Hey all, I'am at wits end with this thing. Here's what I have. 283 w. rochester 2g carb (rebuilt)#50 mains not sure of pump jet size, stock dizzy w/pertronix. Base timing set at 4deg. w/vac adv total around 22deg. Now for my problem, it seems to fall on it's face about 1/4 throttle in. The quicker I mash it the harder it falls. I know some of you will say it's an accel pump problem but both jets fire a nice steady steam off idle and it did the same thing before I rebuilt it. Now if I'm on the highway at any speed and just hold the throttle steady I can kinda feel an "on off" surge. If I give it a bit more throttle it goes away until it reaches the new speed and starts to do it again. I have tried playing with the timing and it has made no change what so ever. There don't seem to be any air leaks anywhere and it pulls about 20in of vacuum. Any suggestions would very helpful.

Symptom sounds to me like 'lean surge'. That would be during the transition from the idle circuit to the main or power systems. Could be lower idle air bleeds or power system. Here's an online site with really good manuals.http://www.tocmp.com/manuals/Carbs/Rochester/2-Jet/Manual/MCarbRoch1973__2G_0011.htm

Thats a great reference article,but it doesn't really get into trouble shooting and reapir. I saw this power system when I disassembled the carb during rebuild and blew out all of the passages with compressed air. My question is what is my power system doing that it shouldn't be or what is it not doing that it should be?

Click INDEX at the top and go down to TroubleShooting. There are flow charts by symptom. These are the 'manuals' used back in the day by dealerships for repair.

Yep, now I see it. Thanks Dave.

Yes, sounds like lean surge. First thing -- is the float height right. Second, a couple more degrees of timing probably wouldn't hurt.

I follwed the instuctions on the rebuild kit to get the float height to what I believe is the correct height. As far as timing goes what I figured if all else failed go back to factory recomended specs. Currently I have it based at 4deg. with the vac/adv hooked up it's at 22deg at idle. Zeke what do you recommend I change my base timing to?

With a stock cam, maybe go to 6-7* degrees of initial timing. If you have stepped up the cam at all from stock you'll need more than that -- maybe 10-12* keeping your total under 36*. It could require even more, but I'm guessing not much cam if you are still running the 2G carb. The idea here is that more timing will help it to idle with a less throttle blade opening. This will keep you out of the transition circuit at idle, and allow it to do it's thing as the throttle opens more.

Frankly, you can play with timing a lot as long as it isn't pinging under acceleration. If it likes more timing, give it, put don't exceed 36* total mechanical plus initial.

BTW, all this assume good cap, rotor, wires and a strong coil. I assume that the condenser is gone if you have replace the points with Pertronix.

I'm picking up on the part of your problem where you have the lean surge at steady state highway speed where you have some throttle opening, but are not using the accelerator pump circuit to deliver fuel.

I'll give the timing another go. it used to be at 16deg initial and it did have a bit more spunk,no ping, but it seemed to have a "rich" unburned fuel smell from the exhaust and still the hesitation was there. when I backed it off to 4deg it got better milage, the exhaust richness was gone and so was the "spunk" through all of this trial and error the hesitation problem remained.Let me bring another idea, what if when I come off idle and lose vacuum and all of the advance from the vac-can, could that sudden loss of advance cause a hesitation?

That little bitty fuel filter in the front of the carb clogs up quickly. I put a big metal one ahead of it in the rubber line between frame and engine.
Whenever servicing carb or filter, I find a way to blow the fuel line backwards into the tank, in case the pickup screen there is getting clogged.

Your vac/ adv should be hooked to ported vacum not manifold vac. check timing with it disconnected 10 to 18 should work good for idle and when you bring the r's up the timing should advance ..
Then hook up the vacum line and see what total is.it should not be more at idle , if it is then the throttle plates are opened too far , and you have another problem.
Personally i do not like vac/ adv.. i prefer a well laid out mechanical curve that is all in by my cruise rpm. Usually with a total of 35-41 depending on vehicle and application..

