Idle adjustment question, take two, Idles high or slowly stalls Options

[BillC]

The old thread asking this question got too long and disjointed, so I figured it would be best to start again with all the data stated concisely and in one post.

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The problem: I can't get my car to idle reliably under 1500-1600 RPM. If I adjust the idle bleed screw to lower the RPM, the engine will slow down and stall.

The car:

• 1973 2.0
• Engine was rebuilt a few hundred miles before I bought it, and I've put less than 100 miles on it since then.
• Cam is unknown. I called the shop that did the rebuild, but they don't remember what cam they put in it. They thought it might be a WebCam, but I called WebCam and they don't have any records for this shop buying a cam around that time.
• Factory D-Jet fuel injection. Correct ECU, MPS, CHT and ballast resistor. New FJ67 fuel injectors. New fuel pump and fresh gas. Fuel pressure is 29 psi.
• MPS holds vacuum all day, no problem.
• All vacuum hoses and fittings have been replaced with new ones.
• Freshly rebuilt throttle body, and TPS adjusted properly. New circuit board installed in TPS.
• New 123Ignition distributor, currently set to setting "1", which is listed in the manual as being the correct position for this car without vacuum retard.
• Valve lash adjusted to .006" intake and .008" exhaust while sitting on engine stand, shortly before the engine was reinstalled in the car (so less than 100 miles ago).
• The car drives well at all RPMs and throttle positions, except idle. However, drivability is a little better on setting "A" with vacuum retard connected than it is on "1", but leaving it on "1" for testing (without vacuum involved in timing, it's one less variable).

The symptoms:

• At 1500 RPM, the car idles about 11-12" vacuum and 7 degrees advance.
• At 1100 RPM, car idles at 7" vacuum and 4 degrees advance.
• At 850 RPM, car idles at 3-5" vacuum and 1 degree advance, shortly before it stalls.
• Car idles noticeably rich below 1200 RPM and is very rich at 900 (by smell).
• If the car has been running less than 20 minutes, it will idle (poorly, but stays running) under 1100 RPM. However, after being driven/running more than 25 minutes, it just wants to slow down and die unless idle speed is kept above 1500-1600 RPM.

Diagnosis work:

• I hooked up a smoke machine and verified there are no vacuum leaks.
• I can make the engine stall by turning the idle bleed screw in.
• Performed a leakdown test with the engine completely warmed up by a 30 minute drive. Leakdown was less than 5% on all cylinders.
• If I connect a MityVac to the MPS and set it to 17" vacuum, the idle smooths out and the exhaust cleans up (doesn't smell rich).
• If I create a vacuum leak by pulling a vacuum hose off the intake manifold, RPM goes up several hundred RPM. Replacing the vacuum hose makes the idle go back to where it was beforehand.
• Timing advance is being measured by adjusting the timing light to put the "0" back in the notch in the fan housing.
• I have a new vacuum gauge, and confirmed that it matches the gauge on the MityVac.
• I put a dial indicator on the #3 cylinder exhaust rocker, and confirmed that this rocker has approximately the same lift as stock (.364" at the valve). Couldn't check any other rockers, since there is no room to put the dial indicator on any of them without dropping the engine.

Current diagnoses/hypotheses:

• Cam not compatible with D-Jet?
• Hidden vacuum leak that only shows up at low RPM?
• Others?

Suggestions?

• Convert to modern fuel injection?
• Rebuild engine with correct cam?
• Convert to carbs? <-- No, not gonna happen
• Go back to a bicycle?

[Superhawk996]

I’ll also repost this:

The thing is even some of the most aggressive cams with huge overlap still wouldn’t account for 7-8” Hg of vacuum at idle. Hard to believe that would be in his case unless the builder was incompetent. I’m not even sure if a cam that aggressive is even commercially available for the T4 engine.

This comment is predicated upon an assumption that 7-8” Hg is an accurate gauge reading.

Gotta figure out why the engine isn’t pulling vacuum at idle.