Carburettor dies under throttle

Discussion in '206/246' started by Andrew McCrae, Mar 5, 2021.

  1. Andrew McCrae

    Andrew McCrae Karting

    Jun 5, 2016
    Balancing the carbs today. Everything looking good, idle speed good, Airflow similar on all 6 inlets. Very steady idle. But.... Carbs not connected to each other or loud pedal yet! Carb 3 open the throttle manually quickly and the revs dip slightly before accelerating. Carb 2 the same. Carb 1 open the throttle and the whole engine dies.

    Any educated ideas of where to look. All the floats are set perfectly. The emulsion tubes and the jet into the inlet were all removed and cleaned when doing the float.

    Thanks for your kind help in advance
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  3. Andrew McCrae

    Andrew McCrae Karting

    Jun 5, 2016
    Or if somebody could point me to a site that could help please. I have the Haynes Weber manual but not great for troubleshooting.
  4. Nuvolari

    Nuvolari F1 Veteran
    Sponsor Owner

    Sep 3, 2002
    Toronto / SoCal
    Full Name:
    Rob C.
    I'm not quite sure why you would not have the carbs connected when doing these transitional acceleration tests so linking the carbs is the first thing I would do. Once this is done re-check the balance between the carbs and test the assembly as a whole. That said your problem is most likely one of fueling and because it is happening just as you quickly open the throttle after idle then the high speed circuit is most likely not at fault. Your attention should be on the idle jets (that control fuel flow to 3,000 rpm), the accelerator pump, the float bowl, and the idle needle. I would try the following:

    1. Before re-connecting the carbs together move the #1 carb to another cylinder and see if the problem moves with the carb.

    2. Re-do all of your tests as you did previously and look down the carb as you quickly crack open the throttle (wear a safety mask just in case there is a flare back). Check to see if the accelerator pump nozzles look like they are giving an equal spray. You are looking for one weak one. You can also try this test with the engine not running just to inspect the accelerator pump operation.

    3. Remove the top from the carb and check the fuel bowl to see if there is any sediment there that could be blocking something. Even if you just had the carbs apart it is possible some debris has found its way free and the bowl is a good place to look first.

    4. Remove the idle jets and check them closely for debris or blockages

    5. Remove the idle needle and check it for abnormalities also blow compressed air in its seat to ensure cleanliness

    Somewhere in these tests you should find something that is different or out of place. Let us know what you find.
  5. Ferrari_250tdf

    Ferrari_250tdf Formula Junior

    Mar 3, 2005
    Isn't the brake booster connected to this carb? If so maybe there is the problem.
    Andrew McCrae likes this.
  6. pshoejberg

    pshoejberg Formula 3
    Silver Subscribed

    Dec 22, 2007
    Full Name:
    Peter H
    I'm not even sure that there is a carb problem here and I'm not sure I know why you try to run the engine on 2 cylinders only (If i understand you description correctly). When I adjust my carbs I perform the following two steps:

    1. Adjust set the idle mixture on each barrel by loosening the linkage slightly and turning down the idle on 2 of the 3 carbs until the engine is just about stumbling. Then I adjust the idle mixture on the third carb by observing for max engine rpm. Same procedure for the remaining two carbs.

    2. Adjust and balance the air flow between the carbs by adjusting the idle screws (I have no air bypass screws on my carbs)

    Then I tighten the linkage again taking care not to affect the idle setting on each carb.

    Hereafter I test the car by manipulating the throttle linkage by hand first and then a test drive. Every each issue I have had with reluctant engine response have been related to debris in the idle jets or wrong jet size. I have gone one number up in jet size from standard jet to compensate for modern fuel quality upon recommendation from Rob and that improved engine response in the range from 1000 to 3000 rpm tremendously.

