No spark :(

[Hario]

I was on my way from london to bristol a couple days ago on my 400 superdream when, as i decelerated into a services, my engine cut out
Despite several hours of tinkering i couldnt get her started again because i was getting no spark on either plug, and ended up getting taken back to london in a van.

The RAC guy who looked me over at the scene was of the opinion it was probably my ignition coil, he used a spark tester and it showed a very weak spark on each cylinder (i can see no spark with a plug held against the engine). I had never trusted the cheap coil on there so i was inclined to agree and ordered a replacement, however it arrived this morning and hasn't solved my problem, nor has pouring over every connection and wire between the alternator and ign coil, i also took apart and cleaned the kill switch just in case, but no joy.

Nothing looks obviously wrong with the alternator, and my multimeter registers a good jumping voltage when i bridge the two wires from the CDI to the coil and run the starter motor. Also when i first broke down my battery seemed fully charged and all my lights and starter motor were working fine and continued to do so for some time as i tried get her started again, so i presume the alternator was charging the battery well. The battery is also quite new, about 2 months old.

So now im at a bit off a loss about how to get her running, i would presume that given that it cant be the ign coil, either the alternator or CDI are at fault, or failing that that its a wiring/earth issue. But as far as i can tell all of these things are fine, can anyone think of some testing i can do to try and figure out where the problem is?

If i could afford to i would just replace the CDI and alternator to be sure that all is well, but im out of work at the moment and really cant afford any costs that can be avoided.

Please help!

[Paxovasa]

Have you checked the HT leads and / or the earth?
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Suzuki GSF600 K3 (in the fastest colour, black).

[whitedevil]

start simple first, service the ht leads and clean the boots, and change the plugs.

[Hario]

the new coil came with new HT leads, and ive been testing with a working plug (one i removed at the bikes last service for no reason other than to be thorough). The boots look fine, and the problem is on both cylinders, so presumably if one were a bit dodgy the other would still work.

I think the coil is earthed ok, it completes a 12v circuit if i put the multimeter between the coil mounting posts and the positive battery terminal.

Im pretty new to debugging electrical problems so im not that experienced with a multimeter, is that the right way to check an earth?

A google search led me to a list of expected resistances for every alternator wire on this model, so i guess my next step will be to check those. Apperently the CDI's on this bike are pretty invincible so im leaning toward a problem at generator/pickup/alternator coil area now. Am i likley to need something like a slide hammer to get the alternator off or will a socket set suffice?

Either way it will need to wait until next week now, im going to make another attempt at reaching bristol tommorow, this time on my old 125, so if your on the M4 tommorow and see someone hanging for dear life onto a lorry radiator that will be me

[stinkwheel]

If it's a twin coil, one bad HT lead, plug cap or plug can make both go bad.

You can test the resistance between the two plug caps which will give you an idea if there is a problem in that area. They have an internal resistor and you can't tell by looking if it's gone bad. Don't know the expected reading though.

I presume you've replaced the plugs?

You can also check the resistance of the pickup coil.

If they're the same as a 125 superdream, there is a seperate coil in the alternator just for the ignition. The battery charging does not mean this is working properly. Again, you should be able to test this.

I had a weak spark on an old kawasaki once. I thought it was all earthing properly but out of frustration, I ran a big, thick wire from the coil earth to the battery negative and it suddenly clattered into life. Maybe worth a try to definatively rule this out.

Also consider the ignition and kill switch wiring.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Hario]

So i survived my ordeal on the motorway and now im puzzling over this problem again.

So far i have new spark plugs, ignition coil and HT leads. Ive also had a look at my generator setup which looks pretty crappy and old but returns the correct(ish) resistances, although the glue over the coiling of the stator is pretty chipped and there is a bit of rust on the magnets. I disconnected the wiring to the kill switch etc too.

This only really leaves the CDI and possibly the plug boots, i hear that CDI units can only be tested by putting an alternate one and seeing if it fixes the problem :/

As far as the boots are concerned i tested them for resistance and they gave back slightly different values 10.something verses 12.something is this possibly the problem? i cant find any guide values to test against anywhere.

Can anyone think of something im missing before i fork out any more dosh on this?


on a slight side note, i picked up a whole NOS generator setup, including the pickup and flywheel etc. on ebay thinking to replace my dodgy looking one but now it's here it turns out it slightly different to my one. Apparently its from a 400t but they had it up as a 400t/n setup so i figured they were the same.
my old one looks like this: https://www.cmsnl.com/products/acgen-flywheel_31100443005/
the replacement like this: https://images.dk-parts.co.uk/Item12096-2.jpg and https://images.dk-parts.co.uk/Item12096-3.jpg

the wiring etc. look the same and im sure it will fit, but is it likley to work do you think? its going to be next to impossible to find another NOS superdream generator if it doesnt :/

I really appreciate your help, im feeling serious motorbike withdrawal setting in at this point

[cb1rocket]

NOS? looks used to me.

On the other hand I can't see any problems with the swap. They look identical and as long as they fit and you can wire it then good.

Having experiance the same problem on a more modern bike, the pick up sensor can usually go duff while still showing a good reading. If you can just change the pick up sensor and test, then you might get away with doing the entire swap.

[Hario]

the actual origin of the flywheel is that it came from a cb400t in a honda training school of some kind where the bike was used to practice mechanical jobs, but where it was never ridden. so it was in a complete bike but has no miles on it.

as far as similarity is concerned the one on the bike at the moment doesnt have copper windings on each protusion of the stator as the replacement one does, also the flywheel is a bit different. And i dont really want to pull the pick up off as it means cutting the wires and that rules out sending the lot back if its not a useful swap.

i think i will email the seller and ask them what their source on it working in a cb400n engine is.

[Gazdaman]

Have we ruled out kill switches? Kill switch, ignition switch, side stand switch and clutch switch if fitted?

They would cut the spark without any other symptoms. Especially if it was a short somewhere not particularly obvious. Worth half an hour with the multimeter.

Gaz

[Hario]

Apparently isolating the kill switch from the circuit is a matter of unplugging a single bullet connector by the CDI which ive tried and made no difference, i also dismantled the switch itself and tried then, again no joy. there is no kickstand or clutch switch. As stinkwheel suggested i will try a length of wire from the CDI and coil to the neg battery post tommorow to eliminate the chance of it being an earth, but testing with the multimeter the earths do seem ok.

at this rate i will soon have replaced the whole ignition system :/

[stinkwheel]

10 and 12 kiloohms seems on the high side for a plug cap.

They are usually 3, 5 or 8K. If it's an NGK one, it'll usually have the resistor value stamped into the plastic moulding somewhere.

Pretty sure a superdream should have a 5K plug caps so yours are reading double what they should be. Not that this is definately the problem but it warrants a closer look.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Hario]

well i have new plug boots coming so i can hopefully eliminate those in the next few days, i also found an old but working CDI on ebay so i can try that when it arrives too.

Which just leaves me with my alternator dillema, not sure if i should send this one back or try it out
I dont really know enough about them to make a decision on wheter slightly different models will be interchangable or not, my old one could just be an updated but essentially same device.

[Hario]

Just for some closure i finally got her running!

Turns out it was the alternator, although im not sure which part as all the resistances checked out, oh well.

Also it was the older model alternator that got her started in the end, i guess they didnt change the timing or output, having said that since she started she is running a tad hot, and is a bit more clattery than usual so im concerned the timing may be off, but it will take more looking into to be sure (i dont have a timing light).

Thanks very much to everyone who contributed in this thread, you have been very helpful!