78 911sc CIS problem cold start low rpm power loss idle hunting intake backfi- SOLVED
 

Symptoms

Cold:
First crank idles up and stalls
Second crank might run or might stall
Third crank it will start idle hunting until up to a reasonable operating temp if revved will backfire through intake and not rev up
Terrible gas smell/fumes - hurts eyes when close
If fuel is squirted into intake Itl rev freely

Hot:
Idle is now smooth but higher
Down on power in lower rpm until around 3500rpm then pulls like it should.
Lower rpm it really is dead like you need to change down a gear to get up any small hill even a 3rd gear hill requires 2nd
Once left for an hour it won't start at all unless some kind of ignitable vapour is sprayed through intake manifold.


What I've done:
General tune up plug filters etc
I removed AAV and that eliminated most of idle hunt but no other symptoms

sounds like the cold control pressure is too high making it too lean.
cold start requires rich mixture and extra air.

if you want to fix it youll need the fuel pressure tester then you will need to modify the WUR. if you work on your own car the WUR mod is no big deal.
that way you can set both the cold and warm control pressure, its really a must

you mentioned that the AAV is removed, so the AAR- the one with the elec plug, is still there right? cause you need that one.

ok so i read the entire post- sounds like the warm CP is probably high as well

What jason said.

Tell us about the degradation of performance. Was it right away or did it get crappy over time?

Did you fuss with distributor timing?

No start after an hour is residual pressure. You have a leak that kills pressure and it's still hot enough to not fire the cold start injector.

Fuel accumulator is suspect but not guaranteed as the culprit. You can have a leaking injector, for example.

Buy the gauges. Best $150 you can spend right now.

Bought it with problems and blown intake gaskets and airbox. Timing is in spec. Airbox glued and screwed back with pop off and all gaskets done. Will make wur adjustable and see if that helps then will purchase some gauges

first get the gauges, then modify the WUR. you need the gauges to see if in fact you have a fuel pressure problem.

one thing you can do without any of that is take out the WUR and clean the screen on the inlet port, if thats clogged it wont run right

Make sure the boot from the air metering plate to throttle body is properly seated and sealed and not torn.
I bought my car as a non-runner that required the same gas poured on air filter to start you described and found the boot was off on one side at the back where it wasn't visible.
10 minutes with a screw driver later, I had a running car....

Problem solved!!!!!

Reconnected Aux air valve
Made WUR adjustable. Tapped pin down 2mm.

Started up and idle hunts for 1minute on warmup but can rev it freely when cold. Awesome. When warm everything is as it should be. As I don't have pressure gauges to check the cis pressures but have access to a friends wide band afr setup I will check with that shortly but it is waaaaay way better. Now I will try and trace the idle hunt problem but that's the least of my worries at the moment as its drive able while cold now.

Idle hunt problem is typically a too rich mixture, probably because you tapped down the pin without using test gauges. When the car warms, the wur is not affected by the cold start pin. Once you have the fuel gauge set. the first step is to check your cold control pressure against the graph for your particular wur (which shows the pressure at specific ambient temperatures) and set it using the adjustment you installed. If that doesn't solve the hunting issue, you will have to move on to another possibility.

As per first post its had idle hunt problems since owning. Both lean and now richer. Will track it down at a later stage