A couple questions

[Castle]

hi everyone, I have been working on a 1968 Newport....bought it last July. In adjusting timing and idol, I can’t seem to get the curb idol at the spec from my FSM. It calls for it to be set at 600- 650rpm.... but as soon as I get it below 725, the carb starts to have a high whistle sound. Any ideas as to what causes it to do so?
Next question has to do with the gas tank.... there are two vent tubes that come out of it, do they perform the same function, and what is that? Is it bad for engine performance if one of them is plugged up? Why do they go into the trunk cargo space?
Timing question.... I upgraded to a Pertronix mechanical distributor, it seems as though the base timing is way advanced from the FSM spec, is this normal.... the manual spec says base is At 7.5 and it seems to run pretty well between 24 and 28.
Thanks for your input!
ps.... I have a new timing light, digital, with advance and rpm.... among a coiuple of its options.

 

[BLUPORT]

hi everyone, I have been working on a 1968 Newport....bought it last July. In adjusting timing and idol, I can’t seem to get the curb idol at the spec from my FSM. It calls for it to be set at 600- 650rpm.... but as soon as I get it below 725, the carb starts to have a high whistle sound. Any ideas as to what causes it to do so?
Next question has to do with the gas tank.... there are two vent tubes that come out of it, do they perform the same function, and what is that? Is it bad for engine performance if one of them is plugged up? Why do they go into the trunk cargo space?
Timing question.... I upgraded to a Pertronix mechanical distributor, it seems as though the base timing is way advanced from the FSM spec, is this normal.... the manual spec says base is At 7.5 and it seems to run pretty well between 24 and 28.
Thanks for your input!
ps.... I have a new timing light, digital, with advance and rpm.... among a coiuple of its options.

Have you confirmed there are no vacuum leaks at the carb? All ports used/plugged, base gasket is good?
If you have a divorced choke, make sure the diaphragm in that isn't rotted out, creating a vacuum leak.
Also check the vacuum advance diaphragm on your distributor, make sure it's not bad.
Make sure you're disconnecting the Vac Advance when you're setting your base timing, and plug the port.
I LITERALLY just went through all of this on my 68 'Port with a 383/2bbl. My timing is happiest around 12 degrees, and when it came to tuning the carb, I found out mine was all gummed up inside, so I ordered one from @Dana which has been the BEST money I've spent on the car so far. A vacuum gauge hooked to manifold will be a huge help too.

As for the vent tubes, I am following along for that answer, because I thought one line was the actual fuel line to carb, and one was a vent, and I'd like to be corrected if I'm wrong.

 

[Buster Hyman]

When I replaced my 67 Newport's fuel tank there was several different vent configurations all in the same fuel tank shape , I think they may have used this system because of the low angle of the fill neck, maybe fuel would block a vented cap especially on acceleration. In Roch-chacha you can have some extreme temperature swings and I would think good venting would be important.

 

[mr. fix it]

my 66 Polara has the dual vent tubes as well.
Just make sure they are in the correct position so they work properly.
With mine I actually shortened them up & had them go up for about 6 inches then a nicely curved 180' bend downwards

here's the FSM diagram of the proper length vent lines..