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Thread: carb "dead spot"

  1. #16

    Default Re: carb "dead spot"

    Quote Originally Posted by christofloffer View Post
    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.
    No, if it doesn't work, forget it; it's just a quick fix that sometimes works. You might still end up stripping the carb.

  2. #17
    Forum Approved Trader christofloffer's Avatar
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    Default Re: carb "dead spot"

    right then, back into this one. i got the carb off today to take a look at this dead spot and eliminate it. i left the bowl full so that i could see the jet from the accelerator pump and i noticed something odd. if i push the arm itself the jet is much stronger than if i pushed it via the linkage. i had it connected on the lower hole of the arm of the accelerator pump, simply because thats where it was before. i am wondering if perhaps it should be on the higher hole. would that make it turn faster and therefore be more effective? i hadnt thought about doing that whilst it was on the motor but its a thought.
    i am not sure that the problem is the accelerator pump itself as the jet seems clean and strong and works every time. it seems consistent and doesnt stutter either. i would have thought that if there was muck in there then it would be stuttery. could there be an air leak somewhere causing the same effect? i lapped everything on a glass plate before refitting and it looked pretty flat.
    with the engine hot and stationary there is no dead spot, it revs smoothly and quickly. its now only under load that its bad and its quite a difficult thing to handle on the road as low speed gear changes trigger it badly and to the point where it almost stalls and then kicks my butt when it picks up again.
    any thoughts?
    Chris
    1977 series 3 ex mod (FFR) 109 2.25p 12v HRTC hard top "phil"
    My work: The emporium of the useful and curious http://emporiumoftheusefu.wixsite.co...umofthecurious
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  3. #18
    Engineer phoenixdave's Avatar
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    Default Re: carb "dead spot"

    Does it still have a flat spot if you operate the accelerator really slowly? The accelerator pump is only required when the pedal is pressed quickly so if the flat spot is there regardless then you have a different issue.

    The linkage can be fitted to the upper hole, but not sure it will cure the problem.

    You could try turning the mixture screw out a little to ensure there is enough fuel going in just above tickover before the main jet gets going properly. 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.

  4. #19
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    Default Re: carb "dead spot"

    Give it a try, if its bogging down on the road you could give the accelerator a quick prod.

  5. #20
    Forum Approved Trader christofloffer's Avatar
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    Default Re: carb "dead spot"

    no flat spot when operating things slowly, and not when the engine is hot but not under load.
    my main thought with the top hole is to alter the ratio. if it turns faster then it put the fuel in quicker and possibly that is where the problem is. i did try prodding the pedal but it was a bit hit and miss as to wether it helped.
    i think i will not open it up just yet and try the mixture screw and other hole first. if they dont work at least its a couple of things ruled out for further work.
    Chris
    1977 series 3 ex mod (FFR) 109 2.25p 12v HRTC hard top "phil"
    My work: The emporium of the useful and curious http://emporiumoftheusefu.wixsite.co...umofthecurious
    Proud member of the YCHJCYA2PDTHFH Club

  6. #21
    Engineer solemnwarning's Avatar
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    Default Re: carb "dead spot"

    It should be in the top hole where it will inject more fuel when the throttle opens.
    1970 Series IIA 2.25 petrol - WIP

  7. #22
    Engineer holly76's Avatar
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    Default Re: carb "dead spot"

    the two holes are to compensate for altitude, i cant rememeber whish is which though. i have mine set for more fuel

    when you describe the symptoms, it does sound more like an ignition issue. an engine loaded at low speed has high compression pressures which a weak spark will struggle to jump accross.

    do you have a spare carb you can swap to eliminate it?

  8. #23

    Default Re: carb "dead spot"

    Bearing in mind Holly's wise words above, it might be worth trying another condenser on it. Are the points burnt and nasty looking or blued with excess heat?

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    Default Re: carb "dead spot"

    Good point, check the plugs and set the gaps to the minimum end of the range as well.

