short chain runs overheat and yes, a tensioner on a run like that is just about a no go because of space. You're running 8:1 ratio, what cc engine do you have? If you have the tools and ability, there are two options that might fix the issue by eliminating the short chain run. You could shift the engine back or upwards, move the axle sprocket inline with the driven pulley and chain direct that way. Or you could get rid of the driven pulley mount, raise the jackshaft and put the driven pulley on the jackshaft--this 2nd method is the type used by nearly all the 30 series go kart setups. Both will get rid of the short chain. Depending on the driven pulley shaft diameter, you'd have to drop down to an 8T drive sprocket to get a 7.5 ratio, a 10T would give you a 6:1. My other comment/question would be as to what size sprockets/chain are you using? If it's 50, you can get KMC HD non-oring chain for relatively inexpensive price. just make sure it has the HD on the part number, has thicker side plates on the links.
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