Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: Gary400/4 on October 07, 2019, 11:46:44 AM
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Hello all, I have been toiling away over a couple of years in my shed resurrecting a forlorn cb400/4. The problem I now have is trying to get it to run. Engine is rebuilt, good compression engine turns over on the starter motor. I have 11.7 V measured at my ignition points terminal. This means current has flowed through my primary windings in my coils then out on to my yellow and blue wires. I can manually open the points to obtain a spark across them. The points are timed to start to open at the F1-4 and F2-3 marks. I have fitted new capacitors. Points gap set at 14thou of an inch.
The coils are Tec FL703 type, the primary resistance measured at approx 5.5ohms the secondary windings measure at approx 12Kohm. Without suppressor plug caps.
I have also noted that when the points open, if a analogue meter is used the pointer will drop towards zero. If I use a digital meter. The low resistance across a closed set of points should go quite high in terms of open circuit resistance, when the points open. But I am detecting 7 ohms of continuing continuity. Suggesting a resistive open circuit short circuit to ground. This has me thinking this is offering my ignition voltage a low resistance path to ground, lower than having to energise a collapsing electrical field inducing an emf into the coil secondary windings. It's all well and good putting new coils on, I am not 100% convinced that will solve my lack of spark problem.
On the points, I do wonder if the tiny insulting ring that passes through the the terminal screw tab, and has insulated discs either side, can loose some resistance.
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It's difficult to diagnose the problem without seeing it up close.
I would check that the points wiring and insulators are connected in the right sequence. Then check continuity on the wiring all the way from points to the coils and double check that the coil connections are the right way round!
It's unlikely that both coils are faulty, but they might be. If you can get hold of a known good coil, try that in place of one of the existing coils. Hope that helps.
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Did you say that you get a spark at the plug when you manually open the points?
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You can simulate points by shorting the blue (or yellow) to ground and then disconnecting. When you disconnect, you should have a spark.
This test should prove the coils to be good or bad.
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Sounds like maybe corrosion or preservative on contact surface.
If not and spark occurs when you manualy open the points with everything connected the cam is either/or/both not opening points enough or at correct time
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Thanks for the input. I am still non the wiser. The points are still serviceable given that they are gapped and the fibre heels mean that they are still within adjustment to time the moment when the points to the F1-4 and F2-3 marks start to open. The blue wire is paired to coil serving cylinders 1-4 and corresponding set of points. Conversely the yellow one serves the coil to cylinders 2 and 3. The black and white wire which carries ignition voltage common to both coils. The black wire within the loom is a positive supply to all of the ancillary components. Because continuity exists between live and ground via the resistance of bulbs in the console, there are 3 common green ground wires between hi beam and the indicators. I am wondering if this is causing me to detect this at the points? I have seen replacement coils for the FL703 on eBay at around £25 each. They are push fit HT leads with screw locking caps.
Should I be detecting continuity of around 7 ohms between the points terminal wire and ground when the points are open or should this be closer to infinity? To know for sure will help me with my investigation.
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There are many possible points of corrosion in the ignition circuit, and as a first port of call you might try hot-wiring the coils directly to the +ve of the battery.... ie cut out the ignition switch, kill switch, and old wiring / bullets in the original feed. Of course, also ensure the points back plate is securely earthed. If this solves your problem you’ll then have to work backwards checking the aforementioned components in turn.
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Welcome to the forum. I see your already having fun. ;)
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... at some point today I’ll measure the resistance figure you seek on my (working!) 400F.
Addendum... Doh, and sorry Gary, my 400F has a breakerless ignition, and to get what you want would mean a fair bit of disturbing things. My excuse is 7am writings and 5 different bikes... forgive me!
I agree the ‘idiot’ lights would induce a reading, but don’t forget that the test you describe will also include the respective coil primary winding and the voltage regulator resistances. (The VR, I think, will be feeding the alternator field coil at 0 rpm. (My ‘750 book suggests this field coil is rated at 7 Ohms & I’d guess the 400F same-same). Anyways, point is, if you disconnect the Regulator, take out the Oil Pressure lamp, put the bike in gear (Neutral lamp out), all lights off etc... then your yellow / blue test circuits should only be measuring their respective Primary coil resistances. And if these 2 values (ie Yellow & Blue) are the same, then that’s a good sign.....though not proof of a healthy ignition system.
