Carrera 3.2 Motronic - Is retarded timing at idle normal?

[RobFrost]

Still ironing out the tuning my 1988 Carrera 3.2 with the Bosch Motronic system. I got around to looking at the timing yesterday and I learnt that it just has rotors in the distributor with no points. This much should have been obvious because I kind of knew the Motronic controlled ignition timing, I just hadn't thought it through. Anyway... after working that out, I could see (depending upon my correct interpretation of timing marks) that the timing at idle is constantly twitching (at very high speed - many times per second) between about 0 degrees and 10 degrees (estimated) retarded.

This can't be some play or malfunction in the distributor because well, timing isn't controlled there. I can't find reference to it in the Bentley manual. Does anybody know if this is a normal part of the car's maintenance of a steady idle?

[911hillclimber]

Does it run /idle well?
My 3.2 is stock and 140K miles or so old, never been apart so factory. Never looked at the timing because the DME sorts it all.
Tick-over and running is faultless so...if it isn't broken, don't fix it?

[Bruce M]

The advance curve increases below 800rpm (or thereabouts). That way if the idle falls, the advance increases which generates more torque and increases the idle, then it retards back. It’s a self stabilising system and fairly common.

[RobFrost]

Does it run /idle well?
My 3.2 is stock and 140K miles or so old, never been apart so factory. Never looked at the timing because the DME sorts it all.
Tick-over and running is faultless so...if it isn't broken, don't fix it?

Surging idle at times, especially when warm. Setting fuel adjustment screw back to stock greatly improved it. 150k miles. Just sorting out vacuum leaks, etc. Will post a tech thread re one of them when it's all done. I also just cleaned all the gunk out of the throttle body - apparently that's a common cause of rough idle. Will know if it's better once the vacuum hoses are all back on. I was more concerned it appears to be up to 10 degrees retarded... Surely it should never be retarded? But I guess I need to get more certainty on which timing marks I'm looking at as user error is the most likely explanation.

[RobFrost]

The advance curve increases below 800rpm (or thereabouts). That way if the idle falls, the advance increases which generates more torque and increases the idle, then it retards back. It’s a self stabilising system and fairly common.

Seems strange that it would appear up to 10 degrees retarded then. Mike B said it's impossible for the flywheel to be fitted at the wrong angle - suggested damage to the position sensor. I wonder if the pulley could simply be on at the wrong angle? Most likely explanation continues to be user error.

[Bruce M]

Ah missed that detail.

[RobFrost]

Ah missed that detail.

Thanks for the point re it being used to moderate the idle though. That explains the jitteriness.

[911hillclimber]

The flywheel and the front pully can only go one in one set position, ie flywheel bolts: one is off-set to the others that are evenly spaced.
The ring on the flywheel is thus stable (not the starter ring, there are 2 rings!)

[gridgway]

How are you measuring the timing to see the flitting to retarded many times per second? Just wondered if there was an issue with the measurement rather than the timing itself?

[RobFrost]

How are you measuring the timing to see the flitting to retarded many times per second? Just wondered if there was an issue with the measurement rather than the timing itself?

Timing light on cylinder 1, strobe on the timing marks.

I came to the conclusion (based on what some sources said) that to maintain a stable idle, in addition to the idle control valve, Motronic rapidly adjusts the timing retardation - so that accounts for the *rapidly fluctuating* retardation. What I'm not so sure about is why the base timing is retarded. Maybe I have the wrong timing mark - it's hard to see.

[gridgway]

Regarding the timing marks, can you measure at a higher rpm to see if you get the advance expected?

Be fun to find details of the motronic algorithm for retarding. How many sparks does it retard for?

[RobFrost]

Regarding the timing marks, can you measure at a higher rpm to see if you get the advance expected?

Be fun to find details of the motronic algorithm for retarding. How many sparks does it retard for?

yes it advances as expected, I could already see that. It's just retarded at idle. I assume it's the same for all plugs because it runs beautifully and it'd vibrate if it didn't.

[gridgway]

Cool, then you're probably in the territory of not fixing what's not broken!

[RobFrost]

Cool, then you're probably in the territory of not fixing what's not broken!

If there's extra horsepower to be eked out I'll be after it - whilst trying not to break anything. e.g. may go for a MAF conversion. Also, there is an issue to be resolved - I was lucky to get through the MOT emissions with high CO, so I want that fixed for next year. Excessively retarded timing could be a cause of unburnt fuel escaping the cylinders.

[gridgway]

Ah, that's a very different story then! More BHP plus a valid MoT. Agreed, both need sorting!!