Does the engine run really smooth?
I had a truck that ran great except under power, the plug wires were breaking down under a load(?).I replaced the wires and problem solved.
How many miles are on the truck?
I would not think fuel wold cause such a drop in mileage but I guess anything is possible.Maybe swap motors out of your 51 !!!!! eek grin

There is enough good in the worst of us and enough bad in the best of us that it does not behoove any of us to criticize the rest of us.
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Be yourself. If you are ever lost, It will be much easier to find yourself if you know who you are!