"Steve" > wrote in message
>
> 1987 Japanese truck.... that'll most likely be carbureted rather than fuel
> injected. And they tended to use Mikuni carbs, which are notoriously
> ill-tempered as they age. Power loss at speed coupled with getting worse
> when turning one direction and better in the other sounds like a fuel
> delivery problem- stuck float valve, plugged fuel filter, etc.
>
> Check the simple stuff first.

Worn or broken motor mounts classically make problems like this even
worse.