Headlight on, right cylinder off. . .?? - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 15 (permalink) Old 07-21-2019, 10:57 AM Thread Starter
Supersport 600
Main Motorcycle: 1969 650
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Headlight on, right cylinder off. . .??

Hello, folks.
Before I start testing and dismantling, has anyone come across this odd situation?
The bike is a '69 T120. The charging system seems to be working fine and the battery is good. The bike always starts first or second kick. I'm still running points ignition and it has a new Amal Premier carb.

The old girl runs like a champ and idles well. . . .until the headlight is switched on. Once you flick the switch over, on idle, the right hand cylinder cuts out. Switch the light off, and it fires back up.

I'm guessing I have a short somewhere
Anyone experienced this weird problem?

Thanks in advance.

John
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post #2 of 15 (permalink) Old 07-21-2019, 01:11 PM
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Hi John,

Quote:
Originally Posted by suzyfan View Post
I'm guessing I have a short somewhere
Nope. "short" is an abbreviation of "short circuit", which is a connection between battery -ve and battery +ve without the electricity passing through a resistance. Because a short-circuit draws a huge current (Amps) from the battery, you better hope it blows the fuse, because the alternative is melted wiring and the bike catching fire. Components like ignition coils and lamps bulbs are "resistances" so, because they're working, the bike doesn't have a "short" (circuit).

Out to dinner so, unless someone else has posted in the next few hours, I'll post some tests to make.

Regards,
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Stuart
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post #3 of 15 (permalink) Old 07-21-2019, 01:56 PM Thread Starter
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Thanks, Stuart.

John
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post #4 of 15 (permalink) Old 07-21-2019, 05:26 PM
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Question

Hi John,

Quote:
Originally Posted by suzyfan View Post
battery is good.
idles well. . . .until the headlight is switched on. Once you flick the switch over, on idle, the right hand cylinder cuts out. Switch the light off, and it fires back up.
Just as a matter of interest, how do you know "battery is good"? You have a multimeter, set to the Volts range just above 12V, connected across the battery? What did the meter indicate:-

. When everything is switched off?

. Just the ignition is switched on?

. Ignition and lamps switched on, but engine not running?

A fully-charged battery should read ~12.6V; significantly above, you're likely reading 'surface charge' after battery-charging; significantly below, e.g. 12.3V is a partially-discharged battery.

Otoh, if you're seeing 12.6V or close to it even with all lamps on but engine not running, leave ignition and lamps on for at least 15 minutes, meter across the battery should still indicate above 12V.

Also use the multimeter's Ohms range to measure the primary resistance of each coil - i.e. between each coil's "-" and "+" terminals; depending on the make, I'd expect the meter to indicate between ~3.5 Ohms (original Lucas) and ~4.2 Ohms (PVL).

Post how you get on?

Regards,
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Stuart
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post #5 of 15 (permalink) Old 07-21-2019, 06:03 PM Thread Starter
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Thanks, Stuart.
I will carry out your investigations when I can get to it. I left the bike at my dad's place today, which is around 7 miles from me.
A couple of other observations from earlier.

a) If I switch the lights on with engine switched off, the headlight still shines at full power on both dip and main beam.
b) When I switch the ignition on, with the lights switched off, the ammeter flicks left (negative), further than it did before the problem started. If I then switch on the lights, the ammeter hammers over to maximum negative, really hard.

The battery is a yellow Motobatt, around 7 months old.
Thanks very much for your input. Hopefully, I can get off work a little earlier in the next couple of days and do your tests.

On a positive note. . (see what I did there)?. . the bike won "Best British" at a fairly big show today, then it started misbehaving on the way home.



John
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post #6 of 15 (permalink) Old 07-22-2019, 05:23 AM
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Hi John,

Quote:
Originally Posted by suzyfan View Post
won "Best British" at a fairly big show


Quote:
Originally Posted by suzyfan View Post
a) If I switch the lights on with engine switched off, the headlight still shines at full power on both dip and main beam.
To be clear:-

. "engine switched off" is ignition switch 'off'?

. "headlight still shines at full power on both dip and main beam" is both filaments are on at the same time, or only one or other of the filaments is on, depending on the handlebar dipswitch setting?

. Both headlamp filaments on at the same time would be wrong normally and could be the reason for "If I then switch on the lights, the ammeter hammers over to maximum negative, really hard."

. Otoh, only one or other of the filaments 'on' depending on the handlebar dipswitch setting could be normal given certain wiring connections and switch settings:-

.. If, by "lights on with engine switched off", you mean the ignition key switch is 'off' but you can still turn on the lights with the toggle switch on top of the headlamp shell, that's normal if the wiring has been connected to the ignition switch following "Fig. H32. Wiring diagram all models from DU66246 (Home)" on page H31 of the workshop manual.

.. In this case, the Brown/White wire supplying Lighting Switch terminal 4 is connected to a 'double' spade terminal on the ignition switch and the Brown/White wire from the Ammeter is connected to the same 'double' spade terminal. As the Ammeter is connected to battery -ve by the Brown/Blue wire, simply switching on the Lighting Switch will turn on the lights.

