BAYS BEACH

If your valve seals were bad you would've been getting smoke thru every gear when twisting the throttle not just top end 5 the gear. Blue smoke running hotter than should top end is a lean running condition to much air not enough fuel

Sorry I didn't respond before you tore into the motor but I bet your fue/airl mixture screw in bottom of bowl was just off a bit sounded like original poblem was running lean this would explain the burnt metal smell the blue smoke just a lil bit and the impossible cold start problems. I had same prob with my 87 atv of my neighbors he said it hadn't ran without a bogg on top end since he got it 15 yrs ago after cleaning the carb it would start bogging at 3/4 throttle to pinned in last gear like it was running outta fuel if I continued to hold throttle it would die and be hard to start but if I let off it would go back to idle. Turned bottom air screw in to seat then back out to 1 3/4 turns it was previously out 2 1/2 turns immidiately fixed. My neighbor said this is best it's ever ran and he's had it in three different shops trying to fix it. The new carbs you buy today are cheap Chinese version of the real aluminum cast that came on it I would clean original and set properly and stay away from ethenol mixed gas only run 100% this will solve your rusted issues when storing either absolutely full tank of gas with fuel shut off and bowl drained with a lil seafoam placed in bowl and in cylinder or completely empty tank when there is room for moister to get in tank with ethenol mixed gas the ethenol pulls moister in as it seperates from the gas and settles to bottom of tank so like I said all the way filled or all the way empty is storing. I always use seafoam instead of stabil it last all year just but it in the can that you fill toys with each time getting gas and your good