3.5 on SUs - lag

Bit of advice needed for a friends CCV truck that I'm trying to fix;
The problem is:
When on throttle (like a hill etc) it lags/loses power/won't rev. It's a 3.5 running on SUs with an autobox.

I've had the carbs apart and serviced them with new parts where needed - when that didn't work he changed the carbs to another set and changed the dizzy but the problem is still there

He took it a rolling road - they tuned the carbs some more including changing the needles, changed the fuel pump, put a fuel pressure limiter on and restricted the fuel return pipe (made it smaller) but it still has the problem.

The only thing I can think of is a inlet manifold leak that is intermittent????? Has anyone else had a problem like this and found a solution? Right now he's feeling like changing the whole engine..........

As it's an autobox is it just doing this because it's needing to change down a gear & isn't?
Any improvement if he selects a lower gear?
Is the kickdown linkage in place & correctly set up?
What autobox is fitted? If it's the ZF4HP22 as fitted to Disco 1/RRC/Defender then Ashcroft's advice is:
"If the vehicle is a Disco I or Range Rover Classic under no circumstances drive with the kickdown cable disconnected, as this will cause premature autobox failure.
Ensure that the shift assembly is set correctly and full travel of the shifter and engagement of all gears is positive, partial shift will cause premature autobox failure.
If the vehicle is a Disco I or Range Rover Classic you may want to adjust the kickdown cable if you want to fine tune the shift pattern, the looser the cable the softer and earlier the shifts, the tighter the cable, the harder and later the shifts."

Any idea on the engine's mileage? Could be a worn cam

Ratae - good advice but it will kickdown normally - it's an older 3 speed so I will check the cable is properly adjusted

Kev12 - it's out of a late LDV so could be high. I maybe thought the same so will have the manifold off anyway to change the valley gasket and do a little inspection

Any other thoughts?

Only other things that spring to mind would be plugs/leads and such

Might sound stupid but there isn’t anything in the fuel tank that could be blocking the outlet pipe when you put it under load? We had it with a tractor would run normal and rev fine but put it under load it would lag/loose power changed everything on it fuel pumps filters in he end it ended being up being a small but if electrical tape in the bottom of the fuel tank.

This sounds weirdly familiar. Haven't fixed my problem either. Following!

Toms820 - The Rolling Road apparently checked the fuel pressure and the feed from the tank and couldn't find a problem. It might be worth tanking the tank off though
He's already changed the Dizzy, leads, plugs etc
I'm gong to do two things - change the carbs for a know good pair (my SUs from my own 3.9) plus change the valley gasket and check the cam lobs at the same time. I can't think of anything else.............
I'll let you know how we get on