My 1978 TY175 was last started several weeks ago and ran like a dream.

Now it won't start - there's no sign of a spark at all.

I'm no electrical expert but so far:

I'm seeing about 3.5 volts AC on the black/white wire to the Low Tension side of the ignition coil.  For this I'm turning the engine over with a drill using the 19mm nut that holds the flywheel/magnets on, with the plug out so that I'm not fighting the compression.

I'm using a digital volt/ohm meter.

So I'm assuming the source coil etc side of things is okay? The ignition "source" ohms are as per the book.

I've tried my spare ignition coil (can't remember where I got it, so it could be duff) and two spark plugs.

Both ignition coils show several thousand ohms on one side but open circuit on the other, which seems a bit odd.

Could both be duff?

I've taken all the connectors apart and reconnnected them, and bypassed the kill switch. 

One other thing that may be a clue (or unrelated!!), the last time I tried the headlight it was really dim, even though the bike ran fine.

The electrics appear to be the standard (super-simple) set up so I'm confused I can't work out what's wrong - how hard can it be !!?? :-)

 

A coil should have cont. Primary side to earth.

Should be lots more than 3.5v. Hard to say what's faulty. Perhaps the condenser is faulty

HT coil primary 4.5 ohm

HT coil secondary 6000 ohm

Open circuit on primary means its no good.

Edited October 9, 20169 yr by andy.t

Might be best if I replace both the condensor and the coil as they're unknowns.

What needs to come off to replace the condensor?

 Usually the flywheel, unless it has been relocated. Condensers are made to die. Coils usually not. New points never hurt. P and C was the drill back in the day.

Sorry I should have worded that better - the manuals aren't clear on this - do the source coils+anything else have to come off?

It's in the standard position and I'm happy to solder.

Sorry I should have worded that better - the manuals aren't clear on this - do the source coils+anything else have to come off?

It's in the standard position and I'm happy to solder.

not normally

you should be able to change the condenser on a Ty175 without taking any stator coils off or taking the stator backing plate off the bike

 

When you spun the motor with the drill and got 3.5V AC, did you have a sparkplug connected between HT and earth, and what RPM did you spin it at?

 

You used a term for what you did to the killswitch that I thought I had better ask about. Did you bypass it or did you disconnect it?

 

For what it is worth the most common loss-of-spark issues with TY175s are caused by (in no particular order)

Worn main bearings

Not enough points gap

Dirty or wet or stuck or worn points

Failed condenser

Broken or cracked points or condenser wire

Broken or cracked LT wiring

Sheared flywheel timing positioning key

Failed stator winding insulation (the last few inches) causing a leak to earth

 

Less common:

Loose stator coil mounting screws

Magnets rubbing on coil laminations

Repaint of frame causing poor earths of motor casings or HT coil

Pinched or rubbed through insulation on killswitch or LT wiring

Faulty HT plug cap

 

Good news - it's running again :-)

I thought it must be the condenser or ignition coil so changed those but still no joy. Much head scratching.

So I went back to basics, checking everything.

I used my old 1970's Avo meter this time, rather than my digital multimeter (young people may need to google Model 40 Mark 2 Avo meter at this stage!) and everything checked out except the resistance across the points was a bit odd and inconsistent, which wasn't really visible with the digital meter.

So I gave the points a clean and hey presto, ring, ting, ting  :-) :-)

 

Thanks for all the advice everyone - it made me re-think my strategy - and Feetupfun I will print off your list of common issues and put it inside my Haynes manual for next time.

Edited October 29, 20169 yr by dpyam