Uneven idle on Pierburg Carb

[surnameforename]

Hello all,

The Van is a 1990 1.9 litre WBX with various carbs and originally a DG designation.

I've been busy this evening retro fitting a Pierburg 2E carb back onto the van. It had a Weber on it for a several months, which hasn't been very reliable. After reading many of the Weber related post on here I was determined to try the Pierburg again to see if I could get it running. This is what I've done...

* checked and cleaned the Pierburg
** all the passageways were clear
** the float works OK
** the cone fuel filter was clear
** the diaphragms are good (but the vacuum pipe to the choke pulldown was cracked and I taped this up as a temporary repair not having any spare vacuum tube)
* refitted it

Initially the van started up fine - I was frankly amazed! But its not idling very well at all. After starting it up from cold the revs are about 1300, which rise to 1700 and then fall back to about 1500. I leave it for some time and the engine is fully warm at this point - the temp needle is in the middle of its range and the water pipes are nice and hot. So the first problem I have is that when the engine is warm the choke isn't coming off properly. I'm really not sure why this is - could the auto choke unit be knackered? Is there any way I can tell?

Anyway I then press the accelerator to give it some revs and the idle speed falls to a more normal amount - about 800. Adjusting the idle screw seems to lead either to the revs falling to 700 or so and the engine stalling, or going up to about 1200 which is a bit on the fast side. There is a very narrow range in the screw adjustment to go between one and the other. It isn't very consistent between startings - I did manage to get it to level off at about 900 on one go but after stopping and starting the engine they had fallen right off again. Or, for example, I'll turn it 1/8 clockwise and the revs jump up but I have to turn it 2 turns anti-clockwise to get the revs to fall off.

This is what I'm thinking of doing...

* replace cracked vacuum pipe
* check the wiring to the carb (not entirely convinced the mecahnic wired the Weber up correctly and I've been basing my wiring up of the Pierburg off of that)

Does anybody have any further suggestions? Its been a long evening and my brain is now getting fuzzy as I've been checking the wiring diagrams and am in need of some suggestions and pointers as I'm getting lost with it all.

Cheers,

Gavin

[Fritz]

Gavin, in your description of what is happening, you fail to mention the mixture screw ?


Locate this important item and make sure it has not been screwed right in so it bottoms on it's seat..........In an ideal world using an exhaust gas anyliser to set up the mixture @ idle will help you no end.

Auto choke may be also at fault if it has been adjusted incorrectly ( if it is holding choke flap closed once engine is warm) .

Regards


Fritz,,,,,,,

PS, you don't need us to tell you to replace any cracked hoses or useless diaphragms.......

And lastly make sure the secondary choke flap on the throttle body is not sticking/siezed as this can lead to problems if it is not fully closed at idle.



PPS, choke pull down units are a common replacement item, and can be found at Dr Baxters Emporium...

[surnameforename]

Thanks Fritz,

I'll check the mixture screw on the carb and replace the vacuum hose later today and see how it is running. The carb had been off of the van for a while and I wanted to make sure it was definitely faulty, and of the ways it was faulty before diving in and changing lots of things. It is working better than it did though - before it didn't idle at all!

Long night last night so needed to make sure I was going in the right direction,

Gavin

[surnameforename]

Hi Fritz,

Well I've had the afternoon off and have been a twiddlin' but have one very important question...

which screw is the idle mixture screw?
is it the one on the top of the carb next to the primary and secondary jets (labelled as the Pilot Jet in the Haynes manual)?

I have looked all over these forums and the internet to try and find out for sure. If it is this one then it is definelty screwed down, well was as I tried undoing it a little bit to see what the effect was. And the main effect of taking it out half a turn was to even out the idling and allow me to adjust the idle speed to roughly 900 (ended up more like 950). (I trimmed off the split bit of the vacuum hose and checked the wiring but have some other questions on the latter for another thread).

Anyway, I'm going to leave it on the drive until I know for sure.

Gavin