The Dreaded Idle Mystery, Idle Me This Options

[Apexster]

Looking for a little insight on the dreaded idle mysteries of a 1974 2.0 914. I've searched the archives for a couple months and tried just about everything.

Valves freshly adjusted. Compression is above 135psi and within 2% on all cylinders. Original D-jet intact with new hoses and all components tested and working. Fuel pressure tested (30-31psi from original 3 hose fuel pump). New plug wires, newly rebuilt distributor, recent fuel filter. I'm about to replace the 20 year old CHT sensor as well.

The car revs smoothly and pulls very strong. Engine runs great anywhere above 1200 rpm.

Problem is 3 or so minutes after a cold start it drops off the high idle and transitions to a rough idle, like it's running on three cylinders. When I spray carb cleaner in the air cleaner the idle smooths out immediately.

Has anyone else come across this issue and perhaps suggest a plan of action to resolve it? (IMG:style_emoticons/default/confused24.gif)

Thanks and Happy New Year to All.

[Rand]

I appreciate Brad as much as we all do.

Spacers that were never factory are band aids. Can we stop the band aids and get to the real problem? We need a baseline. Obviously there's something else wrong. Please don't try patching it other ways. Fix the source of the problem before you chase mysteries the rest of your car life.

[rjames]

I appreciate Brad as much as we all do.

Spacers that were never factory are band aids. Can we stop the band aids and get to the real problem? We need a baseline. Obviously there's something else wrong. Please don't try patching it other ways. Fix the source of the problem before you chase mysteries the rest of your car life.

Fully agree with Rand.

Provide some more info: Has the car always done this or did the issue just start?

Things that immediately come to mind:
Verify the correct model year DJet parts are installed
Injector wiring harness is good
Vacuum hoses hooked up incorrectly
Vacuum can on distributor leaking
intake runner hoses
throttle body gasket
Leaking plenum (only way to do this is to remove it from the car)
bad injector seals
Idle mixture knob on the ECU is not set correctly
Bad MPS

...to name a few.

If the idle is surging I would start by adjusting the ecu knob to see if that changes anything, then test the MPS and dist vacuum canister to see if they hold vacuum. If it's not surging the MPS could still need to be adjusted.

My fluctuating idle issue (from 900-500 rpms) turned out to be a combo of a throttle body with a worn shaft, a bad vacuum canister on the distributor and a bad MPS. (And I'm probably forgetting something else.)