Strange misfire

[DuncanC]

Hi,


First of all my bikes a 2000s.
A couple of weeks ago I was out and about when having pulled away from a junction I revved it up to the red line and it cut on to one pot. Due to missing allen keys I couldn't do much with it so ended up limping home on one. As I was limping along I noticed that when I closed the throttle it was popping and banging burning fuel in the exhaust.
When home I checked which pipe was hot and determined that the rear cylinder was the dead one.


Next morning I checked the plug cap was on, pulled the plug (a couple of thousand mile old iridium number) and checked all the connections a round the coil. Nothing was wrong. So I fired her up and went for a 30 mile test ride. She was fine I tried high revs chugging along at low revs all was well.


Fast forward to yesterday I went for another blat. I noticed that after a couple runs up to the red line she was reluctant to rev past 7500. Fine up to there no matter what throttle I used but not revving past 7500/8000.
My tacho is an after market one that uses pulses from the rear coil (easier to get to) I noticed that it started fluctuating when misfiring, several hundred revs either way, so not reflecting the engine speed.


I'm now wondering is the tacho fluctuation caused by a lack of voltage to the low tension side of the coil or would a lack of spark cause there to be no signal from the coil?
If the former is it likely that one signal from the ignition box could break down under load/ when hot?
If the later can I change the HT lead on this type of coil or are they moulded in?
Any other idea about what might cause this sort of thing?
Thanks for your help.

[Red ones]

Why are you redlining it?

[Dave-the-rave]

You're right to be focussing on the rear cylinder as all symptoms point that way. Don't change that HT lead yet. Have a look on you tube for how to test the whole coil first. You'll need a multimeter. Maplin do a decent enough one.

Riding around on one cylinder could damage the bottom end so stay close to home when doing test runs.

[DuncanC]

My first response would be why not? Or because I can. The engine is designed to be safe to redline and probably a bit beyond. However in this case redline isn't actually where I went, I went to shift light set at 9500. I don't normally bother with the red line or full throttle for that matter but occasionally it is nice to go a bit nuts. Having found this problem the only way to see if it fixed or not is to replicate the failure mode. If it is a load/heat problem I assume it will continue to deteriorate until it becomes a problem in my normal riding mode and as such I'd like it fixed before it does.

[DuncanC]

Dave, sorry I didn't see your post when I started mine. I have a multi meter and will test the coil. I suspect it will be fine cold as the issue is only at high revs when hot. I don't do high revs on a cold engine.

[R1ffR4ff]

Also check the resistance of the plug cap.Standard it should be 5K ohms but are usually a bit higher around 9k ohms but anything above that get a new one.

These are the red version plug caps,

http://www.ebay.co.uk/itm/Spark-Plug...wAAOSw7NNT6WRf

and these are the black,

http://www.ebay.co.uk/itm/1x-NGK-Res...EAAOSweW5VX6ll

You can make a rubber skirt/seal to emulate the originals from some old inner tube.

[Seeker]

The only bike I've had a plug fail was on a 02 curvy, it would sporadically fire then miss for a few revs. In your situation I'd try a new set of plugs first (non iridium) and then suspect the coil. Not sure about the significance of the rev counter variance.

[Biker Biggles]

Id agree that substitution of first the rear plug,then the cap/lead and coil for known good items is the way to find the culprit.Can you borrow any of those bits from anyone?

[DuncanC]

Right well I've tested the coil primary and secondary and loo to be Ok at 4.7 ohms on the primary and 28Kohms on the secondary. I'll have a play about with the plugs and caps and bits during the week.
As an aside whilst searching for R1ffR4ffs plug cap numbers (thanks for posting the link) I came across an old post about restricted SVs. My bike gets up near a ton easily but then runs out of puff. I'm not really bothered about this as with no fairing hanging at those speeds is not fun. I also put it down to an old bike and a fat pilot. However reading that thread made me think "33hp struggles to break a ton" Humm I wonder. So assuming the restriction is in the ignition module does any body know the part number for a normal module? My part number is J54 20F0 BB7708 OB29.

Restriction was carb washers on the Curvys, not the ignition module.