Idle Adjustment Screws

[bullfrog]

I have a 1968 Polara with a 383 and a Carter BBD carb (tag #4578). I am having an issue with a lean surge (miss) at cruising speeds. It runs perfectly at idle and under load. I was wanting to adjust the idle adjust screws but this carb is one where the adjustments have been filled with lead from the factory. Have any of you drilled out the lead and installed screws? Is that even an option?
The car is 100% original and I really don’t want to replace the carb if I can don’t have too.

 

[Big_John]

They can usually just be popped out by poking something sharp into them and prying them out.

But that's not going to help. The idle screws adjust the mixture at idle, not at cruising speed. If you are having a lean surge, chances are you have a vacuum leak.

 

[bullfrog]

They can usually just be popped out by poking something sharp into them and prying them out.

But that's not going to help. The idle screws adjust the mixture at idle, not at cruising speed. If you are having a lean surge, chances are you have a vacuum leak.

Thanks for the reply. I’ve replaced the hoses and replaced the base gasket as well because I was thinking the same thing….I was thinking that I might have a leak at the intake manifold gaskets as well but it runs too well at all other speeds and loads. I have also sprayed carb cleaner around manifold checking for leaks.
As background, ignition system has been replaced as well (plugs, wires, condenser, points, even coil). Just has me stumped!

 

[volksworld]

unfortunately the gasoline we buy today is not the gasoline that was available 50 years ago...in my experience it burns leaner, especially if it contains ethanol...and if it has a "non adjustable" carb that generally means it's from a year where they were trying to minimize emissions so it was set up very lean at the factory to begin with...I'm not familiar with your particular carb or what specific modifications it may require...on some carbs transition and cruise are still part of the idle circuit (not the idle mixture screw position but the idle jet ...which may not be replaceable ) others depend on the main jet...so I'm just giving you generalities here...being able to adjust the idle mixture should certainly help but you may need more

 

[bullfrog]

unfortunately the gasoline we buy today is not the gasoline that was available 50 years ago...in my experience it burns leaner, especially if it contains ethanol...and if it has a "non adjustable" carb that generally means it's from a year where they were trying to minimize emissions so it was set up very lean at the factory to begin with...I'm not familiar with your particular carb or what specific modifications it may require...on some carbs transition and cruise are still part of the idle circuit (not the idle mixture screw position but the idle jet ...which may not be replaceable ) others depend on the main jet...so I'm just giving you generalities here...being able to adjust the idle mixture should certainly help but you may need more

Thanks for the information. You definitely game me something to research on the idle jet circuit!
I just acquired the car this summer and got it running again this fall. I’ll try some ethanol free gas as well just for the heck of it too (albeit $5.50/gallon)…

 

[Big_John]

FWIW.. Just occurred to me that that there were issues with the BBD body permanently distorting when the air cleaner wing nut was tightened too much. Not saying you did it, but in a 57 year old car, someone else could have.

I think @CBODY67 has had some experience with that issue, but it could be someone else.

 

[MoPar~Man]

I have a 1968 Polara with a 383 and a Carter BBD carb (tag #4578). I was wanting to adjust the idle adjust screws but this carb is one where the adjustments have been filled with lead from the factory. The car is 100% original.

Were they sealing up the idle screws in 1968? I didn't think that started until the 1970's. Maybe earlier for a California car?

Yes I think the gasoline today doesn't get enough blame for performance issues. Are you hearing spark-knock at cruise?

But also - do you still have points distributor? Check the dwell, and maybe get new points - and capacitor. Check the spark plugs. Maybe too old / worn = gap is too high? Coil or ballast resistor is out of spec? What is your system voltage at cruise? Could be alternator or voltage regulator issue. Carb being starved of fuel? Bad fuel filter (at the engine or in the tank). Bad fuel pump. Kinked fuel hose or line somewhere. Tank not vented (can't breathe, creating a vacuum). What is your idle timing set to (degrees BTDC) ?

 

[SIPLOWGUY]

I agree with MoPar Man. I once had a 78 Caprice doing just that. Although it's a completely different design, the knuckle head who put on the rotor somehow jammed the mechanical advance weights closed. I also experienced severe power loss on a 73 Impala because the point dwell was WAY out. Get your ignition up to snuff first.

 

[Gerald Morris]

FWIW.. Just occurred to me that that there were issues with the BBD body permanently distorting when the air cleaner wing nut was tightened too much. Not saying you did it, but in a 57 year old car, someone else could have.

I think @CBODY67 has had some experience with that issue, but it could be someone else.

I've got about 4 complete, runnable mid 1960s 2 bbl carbs, 2 Carter BBDs, two Stromberg WC3s, and they all are afflicted to varying degrees from warped air horns. That's what compelled me to get a 4 bbl carb. No matter who I got these old, rebuilt and ****** over old ones from, the air horns are always warped. I found the Strombergs to be better for running in general.

 

[bullfrog]

A little more background on this car:

It only has 38k miles and as far as I can tell it is the original carb. When I bought it it had sat since ‘93 (last year it was plated). It was originally sold in WV but it does have the “cleaner air” sticker on the inner fender. The only emissions part it has is the non adjustable idle screws.

While getting it running I replaced:

Plugs, wires, cap, rotor, condenser, points, fuel pump, vacuum lines, vacuum advance diaphragm. Replaced all rubber fuel lines and filter. Also disassembled carb and cleaned it and installed new accelerator pump.

Set dwell with my meter and set timing according to sticker on fender (I think it said 7 degrees?). Found that timing to be not enough and advanced it to 10 degrees BTDC. I have no spark knock under acceleration or at cruising speed.

In trying to figure the surge issue, I also replaced the coil and ballist resistor. I’m getting 14.1 to 14.3 volts from the alternator.

Thanks everyone for their input. You’ve all given me some things to check out. I’ll definitely give an update once this snowy weather breaks and I can give it a test drive.