Coils testing and replacing.

[Revvy Kevvy]

My fun with this little bike continues, its really hard to start and its idles like crap so I bought myself a spark tester and the spark is only really there at 1.3mm, in the Yamaha manual I see it should be there at 6mm so I guessed a problem with the coils.

I measured the coils and on the HV side I get 22M ohms and according to the manual it should be 9.5K ohms. Left side is 9.5K ohms, Right side is 22M ohms.

I tried manufacturing my own tester and tried the good coil and the bad coil on it but I cannot make a spark in the sparkplug. I dont really understand why as the circuit is fairly simple and when I press my contacts it should make a spark but its not. Not even on the good coil, did anybody ever make such a tester and did it work for you? Its just a make and break between 12VDC and Ground so by doing that I should get a spark...I get absolutely nothing. In the Yamaha manual it shows a tester....did anybody make one or got one? How does it work?

Now for the coils themselves, many such coils exist has anybody ever tried to just find a set of coils and put them on? Because the spark is 180deg out of phase they have to be seperate coils, I guess dwell angle might affect heating a bit and physical size/fitting might also but is there any issue with just buying some 12VDC coils from any other motorcycle brand and wiring them up?? I found some cheap for a chinese motorcycle brand....can I not use them? or is there some reason why I need yamaha ones?

I ask because one day I connected two fax machines together and tried to get them to talk and I could not, I learnt it needs a exchange between them and maybe I need to learn something about ignition coils too?

Lastly, you can see in one picture I tried to connect a timing light, that dont want to flash either even set on sensitive setting. Works fine on my car but on the bike nothing. Has anybody ever set up a timing light on these machines....did it work??

At the end of this I will be some sort of XS400 expert, winter is comming and I would like to at least get the engine running before I have to put the bike back in the shed.

 

[CaptChrome]

Well, I can give feedback on two items. Regarding replacement coil selection, I would find a substitute coil with a very similar primary resistance to the one on your bike. There are differences. For example, the coils on my TCI ignition bike use coils with 3 ohm primaries, so I could not use the 4 ohm primary coils that are found on breaker point ignition XS400s.

Regarding using a timing light on these bikes, I use one on mine and it works. If it doesn't work, I think it is telling you that you have a problem with good spark from the secondary lead (but I think you already know that.)

It sounds like you are troubleshooting in the proper direction. The coils, leads and plug boots all need careful inspection as you are doing. One other bit of info that might help out is the use of resistor plugs and resistor caps. Some sort of resistance is put into the secondary ignition wiring to reduce radio frequency interference. This is either done by using resistor spark plugs or by using plug caps/boots with resistance built in. If you are using spark plugs with resistance, make sure you have non resistor caps and vice versa. Running resistor caps on resistor plugs can cause diminished spark.

 

[Revvy Kevvy]

Captain Chrome!!

I am a bit lucky in that I have some business trips in china ahead so I went on their version of ali baba and ordered a few different coils for bikes that have points systems. I also found a RD125 coil but I have no idea if 2stroke coils are the same, I seem to recall a wasted spark concept...I had a RD400 a very long time ago.

My ignition coils are 4 ohm because of the points system, I will leave this bike with a points system as I feel its reliable and easy to fix on the side of the road.

Thank you so much for the idea of the resistors in the sparking plugs too...I did not consider that. I think there are resistors in the caps I bought but it never occured to me that a resistor might be in the bloody sparking plug too...... thank you for the heads up.

I have bought a inductive/capicitance pick up for the ignition lead and also a coil tester/ignition tester that I will try connecting to my oscilloscope when I get all these new coils to try and the coil testing device...

Thnak you also for confirming that a timing light will operate on these machines, seems that the spark is too weak to trigger the light so it kinda double confirms that I have a sparking issue. I see in some forums that oil is supposed to come out of the unit where the TDC mark is when you run the engine with the cover off....is it a lot of oil?

When I bought this bike out of the barn the lady said to me...a new battery and it should be quite ok to run again....glad I paid only 800 euro for it!!

....it never occured me to check for resistance in the sparking plugs....damn!!

Well, I can give feedback on two items. Regarding replacement coil selection, I would find a substitute coil with a very similar primary resistance to the one on your bike. There are differences. For example, the coils on my TCI ignition bike use coils with 3 ohm primaries, so I could not use the 4 ohm primary coils that are found on breaker point ignition XS400s.

Regarding using a timing light on these bikes, I use one on mine and it works. If it doesn't work, I think it is telling you that you have a problem with good spark from the secondary lead (but I think you already know that.)

It sounds like you are troubleshooting in the proper direction. The coils, leads and plug boots all need careful inspection as you are doing. One other bit of info that might help out is the use of resistor plugs and resistor caps. Some sort of resistance is put into the secondary ignition wiring to reduce radio frequency interference. This is either done by using resistor spark plugs or by using plug caps/boots with resistance built in. If you are using spark plugs with resistance, make sure you have non resistor caps and vice versa. Running resistor caps on resistor plugs can cause diminished spark.

 

[CaptChrome]

The bikes came from the factory with NGK plugs, BP6ES or BP7ES. These were non-resistor plugs, so the plug caps were resistor caps. NGK adds an "R" in the plug nomenclature if the plug is a resistor plug. So BPR6ES and BPR7ES are the resistor equivalents to the original plugs. The Iridium plugs from NGK are all resistor plugs AFAIK (I use the NGK BPR7EIX). So, if you are using resistor plugs, you need to replace the caps with non-resistor caps like the NGK model LZFH caps.

 

[Revvy Kevvy]

The bikes came from the factory with NGK plugs, BP6ES or BP7ES. These were non-resistor plugs, so the plug caps were resistor caps. NGK adds an "R" in the plug nomenclature if the plug is a resistor plug. So BPR6ES and BPR7ES are the resistor equivalents to the original plugs. The Iridium plugs from NGK are all resistor plugs AFAIK (I use the NGK BPR7EIX). So, if you are using resistor plugs, you need to replace the caps with non-resistor caps like the NGK model LZFH caps.

Yes, I measured them both and the spark plug is 6K ohms and the cap is 5K ohms. I opened up the cap and found inside it the small resistor so I took it out and cut a piece of threaded rod the same to replace it. Now I have a spark of 9mm on the new coil (chinese one) and 6.5mm on the old coil.

I also did the dynamic timing and discovered no oil comes out until you put the bike on its side stand. My left cylinder I can put to LF during the dynamic tune but the right one I cannot. Its close to RT and I cannot more it back further. According to the manual that means I need a new points set.

I put the little chinese coil and its performance is good but during running it gets really hot...I think it cannot handle the lead or maybe the dwell angle is too great. I will get new points, set them up and retry it. If its still too hot I will get a Yamaha version.

 

[Revvy Kevvy]

Bought this engine tester and it has a ignition coil tester so plugged in the coil and set it to a dwell angle of about 20 msec and away the sparks came. Nice, clear blue sparks. Put the old coil on and no spark. Decided to experiment with a 555 timer IC into a 12 V coil and found that also works to mimic the points.

I have ordered a new set of contact points and am going to try and set the timing up using my nice timing light, I guess the coil runs hot because the dwell angle is too much on the right side.