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DT400B Ignition Advice

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DT400B Ignition Advice was created by NickM

Ive got no spark so I'm following the checks in the Yamaha service manual downloaded from the library here. Can anybody confirm that the resistance of the CDI charge coil should be around 130 Ohms. The manual says 12.5 Ohms but I'm sure this is wrong.
My ignition coil checks out OK and the CDI pick up is good. The CDI charge coil is the black and brown wires from the magneto. I've checked for volts whilst turning the engine over and it seems fine. The coil i'm checking is in the photo.
Last edit: 06 Aug 2019 11:49 by NickM.
06 Aug 2019 11:48 #1

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Replied by DEET on topic DT400B Ignition Advice

Check across the pulsar.
White/red to white/green
It should be 90 to 100 ohms.
If it is open, then no spark
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06 Aug 2019 11:56 #2

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Replied by NickM on topic DT400B Ignition Advice

Thanks Deet,
My Pulsar (I called it the pick up coil) is good at 90 Ohms. It’s the charge coil I’m uncertain about.
06 Aug 2019 12:55 #3

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Replied by RT325 on topic DT400B Ignition Advice

Just going by 'my' rule of thumb i'd expect 350 ohms from a source coil. I'll get back later as have the readings somewhere [somewhere lol].
Ok, best i can do is "75 yz400b which will be the same full flywheel cdi mag--same colors--& you're right. Says brown to black 120 ohms. then white red to white green is 90 ohms. Do a voltage test kicking it over with plug out so it'll spin & wiring disconnected as you have it. AC volts. Not sure what to expect as i'm used to up towards 160 volts from the source on a peak meter so half that on an ordinary meter. & a few volts from the pulser coil. See how ya go. Bad cruddy connections are often the cause of much frustration & cost randomly replacing parts.
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Last edit: 06 Aug 2019 16:32 by RT325.
06 Aug 2019 16:19 #4

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Replied by NickM on topic DT400B Ignition Advice

Spot on with that info, many thanks RT325.
I suspected that the manual was wrong as a coil usually will fail open or short, not high resistance. I have checked the volts output which as you say, varies with the rpm and although we don't know what the figures are, I'm sure it's fine. All wiring checks out and connections have been cleaned or replaced. I guess I'm looking at another CDI unit£££ :(
07 Aug 2019 02:42 #5

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Replied by Gr8uncleal on topic DT400B Ignition Advice

Nick

I'd rather run the risk of offending you and possibly save you money, than not say anything and maybe see you go down a blind alley.

Just double checking that you know that the pulser is the black part with the Mitsubishi logo on it (again, really sorry if you knew this!!)? Photo attached.

It's just that these are well known for failing, whereas I'm not sure that the CDI units are.

Regards.

Alan

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07 Aug 2019 05:28 #6

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Replied by NickM on topic DT400B Ignition Advice

Any advice is a good thing and I'm certainly not offended Alan. I have checked mine and it's the right resistance and gives a small voltage pulse when measured. Thanks anyway.
Just finishing off thorough wiring checks before having to find a CDI unit. I'll let you know what the final outcome is.

Nick.
07 Aug 2019 07:10 #7

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Replied by Gr8uncleal on topic DT400B Ignition Advice

Thanks Nick.

It sounds like you're checking it already, but if not, it might be worth giving the engine kill switch (black and white wire, I think) the once over.

Alan
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07 Aug 2019 07:28 #8

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Replied by NickM on topic DT400B Ignition Advice

Well you're not far off the mark Alan! I had checked the kill switch and ignition switch wiring with a multimeter and they both tested OK (both will earth the coil to kill the ignition) BUT, my ignition switch was shorting under load so although it looked good when testing with a meter, it failed in use. I found this by turning the engine over with a cordless drill on the crankshaft with the HT lead secured a few mm away from the engine. I did this to check for dodgy connections and wiggled wires and connections whilst looking for a spark. I found that there was the odd spark which got better when the ignition switch was turned off and on. After excersising the switch and squirting some electrical cleaning solvent around, the spark was constant. Got some fuel in and rode around the garden!!
On the downside, it sounds like a bag of spanners (piston slap) so rebore and new piston needed.
Thanks to you Alan and Deet/RT325 for the pointers.
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07 Aug 2019 12:49 #9

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Replied by Gr8uncleal on topic DT400B Ignition Advice

Good(ish) news Nick, well deduced!

I don't know what the availability of oversize DT400 pistons and rings is like, but as it was a UK bike as well, hopefully not too bad. I got the one for my RT2 from Enduronut here on the forum.
07 Aug 2019 23:52 #10

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