Forums » Help and Advice

Spark Plug failures.

    • 16 posts
    June 5, 2013 4:21 PM BST

    Hi All,


    Can anybody out there offer any advice as to why my Lambretta is going through spark plugs roughly every thirty to forty miles. I'm not sure what she was like before, but since a full engine rebuild, i'm only getting around that mileage per plug. The engine runs sweet for thirty or so miles then total cut-out, change the plug at the roadside, and off she goes again, fine for another 30 miles or so, then same again. I'm not riding her for thirty miles in one go, these are all roughly five to ten mile journeys at a time. I am running her on 4% oil mixture at the moment whilst running her in, intending to drop to 2% after a couple of hundred miles. Is engine running too hot perhaps with 4% oil, therefore burning plug out ? If so, should I change to a cooler running plug, from a B7ES NGK to a B5ES NGK ?, or adjust my air/fuel mixture to run cooler ?


    Any advice would be most welcome,



    • 20 posts
    June 6, 2013 8:26 AM BST
    I'm not 100% sure but if it were me I would go the other way with the plug and try maybe an 8 or 9 in it? What engine do you have in it, tuned or standard? I have a 7 plug in my standard lml 125 2t and a 9 in my T5 Malossi 172 the difference being the T5 has to work harder thus getting hotter and needing a bigger number plug. There was a write up on plugs in Scootering mag a couple of months ago.
    Hope that helps and not confused you even more, lol?
    • 16 posts
    June 6, 2013 10:04 AM BST
    Thanks for the input Pupjoint, it's a standard 100cc (Cento J range) engine with no mods. After doing a bit more research since your advice, it looks like your spot on, i should go up in the number for a cooler running plug not down. I think I'll try a 9 rated plug and reduce the air in the air/fuel ratio slightly and see how long my plugs last then, and go from there. Cheers mate !
    • 20 posts
    June 6, 2013 5:49 PM BST
    No worries mate anytime. I wouldn't fiddle with air/fuel mixture straightaway, unless you're having probs with that now, I would just try the plug change to start.
    • 16 posts
    June 6, 2013 10:05 PM BST
    No, i'm not having problems with the mixture, the plug electrode looks fine, a nice chocolate brown with no deposits or oil. I just thought that with the plugs possibly burning out ? at such a low mileage then the engine must be seriously overheating, so thought i'd try to reduce the heat even more than just changing the plug. I'll take your advice again mate, I can always look at changing the mixture if the new plugs are still failing, take it one step at a time suppose. The only thing that worries me is causing serious engine damage. How many miles should I expect to get from plug in normal cicumstances ? Just checked, I've used three plugs in 80 miles.
    • 20 posts
    June 7, 2013 8:27 AM BST
    Yeah I would just try one thing at a time. There's no hard and fast rule to how long plugs should last, but it is definitely more than you're getting lol What brand of plug are you using? You could try a different brand, or you're buying your plugs from the same shop every time you could try buying just one from another shop incase they have a dodgy batch? It might be worth trying another cdi/coil. See if you can borrow one off a mate just to try.
    I think if your engine was seriously overheating then it would seize??? Maybe, I'm not to sure??? Sounds logical though.....I think lol
    • 16 posts
    June 7, 2013 9:45 AM BST
    Yeah, I've tried Champion and now NGK plugs with the same result. i've got two spare coils, one works, the other doesn't, but I can't remember which one is which by just looking at them. Anyway, as you said, one step at a time and I should hopefully get there in the end. First step : Changing plug this weekend and see what happens, you never know, I might get 50 miles out of her. LOL.
    • 20 posts
    June 7, 2013 7:29 PM BST
    haha fingers crossed for ya then
    • 16 posts
    June 8, 2013 11:24 AM BST
    Now I really am confused mate, i changed the plug for a cooler running B9ES and that plug only lasted about three miles. I changed the plug at the roadside for the original B7ES and rode home. Perhaps i'm going down the wrong road here thinking that the engine needs to run cooler, seems more likely now to be something else. Maybe a coil issue, do you think a faulty coil or dodgy spark plug cap connection could burn out the plugs?
    • 20 posts
    June 8, 2013 5:58 PM BST
    Certainly not the spark plug cap as that would affect the spark ie intermittent or not at all. As the coil/cdi supplies the plug there's a pretty good chance of it being faulty. Try changing it for one you know is good, maybe off a mates scooter which is running ok. The coil/cdi has a good earth doesn't it?
    • 16 posts
    June 10, 2013 10:14 AM BST
    Thanks again for the info mate, I'm going to try a different coil. I have two spares, one works, one doesn't, so i'll try them both out. Does seem strange though, I've never heard of a coil burning plugs out before, I always thought that a faiing coil would not supply enough power for a spark rather than destroy the plug.
    • 20 posts
    June 10, 2013 9:45 PM BST
    Yeah I'm not to sure either lol Just trying to think of different things it could be and as it's attached to the coil I thought it was worth trying another. Let me know how it goes, if that doesn't work then I'll ask a couple of my mates who've been tinkering with scoots a lot longer than me.
  • July 13, 2013 11:35 PM BST

