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Thread: Misfire mystery

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    Senior Member warwick-steve's Avatar
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    Misfire mystery

    Hi all
    Wonder if you could please advise.
    On 3 of 4 recent occasions when I have taken my BT7 for a weekly run of about 12-20 miles it started to misfire after about 8-10 miles. Today was the worst. Having run really well it started to misfire badly for about 1-2 miles and then recovered and ran well again.
    Turning on the ignition starts the fuel pump ticking as it primes the fuel lines. Starting is no problem and it warms up nicely and then runs really well, smooth and responsive.
    My strategy is
    - Clean and inspect the plugs ( new NGK BP6ES plugs fitted about 1500 miles ago)
    -Inspect the distributor cap ( new "quality cap" leads and plug caps fitted about 200 miles ago)
    - Fit new rotor, contact breaker and condenser. (New sports coil fitted about 600 miles ago)
    - disconnect the white and black wire from the cut off switch in the boot/trunk

    I would be surprised if it is a fuel problems a new aluminium tank and electronic SU fuel pump were fitted about 15000 miles ago. Also an in line fuel filter was fitted before the carbs. I also didn't think that todays misfiring would be down to just one of the 3 carbs float bowl needle valve sticking. I could however easily fit a new fuel filter.

    Have I overlooked anything??
    Thanks
    Steve

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    Luke Skywalker Roger's Avatar
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    Re: Misfire mystery

    Is the air getting into your nice new fuel tank as you use up the petrol, and is it getting in fast enough?
    Roger
    Ancient Briton
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    Yoda
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    Re: Misfire mystery

    What kind of distributor cap and secondary spark wires are you running? I've had issues with an off-brand--Bosch, believe it or not--cap and if you're using suppression wires with a stock type cap you'll get similar symptoms. Also, I've heard of poor quality condensers causing misfiring and aftermarket points set quality is suspect as well.

    I don't think it's fuel delivery--pump failing, gas cap not venting, etc.--as these usually cause overall power loss and stalling, but not usually what I'd call 'missing.'

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    Senior Member warwick-steve's Avatar
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    Re: Misfire mystery

    Thanks Roger and Bob
    The tank was fitted in 2010 about 15000 miles ago. Everything has been running fine until about a month ago so I don't think the tank is a problem. The filter is an adequate size and can cope with the flow and it looks very clean inside.
    The distributor cap is a standard aftermarket cap with brass terminals and from a major UK Austin Healey Spares shop. Similarly the wires are standard ones from the same source. The plug caps are NGK ones. As the cap was a fairly recent fitting, I might revert back to the previous one and see if there is any change.
    The new rotor is "New old stock" so should be OK. Also the new contact breaker and condenser come from a quality distributor specialist so should be OK also.
    Guess I'll work through my list over the next few days and see how it goes.
    Thanks for your comments. It all helps
    Steve

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    Yoda Randy Forbes's Avatar
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    Re: Misfire mystery

    If it only occurs after it's gotten hot, my first line of suspected items would be the condenser or the coil (and that's because I've had those components individually give me trouble in the past, mostly with MGBs, but close enough).

    Just by chance, you didn't put any sort of "coating" inside the fuel tank did you; did the fabricator, as insurance against porous welds...? If so, THAT would be the first thing I'd check, even before the ignition!
    http://www.britishcarforum.com/bcf/image.php
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    Darth Vader Michael Oritt's Avatar
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    Re: Misfire mystery

    There is usually no easy way to diagnose intermittent problems and it usually comes down to having known good components on hand to install, one by one. My first guess is that this is ignition related and failing coils often go south as they warm up, but as Randy intimates it could be fuel-related as well.

    Be patient and prepared and you will eventually solve the problem.

    Best--Michael Oritt
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    Darth Vader steveg's Avatar
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    Re: Misfire mystery

    I prefer the coil/condenser evil twins as prime suspects.

    Also the distributor caps - some are bored off-center and the little pointed screws don't pierce the wire enough to make contact. If you've got a timing light, put it on each plug wire in turn and observe whether one of the wires appears to misfire. Not sure this would be heat- or driving-time-related, though.

    Check the little wires inside the distributor. Sometimes they break inside the insulation, causing intermittent running.
    Steve Gerow
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    Maker of most complete Big Healey rear disc kit
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    Re: Misfire mystery

    Also check the plug leads and their connections.
    AJ

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    Senior Member warwick-steve's Avatar
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    Re: Misfire mystery

    Thanks Guys
    Enough to check out for now. I am thinking it is most likely to be the condenser but we will see
    Cheers
    Steve

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    Jedi Knight nevets's Avatar
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    Re: Misfire mystery

    My guess based on my similar experience is the coil, which when hot cause sputtering /misfire

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    Yoda
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    Re: Misfire mystery

    OT: It seems like a lot of owners have had problems with coils; I've never known one to fail (my BJ8's coil has almost 200K miles on it). Am I just lucky? Coils have no moving parts, and the ones in Healeys are not oil-filled, like some of the really old ones. I've heard from a reputable source that the so-called 'sports coils' have issues, but it's more of a quality problem than wear-and-tear.

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    Jedi Knight nevets's Avatar
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    Re: Misfire mystery

    Replacing a coil is quick and easy...if it works Bob's your uncle, if not, not a lot of time /effort invested.

