Author Topic: Misfire problem  (Read 3115 times)

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Offline C Clay

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Misfire problem
« on: 07 Aug 2014 at 20:34 »
Hi,
I took my Mk3 for a spin after a major engine rebuild, including mag rebuild and new plugs etc., it was going great but after about 10 miles it started misfiring on one cylinder then dropped to just one cylinder.
Limped it home and found that on the cylinder that was not firing the plug was wet but there was a spark at the plug, a little weak though.
However, if i removed the plug cap(non suppressed) and hold the plug lead about 1/4 inch from the end of the plug a big spark jumps the gap and the cylinder fires as normal.
Any ideas what the fault could be, plug maybe? Any of you have experienced this issue?
Regards
Clinton

Offline eddie

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Re: Misfire problem
« Reply #1 on: 08 Aug 2014 at 07:10 »
Clinton,
           The first thing I would check is that you are using the correct grade of spark plug - NGK B7HS seems to be the preferred plug these days. From the symptoms you give, it seems that one plug started to foul up, giving the other cylinder more work to do, thus making it run hotter, which in turn prevented that plug from fouling. If you are already running B7HS plugs, it may be a good idea to try B8HS plugs until you have run the motor in and are prepared to make it work a little harder.
  Also, Lucas magnetos are not provided with a 'safety gap', so it is not a good idea to make the spark jump excessive gaps as it can cause the spark to jump across the windings, causing more damage.

  Regards,
               Eddie.

Offline Ian

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Re: Misfire problem
« Reply #2 on: 09 Aug 2014 at 03:45 »
Eddie, why would you move to a B8 plug - I would have thought hotter to prevent fouling when not running hard - B6 ?

Offline eddie

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Re: Misfire problem
« Reply #3 on: 09 Aug 2014 at 06:01 »
 Yes - sorry, I got it wrong. I meant a B6HS - put it down to age (mine, not the bike's!).

  Eddie.

Offline C Clay

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Re: Misfire problem
« Reply #4 on: 09 Aug 2014 at 23:02 »
Hi Eddie,
It has got B6HS plugs fitted.
Not had a chance to look at it today, I am sure its something simple.
Those are normally the tricky ones!
Clinton

Offline C Clay

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Re: Misfire problem - new question
« Reply #5 on: 17 Sep 2014 at 14:40 »
I just got around to looking at it and it was something simple, bad spark plug.
Never had that before but as soon as I put in a new replacement it ran perfectly.
Took it for a fairly long run today and it ran very nicely. However after a few miles I found the tension of the throttle springs was somewhat tiring.
My Royal Enfield Interceptor has Light throttle Springs (made specifically for twin carbs) which makes it much more comfortable.
Do any of you know if there is something similar for the 274 carbs?
Regards
Clinton

Offline eddie

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Re: Misfire problem
« Reply #6 on: 17 Sep 2014 at 14:50 »
Clinton,
            As you are in Berkshire, it might be worth taking one of your springs round to Lee Springs in Wokingham to see if they have something of the same size but about half the poundage. I have always found them very helpful.

  Regards, Eddie.

Offline dalgrae

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Re: Misfire problem
« Reply #7 on: 19 Sep 2014 at 06:14 »
Hi if you take up Eddies advice about the throttle springs please if possible order a pair for me as I get the same problem and I would obviously cover all the costs involved

Regards Colin

 

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