1978 YAMAHA RS125DX no spark (Please Read)

Hi everyone,

I'm completely new to forum posting but I have read many whilst restoring my 1978 Yamaha.

Up until yesterday the bike ran and had no trouble at all other than it ran a bit sluggish. I decided to clean out the carburettor, air filter and fuel tank to sort the issue. I noticed when taking the bike apart that a wire had come loose from a 2.4kOhm resistor under the handlebars which must have been disconnected months so I reattached this. Once I had completely cleaned everything and reassembled the bike I put some fuel in and started it up. It fired up instantly and was sounding much better once it had warmed up. All of a sudden the bike started revving quite highly on it's own and I couldn't stop it. In a panic I turned off the ignition which didn't seem to work and then I disconnected the ignition coil (see picture) as I was worried I could cause engine seizing problems. The bike cut out after 3-5 seconds (I thought with the coil disconnected it would cut straight out). Now I can't get a spark at all and I've no idea what the issue could be. I'm no electrician but using the haynes manual I have ruled out the possibility of it being a faulty coil, plugs or plug caps. I've taken the LHS engine cover off and there's no spark between the points either when the kickstart is operated. If anyone has any help or information I would hugely appreciate it! I'm guessing that the sudden revving could be to do with the carb but will I have caused any damage to the electrics...?

Thanks,

J

Hello and welcome. Sorry can't help with bike though

Hi Mike not to worry

For anyone who may have the answer i've spent another couple of hours on the bike today...

The neutral light and indicators all still work like normal. Using a homemade tool and flywheel puller I could take the generator flywheel off. I've changed the ignition points and inspected inside, the only thing I didn't know how to check was the Condenser, would this cause no spark if faulty?

Please see attached images for reference!

Thanks in advance!

J

Yes it could, if shorted to earth.

Thanks for the reply megawatt!

How would I test to see if it's at fault? There's 3 wires going in to it... 2 to each of the coils and the third to the points.

I've traced all the wires back and there's no breaks or blown fuses anywhere. Could me disconnecting the coil while the bike was running have caused this?

Thanks,

J

Hi Jacek - Just a wild shot in the dark, but do you have a manual with a wiring diagram for this bike? You've probably just dislodged a wire (or two) when you grabbed a handful of coil. What does that resistor do? Can't see how it would cause a runaway. Condenser just goes across the points (+ to gnd) to reduce sparking which will cause the points to erode rapidly. Cheap enough to replace.

Addition: test the condenser to see if it's gone closed circuit. It should not pass 12v DC. You can test this with a mulitimeter or even a bulb on a flying lead. 2 leads to coils + 1 to gnd means it is probably 2 condensers in one housing. Check your Haynes. If either is showing closed circuit, then that could be your problem. As I've said before, cheap enough to replace. ....

Hi Speedy23,

The haynes manual I have covers the bike however there's no exact wiring diagram for it, only similar but all of the wires are different colour etc so it is really difficult. I'm going to replace the condenser this weekend I think. I will let you know how I get on, Cheers!