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Discussion Starter · #1 ·
Hi All,

I’ve got a UK -spec CB400t that was working fine until I jet-washed it 6 months ago. Never re-started. Been fiddling since then replacing CDI and ignition coil, plus plugs and plug leads.
finally got multimeter on and followed Honda maintenance manual, re resistances. Results:
Green-Pink 130ohm (allowed 120-180)
Green-Brown - 88ohm (allowed 75-130)
Green -Light Blue 90ohm (allowed 75-130)
White-Blue 80ohm (allowed 75-130)
Green-white open circuit (allowed 400-500).
Obviously green-white is problem - any suggestions? This seems to be a weak link from the coil white wire to ground( green)?
I haven’t taken the rotor off to check windings, but circuit diagram shows white-blue a separate winding, but no indication how the white gets to a weak ground?
Help!
Do I replace whole core and windings or is there a work-around.
cheers
 

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This is a perennial problem due to age and a poor design.

400 system is different to 250.

You have a dmm, good, but get a pair of the red neon plug caps that flash when HT passes thru, but you will have to make an adapter so they will fit/connect. A chore well worth doing as the flashes can be seen ( more easily at night or in shade ) whilst riding the bike.

No flash, no HT, misfire.

A strobe is also good to check ign advance.

A scope would be brilliant but dmm will suffice.

New HT coil with non detachable leads, but research the GM car coil modification.

You are aware that the cam cover is insulated ?

Using two old serviceable plugs welded ( drill holes and tap if you wish ) to a suitable metal strip, which has a flying earth lead and crochet clip to earth is also a good bit of kit t for these and other applications. Much safer for the fragile ign system and less hassle/more reliable HT test.

New CDI unit, the proper one ?

I would have to look up stator resistances, but the stator green wire is a dedicated earth to the stator.

I assume the starter motor whizzes the engine over well ?

As you know, the battery is completely independent of the ign system

Your bike has a kick start also ?

Resistance check cold and hot, record the values.

Dmm AC volts output check, increase in revs, increase in ac volts.

But your will not start.

Disconnect the cdi black/white wire and try again with good clean plugs and fuel. Black/white wire earthed Via ign switch and handle bar kill switch to kill ignition.

Remove oil filter bolt and take it to a nut/bolt supplier, flywheel extracter bolt same thread, the size and pitch of which I cannot remember, 20 X 1.25 ????

You need the extracter bolt to get to the stator...

Old insulation, flywheel area water drain that doesn't and stater windings covered in epoxy that cracks due to age and heat, taking out windings, make and break and brake and make connections.

The dedicated green earth may also be a bad connection.

Try cdi kill wire disconnect first and get a flywheel extractor bolt, you are going to need one.

Over....
 

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Discussion Starter · #3 ·
Hi Drydreamer,

many thanks for your detailed reply. If you don’t mind my long reply, I re-attach your post and answer one-by-one:


400 system is different to 250.
So I have discovered! I was looking on Dubstep for replacement statues and pickups, but no luck for CB/CM400 twin….and 250 looks simpler….

You have a dmm, good, but get a pair of the red neon plug caps that flash when HT passes thru, but you will have to make an adapter so they will fit/connect. A chore well worth doing as the flashes can be seen ( more easily at night or in shade ) whilst riding the bike.

No flash, no HT, misfire.
I will buy and fit - tho current setup no spark, even weak one …even in total darkness - so it looks like no HT.

A strobe is also good to check ign advance.
Will buy….

A scope would be brilliant but dmm will suffice.
I have numerous analog and digital Tectronix scopes - what would you advise hooking up to, with ground on Green, presumably….?

New HT coil with non detachable leads, but research the GM car coil modification.
No ac voltage at coil.

You are aware that the cam cover is insulated ?
I earth against bottom of block ( shows 0ohm continuity to green wire on stator plug….so a good ground….at least at the green wire.

Using two old serviceable plugs welded ( drill holes and tap if you wish ) to a suitable metal strip, which has a flying earth lead and crochet clip to earth is also a good bit of kit t for these and other applications. Much safer for the fragile ign system and less hassle/more reliable HT test.

New CDI unit, the proper one ?Not new eBay second hand, so could be faulty……but my stator wire resistance checks are all done with plugs disconnected, so ‘open’ connection to CDI, so shouldn’t matter for the suggested maintenance book resistance- checks.

