Need an engine guru

[Solver]

got thee longhorn ready to fire up. older 350,hei- new plugs,wires,cap,rotor.
Bought a rebuilt quadrajet at parts store. I have a surging problem just coming off idle.Highway speeds seem ok. Idle is kinda crappy also. It runs better when I leave a vacuum port open (ie no hose on carb port)seems to smooth out somewhat.The vacuum thing is probably my biggest clue to problem. The idle mixture screws don't seem to have any affect on vacuum at all. They really don't even seem to affect the engine speed. I am thinking its a bad carb thats why all carb info.
Whats your ideas?

[JIMs70GMC]

sounds like the idle is too high, if the idle screws have no effect. They only work when the throttle plates are almost completely closed. Disconnecting a vacuum line is also leaning the carb out so the carb could be dumping gas. Take it back and get another.

[MACKL]

Jim's right on target. See if you can get the idle down. Then try to adjust the air fuel ratio. The air fuel adjustments only control fuel mixture at an idle.
The rest is controlled by a number of items, all internal. Float level, jets and metering rods. The primary metering rods have an adjustment also. If out of adjustment, the vehicle will hesitate at low speed acceleration. On late model carbs, there is a small plug on the top of the carb, just in front of where the air cleaner gasket sits. If you remove it, you can adjust them. On the older carbs, that adjustment requires the top of the carb to be removed. If the carb wont adjust after getting the idle down, take it back.

[Solver]

Ok I took the carb back and got another. So now I am kind of ruling out the carb. Got the idle and timing set fairly close but still not right in the run dept. When I pull the vacuum line that goes to the tranny comming off the manifold the engine runs alot better . I was sure this should decrease engine rpms. Tried same thing with brake booster and same results. Any vaccum line pulled increases rpm and smooths out the idle . Any ideas? I have messed with two front screws but they don't seem to have much effect on engine. Vaccum guage only gets up to about 15 no matter what I adjust.Don't know if this matters on solving problem but the only gasket that fit was a really thin one cause it was the only on wide enough to cover some channels in manifold but it does line up with bottm of carb right and did come with new carb
Help!
Thanks
Shannon

[hobby67stepside]

Shannon, I don't know squat about carbs, wish I could help you, but its good to see you posting again!

[Class of 69]

I will suggest getting a timing light on it and getting the timing right, before you go any further. Sounds like the timing is retarded. Remember to remove and PLUG the vacuum line to distributor, if there is one. Good Luck !!

[Solver]

Timing set at 12 with dist. line plugged

[71Rat]

did you try and play with the mixture screws in the front? or the idle screw? if i remember correctly a good base point for those mixture screws is if you go in all the way and then back them out 2 turns... gives you something to start from and they both should be about even, dont want one in a turn more then the other.

Rat

[72C203503ONTHETREE]

Sounds like you either have bad cam timing or the fuel pump is screwed up and putting out too much pressure and overfeuling the carb. You should have somewhere near 17-20 inches vacuum if you have a stock type cam and your getting the plugs to fire good at idle.

[MOTHERTRUCKERS]

Is this a Tomco rebuilt carb? If so check the throttle shafts for excessive play, they don't rebush the bottom plate shaft holes.

[Jeepster376]

Originally Posted by Solver

Don't know if this matters on solving problem but the only gasket that fit was a really thin one cause it was the only on wide enough to cover some channels in manifold but it does line up with bottm of carb right and did come with new carb

These channels? The channel in the manifold could be a heat riser passage (exhaust) If so it has to be sealed up, either with the correct carb with matching passage or plugged or covered with a plate that will seal it up. Otherwise exhaust could be leaking into the intake plenum.Causing royal mixture havoc. I had a manifold like that once, that I sealed up by screwing a couple pipe plugs into the holes.

On the idle mixture screws, if you turn them in all the way the engine should die, If so they're working. Adjustments need to be made slowly to let engine adjust and stabilize to each change. If it doesn't die when you screw them all the way in, then something is wrong. Idle too high, or float level too high, due to float adjustment, bad needle and seats, or fuel pressure too high.

[Perchjerker]

Timing should be set with the distributor vaccum line disconnected.

[Bowtie67]

Fuel mixture screws wont do anything if there is a vacum hose off, with vacuum lines all on spray around carb and intake and see if engine RPM raises if it does you have a vacuum leak and needs to be fixed. if you dont have any vacuum leaks and turning the front screws doesn't do anything you have another bad carb. I have had 4 bad carbs in a row replacing with rebuilts.

[Solver]

Thanks for all the input guys!!!
some of you were reallly close to problem and all help was greatly appreciated
Jeepster hit it exactly.
Got exhaust coming back into carb from a special casting manifold.
see my updated post (jeepster wins guru) for my next problem to be solved.
Once again this board save the day!