Hey Scotty. I have a 1972 ford LTD Brougham with a 429/C6 trans and 75000 miles on it. It has an automatic trans and for the most part is bone stock powertrain. I swapped out that battleship of a factory intake with an edelbrock high rise dual plane intake. I also ditched the points for electric ignition. I did pickup a new double roller timing chain set as I was told the factory ones with their nylon coating don't last very long. When changing the timing chain out i was wondering if i should pickup a new light cam as well as setting the timing to 0 as I believe the factory has it at -4 degrees. I haven't had the time to do the swap yet as I'm kind of in limbo with the carb. I had a 750 holley with vacuum secondary which was ok but my friend let me try a 650 double pumper with mechanical secondary which made the car lunge like crazy. Quite a change in power off the line. I swapped the jets to a 72 and the motor starts and runs fine without using the choke at startup. Living in Canada and being close to freezing when I tried I found it strange. Is it possible that I might be leaning out the fuel to much with the 650 eventually blowing up the engine? Sorry for the essay I just want that 429 to work the way it was designed to, without all the emissions that are choking it out. Thanks.
Then why does a carburetor have idle jets? Of course they're involved. The best way to see if your main and transition jets are the right size is to do what racers do - it's called a plug chop. Accelerate under load for about half a mile and kill the engine. Pull out a spark plug and check its color. This is a good indication if the mixture is right. Another alternative is to take it to a professional tuner and for major $$, they'll do it for you. Remember, if your running jets too lean, it can burn the pistons.
I don't believe that the jets are involved at idle.
If they're not involved at idle, then how does the engine get the fuel/ air mixture in order to run? This is the reason why there are typically multiple jets (barrels) in variable speed engines.
A 2-barrel has an idle/low RPM jet, and a higher RPM jet. Quadrajets have 2 low-end jets and two high end jets, if I'm not mistaken. They're essentially venturis, only for fuel, not air, and flow differently based on the airflow through the carburetor. The conditions at idle (high manifold vacuum) prevent the high RPM jet from flowing, as the engine speed increases, manifold vacuum decreases, which eventually changes the pressure in the carburetor to a point where the low-speed jet stops flowing fuel and the high RPM jet takes over, just from flow physics, not computers.
I don't think you'll be leaning out the fuel mixture by putting a smaller carb, generally speaking. You're just limiting how much your engine can breath. That said, you may still need to set the jets so that your engine is getting the correct fuel mixture with the setup that you have and the environment. If you have headers, one indicator of a lean mixture might be a red-glowing header pipes while fast idling at around 2000rpm or more.
I don't believe that the jets are involved at idle.
They are the main jets. Alot of people suggested that anything smaller than a 750 is too small. When I had the 750 on it got decent fuel mileage, considering it is a boat of a car but didn't have the get up and go the 650 had. I will do a spark plug test when spring hits. I just found it odd that in cold temps, say 40F, it started and idled great WITHOUT using any choke whereas with the 750 it needed the choke on until warmup was achieved. With the 650, the engine started up quickly and ran perfectly, as though it had been running for hours. The exhaust didn't smell rich.
When I bought the 650 used it would not idle and stay running unless the choke stayed on. Even after the engine was at operating temp it still needed the choke or it would stall. The jets that I changed I think had 34 stamped in them.
Like I suggested, get in touch with Holley and ask them, please.
If they're not involved at idle, then how does the engine get the fuel/ air mixture in order to run?
I'm not sure why you're going on about other carburetors. But any case, it sounds like I may be way off, so if the size 72 jets are affecting the idle, I'd like to know how that is the case.
My Holley 650 dbl pumper only has the screw-in jets and the idle mixture screw. I'd doubt that the jets will affect the idle mixture. How much fuel is needed for idle, as opposed to amount of fuel that the jets are meant to flow? Besides, size 72 jets doesn't sound like idle jets to me, even if there were idle jets on his carb.
You are completely wrong. That's why they are called "idle jets." Why don't you contact Holley and get some help from them on the jetting and tuning of your 650? They made them. The idle jet size is critical to a smooth transition from idle to WOT.
It seems that what you're calling "idle jets" are the idle feed restrictors. I'm not sure if these are the jets he changed. Based on what he's working on, the 650 is likely the low end carb with fixed "idle jets" and he just changed the main jets that are the screw on type--not the "idle jets."
@bokeoyaji - Wrong again.