  10. #25
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    Default Re: carb "dead spot"

    put the carb back on with it put on the top hole. it seems a little better but its definitely still there.
    i did try taking the vacuum advance pipe off to see if that made any difference, but nothing changed.

    i will check out the dizzy and see if anything is going on in there. i will also check the plugs and see if there is any clues there.
    Chris
    1977 series 3 ex mod (FFR) 109 2.25p 12v HRTC hard top "phil"
    My work: The emporium of the useful and curious http://emporiumoftheusefu.wixsite.co...umofthecurious
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  11. #26
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    Default Re: carb "dead spot"

    took a look in the dizzy and didnt see anything amiss. no signs of burning, scorching or overheating. spark plugs were sound, all a uniform tan colour. i set the gaps to 0.3mm as i had been told that was right, though there seems to be different opinions for 8:1 and electronic set-up's. is there a perfect size to get to?
    i tried messing with the mixture screw but it didnt seem to do much. it went lumpy with it screwed in but it didnt make too much difference screwing out past a certain point. its hasnt had a big impact on the dead spot either. it feels like its getting a little better but i havent had it out on the road again yet.

    what should i do about investigating the timing? it was set up spot on with the timing light before it went in but i can check it again and make sure it hasnt wandered.
    Chris
    1977 series 3 ex mod (FFR) 109 2.25p 12v HRTC hard top "phil"
    My work: The emporium of the useful and curious http://emporiumoftheusefu.wixsite.co...umofthecurious
    Proud member of the YCHJCYA2PDTHFH Club

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    Default Re: carb "dead spot"

    0.3mm sounds very narrow to me; standard gap is around 0.8mm off the top of my head.
    1970 Series IIA 2.25 petrol - WIP

  13. #28
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    Default Re: carb "dead spot"

    Quote Originally Posted by christofloffer View Post
    i did try taking the vacuum advance pipe off to see if that made any difference, but nothing changed.
    so youve proved that disconnecting the vac advance doesnt make a difference to your problem......

    ....but has this inadvertently proved the vac advance isnt working when it IS connected ?

    from my observations (i actually have 2 videos on youtube with a strobe on the crank pulley) a quick prod of the throttle should give you a spike in advance without a significant rise in engine revs. the other video with vac disconnected (to demonstrate pure rpm advance) also inadvertently shows the engine bogging down as a result of not having the vac-advance connected
    Drives: 1982 series 3 88" station wagon

  14. #29
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    Default Re: carb "dead spot"

    had the strobe out this morning. at idle its running a little advanced (with the mark to the left of the indicator right?) with everything connected. when the revs are up it creeps forward as it did before. with the advance disconnected it sits further back but still a little ahead of the mark (due to centrifugal advance?). when the dead spot appears the mark falls right back to the mark and then once its picks up it shoots back up to where i would expect it to be. it is very much hand in had with the drop off. with the pipe disconnected it still drops back but the gap is not so great as its further back anyway. so would this indicate that the dead spot is due to a lack of vacuum pressure in the carb/manifold? i sprayed all over the place with both easy start and brake clean and i couldnt find any obvious air leaks.
    so many bits to think about with a petrol motor.
    Chris
    1977 series 3 ex mod (FFR) 109 2.25p 12v HRTC hard top "phil"
    My work: The emporium of the useful and curious http://emporiumoftheusefu.wixsite.co...umofthecurious
    Proud member of the YCHJCYA2PDTHFH Club

  15. #30

    Default Re: carb "dead spot"

    Check first if the vacuum unit is operational: distributor cap off, vac pipe disconnected at the manifold end; suck the vac pipe and observe if the distributor base plate moves. You should see it move as you suck, if it doesn't, there is your problem.
    Thinking about what you say above: when the engine revs reach the dead-spot, surely the engine revs drop? So you should expect to see the timing mark move back too.

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