RPM drops below idle when coming to a stop sign

[1969911T]

Engine (911T 1969) will slow then eventually stops when braking for a stop sign. If I blip the gas pedal before the engine dies, the idle will stay forever at about 1200 RPM.
Have MSD ignition, engine starts immediately if it stops. This just started a few days ago and the idle rpm also jumped to 1900 RPM. Choke is not engaged. Lowered the idle RPM to 1200. I think the engine is running as good as ever(owned since 1972) except for the engine stopping. Maybe the engine is getting smarter, sees the stop sign and thinks the engine is supposed to stop instead of the car???!!!

Have original Marelli distributor with a new stops so there is no wear in the advance mechanism. timing is correct. Have Pierburg fuel pump with pressure regulator set at 3.5psi and all floats in the Webers checked for proper level.

The engine is warm (oil 180F). This issue almost has to be related to the idle RPM jumping up, but no idea how that can happen.

Any suggestions?

[1QuickS]

I was expecting a more sage response than what I can offer but since none have been posted I opine:

Since engine was running OK and suddenly you have an issue I expect the issue is not carbs (I'm a carb guy so I always think carbs are the issue). Distributor is suspect to me, perhaps a spring in the advance mechanism broke or the ball bearing that is between the plates jumped out of its groove or there is stiction in the advance mechanism. Pop the top off of the dizzy and put some motor oil onto the felt in the hollow driveshaft. Then, grasp the rotor & do the Luke Skywalker routine while manually twisting the rotor CW & CCW to see if you can feel any "Force" that is not smooth. Also check to see if it returns willingly when manually advanced.

There, that empties my brain on this topic. Synopsis is: mechanical issue within distributor is ruining your driving experience.

[Dexterpix]

Are you still running points? If so check the gap, I had the same problem as the points wear and need readjusting. I compensated on the road by raising the idle stop till I got home and could adjust properly. Hope it helps.

Joe D

[1969911T]

Checked the distributor advance and it is smooth. Just put in new points and set the timing a couple of weeks ago. Will check the timing and the air flow balance on the Weber’s and see if anything has moved. Thanks for the ideas of things to check.

[edmayo]

You need to start isolating systems. Hook up your timing light, run engine to warm idle, disconnect throttle rod from cross bar, now you will need to start correlating carburetor settings with timing settings. First you must start with your basic idle speed mixture and synchronization settings As you carry out your carb adjustment procedure you must constantly check your timing. As you change idle speed the timing will also change, so you must constantly recheck and/or adjust the timing. As the timing is reset that affects the idle speed which affects your previously set idle mixture and volume screws. It is a back and forth until you can get the engine to return to idle each time you blip the throttles. AND the throttle cross bar must be friction free and throttle linkage arm must ALWAYS return to bear against the stop screw. Your solution will lie somewhere in these settings.

[NZVW]

The Oracle speaks.
Where would we be without our Mr Ed

[dmaddox]

In addition to Ed's wisdom - Paul Abbott has also documented the above procedure that will really help. http://www.performanceoriented.com/special-procedures-1

Here to help. Also - as Ed mentioned and you'll read, this is a game of back and forth for quite awhile, so do it outside on a nice day or you'll get a headache sitting at the exhaust pipe messing with all of this. Pick up a synchrometer and have fun.

Last, don't rule out the distributor. A shameless sales plug here, I have a rebuilt 911T distributor if interested, you can PM me.

Have a great weekend -

[1969911T]

Ed, I do not understand about adjusting the timing based on idle speed. I always thought
I need to set the timing at 35 BTDC at 6000 and the idle timing will be whatever the distributor says- but this apparently is not correct?
I have an advance curve for the Marelli, which says, for example, I should be at 0 advance at 1400rpm. So, I set the idle timing based on idle rpm, then when done adjusting, then set the 6000rpm timing?

[edmayo]

When the car was brand new that is what we did. It is no longer brand new, distributors are not perfect (Marellis were difficult even brand new) carburetors had perfect airtight shafts, so generally you followed factory procedure. Well that may not work as well now, so you have to be adaptive to get the results you want, a 900 RPM idle, that consistently returns, which we don't have now. So you may need to deviate slightly to get there. And I have no idea of your skill level in carb synchronization and mixture setting. And I wanted you to understand that ignition timing and carburetor adjustments are interrelated. In your first post you didn't say WHEN this started. Has it always done this? I think you indicated this was a recent problem. Since you put on the MSD? Since you fixed the distributor? Mechanics first rule, go back to the last thing you did before it started running badly.

[1QuickS]

Back to carbs, if you use a STE synchrometer then that can be a good tool for evaluating if your idle mixtures are rich. A rich idle mixture may be the result of compensating for throttle shaft wear. Throttle shaft wear becomes troublesome around 90k miles for Webers as my rule of thumb. Throttle shaft wear allows uncontrolled air to enter the carb at low RPM when intake manifold vacuum is high due to nearly closed throttles.

This issue of worn shafts and rich idle mixtures is gradual and a sudden change in driving characteristics is suspect which is why I think it is a dizzy issue.

However, you may be at a tipping point.

Proper idle mixture requires throttles to be opened enough to allow air for combustion so a rich mixture will require more air passing by the throttle valves which requires throttles to be opened more than desired. This increased air flow as set during a garage tuning effort may be be randomly augmented by the throttle shafts moving around in the journals during slow speed driving thereby adding or decreasing air into the engine. An increase in air flow would increase idle RPM due to improved air/fuel mixture and possibly cause the dizzy to advance which would result in a large RPM increase. A decrease in air flow would cause the mixture to become very rich which would suppress the idle RPM. During deceleration the vacuum in the intake tract would cause the throttle valves to close more completely which would create the excessively rich idle RPM.

The reason why I said the STE synchrometer is a good tool for evaluating idle mixtures is that a typical STE air flow reading at idle is somewhere around 5.5 Kg/hour when "Lean Best" idle mixture has been achieved. Lower readings and the engine will be idling too slowly and higher readings indicate the throttle valves are opened up a fair bit, possibly to the extent that they partially expose the first progression hole in the carbs.

So, if the dizzy is behaving itself (Marellis have a poor reputation as Ed mentioned) then address carb tuning for Lean Best idle mixtures, air flow balance between barrels at idle and side-to-side balance at 3000 RPM. As a quick sanity check, observe your throttle shafts at idle, if you can hear clicking like a sewing machine or if you can see obvious up and down movement of the throttle shaft ends with respect to the carb housing then you most likely have a carb wear issue to consider.

As a personal comment: Ed built a trick 2.3 liter, dual plug, big valve, S-cammed engine for my 67S and when I installed my old Webers the engine ran erratically to the point that I was embarrassed to drive it. It was only after I re-bushed my Webers (I was my own first customer) that they behaved properly and provided the driving pleasure that engine promised.