1.9Dg was fine now very temperamental!

[Mr Bean]

For what its worth I did a load of work on my DJ WBX recently and after the engine was re-installed everything was perfect then out of the blue I got almost the exact symptoms as you are suffering except the ECU/whatever wouldn't let the motor die . Yes I know a WBX is different. But after hours of faffing I found one of the small hidden rubber pipes from the vac system unplugging itself mysteriously every time after I plugged it onto the air box gizmo. A tie wrap fixed it.

Back to your problem:
I thought that 950 was a tad high for a tick over. Plus that erratic and tenuous (stops at the slightest chance) tick over and popping exhaust can be lean mixture due to air leak. A really lean mixture can cause backfiring in exhaust when throttle opened suddenly or under load as can retarded ignition. For blocked jets we used to whack throttle open then strangle engine by putting hand over carburetors to suck on possibly blocked jets. And for checking for air leaks use thick oil on suspect gaskets/fittings which is more messy but more viscous than WD40 . A bad air leak can gobble up WD40 far too quickly. We also used to restrict air entering carburetor by slowly closing hand over intake while engine ticks over. Engine may smooth out just before you flood it to a halt. You didn't mention state of exhaust pipe - sooty or whitish or the colour of of the spark plugs. A trick for weak sparks was to close the plug gaps up a bit but not so relevant to more modern systems.
Dead keen to know how you fix it.

[PeteandNikki]

Next instalment!

Checked timing with vac adv disconnected and plugged. 5 deg btdc . Reconnected and revs rise a bit and timing advances about 5-7 deg All good. (Already Removed and replaced short length of braided flexi pipe off inlet manifold for brake servo) I left ticking over for about 10-mins.

Turn off and re-start. No chance! Back to the same symptoms. Will only stay running with constant slight pressure on throttle to keep revs up at about 12-1300ish take footboff throttle and it dies a death.

[PeteandNikki]

Just to add. Spark plugs are light grey/beige colour and exhaust is black but not excessively sooty.

[937carrera]

Great, so timing is 5 deg BTDC vac disconnected and 10-12 deg BTDC vac conected. Seems like timing and vac advance is working correctly.

The fact that it runs fine at 1200 rpm + and the plugs are a good colour does seem to indicate a weak mixture on idle, when you raise the engine speed the main jet comes in and provides the right fuel for the extra airflow.

It is looking like an air leak or blocked pilot jet, but I'm not that familiar with the Pierburg carb so will let others offer more specific advice.

[PeteandNikki]

[quote="937carrera"]Great, so timing is 5 deg BTDC vac disconnected and 10-12 deg BTDC vac conected. Seems like timing and vac advance is working correctly.

The fact that it runs fine at 1200 rpm + and the plugs are a good colour does seem to indicate a weak mixture on idle, when you raise the engine speed the main jet comes in and provides the right fuel for the extra airflow.

How come I can occasionally get it to run fine but as soon as I switch off and re-start it all goes pear-shaped again?,

[937carrera]

Got me head scratching on that point too.

Had a similar issue on my CU which I solved with re-doing the timing, but that engine has other issues and is due a rebuild anyway.

It's almost as though the mixture is so weak the engine only has enough power to idle when it is already idling. Throttle off, the engine slows below the idle speed and the mixture is not strong enough to pick up to normal idle ???

Can you get it idling and increase the idle speed on the carb, does this "solve" the problem ?

[PeteandNikki]

I tried increasing the throttle stop screw on the carb and got it running ok. I took it for a run and the revs gradually built to 3000 plus with no throttle so had to pull over and back off the screw till idle speed was ok. Drove home, idled ok, switched off, re-started and back to square one!!

[937carrera]

OK, so that seems to be confirming the weak mixture idea, I'm not sure that the throttle stop is the right way to adjust idle speed on a Pierburg, but as I said earlier, i don't know that carb

Have a read here, Itchyfeet has another picture with labels, but I didn't find that immediately

http://forum.club8090.co.uk/viewtopic.p ... p=8217684&

Also, are you sure the shaft for the butterfly, throttle cable is turning freely and not sticking

[PeteandNikki]

I’m sure the carb is operating mechanically ok as it nearly new. It’s a Brosol replacement for the Pierburg which is a direct copy. There are only 2 general adjustment screws. The throttle stop and the CO2 idle mixture screw. As I’ve said. I’ve adjusted the idle mixture to within .5% of the VW setting so that should be fine.

[937carrera]

Ok, just throwing some ideas out there for you

[PeteandNikki]

Ok, just throwing some ideas out there for you

I’m grateful for any suggestions as I’m totally mystified ( not to mention P#*§ed right off!,)

[937carrera]

I tried increasing the throttle stop screw on the carb and got it running ok. I took it for a run and the revs gradually built to 3000 plus with no throttle

That shouldn't be happening either should it. Can you adjust the stop again and replicate the problem when stationary, blipping / holding the throttle perhaps, both at the accelerator pedal and at the engine end, or maybe just leaving it alone and seeing if the idle creeps up to 3000+ ?

[PeteandNikki]

Yes I’ve done that, got it running in it’s temperamental state then wedged throttle pedal with a stick, run round to the engine and screw throttle stop in to the point that the revs are held steady enough to remove stick. Revs will gradually rise so I backed off the screw until tickover at about 950-1000. All ok until I turn off and start again then we return to the problem!

[937carrera]

That's slightly different. Just get it idling using the throttle stop and then see if the revs rise, without making further adjustments or touching throttle.

The purpose here is to confirm that over time the idle continues to increase. Then switch off and see if it will idle "normally"

This "shouldn't happen", but if it does it gives a further piece of information that may help in diagnosing what is actually wrong

[937carrera]

Have a read here - similar problem and solved

http://forum.club8090.co.uk/viewtopic.p ... 64&start=0

Have a good read of the linked document by CJH on page 1

James also has a useful resource here

https://vwt25.blog/2016/09/15/pierburg- ... justments/

Obviously you'll need to adapt to your Brosal setup as appropriate