CB 750 K5 Hot Exhaust Pipes (MICE?)

[Harper.]

Hi everyone,

I have a 1975 CB750, and a couple weeks ago the main fuse blew out while I was riding up a hill.
I finally replaced it (after it sat in an outdoor shed for three weeks) and it started right up.
However, it idles very low and rough, and stalls if I don't keep on the throttle. More worryingly, the outside two exhaust pipes (Far left and far right) got smoking hot within seconds. I have a four into one exhaust, so I don't know how to feel to see if everything is firing. And, I saw a tell-tale gnawed walnut sitting on the engine case.
I'm new at this, and I hope I haven't made a boneheaded move already. Should I have changed the gas and oil that was sitting in there? What is the next diagnostic step? What is the standard procedure for starting a bike that has sat for a while (if there is a standard procedure)? What causes pipes to overheat?

Sorry if any of these questions are inane or unanswerable,
Thanks,

Harper

[Re-run]

Well 3 weeks will not make gas and oil go bad. Gas can lose a little potency but oil will be fine and neither will cause your issue. You first step would be to check your wiring. the outside pipes fire on the same coil so they sound fine. You want to check all the wires for the 2&3 coil. A wire might been chewed through if you found a walnut. Once the wiring is checked out, try riding the bike around the neighborhood some. If the carbs are just a bit dirty, than might clear it up. Give this a try before tearing into everything.