Don't play the mileage card on me .. my vehicles with carbs and no vacum get better mileage than they did from the factory and have more power..
Key phrase "well laid out"

Your vac/ adv should be hooked to ported vacum not manifold vac. check timing with it disconnected 10 to 18 should work good for idle and when you bring the r's up the timing should advance ..
Then hook up the vacum line and see what total is.it should not be more at idle , if it is then the throttle plates are opened too far , and you have another problem.
Personally i do not like vac/ adv.. i prefer a well laid out mechanical curve that is all in by my cruise rpm. Usually with a total of 35-41 depending on vehicle and application..

Don't play the mileage card on me .. my vehicles with carbs and no vacum get better mileage than they did from the factory and have more power..
Key phrase "well laid out"

There may be somthing to that Jeff, this vac/adv definitely gets vacuum at Idle. I can hear the diaphram snap open as soon as I hook up the vac hose to it. So maybe the plates are open too far. I'll play with the timing and just recurve the Dizzy to eliminate the need for the Vac/adv.

That little bitty fuel filter in the front of the carb clogs up quickly. I put a big metal one ahead of it in the rubber line between frame and engine.
Whenever servicing carb or filter, I find a way to blow the fuel line backwards into the tank, in case the pickup screen there is getting clogged.

Great suggestion, I was always skepi as to how much fuel actually gets through that brass pumis stone.

Great suggestion, I was always skepi as to how much fuel actually gets through that brass pumis stone.
My aftermarket variety filter in the carb nut is pleated paper construction with redish rubber (?) ends.(71 Malibu 350/350 two barrel single exhaust)

Your vac/ adv should be hooked to ported vacum not manifold vac. check timing with it disconnected 10 to 18 should work good for idle and when you bring the r's up the timing should advance ..
Then hook up the vacum line and see what total is.it should not be more at idle , if it is then the throttle plates are opened too far , and you have another problem.
Personally i do not like vac/ adv.. i prefer a well laid out mechanical curve that is all in by my cruise rpm. Usually with a total of 35-41 depending on vehicle and application..

Don't play the mileage card on me .. my vehicles with carbs and no vacum get better mileage than they did from the factory and have more power..
Key phrase "well laid out"

Well I brought the base timing up to 10deg. and it certainly livend up the response. I hooked up a vac gauge to the vac/adv hose and it shows vacuum at idle no matter what the timing or how much i reduced the idle speed to close the butterflies. I'm not sure what to do to elimnate vaccum from the vac/avd line at idle. Anyway the added timing did help some with the hesitation so I think I'm on the right track to solving that issue.
Thanks to everybodys helpful suggestions.

I really like manifold vacuum for the vac advance. Ported vacuum is the advent of emissions tuning, where they wanted a more retarded timing at idle to keep combustion temperatures down to keep smog (NOx) down.

At slight or partial throttle opening, you still have some vacuum advance adding timing. That's okay because you don't have a great amount of combustion pressure and need to get the flame front started early. At full throttle opening, vac advance drops off but as RPm comes up, mechanical advance comes in.

For sure, you can live without it and rely on a well tuned mechanical advance. But it so easy to add up to 20* of additional advance based only on engine load.

I really like manifold vacuum for the vac advance. Ported vacuum is the advent of emissions tuning, where they wanted a more retarded timing at idle to keep combustion temperatures down to keep smog (NOx) down.

At slight or partial throttle opening, you still have some vacuum advance adding timing. That's okay because you don't have a great amount of combustion pressure and need to get the flame front started early. At full throttle opening, vac advance drops off but as RPm comes up, mechanical advance comes in.

For sure, you can live without it and rely on a well tuned mechanical advance. But it so easy to add up to 20* of additional advance based only on engine load.

Currently, I have it at 10deg base around 27deg w/vac adv 36deg total at 3000rpm. The hesitation is almost unnoticable, and it's fun to drive again. Only other item I wanted to add was a hotter coil just for *****s & giggles. Otherwise, problem solved.