Poor idle with vacuum adv plugged in, 1971 914-4 Frankenstein Options

[mikesjunk]

Ok so I've went back to basics to start going through things looking for the poor warm idle issue. I've read enough on the D-Jet to know this could be a challenge but it pretty much dies when you let off the gas to come to stops. Not so fun for drives.

So here's where I'm at
New plugs, wires, cap, rotor, condenser, points, intake gaskets, injector orings,
Carb Spray tested intake and all vacuum attachment points
Valves just set to .006
Dwell is 49 degrees
Timing is a tad over the 37 degrees. It bounces a few degrees from just out of the notch to right on....advanced side.
MPS does have small leak. It holds vacuum for about a minute....not the 5 minutes I've seen in specs.

So when I was checking the timing I had the two vacuum lines from the distributor unplugged and capped. Got it checked and went to plug in the one on the front of the distributor that points back at the distributor. As soon as I plug it in the idle drops a couple hundred RPM. It idles much better unplugged even sucking air.

So any clues as to what's next. Not sure I've seen/read what the two ports each do on this distributor.

[TheCabinetmaker]

D jet timing is set at 27°, not 37°'. Engine above 3200 rpm. Idle drop when plugging vacuum in is normal.

[mikesjunk]

See old man memory kicling in. So it's slightly more than 27 degrees or the red mark on flywheel.

Unplugged it's about 700 rpm and plugged in about 500 rpm per my dwell/tach gage. The knob on ecu doesnt seem to do anything.

[PancakePorsche]

If you are talking about the backside connection of the vacuum can than yes it is retard and slows the idle speed down. You should be able to reset your idle speed with the idle air bypass screw. This is a EPA/smog related feature. Also make sure your TPS is properly adjusted to turn on the idle mixture circuit. If not adjusted properly the knob on ECU will not function at all.

[914_teener]

If you know that the MPS is leaking......that is problem one.

The rest is secondary. Fix that first.

[dlee6204]

If you know that the MPS is leaking......that is problem one.

The rest is secondary. Fix that first.

(IMG:style_emoticons/default/agree.gif)

You already found a failed component that could cause your issues.

[mikesjunk]

From my research on the MPS (from a reenlist article on MPS)

At idle, the manifold vacuum is about 10 to 15 in. Hg (inches of mercury (Hg), a common unit used for vacuum), and at full-load state the vacuum is near 0 in. Hg. The MPS is designed to respond to two fundamental load conditions, part-load and full-load. Idle mixture is controlled by a special circuit in the ECU that senses the idle condition from the throttle position sensor (TPS). Part-load operation requires a linear response to load, and the mixture is a compromise between performance, fuel economy, and emissions. Full-load enrichment requires a transition from the part load response for enrichment to provide maximum power output, as the manifold vacuum drops to near zero.

So based on this idle should not be impacted by the MPS....assuming the information is correct. Yes I know it has an issue but not like they are readily available.....or cheap. (IMG:style_emoticons/default/smile.gif)

I do though need to check to make sure the TPS has the right signal at idle for the ECU. On the todo list.

[914_teener]

From my research on the MPS (from a reenlist article on MPS)

At idle, the manifold vacuum is about 10 to 15 in. Hg (inches of mercury (Hg), a common unit used for vacuum), and at full-load state the vacuum is near 0 in. Hg. The MPS is designed to respond to two fundamental load conditions, part-load and full-load. Idle mixture is controlled by a special circuit in the ECU that senses the idle condition from the throttle position sensor (TPS). Part-load operation requires a linear response to load, and the mixture is a compromise between performance, fuel economy, and emissions. Full-load enrichment requires a transition from the part load response for enrichment to provide maximum power output, as the manifold vacuum drops to near zero.

So based on this idle should not be impacted by the MPS....assuming the information is correct. Yes I know it has an issue but not like they are readily available.....or cheap. (IMG:style_emoticons/default/smile.gif)

I do though need to check to make sure the TPS has the right signal at idle for the ECU. On the todo list.

Ok...I'm going to be Krusty here:

At idle, vacuum is highest. So the statement that you posted is just stating that fact about the MPS. If that is the Brad Anders site then you haven't read the whole thing.

If your MPS is leaking then you have a vacuum leak. On D-jet this makes the ECU think the motor is under load increasing the injector duration inducing a high idle speed.

Fix the MPS. Chris Foley at Tangerine Racing has a kit and other's here will help if you want to do it yourself.

If you want to be "cheap" and throw money at other stuff then that is a choice.