Rochester 4GC on a nailhead- I need a diagnosis

Discussion in 'Other' started by buick64203, Jul 27, 2010.

  1. buick64203

    buick64203 Just plum crazy Staff Member

    Heres the problem- the new 61 Invicta I bought developed what I think is a carb problem. The car starts right up and runs fine for maybe 20 seconds/ half a minute. Then it starts to die like its running out of gas. If I work the throttle a little it will stay running. I can see two nice streams of fuel shoot out when the accelerator pump squirts.

    The carb has been previously rebuilt. And from what I see, just recently. I took the carb apart and found very fine sediment in the bottom of the float bowls. I took the carb apart, cleaned it as best I could and put it all back together. Same problem. I removed the fuel filter in the clear glass bowl and it was clean as a whistle. I did not check fuel pressure as I didnt have a gauge handy. But I dont belive its fuel pressure problem because I can keep it running off of the accelerator pump. Plus the pump looks brand new.

    Before I drop a couple of hundred at the carb shop, does anyone know what specifically might be wrong that I can look at? It would really suck to spend 200 bucks and have the same exact problem. Is this a problem possibly common to a 4GC? As you know, I have little experience with these early cars.
  2. CameoInvicta

    CameoInvicta Well-Known Member

    Jay, I can't comment directly on your problem, however I had my old 4GC rebuilt and it still gave me nothing but problems. I don't know how important originality is to you, but you might want to switch to an aftermarket Edelbrock or Holley before you go dumping more money into the Rochester.
  3. BillA

    BillA Well-Known Member

    I had similar problems on my '64 last year and opted to go with an Edelbrock 1406. Problem solved. I'm holding on to the 4GC just in case I decide to go back to the original setup.
  4. telriv

    telriv Founders Club Member

    Yeah Jason, those 4GC's can be a PITA. They either flood or run lean. Best I can tell you is to find an original AFB for it & do a rebuild on that. Much easier & if you can find an original it will have all the correct hook-ups for throttle & trans.

    Tom T.
  5. buick64203

    buick64203 Just plum crazy Staff Member

    Thanks for the info guys. It must be running lean because it seems to run rough and develop a miss when it starts acting up.

    I have an old school carb shop in the area Im going to call on when he re-opens from vacation next week. Maybe he can hook me up with a Carter instead. At least Im on the right track.
  6. Schurkey

    Schurkey Silver Level contributor

    Does the choke work? Sounds like the choke is opening too fast.

    When was the last time you verified that the idle mixture adjustment was correct?

    Does the heat riser valve on the exhaust manifold work properly?

    Do you have the sheetmetal heat-shield between the carb and the gasket on the intake manifold?

    Have you verified that there are no vacuum leaks; including a defective PCV valve?

    Engine tune-up is good? Dwell, timing, points, cap 'n' rotor, plug wires, plugs, compression etc. are all verified OK?
  7. telriv

    telriv Founders Club Member

    A 4GC DOESN'T need a heat shield under the carb. as the base is CAST IRON. 1961 has NO PCV Valve.
  8. telriv

    telriv Founders Club Member

    Unless a PCV was added at some time by the dealer or an after market one was installed at one point in the valley pan. There used to be a kit available for the valley pan from Buick & also the after market.

    Tom T.
  9. buick64203

    buick64203 Just plum crazy Staff Member

    Choke is operating OK. When the problem happens the choke is still closed. Manipulating the choke blade doesnt have any effect on it.

    I did play with the mixture screws. They were turned out 1 1/2 turns. I screwed them in and out while a buddy worked the throttle to keep it running. No luck

    I held my hand over the carb and no vacuum leaks. I didnt have my vacuum gauge with me. I was "cooking in someone elses kitchen" so to speak. There is no PCV, only a road draft tube

    Aaahhh...the sheetmetal gasket. There is not one there. Only the conventional style composite/ fiber gasket. The gasket thats on there does have the same slot on it that matches the manifold and the base of the carb.

    As far as tune up parts, everything is new- wires, capp rotor plugs, coil, etc. All replaced from the prior owner
  10. buick64203

    buick64203 Just plum crazy Staff Member

    Im far from a carb expert, but it seems to be in the idle curcuit. If I were to speculate, I would guess that there is orifice or curcuit that is clogged with that sediment that I found in it. There was a good amount in there. Enough to cover the bottom of the float bowl area. Probably what the inside of the tank looks like too.

    Im thinking about running an additional inline filter before the fuel pump to catch any fine debris the stock paper filter is allowing to pass through
  11. telriv

    telriv Founders Club Member

    Sounds lean. I have done this in the past. Sometimes it will "Suck" out the debris/clog. Bring the engine to about 2-2.5K RPM's & at the same time use a screwdriver to hold the choke blade closed tight. As the engine starts to die release the choke & let it "Race" up again. Do this a few times. It may just clear things up. In the not so distant past there was a piece of "Lint" caught in one of the idle tubes & another caught in the tube of the venturi. If I had done mentioned above I would not have to had taken it apart. Sometimes you have to look very close to find these kind of problems. I remember one time I had a'70 MKIII Lincoln. Got it for cheap because the C6 tranny kept going bad shortly after 3/4 rebuilds. I did a rebuild also & it wasn't right. Had it out a couple times & apart all over the bench thinking "I" did something wrong. Well, come to find out, I'm sure, whoever did the original rebuild was using one of those "Red Shop Rags. A very small piece was inside one of the passages. The way I found it was by blowing through all the passages in the case with 175PSI shop air & air gun with a rubber tip. I heard this "Poof" & saw something fly out. Was a small piece of the rag. Put it all back together & drove it finally fixed after about 6 previous rebuilds for 5+ years. Had another C6 in a Police car kept burning up tranny's within a week of overhauling. This car was new, less than a week old at the time. Come to find out the cooling line fittings screwed into the tranny & one was not drilled through.

    Tom T.
  12. buick64203

    buick64203 Just plum crazy Staff Member

    Since the car isnt at my house and all I have is the carb, I got to thinking how I could test this carb. It dawned on me that Jennifer's 73 Mach has an aluminum manifold that double drilled for both carb bolt patterns. :idea2:

    I could bolt the 4GC on the Cleveland and run some tests. It bolted on with out a problem and the car ran fine. I can only surmise that I have either bad gas in the car or the fuel pump is not up to snuff. I'm going to bring my fuel pressure gauge over Eric's house and then take a gas sample.

    If its bad gas, what would you do short of dropping the tank? The car had very little gas in it when I picked it up. I was thinking about filling the tank to dilute whatever is in there. Although that could compound my problems if i need to take the tank down.

    Here's a picture of what a 4GC looks like on a Ford 351C. I think its an improvement:laugh: . Jennifer didnt seem to agree. :bla: Im gonna throw the Holley back on it before she starts getting upset!

    Attached Files:

  13. doc

    doc Well-Known Member

    Steal the holley,,, steal the holley,,,,:laugh: :laugh: Actually that dont look like a bad carb.... I dont think that it flows a whole lot.... it will be about 500 or so cfm.... so now you know that the carb was good..... start checking everything else.....Yes you can dilute the old gas with some good gas with good results... that is the easiest thing to do.... and wont hurt anything....
  14. Cliff R

    Cliff R Well-Known Member

    4GC's are OK, but not much cfm's.

    There aren't any floats avaiable for them that I know of, so we've had to steel them from used cores for quite some time now.

    They are plagued with accellerator pump issues since ethanol hit the gas pumps.

    We just got in our new Viton pumps a few days ago. We had already been sent samples from the supplier and tested them.

    We see at most about 1-2 of these a year in the shop, so we don't stock too many parts for them.....Cliff
  15. buick64203

    buick64203 Just plum crazy Staff Member

    One thing I did notice when I bolted the 4GC on Jen's 351C is that the engine did seem to run a little rough with it on there. In hindsight, I should of enriched it with some propane to see if it helped.

    I like the idea of swapping it to a Carter. If I was going to keep it, I would probably go that route.
  16. bhambulldog

    bhambulldog 1955 76-RoadmasterRiviera

    Hey! That's nice of Ford to put a wrench tray right there under the hood!:laugh:
  17. buick64203

    buick64203 Just plum crazy Staff Member

    Ok, here's an update-

    I put my fuel pressure gauge on the line and I got only 1/2 lb. I took it off ands was able to have it squirt by pressing the arm down. Good or bad? I dont know. So I put on a new rebuildable stlye fuel pump. Only problem is that the pump has been laying in a box for 20+ years. That one, the pressure gauge flucuated between 1/2 and 1 lb. Ok, lets try one more pump we had laying around which was supposedly a "good used" one. Third pump was completely dead and was leaking from the bottom. All three attempts were made using a boat tank to the fuel pump to eliminate the tank and lines as culprits.

    So after that barrell of monkeys, I quit for the night smelling like a filling station. So either we have three bad fuel pumps ( a good possibility) , or the eccentric is no good. So I walked away more confused than before.

    For my next trick, Im going to procure a brand new pump and try that. Im also going to bring my boroscope and see if I can get a view of the fuel pump eccentric with it. Its an 80k mile car, so I would be surprised. If thats the case, Im welding an 1/8" piece of steel to the fuel pump arm.

    So after the emotional scarring has healed a bit, I'll go after it again.

    On a good note, I did reinstall the door glass after getting the lower glass channel replaced. What a PITA! Took me two hours and the removal of the regulator and P/W motor. whew...
  18. Schurkey

    Schurkey Silver Level contributor

    Did you test the pressure WHILE THE ENGINE WAS RUNNING?

    If you did this at cranking RPM, IS THE PUMP PRIMED?

    First Guess: The pump(s) are full of air, you're not going to get an accurate reading of pressure when the pump is "vapor locked".
  19. buick64203

    buick64203 Just plum crazy Staff Member

    Yes, engine running. It was running on what was in the float bowl.

    Im about *this close* to trailering up to Tom's shop and just droipping it off. Either that or pouring gas on it and lighting it on fire :blast: :spank:

    I located a new pump at a local parts house, but they want 60 bucks. Seems a bit high, but I'd like to get one by the weekend
  20. doc

    doc Well-Known Member

    Jason,,,, just a suggestion... carter makes a electric pump that puts out 7 psi. and will last for years... they are about $75 plus the cost of a block off plate.... it is really nice to turn the switch on and wait a few seconds while the pressure comes up and then hit the starter for an immediate start.... really nice in the winter....

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