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    Darth Vader steveg's Avatar
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    Re: Misfire mystery

    Quote Originally Posted by Bob_Spidell View Post
    OT: It seems like a lot of owners have had problems with coils; I've never known one to fail (my BJ8's coil has almost 200K miles on it). Am I just lucky? Coils have no moving parts, and the ones in Healeys are not oil-filled, like some of the really old ones. I've heard from a reputable source that the so-called 'sports coils' have issues, but it's more of a quality problem than wear-and-tear.
    Your coil is probably OEM. The ones failing are newer. My sport coil that failed was oil-filled, as are my two Pertronix coils. Am currently running a resin-filled Bosch blue coil from Jeff Schlemmer.
    Steve Gerow
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    Maker of most complete Big Healey rear disc kit
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    Yoda
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    Re: Misfire mystery

    Quote Originally Posted by nevets View Post
    Replacing a coil is quick and easy...if it works Bob's your uncle, if not, not a lot of time /effort invested.
    That's kind of what I expected (although you're out the cost of a coil). Although, being mounted on the generator subjects a coil to more vibration and movement than mounting it on the body somewhere. Then again, mounting on the generator or elsewhere on the engine means the entire ignition moves as a 'unit,' which reduces stress on the wiring (one of the more likely culprits, in my experience).

    Steve, are you saying you've had Pertronix coils fail?

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    Darth Vader steveg's Avatar
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    Re: Misfire mystery

    Quote Originally Posted by Bob_Spidell View Post
    That's kind of what I expected (although you're out the cost of a coil). Although, being mounted on the generator subjects a coil to more vibration and movement than mounting it on the body somewhere. Then again, mounting on the generator or elsewhere on the engine means the entire ignition moves as a 'unit,' which reduces stress on the wiring (one of the more likely culprits, in my experience).

    Steve, are you saying you've had Pertronix coils fail?
    Bob - apologies for being unclear. Neither pertronix coil has failed. I bought the Bosch blue coil in a spirit of belt-and-suspenders and because I think it's better made than the oil-filled pertronixs.

    I see the Bosch coils are still available through Advanced:
    http://advanceddistributors.mybisi.c...osch-blue-coil
    Last edited by steveg; 11-02-2017 at 10:36 PM.
    Steve Gerow
    Altadena, CA, USA
    Maker of most complete Big Healey rear disc kit
    Check out my galleries:
    http://www.pbase.com/stevegerow

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    Obi Wan Patrick67BJ8's Avatar
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    Re: Misfire mystery

    Quote Originally Posted by steveg View Post
    Bob - apologies for being unclear. Neither pertronix coil has failed. I bought the Bosch blue coil in a spirit of belt-and-suspenders and because I think it's better made than the oil-filled pertronixs.

    I see the Bosch coils are still available through Advanced:
    http://advanceddistributors.mybisi.c...osch-blue-coil
    When I had my BJ8 dizzy rebuily by Advanced I also got the Bosch Blue Coil plus top loader dizzy cap and new plug wires. Jeff said there was cross firing sometimes with the side loader caps when using a pertronix.

    If you have Advanced rebuild your dizzy send the cap/wires and Jeff will check them out to. Don’t forget to get his red rotor, actually get two. One for you and one for a friend still using the original or inferior replacement type for the roadside Tech Session.
    Patrick
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    Yoda
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    Re: Misfire mystery

    Quote Originally Posted by Patrick67BJ8 View Post
    ... Jeff said there was cross firing sometimes with the side loader caps when using a pertronix ...
    To be clear, (I hope) he didn't blame the Pertronix for cross-firing. An Ignitor--or any other electronic points module, or points for that matter--only dictates the timing and, conceivably, the duration of a pulse of current to a coil. How the resultant spark gets routed is out of the Ignitor's hands. Presumably, Jeff meant that the increased spark potency that electronic ignitions achieve can overwhelm old school stranded wire secondary wires, and it's possible the arrangement of the terminals in a side loader cap can contribute to cross-firing. That's where suppression secondary wires come in: they usually are wound with a a spiral metal coil, which absorbs and grounds the induced electromagnetic field around the wires, and even those can cross-fire if you arrange the wires too close together. And top loader caps generally send the wires on their separate ways, whereas with a side loader the wires are close together and parallel, good conditions for cross-firing.

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    Senior Member warwick-steve's Avatar
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    Re: Misfire mystery

    Hi again
    This thread has moved on and I am learning much from it.
    Now, I am reporting back on the misfire problem. I have just returned from a run having replaced many ignition components and the car ran without missing a beat. I suspect that the condenser or the coil was the problem.
    I replaced
    - Plugs with a new set ( those removed were all clean and dry)
    - New Contact Breaker (good quality from the Distributor Doctor)
    - new rotor (not specifically for a Healey but original old stock for a 6 cyl Lucas distributor)
    - new condenser
    - new coil
    However, I did notice that there was some pinking/pinging under hard acceleration. I have never had this before and wonder if the new contact breaker might have changed the timing slightly. I was proposing to retard it a little using the vernier on the vacuum unit or slacken the distributor and use a timing lamp on the LT terminal and earth. Is this the way to go?
    Thanks in for your help
    Steve

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    Yoda John Turney's Avatar
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    Re: Misfire mystery

    Coarse adjustment of timing is done by slackening the distributor clamp. Fine adjustment is done with the vernier on the vacuum unit. I would use the vernier.
    John, BN4

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    Senior Member warwick-steve's Avatar
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    Re: Misfire mystery

    Thanks John
    I will adjust using the vernier
    Steve

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