1974 2.0L - Rough Idle/low idle hunting Options

[Rand]

If you pull the MPS hose off while it's running, does it change? If not, the hose or the MPS is completely shot. Pulling the vacuum hose off the MPS will cause it to run full rich. Which at an idle will cause it to drop, stall, and stumble over the super rich mixture.

[srb7f]

You need a new or rebuilt MPS if it doesn't hold vacuum. I just replaced a leaky one, and the throttle response is much more smooth and the engine develops full power the way it should.

[frankc]

I did retest the MPS last night and it did not hold any vacuum (O Hg).

I know I need to replace it - but the ultimate question is : Will this impact my ability to idle? In other words, should it still be able to idle at 900-1000 rpm, but just have a rich mixture?

According to the D-Jetronic troubleshooting guide - if the throttle body idle screw does not alter the idle - then it is probably a lean mixture, timing, etc... . So if the MPS is blown - it should be running rich - correct?

Perhaps, I have a compound problem.

Frank

You need a new or rebuilt MPS if it doesn't hold vacuum. I just replaced a leaky one, and the throttle response is much more smooth and the engine develops full power the way it should.

[Bleyseng]

too rich at idle and the engine bogs due to the too rich mix.

Also how is your engine vacuum at idle?

[r_towle]

a bad mps will make it run like a dog all the time.
Get a new one.
A blown MPS could and probably will create a pretty big vacuum leak...
So, the car will not idle, nor will it run correct anywhere , at any speed.

Rich

[Dr Evil]

Yup, two prong problem.
1. your MPS is toast (I am sorry to hear that) so your mixture is crap (rich possibly)
2. Now you have a big vacuum leak into your air box after your TB so it leans it out.

Start with the MPS.....come back after that as you will not liekly find/fix anything else with such a major problem looming.

[frankc]

(IMG:style_emoticons/default/confused24.gif)

Update - June 15th:

The MPS on my car was blown. I replaced it this evening (thanks Ron) with one that tested to spec. The car's performance improved (no backfire!), but it did not cure the hunting at low idle. Adjusting the idle screw did not affect the idle speed.

Next, I hooked up a vacuum gauge to the throttle body (did not tee it - just blocked off the hose going to the diagphram (retard side) of the distributor. I measured 13.5 inches of vacuum - so that is a little low. Need 15 inches - correct?

However, the car now idled and I could adjust the idle screw, which adjusted the speed to 950 without a problem. It stayed rock steady at 950. Connect the vacuum hose from the dizzy to the throttle body - the car low idle hunts/dies again. Removed and blocked off - 950 rpm.

I take the Mighty Vac this morning - start the car and pull 15Hg of vacuum on the diaphram. The car idles at 950rpm, but I notice there is leakdown to about 5 Hg within 60 seconds. Even after leakdown the car idles. Recoonect the vacuum hose from the throttle body to the retard side of the dizzy - car hunts/dies.

So - is the problem the dizzy or just the Vacuum Diagphram? I will recheck the timing to see if I set static timing correctly at TDC or the 7.5 BTDC mark. Timing was dead on - 27 deg BTDC - but I will double check. and Dwell at 48.

Question: How to test the diaphram? Pull 15HG and see if it holds it for x minutes?

Looks like I still may have a small vacuum leak somewhere else, unless I am measuring it wrong.

So progress. The MPS needed replacing, but the idle is still the same - although no longer burning rich.

Thanks in advance.

Frank

Yup, two prong problem.
1. your MPS is toast (I am sorry to hear that) so your mixture is crap (rich possibly)
2. Now you have a big vacuum leak into your air box after your TB so it leans it out.

Start with the MPS.....come back after that as you will not liekly find/fix anything else with such a major problem looming.

[ctc911ctc]

I have a similar problem - was there any resolution with the Dist-Diaphragm?