Weber DMTL sudden Idle issues...

Right so last weekend after replacing my trailing arm bushes, I decided to go out for a drive... I pulled up to some traffic lights and could hear a missfire, which got a bit worse and then ultimately it just cut out. I managed to start the engine again but it was rough and cut out again, fortunately I managed to just limp it around the corner from a busy junction. At first I thought it was just low fuel as the gauge doesn't really work too well (if at all)... so off I went with a jerry can to get more petrol. Fuelled up she fired up and seemed to run a little better, still not perfect, cutting out a couple more times in the next 5 mins or so, but after that ran well enough to get me 15 mins home. Up until this point it'd been running fine for maybe about 400+ miles of my ownership.

A bit later on it started up ok, and was idling for a few mins but again started missing and then cut out, and its been doing that pretty much every time since, the longest I've had it running this week is about 5 mins.

I quickly discounted any fuel flow issues as theres plenty flowing to the carb, so thought clear carb issue. First off I cleaned the idle jets and emulsion tubes (I think), whatever is accessible from the top. No real issue to be seen here but cleaned non the less. No improvement, so disconnected the choke and had the top off. Now there was a fair bit of dirt at the bottom of the float bowls, which is odd as its always had a filter in line, but I cleaned that out and stuck the lid back on for a test... No difference again, wouldn't idle. So then I tried the same again, cleaning it out again, had the jets out again and tried spraying some cleaner down the holes. Then I took the idle cut-off solenoid off to try and clean out better. Found the solenoid itself seemed to not be going in, I though I'd cracked it then, a faulty solenoid! But I put one from the old carb I got with the Landy on, put it all back together and again, no difference.

So now I'm at a bit of a loss, what do people make of this? what do I do next? The carb itself isn't that old, it was put on by the previous owner when he couldn't get the original running right, even with a rebuild kit. So now it seems like I'm in the same situation with this newer carb. I feel like this might be one of the reasons why you don't see that many 2.5 Petrol coilers these days... maybe life would be simpler with a TDI

I had similar issues with my 2.5 petrol, which was fitted to my series 3. It came from a 1986 90. It ran fine for several years until I had similar symptoms, which got worse, causing very poor running and stalling when idling. I rebuilt the carb with a genuine rebuild kit, which helped for a year or so. When stripping down the carb, the floats were disintegrating, causing blockage of the jets. The problem returned so I fitted a new carb, which again resolved the matter for a few years until it came back. I think the fuel lines were also disintegrating through age. I ultimately replaced the engine with a diesel.

Knappster said:

I had similar issues with my 2.5 petrol, which was fitted to my series 3. It came from a 1986 90. It ran fine for several years until I had similar symptoms, which got worse, causing very poor running and stalling when idling. I rebuilt the carb with a genuine rebuild kit, which helped for a year or so. When stripping down the carb, the floats were disintegrating, causing blockage of the jets. The problem returned so I fitted a new carb, which again resolved the matter for a few years until it came back. I think the fuel lines were also disintegrating through age. I ultimately replaced the engine with a diesel.

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So at this point I've had the jets out several times, cleaned the float bowls, put it back together. Then on testing it I found the idle cut-off solenoid seemed to be stuck out, so changed that for the one on the old carb that worked fine. I thought I'd cracked it but it then seemed to be running even worse...

Then I had the whole carb off, cleaned out whatever I could without removing the accelerator pump or the other diaphragm. At that point I noticed the float vent solenoid wasn't working as well, which turned out to be the rubber plunger popped off the end. So at that point I was sure I'd have sorted it, having found and sorted 3 problems, but so far no luck. I'm going to dig into it again soon and check other things as well.

As much as I would like to keep this engine, being original etc if I keep having issues it may well get swapped out for something diesel. I know a 200TDI would be easiest if I could get my hands on a decent one

The 2.5 petrol is a great engine when it runs well. Would it be worth sending it for a total rebuild with new gaskets throughout? The rebuild kit if I remember correctly doesn’t include everything but rather the floats, gaskets to separate the housing and jets. My hoses had also perished so these might be worth looking at. It was some 27 or 28 years ago when I rebuilt mine so my recollection isn’t great.

Knappster said:

The 2.5 petrol is a great engine when it runs well. Would it be worth sending it for a total rebuild with new gaskets throughout? The rebuild kit if I remember correctly doesn’t include everything but rather the floats, gaskets to separate the housing and jets. My hoses had also perished so these might be worth looking at. It was some 37 or 28 years ago when I rebuilt mine so my recollection isn’t great.

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I was trying to avoid a rebuild as the carb itself has been on the Landy less than a year... I can see it has '21' in the casting so its not that old either. The one thing I haven't checked with the carb is the float level needle valve though so I will do that. If I need theres an almost new valve on the old carb I could take off. The previous owner did buy the proper rebuild kit for that, put all the bits on but it still never ran right. So he payed the £300 for a replacement carb. Honestly why are these Webers such a pain... there's a good few threads around with issues it seems, but these days with them not being all that common I guess not much help either. Seems most of the useful info I found on these was from VW forums of all places.

Also on the perished hoses... I still have hard lines running down the chassis but I know some of the rubber hoses need replacing, to be fair maybe the tank does too as theres a good amount of debris caught in the filter. It could be a case of new tank and replace the all the pipes, maybe the fuel pump and sender thats not working too, and get the carb rebuilt if I can't sort it

Well I'm not quite sure exactly what the issue was but touch wood I've fixed it! Got her running again this afternoon after giving the float bowls another clean out, checking the float level and blowing out the needle valve. Then replacing the fuel filter and going over the ignition, adjusting the points, cleaning contacts etc and giving the battery a good charge. She was idling fine for about 10 mins so I decided to go for a quick run... no problems in the couple of miles I did which is promising, hopefully things stay this way. (apart from the gearbox oil leak I need to check out as its not stopped since last time I cleaned it 2 weeks ago)

For anyone with DMTL issues in the future finding this thread, scans from the Weber Hayes manual for this carb are out there online which are quite helpful! There's some helpful cleaning/disassembly videos on youtube too, one guy going though a DATR which seems very similar (apart from being auto choke), and of course some info on VW forums too.