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Home Repair And Restoration

CBF 125 Wet Tickover Problem - SOLVED!

By Ren Withnell

Repair Date 27-02-15

The bit you want to know - fit a new spark plug gap.

If you're still reading, well done you. Right here's the tale of woe. All the way through my "Scotland In Winter" trip the usually trustworthy little CBF 125 was giving me grief. It would run fine and dandy above tickover but when I'd stop it would either be lumpy or stall. When I came back I thought I'd fixed it by changing the plug and cleaning the spark plug cap. All has been well this last few weeks but then it started to rain again.

On Wednesday I rode to work in light rain. The bike behaved well enough but I noticed the tickover was a tad low and when I blipped the throttle rather than the usual crisp response it felt woolly, unenthusiastic. I did ponder if something was amiss but it caused no issues so I decided it was just me. The dry ride home that evening was completely uneventful. 

Then on Thursday the heavens opened. While travelling through town it again felt woolly and at one point I though it was going to stall. Then I got onto the motorway. With the rain battering me and the spray off the lorries the bike received a darn good soaking. Then when the traffic stopped due to the countless roadworks it stalled. It was a real cow to get it going again and to keep it going I had to blip the throttle constantly, like a racing driver on the grid waiting for the lights to go out. It was very very annoying.

That evening after work and being dry for several hours I returned to the bike. Would it start? Would it heckers like!. Kaschf, kaschf, kaschf, kaschf, kaschf, kaschf...kaschf, kaschf, kaschf, kaschf, kaschf, kaschf, kaschf, kaschf, kaschf, kaschf, ....kaschf, kaschf, kaschf, kaschf, kaschf, kaschf, ...Nothing. kaschf, kaschf, kaschf, kaschf, kaschf, , cough cough...nothing. I spent perhaps 20 minutes trying, then waiting thinking I'd flooded the engine, then trying, then waiting. Nothing. 

Out of desperate frustration I messed with the spark plug cap. It started. Before it had chance to stop I kept the revs up and got the hell out of there.I took my 125 home via the motorway to charge the battery and to not give it a chance to stop again. I got home safely in the dry and it sat in my back yard ticking over as though nothing was ever amiss and nothing ever would be. Cheeky git.

This morning I had a devious plan. Start it up while dry, let it run, then get it wet. With hosepipe in hand and a warm running engine I started to gently dampen the spark plug cap. The revs fell. A little more water, I can her the odd "CLICK" as the cap shorts out. I soak the cap and the engine dies. I dry the cap, it starts, I repeat the process, it dies. Definitely the spark plug cap.

Listen for a "CLICK" after the water stops spraying. That's the plug cap shorting out.

Needless to say my CBF is now wearing a new NGK "LB05F" spark plug cap. It ticks over fine with a light mist, a wet mist and just a hint of dropped revs are noticed if I douse it entirely. I guess even a new cap wasn't designed to run under full flow from a hosepipe. 

Please also see my earlier CBF 125 tickover problem being solved if you don't think it's your plug cap.

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Reader's Comments

John said :-
That's exactly what got our YBR125 last winter. Also killed the motor if you went to full throttle. Fortunately for me I have a great mechanic who spotted it straight away.
www.justaboutsurvivingbeingme.blogspot.co.uk/2014/02/never-missed-beat.html ...
Ren - The Ed said :-
A good read, Cheers John. So it transpired to be the same problem, nothing more than a dubious spark plug cap?
John Deville said :-
like you said at last nights meeting....................keep it simple, been there done that...................lol. its also worth giving that area a dose of acf50...........
Ren said :-
Hi John. I thought ACF50 was like an anti-rust sort of thing rather than anti-water in places it ought not to be? Anyhow I'll leave it as is for now. I want to get the bike into some serious rain to ensure the problem has been properly fixed.
said :-

bobbikev said :-
Hi, can I just mansion an exalent

Hi, can I note a good spray to help you out if you are ever in this situation again. The spray is called wet start( not damp start as this is just the same as wd40) I had a car some years ago that suffered the same problem so I bought the spray as reccomended by a friend and it worked every time. A quick spray around the distributer cap and it fired up immediately.
It's handy to carry with you as it drives the water away from the electrics rather than seal and lubricate.

Ren - The Ed said :-
Hi bobbikev. My problem was that I didn't know what the problem was! You're quite right there are several good products out there that will keep the rain out and the electricity in. A can of that stuff would have got me home no problem.

But once I'd sussed out it was the spark plug cap a new one doesn't cost much and should fix the problem once and for all. I might get a can of Wet Start because it might help with diagnosis in the future. Cheers :-)
Bob said :-
Plastic ages, most folks don't know it but it does. That "new car smell" is due to the volatiles in the newly moulded plasic parts evaporating out. Over time plastic becomes more brittle and less ...plastic.

The HT cap is charged with the job of separating 20-30KV from the lovely conductive rain and alloy engine. As the plastic ages its dielectric strength reduces and it can actually become conductive at the sort of voltages seen in an ignition system.
The conditions on the top of a spark plug (thermal cycling, corrosive salt and water) are perfect for prematurely aging plastic parts.

So don't blame the humble HT cap, it leads a damn hard life and should be replaced at regular intervals.
Ren - The Ed said :-
Hi Bob. I - being tighter than Scrooge - expect EVERYTHING to last forever. I find it appalling that oil has to be changed at all let alone often. I get very angry when tyres that should last a lifetime wear out. And as for petrol....well....you fill the tank up and ride around for a bit and the stuff just vanishes! I think it is a conspiracy by the government. Food is another problem. I eat, I am full then later on I am hungry again. What is that all about?

So, knowing this now what do you think the chances of changing my HT caps at regular intervals are?

Cheers, useful info.

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