Bad misfire
 

Bit of a puzzle to me, this one. The XT has been laid up for 3-4 weeks while I got the stuff to rebuild the rear wheel. Sunday I got it all together and had a successful test ride. Monday I set off to work and at the first junction it backfires and dies. I assumed it was running out of fuel, switched to reserve, restarted, and refilled at the next opportunity. Work and back, all fine. Tuesday, work and back (25 miles) all OK. This morning, it seemed fine until I switched on the headlight in a bit of drizzle. Within half a mile it had started misfiring, backfiring and losing power big time. I switched my route to side roads away from the traffic and it stalled. It restarted fine, and idled away like nothing was wrong, but when I got going it could barely pull 20 mph in 2nd, with a really bad misfire and a lot of banging from the exhaust. It had no problem idling, but as soon as it was asked to do work it misfired badly. I was about to head for home when I decided to turn off the headlight. Within a few seconds the misfire cleared and it ran like a good'un, and I decided to head for work again. It got me there with no further problem - another 6-7 miles with the headlight off.

My first thought was a blocked jet, but the fact that reducing the electrical load seems to have corrected it makes me think it's electrical. But what? There's plenty of juice in the battery to start it, so it's not a dead battery. But I can't think of any way that turning on the headlight should make this happen. Anyone got an idea?

Hey mate,
My first instinct was blocked jets too. So i would give that a clean anyway,, a few weeks can make fuel go bad and that could have dirtied your jet, so all in all it may need a clean anyway making your problem worse.. HOWEVER like you said there seems to be an electrical element as well. Do you have a fuel pump. If so I would check this next.. if it is working too hard (for what ever reason) it may be needing more power (I admit blind guess here)? Other then that my next guess was plug leads make sure that they are not cross-arcing (use the mist water in the dark tech). Finally,, the amount of Sh*t I have found on my electrical system since riding this winter (salt corrosion) is amazing.. I would also have a good look and clean of all your electrical connections..(my main fuse was corroded so badly that it was no longer red but green!!! and yes i used ACF50 though out the riding!!!:() Good luck and let us know...

Sounds electrical to me...

Maybe the earth on the coil or a crossed earth.. Could also have water in your plug cap. I would trace back everything from the plug and see if its dry and sound.

Hi Xander

Thanks for the thoughts. I'd be surprised if it was a blocked main jet, even though the symptoms point in that direction, as the bike was only laid up for about 3 weeks, and I made a point of starting it up and running it for a while every few days. I had a bit of a problem last year which turned out to be a short in the headlamp cowling, but as I was tracing that one I checked and greased up all the connectors, including fuse and CDI unit, so I am pretty sure that the connectors are all good. It's only done about 1k since a full service with a new plug too. New battery last autumn.

I'm going to treat the main jet as a last resort effort, as I prefer to leave carbs well alone if they are running right (and with the headlight off, it runs completely as normal). The only bits I am not sure of are the CDI unit, the coil and the HT lead. Any of those going bad would cause the misfiring, but why this should only happen with the headlight on is a mystery to me. I have had a similar misfire before (different bike), when the alt decided to stop charging and I was running the ignition on the battery only. When the battery was almost dead, it could give enough juice for the ignition at idle but not under load. But in the present case, there is plenty in the battery for the electric foot, so I don't think that's it here. I'll wait until I get home and run a multi-meter over the charging system and see if there's a problem there.

Thanks for your thoughts - much appreciated.

Update: definitely electrics.

When I got close to home on the way back tonight, I switch on the headlight and sure enough, about a mile later it started misfiring. Finally, it stalled, and there were no lights on the panel. By a lot of jiggling with the key and the kill switch, I got the Rev/Ign and Neutral lights back on, and off she went. I can't say what I did to get it going, and I can't replicate it now I'm back home, but it must be a loose something somewhere.

Battery voltages:
Engine dead, not run for 2 hrs: 12.9v
Engine idling: varies between 13.5v and 14.0v
Engine fast idle: varies between 13.2v and 13.6v
Engine idling, headlight on: around 13.9v
Engine revving high, headlight on: varies between 13.2v and 13.5v

The faster it revs, the lower the voltage seen at the battery, roughly speaking. The headlight actually dims as you speed up the engine. I've never encountered this before. I'm guessing that something must be up with the alt or the reg/rec. Your thoughts?

Seeing that the ignition & headlight are both powered up through the ignition switch in the on position, maybe you've got a bad connection at the ignition switch or burnt contacts/water inside the switch.
Test for correct voltage at the headlight when it goes dim, & if the voltage is low, trace it back til you find good voltage.
Or you could try bypassing the ignition switch & see if that fixes it.
I presume you've checked the various earth connections already?
HTH
Bob

Thanks Bob. I will check the voltage at the headlight and trace it back. However, the wildly varying voltage at the battery says to me that the voltage from the alternator isn't being regulated properly, and that points to a bad reg/rec unit rather than a simple bad connection somewhere up front. A bad connection or switch would account for the misfire and dimming light, but the battery voltage readings are something that I haven't seen before. Do you know any way of testing a reg/rec?

Looks like that this weekend taken care of, then :)

Cheers.

I wouldn't be concerned with thew slightly erratic charging voltage, as the lowest voltage with the engine running(13.2v) tells you the charging system is charging.
I would concentrate on locating the high resistance(voltage drop) causing the headlight to go dim & engine missfire.
Bob

New reg/rec in today, just done 40 miles with headlight on, no problems. Looks as if this was the problem. Charging voltages now normal, i.e. about 13.5v at idle, and goes up when you rev the engine.

Thanks to everyone for the advice.