'72 145E

I can't get a steady idle. Well, I can, depending on when I clutch. Coming to a stop, if I put the pedal down above 1500rpm the car will often stick with a high idle, 1200-1300rpm. If I engage the clutch a little, it will come down and stick at around 600rpm. If I adjust the idle screw, I can get the lower idle up to around 800rpm, but then the higher idle is obnoxious, around 1500rpm or higher.

Things I've done so far, assuming it was a vacuum leak:

Replaced most of the vacuum hose coming off the intake manifold. Those that I haven't are ports that have been plugged with the purpose designed vacuum plugs you can get at auto parts stores. Those ports are for the vacuum retard on the distributor, and the two for the cold idle valve. Well, except the valve cover hose. I just didn't replace that because it looked fine.

I had to replace my exhaust manifold anyway (it came off in 2 pieces), so I replaced the manifold gasket. Visually inspected the intake, no cracks.

Re-mounted the injectors to the head (found leaks with carb cleaner), and tested the seals with carb cleaner.


Assuming it was a distributor or recently installed electronic ignition(the crane system) issue:

Originally a -021 distributor car, I used a -033 distributor when I installed the electronic ignition. I don't know what the difference was, but I switched back to the -021 distributor and installed the EI in that one. The car felt stronger, but the idle still moves around. Re-set the pickup for the EI inside the distributor using the "move it until the test light on the ignition module lights up" method.

So, tonight for the drive home I'm going to re-connect the vacuum spark retard. After that, I'm not sure where I'll be checking next. Any ideas?

After re-connecting the vacuum retard (I still giggle when I say that, you don't want to know the image that pops up in my head) and adjusting the idle screw I got it to around 700rpm. I'm measuring by ear, I don't think my tach is accurate. Anyway, it sounds great. I also unplugged the ambient air sensor... At idle, with that plugged in, the car sounds like it has a miss. I thought it wouldn't have that much of an effect, but guess not. The sensor must be severely out of spec.

Car is running really awesome right now, I'm quite satisified. Next step: Completing the Crane ignition set. I've got the xr700, next is the HI6 ignition box and the coil. New plug wires as well.

I also unplugged the ambient air sensor... At idle, with that plugged in, the car sounds like it has a miss.

Adjust the idle mixture on the back of the control unit to sort that out.

Does the throttle plate have a little spring loaded valve in it? Some did, and you can solder it shut to prevent air passing through it.
Also check the that the auxiliary air valve is closing, and make sure the crankcase vent isn't passing air - remove and plug either/both with tape to find out.
The brake booster can also leak adding air to the manifold.

The throttle plate does have a spring loaded valve in it. When I nudge it to the side at idle it lets in quite a bit of air. What's the point of it?


The aux air valve is disconnected, and the ports on the manifold are plugged.

I'll check the vent on the side... I didn't mess with that hose much.

When the throttle is closed at high rpm - like descending a hill or leaving the highway - the valve opens under the very low pressure created in the intake manifold, and admits enough air to prevent misfires. As the engine rpm drops to idle, the manifold pressure rises a bit and the valve should close. Of course, over time the spring weakens, and it's capable of then opening at normal idle pressures, causing the fast idle.
You might also check the the distributor advance weight springs are pulling the weights in at idle. It's tough to determine, but sticky weight pivots or weak springs will allow the advance to "hang up" and that will also cause the fast idle.

The spring-loaded valve is an emissions thing - they have them on carbs also.
I agree with Rhys that you should solder it shut.
(Joe Curto recommends the same on carb butterflies so equipped.)
(Or get a butterfly that doesn't have the valve in it.)
--
George Downs, Bartlesville, Heart of the USA!