Hitchcocks Forum

I took the Tiger 90 out for a good run yesterday evening, the first for several weeks. She ran beautifully and never missed a beat. Until I switched on the lights, then within a minute or so she started missing quite badly. Turned off the lights and the engine picked up again without any misfire. Lights on, misfire back again. Fortunately I was only a mile or so from home got back in the failing light without my lights on. Anybody got any suggestions as to where to look first? I can see no wiring issues and this problem did not occur before the bike was parked up a few weeks back. As the lights all work fine without the engine running I think the battery is ok. Electrics are original simple 6v and untampered with.

The most likely cause is a disconnected wire, faulty light switch or faulty coil in the alternator, These bikes run one pair of alternator coils without lights on, two pairs with side lights and all three pairs with the headlight on, the switching is done by the light switch, check the battery voltage with the lights off engine running and then both light options this should prove it is a charging fault, then all you need to do is find which one is the cause.

Zenner diode and heat sink ok ??

Eddie, I had a persistent misfire on a Super Meteor once that proved to be a problem with the rectifier. A pal also had a similar experience recently with a Trident. Easy and cheap to swap so worth a try?
REgards, Mark

Just to clarify the early 6v version of this bike does not have a Zenner diode, the charge regulation is carried out as in my post above and to be honest is not "SIMPLE" as the OP states, in fact it is a quite complicated wireing set up using a combination of ignition switch and light switch settings to control the charge.

For simpliity most of these bikes are now modified to 12v.

Thank you for your comments gentlemen. I fitted a new battery today and even with the roughly 80% (dry)charge in the new battery the problem seems to have been sorted. The amp meter is showing a healthy charge so I expect the battery to be fully charged pdq. Lesson learned, look to the obvious possible causes first.You're quite right of course Dennis, there's no zenner on a 6 volt system and most 6 volt systems have been changed to 12 volt. It's not exactly rocket science to do the conversion, it is pretty straightforward, but in the interest of keeping the machine as factory spec. original as I possibly can she will remain 6 volt.

Nothing wrong with that Eddie, I also like to keep old bikes pretty much as they were made.

Have you checked the battery voltage in the three different charge states?, Ie, without lights with side lights and with headlights, it is worth checking as a new battery will mask the fault long enough for you to be caught out one night.

Hmm, lost the last bit somewhere, Even with a failing battery the alternator should have been giving enough power to run the bike with the lights on.