Strange 'pinking' noise

[Trundler]

I realise this is not Syncro specific but I have a problem afflicting the 2.1Dj in my Syncro. Tried to post this in the Mechanical forum but the site won't let me do it for some reason...

I have a strange pinking noise on light throttle openings at mid- revs. It does this on both petrol and LPG so I'm suspecting ignition and specifically the distributor advance retard. Strange thing is that it does it more on (higher octane) LPG than on petrol It seems to disappear at higher revs or as more throttle is applied.

As I understand it, the vac advance advances the ignition at lower revs or lighter throttle when there is less vacuum so if it didn't work the timing would be relatively retarded (perhaps like me!) at those revs so you wouldn't expect pinking.

It is most audible in 2nd gear at the sort of revs where you might change up to third under gentle acceleration, which makes me worry about motorway cruising as revs/throttle must be similar, though you can't then hear the pinking for wind and transmission noise.

Any ideas?

[silverbullet]

Lighter throttle = more vacuum, so the distributor applies more advance better cruising economy.

Pinking under moderate load i.e in lower gears means it's very lean, maybe dangerously so. My SA does this on 95 RON (10:1 compression and the ecu ign. timing is set up for 98+RON) but it doesn't make a peep when on LPG.

Sounds like your LPG needs a tune-up at the very least, or the timing is way out for some reason. Or it's got a big air leak that also making it go lean across the board. Lots to investigate!

[Trundler]

Thanks for the reply silver b .. lots to check indeed. I really need to give it an expert but I have noticed that the problem is much reduced if I disconnect one of the vac pipes to the distributor. I can see that any one of your suggestions could be the problem though.