.. However, the Lighting Switch should still have two 'on' positions - central should turn on just pilot and tail lamps, only fully to the right should turn on either headlamp main or dip depending on the handlebar dipswitch position (and turn off the pilot lamp).

Quote:
Originally Posted by suzyfan View Post
b) When I switch the ignition on, with the lights switched off, the ammeter flicks left (negative), further than it did before the problem started. If I then switch on the lights, the ammeter hammers over to maximum negative, really hard.
Does your bike's Ammeter have "8" or "12" at each end of the scale?

Risking telling you something you know already, the standard Ammeter wired normally is between battery -ve and the bike's 'consumers' - ignition coils and bulbs. So it shows the current being drawn from the battery at any given time - when you switch on just the ignition, the coils draw about 3-and-a-bit Amps, switching on just pilot and tail lamps draws about another Amp, switching on dip draws about another 3A (assuming standard BPF 40W dip), switching on main draws about another Amp (assuming standard BPF 50W main).

Finally here, following another recent thread, does the bike's dipswitch have the correct-colour wires (Blue, Blue/White, Blue/Red and Brown/Black) or different colours/combinations?

Quote:
Originally Posted by suzyfan View Post
The battery is a yellow Motobatt, around 7 months old.
Hopefully then it isn't the battery; unfortunately, from other posts, there doesn't seem to by any rhyme or reason why some batteries last for years but others of the same make crap out after only a short time.

Hth.

Regards,

Stuart
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post #7 of 15 (permalink) Old 07-23-2019, 05:02 PM Thread Starter
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Thanks, Stuart.
Yes lights are functioning correctly (one filament at a time) and the pilot and tail lights are still working.
All lights work with both engine and ignition off.
I've not had a chance to get back to it yet, but hopefully, tomorrow. . . :-)

John
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post #8 of 15 (permalink) Old 07-23-2019, 07:39 PM
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First, I would remove the headlight bulb and see if turning the headlight switch on still kills the RH cylinder. If it does, there may be short may be between the headlight "hot" node and another (non-ground) node that when forced to a high voltage it messes up the ignition for the RH cylinder. With everything off and the headlight bulb removed, I'd then check the resistance between the headlight "hot" node and anything related to the ignition system. Anything showing a low resistance would be a problem.
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Grant

Last edited by StuartMac; 07-24-2019 at 01:10 AM. Reason: PM sent
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post #9 of 15 (permalink) Old 07-23-2019, 10:06 PM
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"The charging system seems to be working fine" How do you know this?

"the battery is good" As above. What tests confirm this?

Just having feelings about things like this and seeming to be OK isn't the way to solve the problem.

In all likelihood I think you have small, if any problems, but until you can report voltages in all switch positions, with engine off and running at low and high rpm, its a guessing game.

The fact that you lose a cylinder when you put the lights on is very likely related to the lowered system voltage (especially as you say this is at tickover). This screams battery.
That it affects one cylinder (does the other cylinder kick in when you rev it?) may suggest that 1 set of points, 1 condenser, coil, lead, cap or plug on the side that drops out is "closer to the edge". This could be a developing failure of one of those components, but before you go on a costly goose chase, check:

1) Battery and all connections (12V+ at coils when lights on say)
2) System voltages at low and high rpm (what are they at tickkover with lights on and when revved?)
I'm guessing you'll find the solution there, but if not:
Assuming that's all ok, swap the LT wires between coils and put the plugs and plugs leads on the opposite sides, to see if the issue switches to the other side.

If the dropout cylinder switches, the culprit will be one of those components in that chain, and you can do individual swaps to identify the pesky bugger, or do as most do and just replace the points and condensers and it works most times.
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post #10 of 15 (permalink) Old 07-24-2019, 04:56 AM
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Hi Grant,

Quote:
Originally Posted by grant.edwards View Post
First, I would remove the headlight bulb and see if turning the headlight switch on still kills the RH cylinder. If it does, there may be short may be between the headlight "hot" node and another (non-ground) node that when forced to a high voltage it messes up the ignition for the RH cylinder. With everything off and the headlight bulb removed, I'd then check the resistance between the headlight "hot" node and anything related to the ignition system. Anything showing a low resistance would be a problem.
'Fraid you're confused.

The only component I can think of that introduces different Voltages ("high" and 'low') on bikes covered by this Forum is electronic ignition with multiple coils connected in series; John's bike is too early to have been fitted with one originally and he hasn't mentioned his bike's been fitted with an aftermarket e.i.

Absent an e.i., all "(non-ground) node[s]" are (should be) at the same Voltage when compared to battery +ve; if two (say headlamp main 'n' dip and right-hand coil "-") are different, the one showing below 'system' DC Volts has a fault between it and battery -ve, the fault causing additional unwanted resistance.

"low resistance" (zero Ohms on an analogue meter, below a nominal 0.1~0.2 Ohm on a digital meter) between "(non-ground) node[s]" (including battery -ve itself) is not "a problem", it's the contrary - measurable resistance between them - that'd be the problem.

Hth.

Regards,

Stuart
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