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Thread: carb "dead spot"

  1. #31
    Forum Approved Trader christofloffer's Avatar
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    Default Re: carb "dead spot"

    ok, just did as you said and the base plate does move. takes a fair bit of sucking to move but an engine should be fine with that.
    yeah the dead spot kills the engine so the revs do drop quite dramatically. if i dont blip the throttle it will just stall. so i guess it is fairly logical. the annoying thing is that once the revs are above the dead spot it accelerates fast and easy regardless of how quick i push the pedal. as far as i can see everything is functioning and its more than a little exasperating.

    by the way i reset the plugs to 0.8mm first thing.
    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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  2. #32
    Forum Approved Trader christofloffer's Avatar
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    Default Re: carb "dead spot"

    still no progress. had the carb off today, stripped it and cleaned it. i made sure the accelerator pump was working and not jamming or sticking and also made sure that the ball valve below it was working. i cant see anything wrong with it. there is a little slack in the rods but there is little i can do about that and spraying easy start or brake cleaner at them seems to have no effect.

    do webber carbs have these issues?
    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. #33
    Apprentice series2steve's Avatar
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    Default Re: carb "dead spot"

    the 34ich i got over 18 months ago has been great ,better on petrol, easy to set up , i did have only one problem , cheap chinese floats ,filled with sponge........................ive got a thread on here somewhere

    anyway ,apart from that,id recommend them
    1967 2a pick up petrol

  4. #34
    Moderator DarrenH's Avatar
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    Default Re: carb "dead spot"

    weber carb uses a ported vacuum, so theres none at idle (when engine vac is greatest) then it takes a slug just as you crack the throttle.

    what distributor have you got, is it the 45 type with no advance/retard adjustment thumb wheel ? just a the vac bulb ?

    Quote Originally Posted by christofloffer View Post
    had the strobe out this morning. at idle its running a little advanced (with the mark to the left of the indicator right?)
    correct. on modern fuel the distributor should be advanced slightly, as a base setting (5-8 degrees, so TDC mark flashing to the battery side of the pointer)

    Quote Originally Posted by christofloffer View Post
    when the dead spot appears the mark falls right back to the mark
    i'd question this behavior, it means its deleting your advance which is what should be giving you your throttle reponse (in tandem with accelerator pump jet)

    Quote Originally Posted by christofloffer View Post
    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.
    with the vac pipe disconnected the advance is completely linear with engine revs, so no advance at idle, then climbing up to 25-30 odd at max revs. if its wandering, or randomly pulling back as you grow the revs, this might be your issue. warn distrbutor /centrifugal advance mechanism
    Drives: 1982 series 3 88" station wagon

  5. #35
    Forum Approved Trader christofloffer's Avatar
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    Default Re: carb "dead spot"

    yeah its the 45D type with no manual adjuster on it. it wont be worn out or anything as its a brand new powerspark unit. rather a module fitted to an old housing i plumped for a new unit. there shouldnt be any wear in any bit of it and there is certainly none to see.
    the timing does sit stable when running and responds to the revs (aside from the dead spot). it drop back when the dead spot occurs exactly in time with the drop, rather than before it. so it is pretty consistent. the idle advance sounds about right. i think extreedoc made a good point about the revs dropping so the timing surely follows.
    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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    Default Re: carb "dead spot"

    ^ Had a similar issue with my power spark, first thing first, lube the rolled over flange the d45 top plate pivots around on, its not unheard of for em to jam hell pop it off, its gotta rotate smooth and any play there is just as bad as worn slop in the centre shaft.

    My problem seamed to be the magnet bit though, the separate magnet ring worked fine, the one with the integral distributor arm seamed bad - uneven, may be worth adding some marks and trying the strobe on all 4 different plug leads, because in my case at least one of them fired early!

    Plug gaps, open em up, 40 thou works well with the power spark modules, no need for a tiny gap like with points, so long as the leads and cap + coil are good. I fitted a gold coil to mine and new leads its not much cost for the certainty of eliminating those problems.

  7. #37
    Forum Approved Trader christofloffer's Avatar
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    Default Re: carb "dead spot"

    ok, i am done with this carb. i rebuilt it again today, with a new kit, re-lapped everything and cleaned out every bit of it. it seemed that the dead spot was practically gone. there was a tiny lag when putting the throttle down but it was literally a fraction of a second and barely noticeable. so i took it out to see how it behaved under load and at first it was perfect. it was responsive and gutsy and exactly as it should be, then all of a sudden it started bouncing and cutting out (we call it kangaroo juice) and no change of gear, speed or throttle position made any difference. which meant it was a very slow and jumpy trip back and i couldnt get out of 2nd. then when i went to turn into our street, which is a hill so i always slow down and go into 1st to make sure i am moving, it simply stopped leaving me blocking the whole lane and street. it wasnt keen to start again and when it did i just tanked it in 1st to get back up the hill and park it up, which was about as difficult as you would expect. got back on the drive and it is revving fine again.
    i literally have nothing else to check now, i went through the timing and ignition again and its all exactly where it should be and stable as you like. i know beyond all doubt that the carb is clean and working. the air supply is fine, the throttle is moving freely and the mixture is fine. so it should have been spot on when it went out. i dont know what has caused that to suddenly kick off but its just a waste of space again. i am getting really tired of fighting with this car now.
    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

  8. #38

    Default Re: carb "dead spot"

    Reading what you've just written, it crossed my mind it may have nothing whatever to do with the carb.
    How old is the coil?
    I ask because it is possible for an ignition coil to have an intermittent internal insulation fault of some sort.
    The fault will stop the plugs firing in certain circumstances, accelerating is one of them.
    (All to do with the different mixture in the cylinder and the ability of the plug to fire)
    Have you tried swapping the coil for one known to work?

