Since I last really worked on my 72 we've had two kids; the last real major drive it had was to Le Mans Classic in summer 2010. Obviously my spare time isn't what it once was!
Before the trip to Le Mans, I had replaced the original fuel pump with a Walbro unit. As these don't have 3 connections like the original pump, I've altered the routing of the fuel lines, I am sure according to a diagram I found on here.
On the way to Le Mans, and in the shakedown runs previously, I've had the following symptoms:
* Car will start fine, whatever temp (MSD helps here)
* Runs as normal for 5 miles or so, normal driving
* Then starts to misfire and pop, quite severely
The first few times I had this obviously I would stop the car. It would not restart, after 5 minutes or so, it would go again, and then would run great. Hard highway runs etc, no problem, great power. So this became a regular thing, and I realised if I kept the revs up (5k plus) through this misfire period, eventually the car would run fine again. Weird right? So we drove it to France and back like this, and had no problem cruising at high speed on the autoroutes. Definitely not down on power; the car would pull hard, and we drove at 150kph most of the way).
I had planned a nice route to La Sarthe, on the historic N138. However, I soon realised that town driving and my 911 didn't mix.
Spending too long at lower rpms would mean the misfire would come back. Of course, in city driving keeping above 5k rpms is very difficult! For the rest of the trip we planned where we could go at high speed (!) and didn't have too many problems - we made it 600 miles home so that's success right?
I want to try and get this fixed this summer as it really hampers the enjoyment of the car; being a sporto it should be at best in traffic - at the moment it is a constant worry!
So checks done so far:
* New tank pickup installed
* All new fuel lines in engine bay and pump area
I haven't had chance to do a full CMA on it, have a fuel pressure gauge to install, but recently have been thinking about the fuel tank vent system. Could pressure be building up causing these symptoms? What are the things to check on the evap system? There's a lot of useful diagrams in this thread:
tank ventilation-MFI diagrams
But where should I start looking - and is this even a feasible source of the running problems?