Tuning carb by Air/Fuel readings

[rumblefish360]

Dennis, a lot depends on the cam.

 

[mopar 3 B]

I have found the above idles condition to be consistent with then low overlap of the 509 cam and very little to do with duration or lift. We are looking at a small cylinder with relatively larger valves. At lower rpms cylinder scavage and low vacuum in the intake can be a problem. Any way this was the explanation I got from a master mechanic years ago.

 

[Bigal75]

Ok so you adjusted the transfer slot. Are you adjusting the idle with the secondaries? Or are you adjusting it with the idle screw?

While idling if you close of the idle mixture screws, does the engine die off?

Now you shouldn't have to open the secondaries very much to get it to idle. If it's opened up too much the you may have issues as well. What happens when you advance your timing at idle?

 

[cdr]

you need to have idle air bleeds that you can change.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0CC4QFjACahUKEwixg8TA04vHAhWIj4AKHUO1DK8&url=http%3A%2F%2Fwww.innovatemotorsports.com%2Fresources%2Fcarb-tuning-hero.php&ei=LbW-VfG4LIifggTD6rL4Cg&usg=AFQjCNGwgcbdjbVvHfJG6xz3-vX-AgrxpA&bvm=bv.99261572,d.eXY

 

[coloradodave]

For info only: The secondary throttle blades are not designed to be adjusted for idle on a vacuum secondary Holley carbuteror. The small screw ( accessed from the bottom of the base plate) that rests against the linkage on the passenger side is there only to make sure the blades don't 'stick' in the bores. You back the screw out until the blades are completely closed and then turn the screw in until it contacts the linkage plus 1/4 to 1/2 turn more. That's it.

 

[Bigal75]

For info only: The secondary throttle blades are not designed to be adjusted for idle on a vacuum secondary Holley carbuteror. The small screw ( accessed from the bottom of the base plate) that rests against the linkage on the passenger side is there only to make sure the blades don't 'stick' in the bores. You back the screw out until the blades are completely closed and then turn the screw in until it contacts the linkage plus 1/4 to 1/2 turn more. That's it.

Yes and no. Lol. It should be a balance. A lot of the issues are caused by timing. With the transfer slot properly set on the primary side it should barely take any adjustment of the secondaries. Worked great on my old 750 cfm 4160 I had.

 

[coloradodave]

Yes and no. Lol. It should be a balance. A lot of the issues are caused by timing. With the transfer slot properly set on the primary side it should barely take any adjustment of the secondaries. Worked great on my old 750 cfm 4160 I had.

Bigal75: Did a little digging and came up with this. Learn something new every day. Thanks.

Adjusting Idle Mixture Screws


"If your car or truck has a camshaft with considerable duration (and as a result, a very lumpy idle), adjusting the idle mixture screws may not help very much. There’s an easy fix: adjust the secondary throttle stop screw. Holley four barrels like this 750 cfm 0-3310C have a small set screw located beside the passenger side throttle bore shaft bore (on the base plate of the carburetor, so you’ll have to flip it over). By turning the screw clockwise approximately 1-1/2 turns, the throttle blade on the secondary will open slightly. This allows a rather large amount of airflow through the carburetor without increasing the “window” seen by the idle transfer slot. At this time, the primary throttle blades can be closed, which tends to enhance the sensitivity of the idle mixture control."

 

[Bigal75]

I'm always learning when it comes to carbs lol.