Spark plug question

[Chimpboy74]

Hi
Been a while since I've been on as I've had no problems.... until recently. My Li125 was cutting out whilst out and eventually packed up a couple of miles from home (ended up pushing it home up a hill !) I know a mechanic guy who helped me have a look at it and there was a problem with the spark plug cap (something had been rubbing against it and caused damage - well, thats what he said to me anyway) and said he'd replaced the spark plug cap and I rode it home 4-5 miles with no issues. However when I went to start it again a couple of weeks later it started first kick and then immediately cut out and wouldn't start again despite my best efforts.

I put it away as I didn't have time to look into it in great detail but I just got it out again and it started first kick... and immediately cut out with no amount of kicking doing anything.

i started looking at what my guy had done and he has fitted an NGK LB05F 5K ohm cap. I did some basic research and found an article that stated I shouldn't have a cap with a resistor and says a "zero ohm" cap should be used otherwise I'd have starting problems. I'm still learning all this stuff so might this cap be the issue? I'm not sure as after he replaced it it ran great until I stopped it.
Theres plenty of fuel and all else seems Ok (to my untrained eye). I havent checked the plug yet but will do that if you think the cap isnt the issue.

Am I barking up the wrong tree looking at this? might the cap be causing an issue or could it be something else?

Unfortunately my contact retired and I cant get a hold of him at the moment - plus i'd like to try and figure this one out myself if i can - with help from here if you can!

Thanks in advance!

[Avantone]

Do the simple stuff first - check the plug (post up a photo - colour, grade etc) and replace with a new one correctly gapped.

[Chimpboy74]

Spark plug looks ok (not fouled or anything) it's a BR8ES. I'll need to get my gap tool back (my dad borrowed it) so I'll check that in next couple of days.

[CHRIS in MARGATE]

Fold and squash a bit of fag packet and that should be a tight fit under the electrode.

[Chimpboy74]

Fold and squash a bit of fag packet and that should be a tight fit under the electrode.

Haha. It will be easier to get the gauges back off my dad. I quit smoking a long time ago!

[Warkton Tornado No.1]

There are varying opinions about suppressed plug caps, but mine is that non suppressed is required for points ignition. Better still to have a fine point plug, my preference now being Champion. There are an awful lot of counterfeit NGK's about plus I think they are far fussier than other manufacturers.

Your HT lead should have plenty of length in it to cut back to allow a fresh part of the core to be screwed into. The same applies @ the opposite end if the coil allows.

It goes without saying that you'd be best to temporarily disable the (normally) green lead to the "kill" or ignition switch whilst you sort this out.....

[Fast n Furious]

Ive commented on this issue before in other threads.
Basically, Radio suppression resistance should only be in one component. Either the HT lead, the plug cap or the spark plug. Any more than one and you can expect problems caused by a very low energy spark.
If there is an "R" in the plug number then it's a resistive plug.
Very often plug cap numbers are illegible and so its best to use a multimeter to see what the continuity reading is.
In my experience, it's best to avoid carbon based HT leads.
I prefer to use resistive plugs simply because, if they fail (open circuit), then I've likely got a replacement plug in the tool box and very unlikely to be carrying a replacement plug cap.

[Chimpboy74]

Ok. So I've been tinkering today. Current spark plug is resistive and HT cap also resistive. Gap was slightly large (but only just) narrowed it slightly checked for spark and looked nice and healthy. Still wouldn't start. Tried a non resistive plug with the HT cap and that didn't work either. Replaced the HT cap for non resistive and it didn't work for either the r plug or non r plug. Checked the HT cable for spark and it seemed ok.

So I'm guessing something else is going on here? The plug seemed to be wet so some fuel is getting through but I'm wondering if there might be an issue there?

As it has sat for a week I was a bit surprised when it nearly started first kick with choke and fuel on (almost but died straight away) this happened as well when I discovered hat I had a problem. It started and just slowly stopped in spite of my attempts to rev it. . Might this suggest something? Choke cable appears to be functioning but I haven't checked anything other than spark plug and HT cap.

Edit: just noticed there is a small split at the back of the HT cable. Not sure if that might be significant given I have been getting good spark

[MickYork]

change your fuel..........

[Martin20]

Are you on 6 volt, or 12 volt?

If 6 volt, could be that your flywheel needs re-magnetizing.

[Fast n Furious]

If the bike normally has bright-ish headlights then the flywheel's magnetism is ok.
I suggest that you carry out a compression test to rule out any problems here before moving on to something else. Could just be something as daft as a blown head-gasket or worn rings which will still wet the plug. The tool is relatively cheap to buy if you haven't got or able to borrow one. A good tool to have in your Lambretta armoury anyway.
Good luck.

[Chimpboy74]

It's 12v. I dont have a compression tester but I've ordered one.

In terms of light brightness I only ever rode it in daylight so never noticed how bright (or not) they were

I tried some easystart and still didn't start. I'll post back when I've tested the compression.

[Chimpboy74]

Checked the compression. Maxed out at 85-90psi.


Not sure what that means.

[CHRIS in MARGATE]

It's not stunning but should still run OK.
Did you try it with the throttle wide open?

[Chimpboy74]

It's not stunning but should still run OK.
Did you try it with the throttle wide open?

D'oh. No I didn't. I will try again tomorrow.

Next step to check fuel line and change the fuel?

[Chimpboy74]

Ok tested again (with throttle open this time!) And for to just over 90 psi.

[Scooterdude]

Ok tested again (with throttle open this time!) And for to just over 90 psi.

It won’t matter weather the throttle is open or closed despite what you may have been told.

[CHRIS in MARGATE]

Almost every compression testing guide tells you to hold the engine’s throttle wide open during a compression test, but most won’t tell you why it may (or may not) matter.

On gasoline engines, the throttle is a valve, which reduces the amount of air going into the cylinder. Closing the throttle reduces the amount for air sucked into the cylinder during the intake stroke. Having less air in the cylinder means that there is lower pressure at bottom dead center (BDC), so that when the piston moves ‘up’ and compresses the air, the maximum (or peak) pressure is lower than it could have been.

I copied this from the internet.

[Chimpboy74]

A bit more tinkering. the HT cable had a small split (which I think was due to cable being too long and being forced to bend when the cap was fitted to the plug. I cut it back and refitted the HT cap. Maybe it was just wishful thinking but the spark seemed stronger after this so I fitted it all back together and still nothing.

Is it possible there is a spark but it's not strong enough? Just want to exhaust the spark plug possibilities before I start draining fuel and checking the lines etc (I've never done this before so would be going in blind!)

[MickYork]

bite the bullet......put your finger in the suppressor cap and kick it over......you'll know if your getting voltage or not.

I'm being serious by the way.