Sometimes has spark but sometimes doesnt.

AnotherWebUser

New Member
Jul 16, 2021
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Me and my friend have put together an 80cc MB, and for a day or two it was running great, then it stopped starting, so we tested for spark and had nothing, replaced it with an ngk and it was running perfect again. The day after replacing the spark plug and after riding about a little we were having problems again, it was misfiring badly and after riding about 30m it stopped. We tried for another 5 or 10 mins to start it but it wasn't happening, so we walked it home. We got home about 15 minutes later and after a couple attempts it started, at first still misfiring a little but then it went away and was back to normal. Now 2 days later we are having the same problem with no spark, it misfired a bunch, enough to move you before stopping but now not much, occasionally it fires once or twice but won't start. Any ideas?
 

AnotherWebUser

New Member
Jul 16, 2021
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Ok so just after posting this it started running again and got me all the way home, but there's a chance its going to do this again. Could it be overheating? Or maybe a faulty cdi or mag? But I can't really understand why it only stops working sometimes.
 

AnotherWebUser

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Jul 16, 2021
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Started of with somewhere between 20:1 and 25:1 for a tank but now its running 32:1. The plug was quite black when replaced but that was still when breaking in.
 
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Testname111

Active Member
May 20, 2021
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Ither to much oil in your gas as venice said or no spark from crappy China magneto ,I suggest this magneto and spark coil personally



 
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AnotherWebUser

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Jul 16, 2021
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I had a look at them but the mag is out of stock, and because I'm in the UK shipping costs more than the coil. Does anyone know where you can get these parts in the UK?
 
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mapbike

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Mar 14, 2010
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Doesnt look like they have the ones with the red wire and a bit larger copper coil, but you do have a source for the standard parts that may get you up and going reliably again.

Be sure and double check all the connections and where wires are soldered to magneto.

a bad soldier could cause your troubles.

hope this helps
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
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Try running it without the air filter for a day or two... that will lean out your fuel mixture a bit.
If that fixes the problem?, you just need to install a slightly smaller jet in the carb (& re-install the air filter).
 

AnotherWebUser

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Jul 16, 2021
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Try running it without the air filter for a day or two... that will lean out your fuel mixture a bit.
If that fixes the problem?, you just need to install a slightly smaller jet in the carb (& re-install the air filter).
Already have had it running without air filter but same issue.
 

Testname111

Active Member
May 20, 2021
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Get a bikeberry mag and cdi dude they great, also tip (Sand the bottom of the magneto contacts on the engine as well)
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
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Los Angeles, CA.
I have some serious doubt that it's your ignition coil... (either they work or they don't).
In all my years working on these, I've don't recall ever seeing one that worked 'on & off' (unless you have a bad wire connection somewhere).

What you're describing sounds like a wet spark plug because you can't see the spark travelling though the liquid on the plug (instead of it jumping the air gap)... & then when the liquid dries up the spark is back.

Maybe your problem is something as simple as the choke lever being slightly on?
Or maybe the wires (or kill switch) are grounding to the bike somewhere??
 
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AnotherWebUser

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Jul 16, 2021
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I have some serious doubt that it's your ignition coil... (either they work or they don't).
In all my years working on these, I've don't recall ever seeing one that worked 'on & off' (unless you have a bad wire connection somewhere).

What you're describing sounds like a wet spark plug because you can't see the spark travelling though the liquid on the plug (instead of it jumping the air gap)... & then when the liquid dries up the spark is back.

Maybe your problem is something as simple as the choke lever being slightly on?
Or maybe the wires (or kill switch) are grounding to the bike somewhere??
Those are some good ideas, ill check it out, it hasn't happened again yet but next time I'll take out the plug and see if its wet. The choke definitely hasn't been on though I have checked that. Also kill switch is disconnected but ill make sure none of the other wires are grounded on the frame as they are zip tied down.
 
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