Lots of carb discussions elsewhere but figured I'd start a new topic in case someone else is having similar issues.

After rebuilding the head on my 73 2.0 with the knocking rod, (only above 45 mph), the carb started acting up by causing severe miss and backfire through the carb on acceleration even after fully warmed, (I have a direct reading water temp gauge).

I initially thought maybe accelerator pump or float setting. When I operated the throttle I could see fuel from the squirter. Took the carb top off and found lots of red powdery stuff in the float bowl. Assumed rust from the tank and or one of the side benefits of our adulterated fuel. There is an in-line filter which was clear. Float setting was correct and needle and seat were good. Cleaned the bowl and jets out and put it back together. No joy still lean on acceleration.

Next time I checked the squirter while moving the throttle slowly and no fuel. Took the accelerator pump apart and couldn't see any problem. Diaphragm looked good, spring was there and no crud. Took the squirter off and cleaned it out. Really couldn't tell if it had anything in it or not but put everything back together and wa-la; joy. Purred like a kitten with absolutely no hesitation or backfire.

Joy that is only when the outside temperature is above about 60 degrees. Lower and it does the same thing although not quite as badly. Continues even after the engine is warm.

I'll wait for some cooler temperatures and check the squirter output again. If it's OK then on to a choke setting adjustment although the engine starts on first crank and goes to high idle as it should and the butterfly is set where it should be. I know that still doesn't mean the choke is operating as it should but never had the problem before the head rebuild. Guess better compression could mean greater air flow and the need for longer choke on time but I'm still suspicious of the Accelerator pump and a diaphragm stiffing up in colder temperatures.

I do tend to think about exotic and unusual causes for problems at times instead of the simple obvious solutions .

If it's not OK then I will put another pump kit in. I suppose I should consider the fact that the crud I found in the float bowl is in the carb passages and nothing short of a total soak of the carb is going to make things better.

Now that I've answered all of my own questions and worked out my process to troubleshoot this issue I will advise of the outcome. Comments welcome.

A lean condition will definitely make it backfire through the carb and a fresh clean head will flow better than a cruddy wore out one, so if the A/F ratio was right before the re build it could very well be lean now especially when cold. Not saying that's the problem but could be part of it, best check the complete tune up..