1979 sc missfire/stutter at 2000rpm

[Gibbo]

Hi all, I'm new to the club and 911 ownership, my 1979sc was running fine, It has developed a strange fault, it starts on the turn of the key, ticks over at fast idle until warm then drops down to the correct speed, it hunts a little but not too bad, when you rev the car through the range there is no discernible missfire, but when driving slowly increasing the revs the car stutters just before 2000rpm and continues until approx 2700rpm, after that its fine, also if you accelerate hard its unoticable, and cruising in 5th at that rpm it is barley noticable, the fuel pump was making a hell of a din, so i've changed that and filter, I've also adjusted the valves(and oil change), and checked the timing(5 deg BTDC), the coil tests ok as do the leads( the HT lead coil to dizzy 980ohms?), the rotor and cap are relatively new. checked the plugs and they are all the same colour, black around the bottom of the thread and a little up the contact but the electrode and tip of the contact is a biscuit colour, do you think its running rich if so why does it only affect running at 2000rpm, any help/advice would be greatly appreciated...

 

[bones]

You've done a lot yourself, so perhaps it is time to put it on a Bosch testing machine and get the mixture and cold and hot fuel pressures tested, the machine also checks the sparks.

 

[sonflowerinwales]

Hi
Have you checked the airbox is sealed? I had a problem with dodgy tickover, it was due to an air leak....
Cheers
Paul

 

[SJW BASIRE]

I know you have checked the leads but are they the 1979 originals? I had a similar thing on my '80 SC and found that the left hand front lead(Number 1?) was arcing onto the heater hose,especially if the weather was damp.A new set of OEM Beru leads from Type 911 and the problem was cured.

 

[Gibbo]

Thanks for that Steve, I will check for arcing(in the dark), the leads are metal shielded with earthing leads and appear in good nick, the only thing is, its not an intermitant problem, it happens all the time hot or cold, thanks again..Tony

 

[SJW BASIRE]

Yes the leads are metal shielded with antistatic earthing to Aircraft specification.It is one of the most distinctive features of the earlier 911 engines like ours.It's a great conversation piece when showing the engine to a fellow anorak and is why I replaced them with the same again rather than use non original parts!