I would have to look up stator resistances, but the stator green wire is a dedicated earth to the stator.
Yes….but it measures 0ohms to chassis, so must be also a common to bike ground.

I assume the starter motor whizzes the engine over well ?
Starter fires well, alternator spins freely, all clean and no inhibitions between brass sender and the pulset pickup ( gap consistent).

As you know, the battery is completely independent of the ign system

Your bike has a kick start also ?
Yes - but new battery and starter/ alternator spin as they should….

Resistance check cold and hot, record the values.
Other than heating outside of alternator with a heat-gun, engine never fires, so won’t get warm. Resistance measured at 17deg C ambient.

Dmm AC volts output check, increase in revs, increase in ac volts.
Max ‘revs’ is the speed of the start motor, I’m afraid - engine doesn’t fire.

But your will not start.
Nope!

Disconnect the cdi black/white wire and try again with good clean plugs and fuel. Black/white wire earthed Via ign switch and handle bar kill switch to kill ignition.
Black/white cable disconnected…..
I’ve tried connecting all connectors, but leaving blue and white wires to chi disconnected and measured ax voltage across the blue-white connectors.
nothing.

Remove oil filter bolt and take it to a nut/bolt supplier, flywheel extracter bolt same thread, the size and pitch of which I cannot remember, 20 X 1.25 ????
Yes - if you’re asking me to remove flywheel cover - I’ll do that.

You need the extracter bolt to get to the stator...
Ok.

Old insulation, flywheel area water drain that doesn't and stater windings covered in epoxy that cracks due to age and heat, taking out windings, make and break and brake and make connections.
Will do, though all the stator winding resistances check out …..apart from white to green ( no 400 ohms).

The dedicated green earth may also be a bad connection.
Green earth buzzes freely at 0ohms to angina block, chassis and to the dedicated ‘free’ terminal below rectifier.

Try cdi kill wire disconnect first and get a flywheel extractor bolt, you are going to need one.
I will extract the flywheel asap ( Saturday) I guess.
I would still love to hear if you know how the white wire connects at 400ohm to Green!

Over....
… but not ‘OUT’ , please - you seem a really great font of knowledge - much appreciated!!!!

cheers,
Manny
 

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The dedicated green wire goes to the stator metalwirk, eyelet terminal from memory. The stator metalwork is screwed via 3 screws to the engine. So a double earth in effect. I replace the screws with bolts for hassle freeness.

You have some scopes, really handy.

Yellow wires from stator are 3 phase to rectifier/regulated for battery charging.

All the other wires are for ignition.

You have a rex rewound stater, a new Rex cdi and a new HT coil.

HT coil from eBay ?

Resistance checks on HT coil which must have a good earth to frame, even though it has a green earth wire.

Recheck all earth's but you say starter motor does spin engine over well.

Plugs in, perform AC volts test on all stator ignition wires, with them connected.

Tabulate your results.

Plugs out, repeat test and tabulate.

Examine wave forms for "irregularities".

2 channel scope, view yellow wire to HT coil....

You have 2 scopes, so can view 4 channels.

You can also view HT coil output but would need to make "potential divider" to bring down voltage to scope, or make a sensor coil for HT monitoring.

So, plugs in, ac v relatively low, plugs out, outputs increased...

Check HT coil resistances...

eBay item, are the HT leads "wobbly" in the HT coil ?

Think about the GM conversion.

Raining again here today so internet may/will be problematic, was dead all day yesterday but came on line at about 6pm.

AC v outputs and pictures please...
 

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Discussion Starter · #7 ·
Hi again,
Thanks so much. Will do on Saturday and send across.
ive looked at replacement new assemblies from DSS…..ouch…..€1100 !!!
I’m sure Problem lies with loss of 400ohm contact of blue-white winding to stator winding…. All will become clear when I pull alternator cover off again!
Btw- the two pulser retaining cross-head screws are seized tight…..looking for impact driver to remove…
Cheers,
Manny
 

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Get the proper flywheel extractor...

Do not as in DO NOT use a 3 leg puller. !!!

Remove the oil filter bolt and take to a nut and bolt supplier, or, search on eBay, machine mart or Toolstation and similar, for a longish bolt the same diameter and pitch, which screws into the flywheel and will extract it with no damage.

Is it you who has the scopes, what did they show, rise in revs, rise in stator wires AC voltage output
 
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