C-1 adjusting dual carbs

[Mike B.]

New to corvette and especially to 2 x 4's and dual point set up w/o vacuum. I get a popping back through the carbs on moderate accel and do not feel carbs are adjusted right and the "feel good method" of turning screws not getting job done properly. I have a '61 270 hp and My question ( and I searched past threads w/o any finds) is how to use vacuum guage to adjust carbs. More in particular, do you use a tee fitting and draw vacuum off both carbs at same time while adjusting screws per the manual or do you do each carb vacuum one at a time. Also I assume I get vacuum at rear base of carb with a nipple type fitting. any help will be appreciated. As a side note there is a loud ticking noise coming from rear carb at the vent bowl cap. any idea what can cause this before I take carb apart??? Thanks, Mike 53700

[Duke W.]

Re: C-1 adjusting dual carbs

The idle speed/mixture adjustment should be in ST-12, but first you should check the distributor for proper point function, and I recommend you advance the initial timing to 10 degrees, but it must be set below the point where centrifugal starts or use a dial back light to set the total WOT timing at above where the centrifugal is all in.

Spark advance map specs should be in ST-12 and are also in the AMA specs available as a free download from the GM Heritage Web site.

Target the idle speed at about 900, and if it still has Duntov cam idle vacuum should be near 12".

Any port that reads manifold vacuum on either carb or the manifold is a suitable connection for the vacuum gage.

The rattle may be metering rods because manifold vacuum (due to too low idle speed or a leak) isn't enough to keep the metering rods fully down, so they vibrate up and down. I've experienced this with AFBs.

Backfire can be caused by a transient lean condition so float and acc. pump dimensions should be checked. It can also be caused by an improperly installed distributor or improperly indexed wires.

Check all ignition system functions to be sure the ignition is to spec before going on to the idle speed/mixture adjustment.

Duke

[John H.]

Re: C-1 adjusting dual carbs

I have a '61 270 hp and My question ( and I searched past threads w/o any finds) is how to use vacuum guage to adjust carbs. More in particular, do you use a tee fitting and draw vacuum off both carbs at same time while adjusting screws per the manual or do you do each carb vacuum one at a time. Also I assume I get vacuum at rear base of carb with a nipple type fitting. any help will be appreciated.

Mike -

Paragon has a handy 2-piece vacuum fitting (their #755) that replaces the hex plug in the rear of the base of the rear carburetor for connection of a vacuum gauge; screw the hex portion in first, then screw in the threaded nipple and you're in business, without having to remove the carb for swing clearance for the nipple.

Assuming you already have the ST-12 manual ("1953-62 Corvette Servicing Guide"), you should also get the 1961 Passenger Car Shop Manual, which has an excellent chapter on the 2x4 carburetor installation and servicing; the ST-12 doesn't cover it, as the components were shared with the passenger cars. Both are available from the NCRS on-line store.


Paragon-VacFitting-755 (2).jpg

[Mike B.]

Re: C-1 adjusting dual carbs

thank you for your time. I'm old and my college days 283 67 chevelle was to adjust dwell with hex key and meter and set timing and go. there is alot more to this than I thought. Both Points adjusted at .016 and dwell is at 35 which is the tolerance of plus 1. No variance with higher rpms and no popping at higher rpms when not under a load. I will get the vacuum adjusted and instead of turning screws and see what happens. thanks again

[Mike B.]

Re: C-1 adjusting dual carbs

thanks for lead on fittings. I do have both manuals and it has been a great source of knowledge but no info about proper way to get vacuum since no manifold access. Also nothing about the ticking noise at vent bowl.