There are several reasons for the 617 engine or others to drop off a bit, and would consider these points, not in any order.Assuming its not a blown gasket or other serious problems.
Assuming brakes are not binding and vehicle run at similar weight to prior.
1. Primary and main fuel filter blocked or water bound. If fitted for more than 10,000 change them or at least the plastic primary filter, blow the flow line back into the tank to see if its clear (low pressure only).
There is a chance the tank filter is blocked but this will need the tank gauge removing to access it advise to replace the rubber hoses at this operation at the same time. Black debris in primary fuel filter means hoses have broken up.
2.The injectors are blocked or restricted with carbon or build ups.
Either use commercial diesel flush or exchange the injectors, ive found that Keith de Costa at www.uniteddiesel.co.uk does a cracking service providing all components and tools via post and Visa card, you then return tools and old injectors, simple.
3. A point to watch is that the valve settings are ok and that the exhaust has not been blocked or kinked by accident especially over the tank!.
4. The air filter can become blocked and it will do no harm to check the filter and the clenliness of the breather pipe from the rocker box.
5. The realistic answer lastly check that the throttle is fully opened when you flatten the pedal and that the rubber bush at the rear of the engine is not collapsed. All the rods need to pulled off the balls and greased then set up to eliminate play and to give maximum throttle action , check the return springs and make sure nothing rubbing.
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