1978 YAMAHA RS125DX no spark (Please Read)

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JackMiszewski
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1978 YAMAHA RS125DX no spark (Please Read)

#1 Post by JackMiszewski » Wed Aug 15, 2018 1:24 pm

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

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Last edited by JackMiszewski on Wed Aug 15, 2018 7:59 pm, edited 1 time in total.

mikestrivens
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Re: 1978 YAMAHA RS125DX no spark (Please Read)

#2 Post by mikestrivens » Wed Aug 15, 2018 6:37 pm

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

Bikes:
Triumph Tiger 800 XCx, 2015,
used to have Honda VFR800 & Harley Fatboy
Cars:
Mercedes SL500 R129 V8 (1999); Mercedes E320 W124 S6 (1995); Mercedes S600 W140 V12 (1995).

JackMiszewski
Posts: 4
Joined: Wed Aug 15, 2018 1:00 pm
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Bike(s): Yamaha RS125DX 1978, Honda CBR500r 2013
Location: Barnsley
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Re: 1978 YAMAHA RS125DX no spark (Please Read)

#3 Post by JackMiszewski » Wed Aug 15, 2018 6:56 pm

Hi Mike not to worry :thumb:
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

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megawatt
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Re: 1978 YAMAHA RS125DX no spark (Please Read)

#4 Post by megawatt » Wed Aug 15, 2018 7:29 pm

Yes it could, if shorted to earth.
Bike work, I come to you and you can learn to do it yourself click the link below for more info
http://themotorbikeforum.co.uk/viewtopi ... 5&p=949652

JackMiszewski
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Bike(s): Yamaha RS125DX 1978, Honda CBR500r 2013
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Re: 1978 YAMAHA RS125DX no spark (Please Read)

#5 Post by JackMiszewski » Wed Aug 15, 2018 7:49 pm

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? :scratch:
Thanks,
J

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Speedy23
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Re: 1978 YAMAHA RS125DX no spark (Please Read)

#6 Post by Speedy23 » Thu Aug 16, 2018 8:03 pm

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. ....
Last edited by Speedy23 on Fri Aug 17, 2018 6:25 pm, edited 3 times in total.
Oh my God, what's happened? I don't know man...I just got here myself!

JackMiszewski
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Bike(s): Yamaha RS125DX 1978, Honda CBR500r 2013
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Re: 1978 YAMAHA RS125DX no spark (Please Read)

#7 Post by JackMiszewski » Fri Aug 17, 2018 8:39 am

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! :roll: