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Thread: Fresh 4776 (600 CFM 4150) Rebuild

  1. #1

    Default Fresh 4776 (600 CFM 4150) Rebuild

    Short version: New-to-me, freshly rebuild 4776, won't idle, and runs really lean (25:1 - 30:1) at 1700 rpm. What could cause such a lean condition?

    Long version:
    I bought a used 4776-6 (a 600CFM 4150) that had been sitting for a number of years. The pumps were frozen and it was heavily varnished, so I rebuilt it using the Holley Trick kit.

    This is going on a stock '77 Chevy 350 with all the emissions equipment removed. That motor was running a modified 750CFM Carter, which is a little rich (as you would expect from that size carb on that size motor) - at around 11:1 at WOT.

    On the Holley, I kept everything the same size as the previous owner - from memory (I'm at work and the car/carb isn't): 66/78 jets, 32/28 squirters, 6.5 power valve (which I replaced during the rebuild). During the rebuild, I soaked the metering blocks and the main body in Sea Foam, sprayed the hell out of all the passageways with carb cleaner, removed all traces of old gasket with a blade and scotch-bright, and scrubbed everything with a brass brush. I didn't soak the throttle body, as it wouldn't fit in my soak "tank," so it was only sprayed.

    For the Holley's test fire/run, I plugged all vacuum lines on the carb, ran the fuel pump to set an initial float level (engine not yet running), and I pulled the idle screws back 1.5 turns. With that, the motor won't idle below 1700 rpm - it just sputters and dies. If I keep it up at 17-1800 rpm, I see 25:1 to 30:1 on the AFR (I'm surprised it even runs)- even up to 3.5 turns out on the idle screws, still no low idle, still really high readings on the AFR. If I bring it up to 3k rpm (in neutral), I see AFR down to around 17:1 - still too lean.

    I would expect the lean condition to be caused by a vacuum leak, but 30:1, to me, means that it's getting almost 2x the air it should be, which means that as much air is coming through the leak as is going down through the main body - which seems unlikely. Is that logic flawed?

    All that said, I'm no expert. While I've worked a bit with the Carter, the Holley was my first full rebuild. Is it possible I left something out? What would cause such a lean condition?

    Notable items:
    - The needle valve under the squirters was completely frozen before the rebuild. Fuel wouldn't even flow by them and they needed to be dislodged with a pick before they would move. They work fine now. Maybe this same thing is happening somewhere else in the carb? Other than the backfire check valve, are there any other moving parts that could get frozen over time?
    - This is a road-race car with no need for a choke, so I removed it completely - only the horn remains. Suspecting there may be a vacuum leak through the choke assembly, I replaced the assembly, minus the choke plate. No difference. I'm suspicious of two holes under the choke assembly that aren't plugged, but are covered by metal on metal. It doesn't look like those holes go anywhere, but it's difficult to see.
    - After all of this, putting the Carter back in place puts the motor right back at the previous rich condition, which tells me the problem exists in the carb.
    - AFR is only monitoring one bank - the car has the stock 4-1 manifolds with straight pipes back from there. no x-pipe.

  2. #2


    Something that has been sitting for so long, and had so much gunk in it, is really hard to say. From the sounds of it, sounds like the idle circuitry still has some gunk in it. Probably in the base plate or throttle body. When something is gunked up that bad, it's a crap shoot on rather or not it gets all cleaned out. Might wanna try chasing all the holes you can with a pipe cleaner or small wire. And go buy a GALLON of carb cleaner, so you can put the entire throttle body in it. Let the throttle body sit in it for 3-5 days. As soon as you pull it out, hit it with compressed air.
    Last edited by 69cam; 09-08-2015 at 01:47 PM.

  3. #3


    Thanks for the reply. I'll give those suggestions a shot.

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