Trials Australia Forums

I'm helping a mate out with a TY175B that he's rebuilding.
He got it in parts and has reassembled everything.
He is not very experienced mechanically, and my last TY was a 175C when it was just a few years old.

On first trying to get it going he decided that the coil was faulty and bought a replacement out of Queensland off a known running bike apparently.

We've just spent some hours going through everything as well as we can:
We pulled the flywheel and checked everything in there, then timed it - probably ended up at more like 20-21mm BTDC when the light brightened, but figured that should be close enough to at least cough.
Removed carby, stripped and reset float (21mm off body of carby), removed and blew out all jets and passages. It's had a new needle and seat installed.
Eyeballed the reed petals while they were mounted on the engine - no obvious signs of damage etc.
Disconnected the kill switch just to be sure.
Cleaned the coil contact area and checked coil had good ground through to engine.
New spade connectors between the stator plug and coil.
Checked spark - it sparks, not a clean blue but more a bright yellow so I'm not sure that all's well with the ignition?
Spark plug gets wet after kicking it a few times.
Put an old, cheap and nasty timing light on it - it seemed a bit erratic getting the light firing - not sure if that's the light or the ignition though. Seemed like it lit up once on the first pass of the flywheel most tries, but maybe not after that.
It's got compression, but I really don't know how good it is - a long time since I've kicked one of these over! I don't have a compression gauge.

Nothing. Choke on/off, no/full throttle ...
Tried roll starting it - nothing.
Tried roll starting while squirting aero-start into the carby throat - nothing.

I've suggested he take it to an auto electrician to get the ignition circuits checked, thinking that weak spark might be to do with a dodgy condensor?
I've seen it suggested the spark cap could be at fault - we didn't have a spare to swap with.

Just wondering what other suggestions are out there?

The replacement coil you fitted - I figure you mean HT coil.
Condenser failure is a very common thing on old bikes.
20-21mm BTDC on the flywheel rim should be fine.
Is the timing points gap reasonable like 0.3 to 0.5mm?
With that sort of ignition don't use a resistor spark plug or a resistor spark plug cap.
You may have a resistor sparkplug cap on there. Some are. The little brown caps are not resistor and the big black caps are. There is no law against taking the cap off and stripping the HT wire back a bit and winding it directly onto the plug for a test if you suspect the cap.
With that sort of ignition set the plug gap to 0.5mm.
TY175 stator coils that are old usually suffer from degradation of the insulation and also from hardening and breakage of any wires that are not held down well. It used to be common to rewind ignition stators but nowadays complete electronic ignitions are usually fitted and may end up cheaper that paying an auto electrician to diagnose and fix the problems.
You can buy a new aftermarket (replica) HT coil for not much $$$ from most motorbike shops or people on ebay. Much cheaper than NOS Yamaha TY coils. Last one I bought was a TY replica coil from my local Yamaha shop and I used it on a Bultaco for years. I think it was about $25.
Most frequent cause nowadays of weak spark on TYs still with the original condenser is failed condenser.

Thanks David,

What's the best solution for a replacement condenser?
The owner told me he couldn't find a source for new ones. I suggested he just go to an auto elec with the spec and then figure a way to mount it in there once he's got something of the correct value.

The plug cap is the big black one, so that sounds worth trying without it.

And yes, it was a replacement HT coil - I didn't take a close look at it so not sure if it's a NOS or aftermarket job.

Yes I buy them from auto electrical shops. Last one I bought was made for a 1960s mercedes car. The capacity of a capacitor is chosen to suit the diameter of and the gap between the ignition points and you'll find that points are all pretty much the same size and run pretty much the same gap so pretty much all automotive ignition capacitors for points ignition will work OK on a TY.
The hard bit can be finding one with a long enough wire and a nice mounting lug. You probably won't find one the right shape to fit on the stator plate and most people mount them externally, up near the HT coil.
I think the original Yamaha HT coil is a big name brand like Hitachi or Nippon Denso.

Thank you once again David.

I'll pass all that on and see where it leads.

Coincidentally another mate told me yesterday he has bought a TY250B and has started down the rebuild road, I'll pass the same info to him also.

This reminds of my first TY rebuild... When assembling the engine, I thought it would be good to cram the main bearings with grease to aid running in. Of course it didn't start, or even try, I checked everything over and over.
Eventually i tipped a spoonful of fuel into the spark plug hole and it fired. Repeated this a few times and away it went