Original restrictor size in swirl pot return line carb 1.8 (DMTL / Pierburg)

Discussion in '8-valve' started by tram-man, Jun 15, 2024.

  1. tram-man New Member

    Feb 9, 2024
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    Over the last 6 months I have had grief with my 1.8 DS engine in my Audi with DMTL.

    Symptoms were loss of power after giving it full throttle for shorts periods. I was also noticing hesitation after longer periods of motorway cruising. In worst case engine dying and dangerous. I put this down to carb icing in the winter even though I know my air cleaner vacuum system works. Now its summer and that theory doesn't cut the mustard. I've had the set-up 10 years and generally happy but always wondered why my lambda LED's were always dancing around. Also I can see that the engine runs nice and rich with full throttle but just before it dies the lambda indicates lean. So I always suspected fuel starvation over icying.

    To cut a long story short today I found out that the car was suffering from fuel tank vacuum. When I disconnected the (return) pipe between swirl pot and tank I heard massive vacuum that surprised me. Thereafter the car ran great again until the vacuum returned. I then tried unscrewing the fuel cap. Again massive suck followed by good running. Eventually a long run with a loose cap meant I had found the cause. This after countless evenings of diagnosis; ultrasonic cleaning the DMTL, ignition, swirlpot delete, larger needle valves, icy red-herring etc etc. I hope you are feeling my pain......

    What actually was causing the vacuum is still not clear. I had already put a prop in the return line to the tank with a 2mm hole in it to act as a restrictor. I read here that there is a restrictor in the return pipe. Mine was never there (had the car 12 years). I blew through this return line but I know this was open as that's how I heard the vacuum. I also located the fuel tank vent pipe and blew through this with and without fuel cap screwed tight. It was open.

    What I think might have been the cause (but unfortunately I can no longer verify) is that the path of the fuel tank vent pipe was such that I had created a fuel lock due to a low point or dip in the pipe. The website explains the theory https://www.tanksinc.com/index.cfm/page/ptype=results/category_id=160/mode=cat/cat160.htm I cant verify as I pulled the pipe to see if it was clear. Any fuel/fluid block would have been dislodged. I never realised that the route of the fuel vent pipe was so critical but it is easy to understand and explains in retrospect ALL the symptoms I have been experiencing. Also worth noting was the problem was worse when the tank was not full. I now that's because the pump needs to work harder to overcome the vacuum.....

    What I would like to know know now is the diametre of the original restrictor in the swirl pot return line. I am keen to keep the swirl pot (several reasons). Would anybody been kind enough to pull the pipe and measure with a drill bit? I now made a 6mm aluminium prop with a 2mm hole but I think it might be too big as one of the things I noticed when I bypassed the swirl pot was that the lambda meter was very steady. I attribute this to the feed pressure then being very constant. With a 2mm hole the lambda is steadier but not as good as direct feed from pump to DMTL.

    Apart from finding out the diametre I hope the post help others with cutting out.
    Last edited: Jun 15, 2024
  2. rubjonny

    rubjonny Administrator Staff Member Admin

    Oct 31, 2003
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    good question, I only saw mention of this original restrictor many years ago on the mk1 forums but I cant remember what size they said it was... I'll see if I can dig the post out but dont get your hopes up. keep us posted anyway
  3. tram-man New Member

    Feb 9, 2024
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    Thank-you. I first drilled a 3mm hole in a 6mm alu bar. But the lambda was dancing quite a bit. Now got a 2mm and is a bit better. I notice that the DMTL's from a landrover have a top plate with a feed and return line instead of just one feed. When I blow through the return I can 'feel' it is restricted more than the feed but cant actually measure the opening.

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