I have a new symptoms, after adjusting the mixture screws (had to drill them out on the base of this 4180c Holley/Motorcraft carb) to get more stable idle and address rich plug fouling issues the front screws seem to be correct. BUT after warming up nicely (though I do need to knock it down to curb idle after 10mins, doesn't lower by itself) if you rev it slow it backfires out the tailpipe and if you rev it quick it stalls. do I have the rear screws set too lean maybe?

any ideas that I think I can act on will probably prevent me selling the truck! hehe

 

Having to manually kick it down is normal. The choke mechanism is not strong enough to overcome the pressure of the throttle spring.

Take the aircleaner off. With it idling nicely look down the carb throat. Without touching the throttle, do you see any liquid fuel going down into the engine? If you do, you float levels are too high in the bowls of the carb. Backfiring in the exhaust is usually caused by too much fuel going into the engine. Liquid unburnt fuel gets out into the hot exhaust and lights off, causing the backfiring.

 

Thanks, I will have a look at that, you really have to find a spot to safely lay on to get a look at down the carb huh... driver's side seems logical if I lay the air cleaner on the other side like I am used to, any other approaches that are good?

 

I can be tough, especially if it has a lift on it. Just be careful. I know it's a little scary when you are not experienced in working around running engines. I still remember when I started out long ago it was a little intimidating working on and around a running engine. I sometimes use a small step stool, one with 3 steps on it. Just make sure you put a rubber mat under the feet, as you are leaning over it will slide out on just concrete or some other hard surface.

 

What is even better the make a step that fits over the wheel.
On my phone so hard to look up bit sure you can get them at any parts store or harbor freight.
Dave - - - -