I have a GM 4 jet carb by Rochester off of a 50 Olds (it's similar to a Rochester 4GC 4 barrel carb) which I had professionally rebuilt and am trying to install on my 59AB Ford flathead. The problem is no matter what I try I can't get it to idle. It runs great at higher rpm, I have 6 pounds of fuel pressure and the vacuum does not appear to be a problem. The engine has an Isky Max 1 cam, it's bored 80 over with a 4 1/8th inch crank and large valves. I noticed that it does not appear to be ejecting fuel when the accelerator pump is activated. Does anybody else have experience with this particular carb? Thanks
I think they didn't start production till around 53...wonder which one you have? Pictures are helpful.
I had one years ago that I couldn't get the accel pump to work. Finally fond that someone had previously tried to rebuildit and had puta too small check ball uder the pump and it had gone down in the passage below the seat for the ball and wedged itself in place almost totally blocking pump output.
I just did one that the acc pump didn't work much...it was missing the ball under the plunger. But that won't affect idle.
Just did a 2GC that had one of it's suction tubes that fell out the venturi cluster. But I don't think that's your issue here. That said, does your carb use a venturi cluster gasket? If so, did the correct one get installed? Example image only.
(1) First Olds to use a Rochester 4GC (4-Jet) was 1952 (on a 303 CID engine). (2) Idle circuits on early (pre-1956) 4-Jets were not the best, even on the 303. (3) Idle air velocity is probably marginal (or worse) trying to use this carb on a flathead. To test the accelerator pump, first start and then stop the engine. With modern fuel, if you make the test without first starting the engine, all of the fuel may have evaporated out of the fuel bowl. If there is no fuel to pump, the pump won't pump fuel Most common cause of the fuel pump not functioning (other than the above) is trying to use the neoprene accelerator pumps found in the cheap FLAPS kits with ethanol-laced fuel. Second most common cause is the check ball under the cluster is stuck in its passage. Occasionally, the issue mentioned in post 3 also happens. This is the inlet check valve, and if stuck, will not permit the pump cylinder to refill. The aluminum ball goes under the pump, the steel ball under the cluster. The aluminum ball is smaller in diameter than the steel ball. I would have suggested a different 4-barrel on a flathead than the one you are trying to use. Not saying it cannot be made to work; just saying much easier to start with something more compatible, and I am lazy! A much easier beginning point would be the Rochester 4-GC used on the Buick/Oldmobile 215 CID from the early 1960's. 6 psi fuel pressure is TOO much! Would be nice to know if you checked everything for vacuum leaks. If there are no vacuum leaks, you may have to significantly alter the idle circuit (much larger idle jets). Jon.
Thank you! I did consider it, but ran out of time. However, there are hundreds of pages on my website free for the looking, and everything there is in the public domain, and may be downloaded if desired. Jon.
I had one that the base was leaking, a vacuum issue. idle sucked but ran great at upper rpms. lactated it with starter fluid with a straw attached to the nozzle. These old carbs also often have wore out throttle shafts.
Aren't the two check balls different sizes? I seem to recall that you can mix them up which creates a problem with the accelerator pump and power valve.
In the for what its worth category: This has nothing to do with the OP's issue, but one common problem (today) with the Rochester G series (2G, 4G) and also Stromberg is the relaxing of the tension of the sealing plugs used to seal the access for internal passageways. Both Stromberg and Rochester used lead balls. After 80 years, these occasionally decompose significantly to fall out. We do include the balls in the Stromberg kits (Rochester used a slightly different diameter, and I don't have a source), but these are easily replaced by anyone. If one is leaking, try using a drift punch and hammer, and expand the plug. If one has actually fallen out, visit your tackle box (or one of a fisherman friend) and find a lead split shot sinker maybe 1/3 larger than the hole. Use the hammer and drift punch, and install the sinker in the hole. Problem solved. Jon.
Yes, the aluminum ball below the pump is a smaller diameter than the steel ball below the cluster. Mixing the balls should not effect the power valve; and the pump MIGHT still work. The reason for the steel ball as a discharge is the mass (weight) of the ball. A lighter ball might be pulled off its seat by negative pressure applied through the pump jet, creating a siphon effect. This would cause the engine to run rich all of the time, and obtain horrible fuel economy. This can be a tuning aid using Carter carburetors. Carter GENERALLY used a pointed brass weight (of varying lengths/weights) but also used a couple of different sized steel balls. The trick is to choose a valve of sufficient mass that the negative pressure is insufficient to pull it off its seat, but light enough to come off (at some time in milliseconds) once the accelerator pump is actuated. The mass can be changed to adjust the timing (not the volume) of the pump stroke. Jon.
Lots of good info here on the posts. I have found that the accelerator pump if installed dry (you said it was rebuilt) can rip if the new style plunger for methanol was used when installed. I oil mine and the plunger hole as well. That would cause the off idle problem, stumble. I don't use the check ball under the pump, but that is just my preference. I would also check idle circuits so see if they are blocked. Welding/cutting torch tip cleaner works well. And as said in above posts, vacuum leaks.
interesting...the one I just did was missing this ball, and it was undrivable. Replacing the ball made it into a car again.
Fuel level could affect both idle and accelerator pump. Sent from my LG-TP450 using The H.A.M.B. mobile app
I’m betting he’s using the carb because it’s a small base and bolts directly to the manifold. I’ve had trouble getting the Rochesters of that era (up to 54) work also and went to the Carter WCFB’s which worked flawless. Ted Eaton in Leona Tx has an article to make the Holley work with a centrifugal distributor but they do not have accelerator pumps compatible with today’s fuel from what an old carb shop I use tells me.
Rochester used two different styles of fuel inlet for these carburetors. (1) Small check ball in pump cylinder under pump which MUST be present for pump to function. (2) Slotted pump cylinder, with NO HOLE UNDER THE PUMP FOR A CHECK BALL. In (1) above, failure to install the ball will completely kill the pump shot, as the ball closing prevents the pump from simply pushing fuel back into the bowl. In (2) above, the slot is the input for the pump fuel. The fuel from the bowl flows through the slot into the pump cylinder, and when the pump is raised, is pulled past the pump skirt into the lower portion of the pump cylinder by negative pressure beneath the pump. As an aside, and quite contrary to popular opinion, the accelerator pump is NOT necessary for an engine to run (but it makes acceleration easier on the driver ). During WWII, it was quite common for farmers, at least in north central Missouri to REMOVE the accelerator pump from the carburetor completely. Gasoline was rationed. If one cannot buy gasoline, one certainly does not wish to waste what little one has. Jon.
I took the carb back to the individual who rebuilt it. He couldn't find anything wrong but he noticed that that the tiny soft plug behind the accelerator pump nozzles was replaced with a brass plug with a hole drilled in it which he plugged and now the carb appears to be working correctly. What I don't understand is why anyone would do this.
If you're good everywhere except idle you either have a vacuum leak or your idle circuit is clogged/way out of adjustment. Sent from my SAMSUNG-SM-N920A using The H.A.M.B. mobile app
A few Rochester 4-GC carbs came from the factory with a small calibrated air bleed in this area. The air bleed had two functions: (1) Add some initial air to the pump shot before it enters the venturii area. (2) Break any syphoning action once the pump shot is complete. Rochester produced many different of the 4GC carbs; some have some unusual features. Jon.
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