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Thread: Engine Missfire

  1. #1
    Senior Member Phill's Avatar
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    Engine Missfire

    My car has a 1300cc Twinport beetle engine and I have tried everything to cure a missfire. The timing has been checked and there are no air leaks any where. The points gap is spot on, new plugs, leads, coil and rotor arm have been fitted. The valve gaps have been checked and are also spot on. The car has a new fuel tank and an in line fuel filter.

    Up until now I have been blaming the carb. The original carb I rebuilt but could only get the engine running smoothly by leaving the choke disconected as with the choke flap fully open the engine had a very pronounced missfire, but with the choke flap partially closed the engine ran relatively well but I could never get a good slow tickover without it cutting out all the time so had to set a fast tickover speed.

    I tried a cheap ebay carb but this was even worse.

    Finally I bought a 30pict1 replacement carb from Machine 7 and whilst this has led to an improvement. (I can now set a slow tickover without the engine cutting out) there is still a tendency for it to missfire around about 2000rpm when trying to accelarate. Above and below 2000rpm the engine runs well.

    I have now run out of ideas of what to do to eliminate this missfire and I am even wondering if the trombone exhaust I have fitted is to blame by having insufficient back pressure?

    Any suggestions?
    "The most beautiful kit car in the world - Motor"

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  2. #2
    Um - have you changed the condenser? It's not on your list and they often get over-looked - modern ones don't seem nearly as good as the originals....
    It's a 50 year old car and everything works, just not always at the same time.......and it's probably about to get jealous!
    *Donate to Euro-Nova today!*

  3. #3
    ***Euro-Nova Supporter*** BlueNova's Avatar
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    Phill,

    I agree with Alex, but if the problem persists I've found an interesting thread on Samba on a similar misfire problem which turned out to be a faulty NEW lead.

    You can find the thread here if you want to read it .... https://www.thesamba.com/vw/forum/vi...er=asc&start=0

    Best of luck!

    Alistair

  4. #4
    Senior Member Phill's Avatar
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    Thanks for the advice. I think I have found the problem. Tonight I discovered my Vacuum advance system was seized. I managed to free it off but sucking on the vacuum tube still doesn't advance the mechanism so I think the diaphragm is knackered.

    This would explain two issues. Stumbling on accelaration, and also if the Diapragm is knackered it will be introducing an air leak into the Carb. First thing I will try tomorrow is to disconnect the vacuum tube from the carb and plug the port to stop the air leak.

    However, It looks like a new distributor is on the cards...But how do I explain to Alison that I'm buying a new distributor when she thinks I've already bought one?
    Last edited by Phill; 01-06-2019 at 09:01 PM.
    "The most beautiful kit car in the world - Motor"

    http://stores.lulu.com/rightsigns


  5. #5
    ***Euro-Nova Supporter*** BlueNova's Avatar
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    It's easy Phill ... just don't tell her ... and get it delivered to your work unit. No, that won't work, she's your partner at work too! ... OK on 2nd thoughts, just get it delivered to me and I'll bring it over at the weekend. Really pleased you've got to the bottom of it! Well done!
    Alistair

  6. #6
    Senior Member Brett's Avatar
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    Quote Originally Posted by Brett View Post
    Sounds like an air leak.

    I assume the car ran fine up till now.

    I would check all vacuum lines first. Make sure nothing is cracked and leaking or heat got to a line and melted it.

    Next I would check your vacuum advance. Make sure the diaphragm is in good condition and not leaking. Test it to see if it holds a vacuum.


    If you haven't found any leaks you can take WD40 or a carb cleaner and spray it around the intake manifold mounting points and the carburetor mount. If the RPMs pick up at the spot where your spraying there is a good chance that the leak is there. .

    One spot everyone usually misses when rebuilding a carb is the shaft that the "inlet choke flaps" are on. If that shaft moves around the bushings are worn out and needs to be replaced


    Edit:

    One other thing not to rule out is maybe dirt got into the carb and plugged the jets


    Edit:

    Another thing that could cause that problem could be that the fuel tank is not vented properly. remove the gas cap and see if that fixes the problem

  7. #7
    Senior Member Phill's Avatar
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    Yes you were right Brett.

    All along I had been pretty convinced it was an air leak and I had checked everything but overlooked the vacuum advance diaphragm. Then I convinced myself the Carb was to blame so bought a new one only to discover there were still issues. So now it was back to blaming the electrical system and it wasn't until last night when I discovered the vacuum advance mechanism was seized that I then discovered the diaphram was also faulty and ultimately the cause of the suspected air leak all along.

    I will listen to your advice much more carefully in future
    "The most beautiful kit car in the world - Motor"

    http://stores.lulu.com/rightsigns


  8. #8
    Senior Member Phill's Avatar
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    So now that I have established that my distributor is faulty I have been looking at buying a replacement.

    I have been looking at this one:- https://www.machine7.com/product.php...1946&xSec=3525
    Which I believe is the same as the one I have

    Or I am considering this one:- https://www.machine7.com/product.php...1944&xSec=3525
    which does not have vacuum advance but relies on centrigual advance only. With this one I would simply blank off the vacuum pipe on the carb?

    The question is which one to go for - the second is a lot cheaper, but is there any real advantage of getting one with vacuum advance?
    "The most beautiful kit car in the world - Motor"

    http://stores.lulu.com/rightsigns


  9. #9
    ***Euro-Nova Supporter*** Spacenut's Avatar
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    Vacuum advance is better for road use. If you are revving at the limit all the time (like a circuit race) then mechanical comes into its own. Because the engine is not under load when cruising on the flat, the vacuum puts more advance on the ignition than mechanical alone, which uses less fuel. When you open the throttles wide the vacuum drops and the mechanical advance takes over.

    Vacuum all the way I say!

    Lauren
    only Pythagoras can save me now!

  10. #10
    Senior Member Phill's Avatar
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    Thanks for the advice Lauren - I will go with your recomendation
    "The most beautiful kit car in the world - Motor"

    http://stores.lulu.com/rightsigns


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