Strange throotle response

[Ruben911]

Need help in diagnozing the following symptom. The car starts fine and achieves approximately 900 rpm idle speed. There is no indication at 900 rpm that there is anything unusual (there is no miss, no backfire, etc). When I slowly increase throttle, the car responds as expected with no unusual symptoms as the rpm increases. However, if I floor the accelerator from idle speed, these is hesitation, followed by backfiring initially until it revs up and then again there is no problem at the higher rpm. Has someone else experienced this problem? My car is a 1976 911s with CIS. I recently performed a valve adjustment and a tune up (new plugs, points, rotor). I have backtracked everything I did and do not see any smoking guns. I'm looking for suggestions as to what the problem might be and how to test for the specific possible problem. Let me know if there is any other info that I missed which may be important. Thanks and look forward to your replies.

One other piece of information, when I was changing plugs, I inadvertently dropped one of the new plugs from about chest height. I inspected the plug and did not see anything unusual (gap was ok), so I used it. Is this a problem?

Ruben911
1976 911s 2.7L

[83930]

My 930 will do this when it is cold, but after it warms up it is fine.Might check your CO and see if you are lean,and possibly check your advance unit to see if it is free of binding.I know the Guru's on the board have had this problem before. Might try a search for a thread of some kind.

[Sunroof]

I am experiencing the same problem and even worse when decelerating. I was gettijng fed up too!!! When warm it seems to be fine, so your problem must be associated with mixture.

I have tried many things, including adjusting the mixture when warmed to eliminate the surging, cleaned the sensor plate (helps anyways) and adjusted pressures at the warm up regulator. I can only pass on my experience and can suggest that you carry a mixture adjustment tool and when warmed up, either lean it or richen it up til it goes away. Often the culprit is the warm up regulator in the cold start mode. Those pressures (high and low) must be adjusted. My mechanic has always said hog wash to the factory setting for CO! News to me, but he claims in my vintaqe (frist year of CIS), they disconnected the enrichment unit and its all by sound and feel from their.

After changing the fuel filter after 3,000 and seeing that fine black river bottom sediment pour out, I believe I may be crapping up the hairline orifices of the fuel distributor. Dirty fuel is the killler of CIS units. Something to consider with an older 911 if your tank is original. Check you fuel filter closely next change.

Good Luck

Bob
'73.5 T

[Ruben911]

Thanks for the feedback guys. I did change the fuel filter a couple of weeks ago when I installed a new fuel pump. One thing I did today while doing some more troubleshooting was to disconnect the vacum hose from the distributor. The revs went up about 300 rpm but when I floored the accelerator I did not experience the missing and backfiring that was evident before. I have not taken the car out for a test because it is raining heavily here in WPB. As soon as it stops raining I will adjust the throttle screw to get down to 900 rpm with the vacumm hose disconnected and go out for a test drive. I want to see if there is something wrong with the distributor vacumn operation. I'll keep you posted as to what I find out. Thanks

[fred cook]

Just a thought, does your SC have both vacuum advance and retard? If so, could you have reversed the vacuum hoses? If your car doesn't still have the two different color vacuum lines, it is easy to do.

[Ruben911]

My car has a single vacumn hose connected to the distributor. I disconnected the other end while the car was at idle and the rpms went up about 300 rpm. I then sucked air with my mouth to simulate a vacumn and the rpms went down. Is this the expected response? Does anyone know the purpose of this distributor vacumn operation? Thanks

[Early_S_Man]

Ruben,

The vacuum retard module only works at or near idle, and only serves for emissions compliance. At idle the vacuum module retard the spark 10°, period!

As your distributor is the same as a 2.4 'T' distributor, I suggest checking out the complete centrifugal advance curve from idle to 6000 rpm, per the info in the following thread:

http://www.pelicanparts.com/cgi-bin/vb/showthread.php?s=&threadid=5432

[Ruben911]

Thanks Warren, I will print the thread and try to check the advance curve this weekend.

Thanks
Ruben
1976 911s 2.7L

[Ruben911]

Warren, I confirmed that the distributor centrifugal advance is working properly. It was almost dead on with the specs that are in the table. I still have the problem with hesitation when I floor the accelerator. It is a momentary sputtering with some backfires. After a few moments it picks up power. One other thing I did was pour some water around the injectors with the engine running and noticed a reduction in RPMs. Is this normal or do I have to replace the injectors/o-rings. It looks like it is sucking a little air through the injector penetrations. Thanks

Ruben

[Early_S_Man]

Ruben,

Yes, it sounds like you need to replace those injector O-rings. You should check the nuts for the aluminum intake runners, too, as they sometimes loosen up. Every little bit helps.