and rebuilt carburetor. The choke is completely vertical. I am unsure how to adjust the fast idle cam. The rods are in the same shape they were in when I disassembled the carb before. I am currently leaning toward some type of vacuum leak, but I have sprayed choke cleaner all around the carb and it does not pick up idle anywhere I've sprayed except directly in the primaries (that's when it's idling low).
I really don't like carburetors with no tags, as this generally means at some time it was "rebuilt" by Dr. Goodpliers (the evil twin of Mr. Goodwrench), and have a mis-mash of parts. But it sounds like it has been working, and it also sounds like you have it together correctly. Perhaps the finger is being pointed at the wrong suspect. Try disconnecting ALL vacuum lines from the carburetor and plug the vacuum sources on the carburetor. If this makes a difference, reconnect one at a time to find the culprit. If this makes no difference, look inside the distributor. If it has an electronic conversion you might consider an upgrade to points/condenser for testing. If that solves the issue, then you can decide which you want in your distributor. And if you kept the carburetor mounting gasket that was on the car, you might compare it to the one in the kit that you used. Jon.
The butterflies don't appear to be hanging, but I'll check again to see if I missed something. As a side question, the secondaries have these flaps on top of them and I'm not sure what purpose they serve. I'm not sure how to describe them other than an extra set of butterflies that are on top of the actual butterflies. That is a good suggestion about unhooking the vacuum lines. The more I work with this the more it seems to strike me as some type of vacuum leak. I'll try plugging those off. Maybe the vacuum advance may have a problem? I also plan to get some map gas and test around all the areas very carefully to try again to see if I can get some type of idle response.
You are looking at a dampener for the secondary barrels . It is adjustable but not causing your current problems.
The Jalopy Journal
