[ChrisTJ]

Hello people,
I'm facing a strange issue with my bike. If I go WOT from idle speed, the spark plug wire is arcing to the engine block.

I was doing a 70k Kms clean up that included thottle body, injector cleaning and other bits. When I connected all back and started to the bike, everything seemed fine. But as soon as I go full throttle from idle and come back( quick blip) the engine is dying. It sounded like it's being starved of fuel.

However, if I raise rpm slowly, it's fine. So, I was confused since now I know the injector is fine. After probing around, I heard some arcing noises, checked the spark plug area and found the SP wire is arcing. This happens only at WOT.

After reconnecting the wire to SP, engine doesn't die anymore, but still arcs sometime and on every WOT(wide open throttle) it feels like engine is missing few beats.

Bike wasn't used for few months, however I've started and idled it twice a month. All the wires are properly connected.

What could be the issue? Help please.

Thanks.

[Jaggu]

There is a leak in the cap or wire that is causing the arc, you will have to replace the cap or wire with new ones.

[ashwinprakas]

Change HT Cable and Cap, don't even think twice, not worth the effort.

Edit:

I just noticed your name and location, are you the same Christo I think you are?



Snap from the old Kozhikode Biriyani Ride.

[ChrisTJ]

Originally Posted by Jaggu

There is a leak in the cap or wire that is causing the arc, you will have to replace the cap or wire with new ones.

Yes, it was the cap. The thing confused me is the arc was only present when going full throttle from idle.

Originally Posted by ashwinprakas

Change HT Cable and Cap, don't even think twice, not worth the effort.

Yes, the same Christo. Glad to see you here.

Btw I got the cap replaced and the arcing is gone. Didn't get the wire, otherwise I would've changed that too.

[Jeroen]

Good job you had the cap replaced.

There is an old mechanic trip to check the state of the HT leads and cap. Let the engine run and spray, carefully, with a plant sprayer fine mist on the leads and caps. If you see sparking you know these need replacing.

Yours were sparking even without the fine mist. Which means they were really bad!

Jeroen

[ChrisTJ]

Originally Posted by Jeroen

Good job you had the cap replaced.

There is an old mechanic trip to check the state of the HT leads and cap.

Thanks for sharing the neat trick. Yes, mine was a goner but It lasted through 72k Kms and 5 years of electrocution

[ArizonaJim]

ChrisTJ

The main reason the spark is breaking through the insulation and jumping to the engine rather than jumping the spark plug gap is due to the pressure inside the cylinder and combustion chamber.

In order to jump the spark plug gap, the electricity needs to overcome the electrical resistance of the air it is passing through.
One might think having more air molecules between the spark plug electrodes, which happens when the pressure goes up, should make the path for the electricity easier to pass through but, actually, it makes it more difficult.

When your just riding down the road at a steady rate of speed, the engine is not working hard and there is a vacuum in the inlet between the carburetor or throttle body and the inlet valve. This vacuum lowers the amount of pressure the piston can produce so, when the piston reaches TDC, the pressure is not very high. In this condition, the spark has a fairly easy time jumping from one electrode to the other.
When you open the throttle wide open, suddenly there is very little vacuum in the inlet tract so the piston can gulp large quantities of air/fuel mixture. This larger quantity raises the pressures in the combustion chamber considerably.
With this higher pressure, the spark finds jumping the SP gap much more difficult so the voltage created by the ignition coil increases dramatically.

The higher voltage looks for any path to get to ground so the poorly insulated area of the spark plug wire that was doing a good job with the lower voltage now can't resist it. The result? The spark jumps to ground through the wire insulation.

Others have given you the fix that needs to be done so I won't get into that.

[ChrisTJ]

Originally Posted by ArizonaJim

The result? The spark jumps to ground thru the wire insulation.

Aha, so that explains it. Even though I fixed the problem, I was keep thinking why does it happen only at WOT.

Thanks a lot for your reply, now I can sleep peacefully.