C85 Backfiring with one cool #3 cylinder?

[43-30013]

I have a C85-12F with Stromberg Carb and Bendix mags that keeps backfiring when throttling off to idle (Aircraft is in restauration so only ground runs have been made as of yet).
After shutting down the engine it is obvious that the #3 cylinder is much cooler than the other three.
Idle is otherwise smooth but over 2000 RPM sometimes there is a sudden spike in RPM as if the #3 cylinder start operating normally and then it falls back.

I have checked the following:
- The valve seems to be ok and is not stuck
- Compression is ok
- Mags are ok
- Carb mixture setting is ok
- spark plugs swapped but no improvement

For some reasons it seems that it runs better with Carb heat on.

Any suggestions for next steps in this trouble shooting?

All and any help is appreciated!

 

[nrpetersen]

 

[Captalbertq]

 

[JimC]

 

[cubpilot566]

 

[J5Mike]

 

[reileyr]

 

[bob turner]

 

[JimC]

 

[NE Cub]