79 bronco 351m c6

I am trying to get the carb to run correctly for Cheyenne area (6100 ft). Up to 2015 it was a flatlander below 800ft. I am tired of it running EXTREMELY rich. Coughs, sputter, will not get out of its own way. When temps are 35 and lower the worse it gets.

I popped the power valve, so I replaced it with, I think, a 65. What pv number should I be using? What jet size?

I bought a carb at the jy from a 79 e150 van with a 302 but the carb body number was the same as mine. 121 and it was for high elevation over 4000ft. This carb is giving me the exact same problems as my original unit.

I did fix the rear intake china wall leak and that helps. It will now ideal smooth and sounds good. But drive above 10-15 mph and loads up heavy unless I shift to 2nd gear a do WOT. It is hard to keep the rpm high and drive the 35-40 speed limit.

Where do I go from here? I want it to stop running rich.

 

Forgot this. Electric choke. I do not feel any binding in the choke linkages. Should the choke linkage where it attaches to the choke plate be in the std hole or alt hole? On the choke dial, what location from 3 rich to 3 lean should it be?

On the throttle linkage side, there is a vertical piece with 4 holes. Mine is in the top hole.

 

If it is running too rich, it should run better in cold weather, not worse.

The key thing with power valves generally is to select one that is not open, even a little bit, when cruising or loafing along on flat ground at part throttle steady speeds. Whether highway at 65 or gravel at 25, this is basically an idle circuit & jetting thing.

A mechanic's vacuum gauge is useful here when plumbed into the cabin, and keep an eye on it to get a sense of how much vacuum your particular engine pulls at idle, cruise, up a slight or heavy grade. Have to at least know what your engine pulls at idle. A 6.5" is a good all around choice for stock engines at sea level, and this is what is included in the kits and carbs.

This is based on a stock engine (no lumpy cams) at sea level pulling 19" to 20" at idle, and maybe 15" to 17" cruising on level ground. When accelerating, the second the throttle plates open the manifold vacuum quickly drops to almost nothing and starts climbing back up almost as quickly. This momentary loss of vacuum is what opens the valve to supply extra juice, keep in mind the engine manifold vacuum is the only thing that holds it closed against the internal spring. Sorry to bore you with the details, but if you better understand how they work it will be easier for you to fix this. A 5.5" or even a 4.5" may be adviseable, but it's important to see what's what with a vacuum gauge.

If the base ignition timing or centrifugal advance is screwy, it will hork the manifold vacuum is what I'm getting at. It's important not to cut power valve sizing too close, or the valve will start to open when cruising into a stiff headwind even when on level ground. This is when people start talking about they could literally watch the gas gauge drop. Manifold vacuum tracks engine load and not RPM.

Then, once you're certain the power valve is not open at cruise you can work on selecting the main metering jet size. Skip two sizes at a time, and jet down. The idea is to select a jet size just above the lean misfire point. If the jetting is too small the engine will surge or misfire at steady cruise on flat ground. Some allowance should be made for seasonal changes, unless you want to rejet a couple times a year. But I'm sure you can dial it in a lot closer than it is now.

Again, it's important that everything is straight before really getting into the carburetor tuning - ignition timing, float height/fuel height. A lot of people mistakenly install hotter spark plugs when the problem is that carburetion is way off the beam or mistuned. Start with the factory stock heat-range plug to get a baseline.

 

That is the throttle pump rod overtravel lever. Top hole is for temperatures below 40° F. 2nd from top is 40° to 80° F. 3rd is above 80° F. and above. This changes the amount of fuel the carburetor accelerator pump "shot" delivers.

 

Thank you for the answers. I have slowly working on it. The bronco is my dd and I work retail and last few weeks my shortest week was 64 and longest 79. I will give follow up has I can work on it.