Fourtrak Engine Problem 2.8td

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Hi, I have a 1985 fourtrak fitted with a DL52 engine. When I let off the throttle the engine revs die down very slowly as if it is over fuelled or injectors are weeping. On the overrun when i have revved it the engine keeps on revving for 4 to 5 seconds before returning to idle, the throttle cable is not sticking, this is definitely a diesel pump/injector problem. Has anyone come across this before? This has got progressively worse over the last 6 months and i have resorted to turning the fuel mixture down to stop it revving up between gear changes! Any advice would be appreciated. Have a look at the beastie in the gallery! Its the big blue one! Thanks Mikey D.

1985 Fourtrak 2.8turbo, Oversize Intercooler, Spring over Axel on 35/12.5/15's.