1977 911s, diagnosing 2 main issues.

[AndreasK]

Advice needed

My 77 911s has had a long lasting cold idle issue. Recently fuel consumption has become painfull. (Norwegian gas prices are through the roof). 1.48 litres pr 10K. Shuld be closer to 1.0. Im also getting popping from unburnt fuel in the exhaust.

Finally got around to getting a fuel tester, and I have run through the tests.

WUR 033
Cold controll 0.75 bar at 10 C. (Spec 1.1 to 1.6)
System pressure 4.8 bar (Spec 4.5 to 5.2)
Warm controll 2.7 bar fron 5 mins and onward (Spec 2.7 to 3.1)
Engine running with vacuum 3.6 bar (Spec 3.4 to 3.8)
Resting pressure 0.5 at 4 min. (Spec 1.3 at 10 mins)

So my cold controll is low. Which explains my cold idle issues? I have to give some throttle until warm. Idles OK avter a couple minutes. Does that mean my WUR needs an adjustment or a reebuild? Or anything else I should try first?

Warm controll is whithin spec. Which means my high consumption and popping from unburnt fuel in the exhaust is another issue. Would changing my injectors be a good place to start? I have no idea how old they are or how they are functioning.

Any other advice for a ever learning air cooled newbie, from the results from my testing?

[AndreasK]

Oh... re checked my ignition timing. Slightly retarded. About 7-8 atdc, shuld be 5 atdc. Adjusted. Could that be causing the increase of consumption?

[PeteKz]

Timing will make a big difference in fuel consumption and torque.

The more important timing measurement is not at idle, but at full mechanical advance. At full advance (without the vacuum connected) it should be around 28-30 degrees BTDC. If that causes your idle timing to be off, don't worry about it. Just get the full advance timing right. When you connect the vacuum, that should pull about another 10 degrees of advance. Thus, your total timing at part throttle and above 4000 RPM, should be about 38-40 degrees.

This may be higher than what the specs call for on your RoW car. Don't worry about that either. Adjust your timing 1-2 degrees more advanced, and then less advanced, to see where it runs best, without getting into pinging (detonation). If you have doubts, it's always safer to run a little less advance, than too much.

Exhaust popping: Does your car have the thermal reactors and air pump? If so, this could be the cause. You need both unburned fuel AND excess air to get popping or "afterfire". The thermal reactor system injects extra air to help ensure exhaust is fully burned, but if you have excess fuel, then you will get popping.

If your car does not have the thermal reactor, then you may have leaking exhaust gaskets.

[jma0506]

It sounds like you're on the right track Andreas and I agree with Pete.

Your CCP being lower than spec will mean you're running overly rich on startup, and depending on your climate, your cold start valve may be firing when cranking creating an overly rich cold state which would make it difficult to start. You should try to get your WUR into spec for the CCP.

As for popping from unburnt fuel, it's possible that your air/fuel mixture for when the car is warmed up is set a little too rich and thus leaving extra unburnt fuel in the exhaust and causing the popping. You should first check for vacuum leaks to ensure you don't have any unmetered air in the CIS system but see if turning your air fuel mixture CCW (lean out) 1/8 or less of a turn at a time and monitor engine running behavior.

[AndreasK]

Originally Posted by PeteKz

Timing will make a big difference in fuel consumption and torque.

The more important timing measurement is not at idle, but at full mechanical advance. At full advance (without the vacuum connected) it should be around 28-30 degrees BTDC. If that causes your idle timing to be off, don't worry about it. Just get the full advance timing right. When you connect the vacuum, that should pull about another 10 degrees of advance. Thus, your total timing at part throttle and above 4000 RPM, should be about 38-40 degrees.

This may be higher than what the specs call for on your RoW car. Don't worry about that either. Adjust your timing 1-2 degrees more advanced, and then less advanced, to see where it runs best, without getting into pinging (detonation). If you have doubts, it's always safer to run a little less advance, than too much.

Exhaust popping: Does your car have the thermal reactors and air pump? If so, this could be the cause. You need both unburned fuel AND excess air to get popping or "afterfire". The thermal reactor system injects extra air to help ensure exhaust is fully burned, but if you have excess fuel, then you will get popping.

If your car does not have the thermal reactor, then you may have leaking exhaust gaskets.

Thanks Pete!

It's a US car, but reactors are long gone. Ssi and 2 in, and some various out mufflers the 7 years I've had the car. I don't have any major exhaust leaks, but smaller ones could be a partial culprit.

I did my timing propperly a couple years ago. Advance was, as far as I can remember, good when at 5 atdc. So yesterday I just did timing at idle to check. Will do the full advance later today👍

[AndreasK]

Originally Posted by jma0506

It sounds like you're on the right track Andreas and I agree with Pete.

Your CCP being lower than spec will mean you're running overly rich on startup, and depending on your climate, your cold start valve may be firing when cranking creating an overly rich cold state which would make it difficult to start. You should try to get your WUR into spec for the CCP.

As for popping from unburnt fuel, it's possible that your air/fuel mixture for when the car is warmed up is set a little too rich and thus leaving extra unburnt fuel in the exhaust and causing the popping. You should first check for vacuum leaks to ensure you don't have any unmetered air in the CIS system but see if turning your air fuel mixture CCW (lean out) 1/8 or less of a turn at a time and monitor engine running behavior.

👍
I may try to adjust my wur, slightly. No-one I know and trust for a rebuild locally. And sending it around 1/2 the globe, will be a last resort😅

My warm-mixture is where it can't go any leaner, whithout it being too lean.

I'm in Northern Norway, so plenty of cold startups. No problem starting, it just won't idle, whithout some throttle input for the 1st minute or so.

Will doo my timing propperly, check for exhaust leaks and just maybe get a new set of injectors. Just to be certain they are OK. Not too expencive and i know they are old. I've seen how to test them with jars and such. But not sure I would trust my own findings😅

[AndreasK]

Looks like ignition timing vas my main problem! I thought it was good at 5 atdc and rughly up against 30 at 6000. I'm now at TDC at idle, and 32-33 btdc from around 5000 and onwards. Vacuum disconnected and plugged. Different car at lower rews! No more popping of any kind. Longer trip tomorrow to check consumption👍🤞 May try a couple more degrees advance...