Help With Backfire Through Carb

[USAF]

Just picked up a 72 Chaparral Firebird SS with the Fuji 440 and the first 1-2 minutes it  runs great but after it’s warmed up, it starts backfiring through the carb at steady 1/4-1/3 throttle.  Has a major loss of power also.  Tried 2 sets of plugs & fresh premium fuel mix but what do I consider next? 
Single digits here in Northern Wisconsin for highs today and have nowhere warm to work on it so any help to narrow this down is very much appreciated.  I’m thinking coils or fuel filter? 

[Vintagesleds.com]

[big danno]

Does that have a tilliotson carb on it or something with adjustable needles for high and low speed? If so try to give it a bit more fuel on the low speed needle, may be running lean in those temps.

[Jetson]

Does that have a tilliotson carb on it or something with adjustable needles for high and low speed? If so try to give it a bit more fuel on the low speed needle, may be running lean in those temps.

X2

[PolCat]

I thought backfire thru the carb is bad timing.

[snowracer]

Is the exhaust obstructed? Probably has a Walbro carb if it is original. Backfiring through the carb does indicate a lean condition, but at 1/3 throttle, not sure about that unless the low speed has been turned in. Carb more than likely needs a good cleaning and possible rebuild. Air leak could also be creating a lean condition.

[Garage Philosopher]

As new it had a Kehin pumper and two mufflers. One exhaust may be plugged...

[Ski of Plenty]

I agree with the lean diagnosis and possible carb work. May also want to try spraying some starting fluid/carb cleaner at the crank seals when the engine is warm. If the idle speed changes, you'll need new crank seals. 

[dirtman]

i didnt know chappys had Fuji motors?

dirtman

[Garage Philosopher]

Different side of Fuji Heavy Industries, later known as Xenoah...

[dirtman]

 

Different side of Fuji Heavy Industries, later known as Xenoah...

ahhhhh.... now that makes sense!     thanks GP!   and i agree.....she's lean!

dirtman