Carb Problem?

[65Impala409]

Hi All,

I am attempting to figure out what is wrong with my carb. My car is a 65 Impala 409 4spd 340hp car all original. The rochester carb was rebuilt and restored by an outfit in chicago. What happens is I can start the car and idle fine but when I start to drive at low rpms it wants to bog down (flat spot). Once you get over the bog down and get the rpms up it wants to take off... Any suggestions? Thanks everyone for your help...
Ned
65Impala409

Bob, do you have a better description of my problem?

[bobs409]

You got the description about right. It's a bog or stumble when you first give it some gas like pulling out or even just giving it gas from idle. (no load on engine) I suspect the accelerator pump or the pumpshot adjustment. (if there are any holes to adjust)

We're looking for any other ideas of things to check/try and any thoughts...

By the way guys, this car is really neato! I got to back it out of the garage the other day. :brow I'll have to get some pics of it soon.

[64ss409]

That carb should have a small check ball under the accelerator pump. It closes a hole when the accelerator pump goes down and then allows gas to be drawn in when the accelerator pump raises. This ball is easy to overlook and loose. (Been there, done that) I wonder if it is in there. Without it, the carb with have a bad flat spot anytime you accelerate.

The bog you are decribing may not be as severe as when the ball is missing, but I thought I would mention it.
Ron

[bobs409]

Excellent Ron! We will "check" that. (pun intended) :D

Bob

[Mr Goodwrench]

I was hoping someone else would chime in here on that, I have been fight that EXACT same problem on a pair of those 4GC's. one on a bone stock 57 chevy 283 the other a 1965 fire truck with a 409??????????????

[Rockfish39]

Ron is right about the check ball!!!

ALSO make sure that the pump cup is a new neoprene one (blue cup) and that the actuator rod is in the correct hole on the lever. (there are two)

Another one of those frequently unchecked annoyances is the gasket between the body and throttle plate. There are several variations of these and if the gasket used on your carb is INCORRECT then you may not have any vaccume at the port. No vaccume advance means bogg too!

Try the simple things first, if all else fails contact Ron Hewitt at Daytona Parts Company. They are masters at 4Gs!

Rock :cool:

[Mr Goodwrench]

thanks rock you have a phone number for them?

[Rockfish39]

Ron Hewitt
Regina Ray

Daytona Parts Company
1191 Turnbull Bay Rd
New Smyrna Beach, FLORIDA!
32168

386-427-7108

Without a doubt, THE FINEST workmanship, parts and service I have EVER SEEN!!

Rock :cool:

[Rockfish39]

Rochester 4GC can be tricky animals to tune. The design was considered to be obsolete by RPD in 1966. Hard to believe that was nearly 40 years ago... :help
You have to be patient with them though :rolleyes:

Anyways, the metering rod/jet arrangements used in Q-Jets made tuning stock GM carbs a whole lot easier, but that is besides the point when dealing with a 4G.

a 4GC has no metering rods. So you have to do some work to tune them

Q: Is your cam stock or not??? IF NOT, then it is very likely you're going to need make JET changes on the primary side. Out of the box 7025125 and 7025124 Carbs use size 67 primary jets and size 80 secondary. Both are considered 'square cut' jets. A neat trick, that I found out about a while ago, is to change the primary jets to a size 74 Q-Jet type. Those are taper cut jets and work wonders in a 4G if your bumpstick has been changed.

Make absolutely sure that you you use a correct gasket set for your 4G. Regardless of what the "Pep Boys " style kits say, one size does NOT fit all !!! :rofl

Another neat trick is to put a LARGER accelerator pump cup in a 4G. The Q-JET pump cup is larger than a 4Gs. So, if there is sufficient material in the carb body to accept machining, bore out the accelerator pump cylinder accordingly. I have to look this up to be certain, but I think the size is 7/16". You also will have to use a PIN vice and some small drill bits to open up the discharge nozzels a little so that they can accomodate the extra fuel flow that the larger 7/16" bore will deliver. Please keep in mind that you have to do some math here. You have to measure the original bore size of your 4G and calculate the PERCENTAGE of volumetic increase the larger bore will give you. You want to open up those discharge nozzles by that same calculated percentage.

I could ramble on and on about 4Gs, but another one of those PESKY things is that you have to ensure is that the throttle plate and the corresponding mating surface on the intake are MACHINED FLAT. I dont know how many stock intakes Ive seen have as much as .010, .015, or even more lateral deviation on, that problem is compounded if the throttle plate isnt flat also.

Induction systems that suck air in beneath the throttle plate will cause you vacuume leak problems when trying to do performance tuning.

Before I forget, also make certain that you USE the new power piston that comes in your kit. Too often, guys dont change them and that causes problems too.

Rock :cool:

[Mr Goodwrench]

thanks for the great info rock! both these projects are on the back burner for me (building a house) the 57 chevy is a bone stock engine and though I cannot remember the carb number it was the correct one for a 57. the fire truck is a originall 18,000 409 unmolested 409 govenor and all. just seemed funny two complete diff engines have the same problem after a rebuild. someday I will get back on them and figure it out though, I do believe a 4gc is a good carb for what it was intended for. intersesting note the carb on our B-25 bomber which has R-2600 engines in it has a rochester products carb on it, and it's a 1944 model.

[65Impala409]

Thank you so much for all your information!!!! I hope to see what sense we can make of it within the next two weeks... Fuel is too expensive to waste now... :dunno I apoligize for not getting back to all of you sooner but I was on a vacation... The carb problem can do it to you... Thanks again

Ned
65Impala409