Outer Limits 4x4 Board

Hi all, i'm posting this for my brother in-law,
He has a 84/85 swb pajero with a naturally aspirated
4D55 eng.It's in really good cond and only owe's him
about $2,000 max.
Now the problem is that it will not go over 80/85kmph
now matter how hard you try.
It doe's have 31" tyres on it and has extractors fitted from previous
owner.
Does' anyone have an idea why this is,and how to fix?
Also any ideas on power upgrades ie turbo,where to get one.
or engine swaps would be great.

Thanks heaps
Cheers Ian.

Is it getting full throttle ?, stretch in cable, wear in linkages, carpet under pedal etc.
Blocked air filter or fuel filter.
J Top

Could also be a blocked exhaust, cat or muffler or crimp in pipe.....

I drive the turbo version of his SWB (the 4D55T), and though I don't know whether there's any injector pump or compression difference between the 2 engines, I do know that when I was having boost problems, it was PAINFULLY slow. Couldn't reach 100, whereas now I cruise at 110 and can push it to at least 130 (there's more in it, but that's getting too close to redline for my liking!). So checking cables, exhaust and general engine health is a good suggestion and will probably help a bit, but for a big improvement I reckon it needs help breathing (especially with the bigger tyres).