Newbie with a 1975 T500 random spark issue

[Fearthepack]

Help. Right cylinder random spark at plug. Points set same. Plugs new and second set new no difference. Swapped In seconded set of coils no change. While bike is running both points show tiny spark. What am I missing?

[tz375]

All sorts of possibilities.

Weak battery
Poor connections or witches leading to voltage drop
Dirty points
Bad ground at points or motor to frame
Condensors going bad
Break in a wire

[Fearthepack]

Battery eliminated. But all other is a possibility. Hard to chase electrical with a battery eliminator in the system.

[jabcb]

How long has the battery eliminator been installed? Did you have it running fine some time ago with the battery eliminator?

Try it with a properly charged battery to see if that helps.

Some of the twins (T250, T350, T500) run just fine without the battery while others won't.

One of my twins is difficult to start unless the battery has a good charge. Also runs just fine when the battery has a good charge.

At the other extreme is my 71 T350.
Last summer I checked the timing & did some other maintenance.
Forgot to reconnect the battery.
It started easily & ran good. Realized what I had done when I saw the neutral light burn out.
(The twins use the battery as a voltage regulator.)

[Fearthepack]

Bought the bike with it installed it looks prehistoric. The bike only ran on left cylinder when purchased. When I got it home I reset the points gap and both fire right up thought I had problem fixed. But right only fires random. PO said points are new. What should the compression be?

[jabcb]

Does the random spark issue occur at all rpm?

Check the voltage at idle & revving.

[Fearthepack]

At idle random, but increase rpm spark disappears altogether but that could be fuel overwhelming what spark was available. Will check voltage Monday afternoon to see if it drops as rpms increase. By random it may spark five revolutions in a row skip 3 fire 1 skip 10 fire 2 skip 3 fire 1 skip 8 ect

[CBWELLS]

Intermittent problems are the worst! Assuming it's only ONE cylinder, & since new plugs & a second set of ignition coils didn't help your problem, I would look at the insulators where the wires attach to the points, & possible contamination in the point area. A careful disassembly, clean & reassembly might help, and a good cleaning with aerosol contact cleaner thru-out the whole point area wouldn't hurt anything. Another possiblilty is the spark plug caps (although these were likley changed with the coils). Sounds unlikely, but I have had spider-web like fractures in plug caps allow arcing to the surrounding fins (you might see this in the dark), & I've also had plug caps actually go OPEN circuit (That was really weird!).

That's the cheap & easy stuff at both ends of the circuit. In between are the wires that go from the points to the ignition coils or even the wires that feed power to the points. Possible OPEN connections due to age & contamination.... or possible poor crimps at wire/contact connections. Worn insulation or pinched wires where the wires pass thru the engine shell, or at clamp areas. Thorough inspection of these might reveal something.

Good luck & let us know what your find. Good education for us all!

[Fearthepack]

Thanks CBwells. It is just the right side random firing. Think I will chase wires from points up to coils. I get good steady arcing at both points. So it seems to me it breaks down after the points and before the plug. Caps look great I did cut off a 1/2 inch and reinstall the plug cap no difference.

[jabcb]

For a quick check, switch spark plug leads & run temporary wires to switch the points/condensers.

Will help narrow things down if the spark issue switches to the left cylinder or stays on the right.

[Fearthepack]

Jabcb, I like your way of thinking will try tonight.

[Alan H]

Thanks CBwells. It is just the right side random firing. Think I will chase wires from points up to coils. I get good steady arcing at both points. So it seems to me it breaks down after the points and before the plug. Caps look great I did cut off a 1/2 inch and reinstall the plug cap no difference.

You shouldn't get much sparking at the points at all. If you do, the condenser is possibly faulty.
A dud condenser will give a weak spark too as most of the energy that should be building up a high voltage spark is going across the points. I've known NOS condensers be faulty as they have dried out. They should have insulating oil inside and if this has managed to escape over the last 40 years or so, it'll be knackered.

[Fearthepack]

Ok I can swap out the condenser and see if anything changes. You may have hit it because left side spark is almost not visible.

[Fearthepack]

Alan H. you nailed it. I swapped out the condenser and bingo solved the random spark issue. Now as a teachable moment was the excessive spark at the right side points the giveaway or something else in the tread?

[Alan H]

I would say that the capacitor failed and started to spark at the points. As well as building up a charge so that the HT spark is bigger, it also inhibits the sparking across the points caused by ANY DC voltage circuit which 'breaks'.
That's why contacts are always rated higher on AC than DC - less contact arcing when the circuit decays due to a switch breaking.
PLEASE do not file or try to smooth the 'tit' off points if there is one due to arcing.
If they are anything other than flat faces, fit new or they just burn away faster as the harder facing is worn away..
Capacitors fail eventually, it was the time for yours. When will this happen? $64000 question!
Might be an idea to change both, as they were possibly twins.
One dies, the other may be sick if ou know what I mean.