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  • More airflow required to the Carb...

    Here we have the Fiat 2.0 TC with its Weber 34 ADF 250 carb:





    I think the car is suffering from insufficient airflow to the carb as I feel it should 'go' a bit more (I might be wrong; haven't had it tested yet - that's scheduled in maybe March or April at a local tuning specialist). While it certainly does pull and I have no trouble putting the hammer down and overtaking... it just feels as if it should go that bit more from a power to weight ratio.
    The air filter is as shown; allowing air in from the perimeter and also through the top - but the top flow is impeded by the bonnet that fits snug over the top. There is a perfect ring shape on the bonnet under side where the filter meets it and I'm guessing the bonnet is sucked onto the filter during acceleration.

    So, I am looking for a solution. On some cars I have seen big air intake cowls on the bonnets; often big ugly things that would look out of place on this style car, I am told there are 100s of different style intake cowls and that there would be one out there which would match the image of the Marlin... I confess I have hunted throughout the internet and drawn a blank. My Dad has suggested that we cut louvre vent holes in that area of the bonnet (just as the engine bay sides have) and this would be ok. I am not so sure that's the best way forward.

    My concern with cutting any hole and getting louvres or an air intake cowl is the chance of water getting in... How do other cars with cowls prevent rain water getting into the filter? Air yes but water no?

    Much obliged for your thoughts / advice!
    Last edited by element; 19-05-17, 05:26 PM.

  • #2
    Re: More airflow required to the Carb...

    pop the bonnet off and whizz up a private road to check thats the problem. Those carbs are renowned for issues with the cold start sticking and messing up the mixture so it might be more fiddly than you imagine.

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    • #3
      Re: More airflow required to the Carb...

      I could do that - but it'd have to wait until Springtime.. The mixture screw is indescribably awkward placed in relation to the engine bay wall and it was extremely difficult to get a screwdriver on it and then be able to turn it - but I think the mixture is about right; as good as I can do myself anyway. However thinking about the open bonnet idea - it would determine whether my hunch is right or wrong - I have a local airfield not too far away & I could time a run there with and without the bonnet - it would definitely prove yea or nay. I'm just SORN over winter for various projects...

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      • #4
        Re: More airflow required to the Carb...

        Originally posted by element View Post
        I could do that - but it'd have to wait until Springtime.. The mixture screw is indescribably awkward placed in relation to the engine bay wall and it was extremely difficult to get a screwdriver on it and then be able to turn it - but I think the mixture is about right; as good as I can do myself anyway. However thinking about the open bonnet idea - it would determine whether my hunch is right or wrong - I have a local airfield not too far away & I could time a run there with and without the bonnet - it would definitely prove yea or nay. I'm just SORN over winter for various projects...
        Or you could place a 50mm block under your bonnet edge, and temporarily strap it down so that you could retain the bonnet whilst testing. Much easier, and safer!

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        • #5
          Re: More airflow required to the Carb...

          Now that is why I enjoy this forum - so many helpful ideas and suggestions; that one is a winner Mike! I am sure I can arrange something along those lines, as long as I ensure the front of the bonnet is equally blocked and strapped that would be a great experiment (air entering the front might stress the bonnet maybe).. Roll on Spring

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          • #6
            Re: More airflow required to the Carb...

            Hot rods run without bonnets so I wouldn't worry about it for a test.

            In my case I cut a hole in the bonnet to allow the air filter some room. I've never had any trouble with water. I think that there is a picture in my rebuild thread.
            Mk2 SWB Marina Roadster with a 2.0L Pinto built in 1986

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            • #7
              Re: More airflow required to the Carb...

              I have a similar set up - I was looking at these

              If you do it let me know how it goes

              This is a new plenum chamber air filter kit to fit the DMTR & DMTL Weber range of carburettors. It is ideal for cars with limited bonnet height & designed for a pipe of your choice to be connected to a remote air filter away from hot air.

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              • #8
                Re: More airflow required to the Carb...

                Originally posted by Alan.c View Post
                I have a similar set up - I was looking at these

                If you do it let me know how it goes

                http://www.ebay.co.uk/itm/WEBER-32-3...MAAOxyNo9SrK1s
                These look good but I'd be concerned that they were more restrictive than the filter..
                Mk2 SWB Marina Roadster with a 2.0L Pinto built in 1986

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                • #9
                  Re: More airflow required to the Carb...

                  I had similar problems on a twin cam engine I ran in a delta in rallycross. the shallow filter was being sucked down almost on to the top of the carb choke under heavy acceleration restricting the airflow and making the motor feel flat. Tested this by removing the filter fitting a pair of the wifes tights over the chokes to keep any big bits out and giving it a blast. The permanent solution was to create more space and fit deeper filters. Demon tweeks recommended at least 2 inches of free space above the chokes. This cured the problem. Or ditch the downdraft carb and restrictive manifold and fit sidedraft weber or similar.

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                  • #10
                    Re: More airflow required to the Carb...

                    Although it's a while away before I can actually test out different ideas and then make changes, given that my mechanical knowledge is perhaps around the level of 'competent diy' and certainly far from professional! I have decided to read and study as much as I can through the winter. I have been reading relevant sections of "Modifying and Tuning Fiat/Lancia Twin Cam Engines" by Guy Croft and he's really made my mind up to make some changes - particularly arranging a cold air feed to the carb instead of the current engine bay temperature feed. In the longer term I would like to go via the twin carb route but funds won't permit that atm. Sidedraft fitted carbs would be nice - typically offering up to 7% more engine power than downdraft carbs. I have a single downdraft carb & have to try to make the best of it for the time being.
                    Also found this very interesting blog: http://www.britishv8.org/articles/mg...-induction.htm - although its from 'across the pond' & speaks of Fahrenheit and inches of mercury, the basic facts are valid. Now I understand why some cars have the air intake set in front of the windscreen - as its a high pressure air zone - there for the taking. At the moment however, it looks most likely that I'll pipe in air from the front & insulate it from the engine bay heat.

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