Missing when warmed up

TerryBPP · 03-16-2003, 11:03 AM

When my 1976 911s 2.7liter reaches 210 deg. it starts missing. It feels like a spark related miss but I have replaced the ignition box (crane), the coil (MSD), plugs (Bosch plat.), and the wires (Bosch). I also checked the vac. lines just in case. Could it be that it is running to rich or to lean? It does not miss until it hits 210. Weird huh.

Any help would be appericiated.

jeff m · 03-16-2003, 07:00 PM

All the CIS people must be out driving!
Two suggestions; fuel pressure test. System and control pressures.
Exhaust analiser, if your co is low or normalish but your hydrocarbons are high you could have a lean miss.
Jeff

Early_S_Man · 03-16-2003, 07:20 PM

Terry,

Check all of your spark plug wires for end-to-end continuity ... missing at ambient temps above 85° F is a classic symptom of a bad Beru connector. Do a search of the 911 archives on key words "Beru shake test" ...

TerryBPP · 03-17-2003, 05:28 AM

What is a Beru? Is there a way to check leaness without an exhaust analizer?

stlrj · 03-17-2003, 05:40 AM

The easiest way I have found to track down an igntion miss is to spray water on the engine while at idle and then listen for the snapping sound the spark makes where it shorts to ground.

Joe

TerryBPP · 03-17-2003, 05:44 AM

I read about Beru problems in the archives and that sounds like my problem. I'll check it. I still can understand why it would only do it while it is hot. If someone can explain it I would appericiate it.

Early_S_Man · 06:35 AM

Terry,

Because of internal losses due to higher Voltage drops accross operating semiconductors at higher underhood temps ... the output Voltage of the Bosch CDI-system is slightly lower above approx. 85° F ... than at cooler ambient temps. When the internal RFI suppression resistor of the Beru spark plug connector is open -- a failed condition that is tested for by checking continuity -- the CDI system spark can jump accross the fault at lower temps when the engine is first started and for the first few minutes of operation. After the engine heats up and the CDI-unit output Voltange falls off ... the gap in the Beru connector can no longer be jumped and provide sufficient spark at the spark plug, and a miss is the end result. I have recommended for years that spare Beru connectors be carried in every 911 that uses them.

TerryBPP · 03-17-2003, 06:51 AM

Thanks for the explaination, I didn't realize the CD decreased voltage after warm up. I love this board. Its like gas, my car would'nt run without it.