I finally got some time to work on the carburetors this week. Lots of things going on that kept me from doing much the last 2 weeks.
So I changed the primary main jets on both carburetors from 72 to 68. This really didn't seem to do much for it, O2 value at idle was still down in the 11's. The cruise test was also about the same as before the jet change. At 3000 RPM it did lean it out a bit with O2 numbers in the 12.5:1 range. I didn't mention in my original post that the car does have a 3.00:1 rear end so the cruise test is generally in the 2000 to 2600 range. 70 MPH is about 2700 and 3000 is pushing 80 MPH. Just to see how the primary carburetor reacts to the RPM I run it up to 2500 and monitored the main fuel system with the air cleaner off, no fuel was coming out of the main fuel system. At 3000 RPM I had fuel coming out of the main fuel system. This was with the car sitting still which may not be a totally fair comparison to what it does going down the road but it would seem like it is running pretty much on the idle and transition circuits when under light load going down the road.
I then changed the Idle Air Bleeds on both carburetors per Quick Fuel recommendation. Primary side of the carburetors went from 73 to 76 and the secondary side of the carburetors went from 33 t0 45. This did make a difference in the idle O2 numbers and more so on the cruise test. Idle O2 value went from high 10's and low 11's up to low 12's. Cruise O2 values went from 11-11.5 up to 13+ under light load, this value was relatively stable from about 2000 to 2600. It was very sensitive to throttle position, letting up on the throttle would drop it down to 12.0, giving it just a little throttle would lean it out for a short time, sometimes as high as 16, it would then settle back to the 13 range. When on the cruise test increasing RPM above 2700 the O2 values would start dropping off, I expect that would be due to the main fuel system coming into play. Today I took it on about a 50 mile run on the freeway with about 15 miles of steady 80 MPH at about 3000 RPM, the O2 settled in right about 12.5 so it would appear I could still put some smaller main jets in the primary side, they were 72, and are 68 now. I was thinking of trying some 66 or 64 jets. Also, thought I might do the primary carburetor rather than both carburetors. Is this a reasonable thing to try?
The idle O2 values still seems a little rich but reasonable on the cruise test under light load. Not really sure if I should just leave it alone or try to improve on it. If I do make more changes would I go bigger yet on the idle air bleed or put some smaller idle feed restrictions in it. It still drives decent and idles fine. It might idle just a bit better and picked up a tiny bit of vacuum with the larger air bleeds. Looking for suggestions on this.....
So now I have a question that I hope this group can educate me on. I have not had a lot of experience working on carburetors (until just recently anyhow) so I have been doing a lot of reading on these types of carburetors trying to gain an understanding of just exactly how they work. Most of the material I have studied is based off Ford shop manuals for Ford and Holley carburetors. I also have one of Vizards book on Holley's that I think it would apply very closely the Quick Fuel carburetors that I am dealing with. My question is about the idle and transition circuits. The old 352 and 390 engines with 2 barrel carburetors have the same type of idle and transition circuits that work the same way, except they just have 2. A single 4 barrel has 4 idle circuits, with mixture adjustments on the primary barrels only. Two 4 barrels are double of the single 4 barrel. So now there is 8 idle circuits sucking fuel while idling and in the transition circuits with 15-18" of vacuum. How is a reasonable fuel mixture and O2 value obtained with 8 circuits sucking fuel? Maybe part of the answer is the 2 barrel carburetor idle air bleeds and idle feed restrictions are sized way different but that is something I didn't have access too. It just seems to me with 15-18" of vacuum and 8 fuel circuits it is almost a no brainer that it is going to be on the rich side. I kind of debated even asking this question because when you give me the answer it might appear to be a stupid question but according to my wife, a retired 911 dispatcher, there are no stupid questions, just a lot of stupid people.....
Thanks for all the feedback already given.
