MFI Tank Vent - source of mystery woes?

[layzee]

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?

[Rot 911]

I suspect this is an electrical and not a fuel problem. If it was a bad tank vent you would get fuel starvation that would become even more pronounced at higher rpm's. I'm thinking like a coil going bad or perhaps the vacuum advance sticking on the distributor. I would insure my ignition system was in order before I started looking at the fuel system.

[HawgRyder]

If you are stopped by the problem...and take the fuel cap off...does it vent pressure or vacuum?
That is the first thing I can think of...and it costs nothing.
Bob

[RSTarga]

Sounds like the warmup regulator is not functioning properly. Have you checked that there are no collapsed discs or that the warmup hose is hooked up properly?

[Porboynz]

+1 on the electrics, but also consider the fuel pump and fuel line routing as well. Make sure that the overflow for the fuel filter console is plumbed as per original. It used to T into the return line with the fuel pump, yours will be missing the fuel pump connection presumably. Too much fuel is as bad as not enough, if you pull a plug when the misfire starts is it fueled up wet? This could also explain the high revs good behaviour and low revs mis behaviour. I once thought it would be a good idea to block off the heater air hoses from the engine to ensure maximum airflow and cooling across the engine. I completely forgot about the warm air hose to the thermopile on the MFI injection pump. It started well and once warmed up was a pig at lower revs. Pulled well at high revs but it sure used up the petrol. Du-uh.

Ah, Classic Le Mans, fantastic. I did the 2008 tour but the 911 would not fit in my carry on luggage, I envy you.

[layzee]

Thanks guys

Kurt - Coil is new, as are wires. All done when I added MSD

HawgRyder - taking the gas cap off is an obvious thing I have never tried - will do.

RSTarga - Will do some more research on the WUR. I've never had any starting problems so reluctant to blame that. Still I suppose I should strip down the themostat and check it. The hose is connected to the heat exchanger and is in reasonable condition.

Porboynz - Going to hook up my pressure gauge on the filter and check that. I should have drawn out the fuel line routing I used when I did it as I have no idea now; another line to investigate I guess.