Inconsistent Idle

[thilmes]

1970 911T Targa is running good but, there is one last issue I am trying to sort out. Always one more...

When tuning the engine in the garage all is fine. The problem comes during driving. Often the idle will drop very low or the motor will stall when I come to a stop. It is more likely if im stopping from highway speeds then during city driving. A couple blips on the pedal and the idle will return to normal.

Has anyone else experienced this?

Engine is basically stock except for installation of 84 mm high compression pistons. carbs have recently been rebuild, zenith 40 TIN. venturi is 30, idle jet is 60, mains are 130

Marelli distributor with points. new wires, new plugs, new rotor, new cap. float level set. timing set per factory manual.

I appreciate your thoughts!

[drwhosc]

My 71 E is ding the same thing. But only after it warms up. I had the MFI pump services and went through the check measure adjust procedure. The wird thing is my my fuel increase lever has little effect. Will be interested to find out what others have to say as I have not been able to figure this one out yet.

So if you have an MFI, you are not alone. If you have carbs, then this is more interesting for me as it is pointing to something other than fuel delivery,

[gsjohnson]

He has Zeniths.

[edmayo]

So you're saying that if you open the throttles with car stationary and then let them snap shut that it idles perfectly, but if you come up to a stop and let off the throttle the engine will stall?? The only difference that I can see is the action of the fuel sloshing in the float bowls. Sounds like float level to me. How did you set the levels, and what is your fuel pressure at the carb.

[thilmes]

Hello Ed. Your restatement is accurate.

It has been awhile since I set the levels, and I might be confusing what I did with a set of webers on another vehicle, but I am almost positive I used the little clear vial you screw into the carburetor. I think I set level to that stated in the shop manual.

With your comment I am guessing that you are thinking the level is too low or the pressure to low, is that correct?

I have not measured the fuel pressure lately. Ill check floats and pressure and reply. Target pressure range at idle?

Have you seen this before?

I also thought it might be a flaky distributor advance. using the strobe light that does not appear to be the case.

All others' comments and experiences appreciated. TJH

[thilmes]

one more comment. The car is a 70T. fuel tank originally had two hoses to fuel pump and one to engine. One hose is now plugged with fuel pump directly in line from tank to pump to regulator to carbs. I doubt that this matters but I wanted to mention it.

[Irish911]

Do you have spacers between your carb intake manifold and the engine? Could be your cabs are running hot when highway driving and when you get off the highway you end up running really lean because you are now pulling in colder and heavier air at lower rpm's, such as when stopped. I am certainly no expert on this and I may have gotten this backward. I had/have the same issue but I have my carbs off right now and will be installing spacers to see if it helps.

[Frank Beck]

You mentioned the "M" word when telling us about your ignition.
Distributor worn? Throttle shafts worn?

Having said that, if idle dips it is usually because the mixture is too rich.

[thilmes]

Thanks for the comment Irish. yup, heat spacers in place...

[thilmes]

Thanks for the comment Frank, M.... not sure if it is worn or not. new rotor and cap... Throttle shafts in very good shape. tell me more about idle dipping if too rich.
Thanks again. Ted