1961 Dual Quad Idle Adjustment

[Joseph F.]

First, thanks to all who responded on my valve lash questions. All is well with lash. Working now on Dual Quad adjustments. Carbs were recently rebuilt - new parts installed, float levels adjusted per specs. Has run all 4 idle screws 3/4 turn out. Spark plugs were black and wet. Now running all 4 at 1/4 turn. Idle seems good with no acceleration or other issues. Any ideas on accurate idle adjustment?

[Tom B.]

Re: 1961 Dual Quad Idle Adjustment

First start with the right spark plug. AC 43's and 44's are too cold and you will fight wet plugs and fouling even with a standard length 45. Put in a set of 45S (R45S) plugs which are extended tip plugs. They will prevent fouling which is something I learned way back in the '60s on a hot 283 I street raced. With the carbs set right and the extended tip plugs you should not have any wet plugs again from fuel.

Now start by setting idle speed on the rear carb at 800/850 rpm. Then back off the mixture screws 1 turn from lightly seated. Adjust mixture screws to highest steady vacuum reading, if you can get a vacuum reading, or rpm otherwise. Repeat mixture screw adjustments for the front carb. Reset idle speed on the rear carb. After setting a good smooth idle turn all 4 mixture screws in 1-2 flats on the screw to lean out the mixture. This will cause a slight rough idle in the shop but will result in a smooth road idle.

Note: I found out years ago to not try to set idle speed on both carbs. Just set idle speed on the rear carb.

Here's a tip I got from John Hinckley back about 2002. I had been simply backing off the front carb idle speed screw until it wasn't touching the throttle shaft arm tab and calling it good enough. John pointed out that the tab that the screw contacts might need to be bent slightly to allow for the primary throttle blades to fully close to make sure it is not drawing idle fuel through the transfer slots which will foul the front plugs. John also recommended using 2 gaskets under the carbs to raise the carbs slightly. When the front carb is closed the primary throttle shaft arm on the passenger side can contact the intake manifold runner preventing the throttle blades from fully closing. I have 3 sets in the garage and all 3 show a mark on the runner where this has happened. One set is on a complete 1961 270 engine I have that has never been apart.

One more tip from John: check that the ears on the front carb linkage slider are parallel to each other and the rod slides freely through them. You may have to bend them or even slightly hone out the holes to get a smooth operation. Also polish the rod to remove any roughness and lightly coat it with Lubricate and you will be surprised how nice it works. This will also keep it from binding and holding those throttle blades open.

Tom

[Bob W.]

Re: 1961 Dual Quad Idle Adjustment

Tom, Great information thanks for posting it as I will be doing a 2x4 car at a later date.

Bob

[Joseph F.]

Re: 1961 Dual Quad Idle Adjustment

Thanks for your reply. Already using R45S plugs. Will work on the procedure you outlined.