Dual Quad 270hp carb adjustment

[Clark Chapman]

Looking for assistance in proper method for tuning idle jets on my origonal 58 dual carb setup. Issue seems to be that it runs pretty rich at an idle after fully warmed up. Front & back carb jets started out at roughly two full turns backout out from closure and turned in util highest RPM acheived but yest still seems to be running rich.
The only non stock change made to the 283 was the change of the cam to a Comp Cam "Extreme" model (Mild RV Cam for small blocks)

Thx,
Clark

[Ed Jennings]

Re: Dual Quad 270hp carb adjustment

Clark, To at least some degree, running rich at idle is sort of the nature of the beast. When I had a car with a 2X4 setup, I had the jets on the front carb almost closed. Ran a lot better, but still a tad rich. You need to be sure that the throttle plates are closed completely at idle. They can be adjusted by loosening the screws and sliding them until they are fully closed and tighten the screws. If they bypass much air it will contribute to the rich condition. Also, be sure you are running plugs that are hot enough. AC 46 is the plug frequently recommended for street use. They are a little hard to come by (though available at a price). AC 45's will work ok, and can be had at the local parts store.

All this said, it's still probably going to run rich. The two carbs together are capable of a flow of about 760 CFM! By comparison, the 290 HP FI unit for that year had a max flow of about 585 CFM.

[James F.]

Re: Dual Quad 270hp carb adjustment

Clark,
Ed's suggestions are good. Tuning for the highest RPM and/or vacuum is also the correct. Tilting at windmills if the throttle plates are not correctly seated.I would expect your "cam" is going to be a real problem! Not so much on the idle circuit but off idle driving. Originally the WCFB's power piston and cam were a matched pair figured out by engine engineers. The power piston would be released (by spring power) at a specified vacuum. Doing so would increase gas to the engine. Your cam is mis-matched with regard to the power piston springs i.e. your engine vacuum is not in the 16"- 17" vacuum range (I'm guessing). I would expect your engine to always run rich on off idle conditions (until you are in the way upper RPM ranges of the engine. Chuck Smith (618) 222-5757 (I'm not connected with yada, yada) has weaker power piston springs and some various rods. You may be able to help your rich idle somewhat by changing rods and off idle condition with weaker springs. Regards,

[Clark Chapman]

Re: Dual Quad 270hp carb adjustment

Thanks for the tips. I'll check the adjustment again.
Clark

[Clark Chapman]

Re: Dual Quad 270hp carb adjustment

Thanks... I'll check the vacuum as well.
Clark

[Bob Herman]

Clark, I may have a similar problem...

... with my '58 dual quads. Also, have a mild Comp cam. After warm-up, the engine runs little rough at idle and tends to bog down at low rpm's when starting out in 1st gear. I was thinking this a lean issue, not a rich one. By comparison, how is your warm engine operating? Why do think it's running rich, plugs? exhaust smell? Thanks in advance.

Bob

[Clark Chapman]

Re: Clark, I may have a similar problem...

Exhaust smell is rich. Off idle & higher rpm it runs pretty smooth.
The jet size was my first thought... not sure if they were replaced at sometime
but the previous advice is worth looking into. I have not looked into it yet.

[Wayne C.]

Re: Dual Quad 270hp carb adjustment

It's been 40 years since I had a Chevy with dual 4's, but my recollection is that I tuned to best rpm, as you stated. I tried to achieve the best rpm adjusting in until the idle roughened, then back out (rich) to best idle; then after achieving best rpm that way I ended the tuning session by turning each idle adjustment screw in one quarter turn (lean) to assist the rich mixture problem and set the idle speed via the linkage stop. I also had good luck back then with a spark plug that had 3 electrodes, but that may have been before I finally arrived at the right tuning adjustments.

[Wayne C.]

PS...

Forgot to mention that tuning was done by starting with all screws adjusted the same amount off their seat, then turning all adjustment screws the same, ie, all in one-quarter turn, let rpm settle, all in one quarter again, etc... never worked with one screw at a time. That may have been a given in your description, but I hate to make assumptions.

[Bob Babcock]

Re: Dual Quad 270hp carb adjustment

Chuck Smith did a wonderful restoration of my 1960 270 hp carbs and it runs beautifully. I don't know what he does, but I like it.

[Roy Braatz]

Re: Dual Quad 270hp carb adjustment

Clark! I wrote many articals for SACE when I was the editor and the one that got most response was metering rods for the dual carbs. For many years I'v seen Dual Carbs runing rich at ideal and haveing poor gas mileage. Most all only needed the metering rods changed. ( adjusting the ideal srews wont help) With out tearing down the carb to see if the jets are 21 or what ever , you just need to come by some other metering rods of various sizes in thickness and by knowing the thickness of the rods your useing you eather increase or decrease there thickness to in large or make smaller the jet hole that they go in. Think of it as hole in your tire,a small hole (less air lose - larger - more air lose)As you stated! I would try thicker metering rods at the front carb, that will lean the carb.
Also by the way, check to see if they are adjusted right by removeing the metering rod Cover and with the choke open and gas pedal closed loosen the rods stop srew, push the metering rods down(spring loaded) and tighten the srew. Now while pulling on the gas pedal rod the metering rods should JUST start to rise. That is the correct adjustment for the rods. This is also true for any CARB one,two or three.