Looking for opinions. I recently bought a 1978 SC with 76k miles. It had a clean PPI but I noticed on hard acceleration a very slight skip tendency above 3000 rpm's. Running characteristics have deteriorated significantly in the 500 to 1000 miles driven since then. One of the oddities of the car is that it requires two key turns to start - every time, hot or cold. First time is a three or four second crank, second time is a two second crank/start.

This has now become three or four start cranks before starting. Also, car stumbles at a 1000 rpm idle but cannot be reved higher until it warms up for a couple of minutes; backfires can be heard from the exhaust outlet; when fully warmed it can intermittently be reved to redline. Sometimes it just won't go over 5000 rpm's the a few seconds later it will run cleanly to redline without the previous skipping, stumbleing, and "flat" running characteristics. The sensation is that it is unable to burn the fuel being fed in, just feels "flat" as if it just isn't firing...any thoughts or similar experiences?

 

BMW....
Ecckchhh!, so we got by that user name start. My personal experiences begin with an 84 Carrera, so I do not profess to be anything more than a helpful fellow Porsche owner. Other SC experts can chime in with their opinions.
I would begin with a basic maintenance tune up. Plugs (use basic copper Bosch, not Platinum, etc), rotor, dist. cap and I believe the SC has points too? Also, before you start, how old are the plug wires? Start the car in your driveway at night and look for arkingat any of the electrical spark ignition points, including the coil.
regards,
Steve

 

Yep, basic tune and then have the hot / cold CIS pressures checked.

 

Hmmm..

Other posibilitites includes, Vacuum leak, tha mean's it's running to lean. Or maybe the fuel pump relay has bad contact. But more likely the misfiring is a sign of lean running or a ignition problem. Or could be the coil, i had a bad coil on my 944 last year, and it stumbled, and i humid weather it would not go past 3000 rpm, lastly it died.

 

i would say fuel pump. my 78 started to run bad, only it did not take 1000 miles to go really south. i limped it home around 100 miles and it barley ran by the time i pulled in the driveway.

you can pull the filter off and bang the inlet side on a work bench with some paper or paper plate and if a ton of crud comes out thats it. i did that and a bunch of sludge $hit came out. new filter and it ran like a champ, better than the previous 1000 miles or so. it was such a slight decrease in power i never noticed it till it went real bad.

good luck

 

Everything everyone else said but don't do looking for points SCs don't have them. Really sounds like a tune up is needed and a check of the plugs/wires and fuel delivery especially.