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Thread: carb "dead spot"

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    Forum Approved Trader christofloffer's Avatar
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    Default carb "dead spot"

    so i have been out tinkering with the carb. i moved it yesterday, under its own steam for the first time in months, and the throttle has a dead spot. if you build up the revs steadily then its fine but if you just put the throttle down it wheezes and dies. i am not sure if its not getting enough air or fuel though. i did the usual trick of spraying easy start around one area at a time and listening for a change in engine tone which would point to an air leak, but i got nothing. then i tried using the priming bulb at the same time as revving to help along the fuel but still came up with nothing.
    if i build the revs up then its ok, and if the revs are already up enough then it picks back up with no issue. its just coming up from idle. building up the revs can be done relatively quickly too, but you cant just put the boot down and let it catch up.
    the timing is fine and the ignition is new right through.
    any suggestions? i am not great with setting this kind of thing up yet as i have mostly had diesels and its been a long time since running with a petrol.
    Chris
    1977 series 3 ex mod (FFR) 109 2.25p 12v HRTC hard top "phil"
    My work: The emporium of the useful and curious https://teotuac.wixsite.com/teotuac
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    Default Re: carb "dead spot"

    Sounds like accelerator pump not working. Could be blocked?

  3. #3

    Default Re: carb "dead spot"

    It is a long time since I tinkered with carbs too but I think the problem is lack of fuel at that particular moment.
    Does pulling the choke out make an improvement?
    Most carbs have either an accelerator jet that squirts petrol directly in the manifold inlet and/or an intermediate jet to overcome this problem.
    I'd check if the metering jets are working/clear and the float level is ok.

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    Default Re: carb "dead spot"

    Agreed. what's happeing is:

    Usually slight rich mix at idle so OK
    Press accelerator and the throttle opens so more air gets in but manifold vacuum drops at first as there's less restriction so its doesn't suck more fuel so it goes weak.
    As the engine speeds up the vaccuum increases and it pulls the extra fuel so you can speed up slowly then settles at higher speed.
    Accelerator pump gets round this by squirting extra fuel or forcing rich mixture (SU) so revs and therefore suction increase when throttle opens.
    Usually some kind of orfice / diaphram that pumps more the faster the pedal is pushed.

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    Mechanic js5d's Avatar
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    Default Re: carb "dead spot"

    Yep, was once explained to me as the air accelerates faster through the carb then the fuel can flow so you lean the mixture temporarily. In any case the accelerator pump on a Zenith is a little plunger pump which is activated buy the second lever coming out of the carb (throttle, accelerator pump, choke) which is connected to the throttle with a linkage. There is a non return valve in the bottom which can get gummed up meaning you push the fuel out at the bottom rather than into the throttle body.

    You can see the accelerator pump do it's job by looking down the throttle and you can see a quirt of fuel coming from a small brass pipe as you open the throttle, probably best visible with the engine off.
    Houston Texas (formally Surrey UK)

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    Default Re: carb "dead spot"

    the choke does stop it from happening. bit of a balancing act once it is warm to stop it from stopping the engine but when its out and when the engine was still cold when i tried it it certainly made a difference.
    this is a little annoying and i am suspecting that some grot has gotten in there as it is a rebuilt carb. i had a similar issue when it was petrol before and we suspected the accelerator pump, but it wasnt as bad as this. so when i was rebuilding it i was careful to make sure that the pump was moving freely and was clean. the jets are all either new or thoroughly cleaned. when it first went on there was no hint of a dead spot at all, which is frustrating. i will pop the intake elbow from the top in the morning and see if i can see it working.
    how any grot got through in enough quantity to cause this is odd though as i have not run it without the filter in place.
    Chris
    1977 series 3 ex mod (FFR) 109 2.25p 12v HRTC hard top "phil"
    My work: The emporium of the useful and curious https://teotuac.wixsite.com/teotuac
    Proud member of the YCHJCYA2PDTHFH Club

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    Default Re: carb "dead spot"

    cant add much, other than to say before i read the other replies i was going to post "accelerator pump" (or its jet)

    wall wetting! fills in gaps of fuelling at transient throttle !
    Drives: 1982 series 3 88" station wagon

  8. #8

    Default Re: carb "dead spot"

    Try this, it sometimes works: take the elbow off the carb and place your hand next to the intake like the cover of a book or a hinged lid. Rev the engine right up and quickly close the lid, (your hand) over the carb intake... like closing a book. Don't just plonk your hand down over the carb but do the hinged lid thing to avoid hurting yourself. If you do it like I describe it, it won't cause any problems. Take your hand away again before the engine dies and rev it again. You can do this a few times if necessary. It sometimes sucks a blockage clear but not always.