    Condenser working OK?  A tricky coil can bleed charge to the electrode through the charge cycle and make it act like a glow plug, an intermittent positive short onto the coil feed line could do that as well and might be worth just checking your stator just in case a rivet hasn't fallen out inside and playing silly buggers on the coils in there, actually saw something like that when a LML rivet had fallen out inside and because it was magnetised it was causing the hall sensor to go crazy lol

    • 16 posts
    July 15, 2013 9:58 AM BST

    Thanks for the response Ian, I'm more mechanically minded than electrical, so any advice I can get is appreciated. I do have two spare coils, one working, one not working, i can't remember which one is which but it won't be too much hassle to fit them and test them out. I hadn't thought of the condensor, is there any way that I can test the condensor, or do I just buy a new one and fit it to see ? I guess the best thing for me to do is look at the coil and condensor first before looking at the stator side of things as both are cheap to try in my case. The only slight problem I have is that even after fitting new coil or condensor, I'm not gong to know if the problem has rectified for thirty miles or so. Hey-ho, that's part of the pleasure of owning a Lammy, I suppose. I'll keep you updated Ian, thanks ! 

  • July 15, 2013 12:28 PM BST

    Don't think you can check a condensor function other than replacing with a known good unit :(   They're not very expensive and at least you know its done.


    Its really annoying when you get such an odd fault like this m8, I had an old Vespa that had frayed its sheating out of the stator area and the wires underneath had cracked with age and would periodically blow all the lights out, put new loom in, bulb holders, checked the stator but that tiny fray in the outer drove me nuts lol

    • 16 posts
    July 15, 2013 1:09 PM BST

    You're right, this issue is driving me insane, she runs superbly with a new plug, but only for thirty miles or so. Even when the plug decides to die, there's no coughing or spluttering, just total cut out. Change the plug, then she's tip-top again. It's been 12 months of this, and I can't seem to get to the bottom of it. Still, i'm never going to get to the bottom of it without trying different things, I just hope it doesn't cost me a fortune in plugs in the meantime. Thanks again for the pointers, I don't think I'll change the condensor and coil at the same time cos if it sorts the problem out, I won't know which of the two was responsible, for future reference. Which would you advise that I change first ?  

  • July 15, 2013 1:35 PM BST

    Condensor is the cheap one to replace, might be worth seeing if your coil is getting hot or its just knacked out if its an old oil filled one.


    Been doing some searching for plug premature death and so many different causes, wrong heat range of plug for driving conditions, jet issues, stator issues, condensor and points, mixing some synthetics and mineral 2T oils, air leak aargh no wonder its driving you nuts lol

    • 16 posts
    July 22, 2013 9:02 PM BST

    Apologies for the delay in responding Ian, i've been away for a week and haven't been on line. The coil isn't getting hot at all, so it might be worth me changing the condensor first as it's minimal cost. From there on, not sure what to try next at this stage but i've tried using a B9ES plug instead of a B7ES and was worse, the plug only lasted around three miles. So far as jet issues go, i've got a standard 100cc with standard jets. I've never mixed synthetic with mineral oil at all, so after changing the condensor it may be worth me pondering over stator, points or air leak issues. AAAAARGH !!!! IT GETS WORSE !!! Bloody Lambrettas !!!

  • July 22, 2013 9:43 PM BST

    Its def something that is causing your plug to go way past its temp range and get very hot, what would worry me is an electrode burning off at some point