283 missing/loses power

[dstarrett]

283 new rebuild,old three speed with granny on floor, HEI, new plugs gapped to
.045, timing set(4 degees advanced I think, mech friend set for me) Starts runs well until it warms up. (on road 50 mph) Miss and carb backfire in 4th around 2100-2500 rpms. Power loss. shift up to 3rd r's come up to 4-4200 and it smooths out a power is better. Ideas? You guys have been great and my 8 year project is getting real close. Pics soon.