    Best, Peter
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  8. Andrew McCrae

    Andrew McCrae Karting

    Jun 5, 2016
    #6 Andrew McCrae, Mar 6, 2021
    Last edited: Mar 6, 2021
    Thank you Guys, in turn
    1. Nuvolari, I've linkage disconnected while trying to balance them to avoid interaction. Just before reassembly I did this quick test to find #1 very different. I have done some but not all of your suggestions so they are now a place to look thank you.
    2. Ferrari-250... So that's what that does . It's #3 cyl for me on my Fiat Dino but now I know thanks.
    3. Peter, I did reverse order to you as that's what the guide says. All 3 running not 2. I get your point on jet size and maybe I need to order new jets from UK or EU. I have floats perfect but didn't remove the carbs to get into the rest of it.

    Thanks, when I get this redone I'll let you know.
    I also thing I need a cheap screwdriver (short length to get in there) as its quite tight around the covers.
  9. Andrew McCrae

    Andrew McCrae Karting

    Jun 5, 2016
    Idle Jets on all carbs are 50. Workshop manual recommends 55. My carbs are 40 DCNF-6 and manual says DCNF-14 so this may explain as progression may differ.

    But definitely I'd be nuts to not try 55s or even 60s. Next post 2 weeks
  10. TonyL

    TonyL F1 Rookie

    Sep 27, 2007
    Full Name:
    I would agree with others and dont really know why you want to rev just one carb up, they all have to run in sync. It is the correct way to tune the carbs though by disconnecting all the linkages and setting the mass airflow thru each barrel with a tickover of no more than 800rpm, any higher and you bring in the 1st progression port and it kind of messes up your idle mixture settings. Then tune the idle mixture screws to achieve best tickover. any increase in revs then adjust the idle stop screws to suit, again checking the mass air flow. Any difference between each barrel then adjust the air correction screws. These should be completely shut before you start, anymore than 1/4 open and it can lean out that barrel.

    The reason why your carb cut out the engine is because when you open the throttle plate there is a sudden pressure drop through the venturi because that carb is running lean. Remember its the vacuum that draws the fuel in.

    I wouldnt go to high on the idle jets, 0.55mm and you may find the car will run rich on idle. If you want to correct for the poor (crap) fuel we get then you better off trying one or two sizes down on the air corrector jet. ie 2.20 > 195.

    However you really need in on a dyno to see what it does throughout the rev range. I put mine on a dyno some years ago and standard jets are not that far off but just needed a little tweaking.
    Andrew McCrae likes this.
  11. Andrew McCrae

    Andrew McCrae Karting

    Jun 5, 2016
    Thanks Tony, That's the procedure I followed quite closely, backing off regularly as revs rose.

    I have, zero idea why people think I ran with only 2 carbs. All 3 were present, correct and fueling.

    But car isn't licensed to leave my driveway so I thought how can I check quickly that all 3 carbs perform similarly? . Tweak all 3 individually even if that is not normally how you drive and instantly I saw #1 performed differently. No other way to instantly, easily check a difference if all were synchronised tightly on the linkage.

    I'm on the right path I believe and everyone is pointing me to the idle jet as a possible issue. So we'll see how it goes when jets arrive in a couple of weeks.

    I'm curious how progression works related to rpm. I mistakenly thought throttle opening was the only influence. I must read that up too. There's so much going on in these things! I'll get a great satisfaction when all back together well.
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  13. Ken Ivey

    Ken Ivey Karting
    Silver Subscribed

    Jan 6, 2013
    Portland, OR, USA
    Full Name:
    Ken Ivey
    I may be missing what you said, but it sounds as if you expect the carburetors to be progressive and open one at a time, like the old Pontiac 3 deuces. In this instance, each carburetor "barrel" feeds one specific cylinder. Opening one carburetor feeds only 2 cylinders and leaves the rest starving for fuel. That's why they need to operate together.

    Sorry if I misinterpreted your last comment.

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  14. Andrew McCrae

    Andrew McCrae Karting

    Jun 5, 2016
    Gosh Ken, no apologies needed. The problem with the Internet and email is its much harder to explain.
    I understood that all 3 should open together but just needed to figure out which one was causing her to bog down. I got my new idle jets 55 instead of 50 so they'll be installed tomorrow along with the 3 checked out carbs. I eventually took them off and also stripped all the silver paint off the exteriors where somebody thought it a good idea to make them look "nice".

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