  9. #39
    Engineer holly76's Avatar
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    Default Re: carb "dead spot"

    `or try the old trick of looking under the bonnet in the dark (pitch black) heat could be causing the insulation to break down somewhere. i'd be tempted to replace the whole HT system, coil, leads, cap, rotor, spark plugs. and if you still have trouble maybe a new ignition module.

  10. #40

    Default Re: carb "dead spot"

    As it ran fine to begin with, it looks like an intermittent fault and is more likely to be an ignition problem. What is good about that is that you can substitute components until you find the problem bit. If it was an ordinary coil and points set-up, I would start with a new condenser but I'm not sure what you have, some sort of electronic ignition? Try what Holly and Series have suggested above.

  11. #41
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    Default Re: carb "dead spot"

    the whole lot is brand spanking new. i doubt any of it has broken 5 hours of use yet, probably more like 3. i would love to be able to replace the works and hope that fixes it but that would cost a wedge and should be entirely unnecessary. the dizzy (all electronic and new) is powerspark with the red rotor arm, the coil is bearmach, the leads and solenoid are lucas. i cant simply keep replacing brand things for more brand new things as it would end up wasting too much money. i know parts can be hit and miss but if all that is dead then i will give up land rovers as a bad job, which is pretty damn close as it is. i dont have any spares or known ones as it all came at the same time. the only spares i have are old leads, plugs and an old dizzy which has play in the shaft.

    it felt to me like a fuel issue. its was the same kind of jumping as if the thing was running out of fuel. it doesnt explain why its absolutely fine one minute and totally useless the next.

    i will go out and look under the bonnet as its dark and an easy thing to investigate. i have ordered a weber 34 ich and a rebuild kit though as my hunch is that there is something going on in the carb. maybe the lapping didnt go far enough, or maybe there is a crack in it or something else entirely. that will be getting rebuilt, and the fuel filters replaced or cleaned (also done at the same time as the rest) and try again.
    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

  12. #42
    Engineer holly76's Avatar
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    Default Re: carb "dead spot"

    I know where you are coming from chris. a miss fire can be a very frustrating problem to fix. getting a new set of ignition components is not a waste of money though, as they are consumables and will be needed at some time. you could fit the parts one at a time to see if things get better. another way would be to swap with someone to see if the fault moves.

    you seem to have done a thorough job with the carb, and to be honest carb faults generally may it hard to idle or poor fuel economy. it could be a fuel pump issue. maybe try feeding the carb with a tank n the roof.

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

    had a look and couldnt see any hint of sparks. i gave everything a shake whilst it was running to see if there was a break which could be found but it all seemed fine. though the dead spot is back with a vengeance and i had to tease the throttle to get the revs up. its almost like its always on the verge of flooding and when the pump kicks in it just tips it over the edge.

    i did think perhaps it was a fuel supply issue, and i worked around it by using the priming bulb that i put in whilst it was running to make sure it was feeding and nothing changed.

    the reason i am condemning the carb is that it is reliable as hell to kick off the dead spot when the throttle moves. when the revs are up and when the revs come up slowly its sounds pretty sweet. surely if the ignition was a problem like that then it would manifest elsewhere too, or be really erratic? especially with the advance pipe removed as it can only respond to engine speed yet the dead spot still occurs unless the revs are built up slowly. if i keep the throttle down when the dead spot kicks in then the engine just stops. the less the accelerator pump would be in effect the less the dead spot manifests, it just seems to tie in. yet when i was working on it earlier i was paying very close attention to the pump and i could see nothing awry.
    i'm not being deliberately dismissive, and i apologise if i seem a bit short, its just been a long day and nothing seems to add up.
    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

  14. #44
    Engineer holly76's Avatar
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    Default Re: carb "dead spot"

    the spark in the plugs is a break down of the insulating properties of the gas surrounding it (air/fuel). at low speed the engine has time to draw in more air which results in higher compression. this means the gas the spark has to break down is denser. as revs increase the gas charge is reduced resulting in lower pressures. which means lower density. which a weak spark can ignite.

    put a spare plug In one lead and start the engine. look at the spark. anything other than white and fat will no do. i'd be temped to first change he spark plugs, for a different make.

  15. #45

    Default Re: carb "dead spot"

    Quote Originally Posted by holly76 View Post
    the spark in the plugs is a break down of the insulating properties of the gas surrounding it (air/fuel). at low speed the engine has time to draw in more air which results in higher compression. this means the gas the spark has to break down is denser. as revs increase the gas charge is reduced resulting in lower pressures. which means lower density. which a weak spark can ignite.

    put a spare plug In one lead and start the engine. look at the spark. anything other than white and fat will no do. i'd be temped to first change he spark plugs, for a different make.
    Yes, I agree.
    Also, the setting of the sparkplug gap makes a lot of difference to how the engine performs under high compression.
    Normally with electronic ignition the spark gap should be greater/wider than with standard conventional ignition.
    However I have seen plugs gaps made so wide the owners are really pushing their luck!

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