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    Engineer holly76's Avatar
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    Default Re: carb "dead spot"

    look in the top of the carb with the engine off. open the throtle you should see the spray of fuel coming out of a jet and hitting the side of the venturi. you will have about 3 good pumps before needing to refill the carb with fuel

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    Default Re: carb "dead spot"

    Agree with all the above, but it could also be caused by the timing and/or mixture controls needing a bit of fine tuning. David
    1982 ex-mil 109 2 1/4 petrol on lpg series III, 'Phoenix'

    1983 2 1/4 diesel 88" series III, 'Willy', currently in bits.

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    Default Re: carb "dead spot"

    i tried the covering the intake thing and it did improve matters. i did it with a book rather than my hand as the suction concerned me, it worked great though. it made it a bit smoky for a minute, which i was expecting, and it has reduced the dead spots effect. it is still there but if you dump the throttle it will catch up a second later and it builds up the revs faster too. as it stands i could probably drive around it, so i think i am on the right track here. the book was wet after doing that though, i am guessing that the sudden suction causes some suck-back right?

    as for the pump itself, it is very easy to see the jet of fuel. it is consistent so i dont think the pump itself is at fault, probably more like the jets. would something like injector cleaner be of any use for shifting any gunk without having to strip it down again?

    the mix is something i am not great with. i understand the principle but when it comes to actually adjusting i am just turning it here and there whilst listening for a balanced engine tone. the timing should be sound, it was done with the strobe before the engine went in and it was right on the dot.
    Chris
    1977 series 3 ex mod (FFR) 109 2.25p 12v HRTC hard top "phil"
    My work: The emporium of the useful and curious https://teotuac.wixsite.com/teotuac
    Proud member of the YCHJCYA2PDTHFH Club

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    Default Re: carb "dead spot"

    you cant adjust anything (regards fuelling) on the carb that would be beneficial to your problem.
    Drives: 1982 series 3 88" station wagon

  13. #13

    Default Re: carb "dead spot"

    Quote Originally Posted by christofloffer View Post
    i tried the covering the intake thing and it did improve matters. i did it with a book rather than my hand as the suction concerned me, it worked great though. it made it a bit smoky for a minute, which i was expecting, and it has reduced the dead spots effect. it is still there but if you dump the throttle it will catch up a second later and it builds up the revs faster too. as it stands i could probably drive around it, so i think i am on the right track here. the book was wet after doing that though, i am guessing that the sudden suction causes some suck-back right?

    as for the pump itself, it is very easy to see the jet of fuel. it is consistent so i dont think the pump itself is at fault, probably more like the jets. would something like injector cleaner be of any use for shifting any gunk without having to strip it down again?

    the mix is something i am not great with. i understand the principle but when it comes to actually adjusting i am just turning it here and there whilst listening for a balanced engine tone. the timing should be sound, it was done with the strobe before the engine went in and it was right on the dot.
    The suction is not going to hurt you if you do it the way I described; it's an old mechanics trick that was commonly used back in the day when all cars had carbs. Yes, there will be some petrol on your hand afterwards.

    Regarding tuning, there is a simple way: once the engine is at working temperature, screw the mixture screw in or out to get the highest revs, when you hear the revs drop, back the mixture screw the other way a little bit. It's easiest with an accurate tachometer to watch, we used to have an old oscilloscope Crypton tuner with a tacho on it. All this assumes the carb is correctly jetted and clean and working properly. Also: I used to find a "Colortune" plug was very useful for setting up carbs, especially twin carbs. Don't know whether they are still available though...

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    Engineer solemnwarning's Avatar
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    Default Re: carb "dead spot"

    In addition to the above, you'll need to reset the idle speed and then tweak the mixture again, if the speed changes significantly while fiddling with the mixture. Repeat as necessary.
    1970 Series IIA 2.25 petrol - WIP

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    Default Re: carb "dead spot"

    so should i just keep on with the suction thing until the problem goes away? i did it about 10 times, then left it to idle for a bit and settle again. i will do it again tomorrow and see if it keeps improving. its certainly a trick i will remember.
    Chris
    1977 series 3 ex mod (FFR) 109 2.25p 12v HRTC hard top "phil"
    My work: The emporium of the useful and curious https://teotuac.wixsite.com/teotuac
    Proud member of the YCHJCYA2PDTHFH Club

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