Weak spark, no. 3

[moto]

After the initial delight in finding a lovely clean fzr, I'm now reminded just how old it actually is....

Firstly it started running on cylinders 2 & 3 (1 & 4 were stone cold), new plugs (CR9EIX) were gap-checked & fitted and now it runs on 1,2 & 4, and just about on 3 which is sparking but very very weakly. The downpipes indicate it isn't really firing well, but it does get warm.

I've cleaned everything & taken the lead back as far as I can whilst leaving just enough room to get my hands in & twiddle the plug cap back on, but that's done nothing to liven up no. 3........ can anyone tell me what's the next step ??

[reelrazor]

How do you KNOW the spark is weak on #3? Did you visually check spark? Or are you going entirely by header pipe temp? There's all kinds of things that will keep the header pipe cold. A single clogged/restricted jet or air passage in the #3 carb will do it too.

The coil cannot be the problem as it fires 2&3 together..there are no separate windings inside the coils for each lead.

So, the problem has to be 'downstream' of the coil...either in the lead-or check this possibility-in the spark plug boot itself. There is a resistor in the boot that can fail, and the plastic of the boot may crack allowing spark to arc to ground. The wire may actually do this too. Often that can be seen if run in the dark.

[moto]

I've had the plug in & out several times for a visual check, the spark is very weak on no.3.

What I don't understand is that no.3 was working fine on the old plugs when 1 & 4 were out of action (I've got the burnt knuckles to prove it!), I only changed plugs as I was going to do so as a matter of course. My initial thought was that I'd dislodged the cap from the end of the lead when changing the plugs, it is a bit of a tight bend to get it on the plug!

I've visually inspected the plug cap, looks fine but I believe it is the original...

[reelrazor]

Inspect it with an ohm meter. The cap should be 5k ohms (maybe 10k?-it should be written on the cap)....anyway, it aint supposed to be 'open' or really high resistance

[bokka]

hi remember the age of ya plug leads and caps,they do deteriate over time being copper inside..

[moto]

I'll get a set of plug caps & see what happens - I took the cap off & couldn't get any reading whatsoever, either regarding resistance or continuity. I cut a piece off the lead & it took ages to screw the cap back on! It still runs just the same, I'm hoping it's the cap, can't see what else it could be as it only affects no.3 which, bizarrely, was running fine before I replaced the plugs.... looking at NGK SD05F-R, anyone used these?