617 engine idles too fast

My 617 engine, a transplant, in my 1981 300SD idles too fast. Should be 800-900 RPM but seems more like 1,500 (tach not working). Sometimes in neutral or park it will start to run away.
Mechanic told me adjusting the idle speed is not a simple matter. You have to remove and replace the injection pump and looking at 4 to 5 hours labor.
You mean there is no simple idle screw adjustment?
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Look between the IP and the engine's block. There's a set screw with a lock nut; it's the throttle idle stop. Aft of it is the throttle full throttle stop screw. With the engine OFF work the accelerator - idle to floored - and watch the lever behind the IP. Be sure the linkage is free and that the lever is in fact bumping against the idle stop and the full throttle stop BEFORE you adjust anything.
I suspect the problem is in the linkage adjustments between the firewall and the levers on top of the engine. Each lever and pull rod has a specified length and changing engines may have thrown off this geometry. Or the two rods on the top of the motor may have been exchanged. I urge you to consult a M-B manual for the specifications or you'll be tinkering with this for some time.
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says...

Thank you. Not in a position to tinker with it myself in that a Manhattan residential street is not tyhe best place to practice shade-tree mechanics. I'll bring your comments to my mechanic.
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Has the engine always been this way? People very very very seldom adjust the idle - there is almost certainly never a need to.
If you know for a fact it had been increased and want to put it back that's fine but if it's idling high there's some other reason why it's doing this - the idle adjustment is most likely where the factory left it and where it should be.
It's between the injection pump and the block and has a lock nut.
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No. It was idling at 900 RPM or so until about a year ago following seasonal maintenance. I mentioned it to the mechanic whose response then was if the engine is otherwise running fine and starts right away (and it is and does) then I should not bother with it. But we do a lot of city driving and it's had an adverse effect on mileage.
People very very very seldom adjust the

fine
idle
The latest mechanic, the one who said it would take several hours to fix, mentioned a weak spring.

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They screwed up the linkage when the valves were adjusted. And, the linkage has ball and socket pivot / connection points that ought to be lubed from time to time.
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It's supposed to be 750 not 900. Has it always been 900.

Har. It takes 5 minutes but you'll need a deep socket.
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