I still reckon your best staring point is as per my earlier post; for my money the culprit might be the kill switch which has weeny contacts, carries a couple of amps, is prone to water ingress, and whose loom is always on the move.
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In other words.... have you got 12V on the black/white wire going to the coils?
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Just a reality check for orientation, if you swap the yellow and blue wires to the points and then try starting.
It's just that sometimes they've been assembled (not factory) with them trying to fire the cylinders at bottom of stroke and swapping the two wires will check that for you.
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Just a reality check for orientation, if you swap the yellow and blue wires to the points and then try starting.
It's just that sometimes they've been assembled (not factory) with them trying to fire the cylinders at bottom of stroke and swapping the two wires will check that for you.
He says he has no spark at the plugs at all.
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Just a reality check for orientation, if you swap the yellow and blue wires to the points and then try starting.
It's just that sometimes they've been assembled (not factory) with them trying to fire the cylinders at bottom of stroke and swapping the two wires will check that for you.
He says he has no spark at the plugs at all.
My mistake, read the spark description and failed to consider it was "across the points" yep different problem.
Edit:- battery voltage would normally be closer to 12.7 in a healthy system as comparison to the level currently. May not make the difference but you'd ideally need to get it closer by charging or using jump leads from another.
Generally if you fully charge a battery and leave it alone, then 11.7 volts residual would suggest it's impaired.
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Just a reality check for orientation, if you swap the yellow and blue wires to the points and then try starting.
It's just that sometimes they've been assembled (not factory) with them trying to fire the cylinders at bottom of stroke and swapping the two wires will check that for you.
He says he has no spark at the plugs at all.
My mistake, read the spark description and failed to consider it was "across the points" yep different problem.
Edit:- battery voltage would normally be closer to 12.7 in a healthy system as comparison to the level currently. May not make the difference but you'd ideally need to get it closer by charging or using jump leads from another.
Generally if you fully charge a battery and leave it alone, then 11.7 volts residual would suggest it's impaired.
The 11.7v is at the points not at the battery. So his ignition is on and warning lights so not too bad really. If it was 11.7v on the battery alone after charging then it would be knackered.
Double check your insulator set up on the points 7ohms is nearly straight down to earth. Disconnect the condensors and try again .
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Many thanks for your further insights, relative to my ignition problem, I take note the kill switch is s potential cause for
concern, grounding faults, chaffed insulation. I will check mine, I agree with the remark that the insulators within the points might be suspect, current might be tracking through to ground? I will remove the items causing resistance within the ignition circuit. As a precaution.
The fact is, 11.7 V DC is detected across the points, given that the battery voltage has flowed through the wiring loom including the primary windings within the coils, switch gear etc. Indicates a healthy Voltage at the points. But I don't think the coil electro magnetic flux and capacitor voltage is being induced into the secondary windings, when the points open, given that the primary voltage has an easier route to ground because of the 7 ohms resistance to ground. I am detecting. I might as well buy a new set of points, to see if that helps. Years ago it was a forgone conclusion to change the points and condensors together. When I used to own a 550/4 I always carried a spare set of condensers, because 9 times out of 10 it was the condensors or one of them, which gave up the ghost causing the engine to limp along firing on 2 cylinders. Which is why I have serviced the points and replaced the condensors for new from Dave Silver spares.
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I’m no electrician (plenty here that are) but you’re measuring the voltage on an open circuit ... and not under load.
(a car battery will often show a healthy enough voltage across it’s terminals, but when asked to crank will go phzzt.
Or consider connecting your multimeter to your battery via two hair dimension filament wires. Yea, you’ll prob see 13.6 volts whatever, but these two wires will never deliver the power necessary to drive lights / coils / whatever.)
When the points close, a couple of amps (/ coil) is needed, and if the circuitry (switches / looms / bullets) are in poor shape then you will not get the power needed (volts x amps) to feed the coil(s).
... which is why I suggested you hot wire the coils directly to the battery.
Good luck Gary, let us know the eventual answer. Oh, and a belated welcome to this great forum.
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More to the point you cant measure resistance(ohms) with a capacitor/condenser in circuit.
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I was checking my insulation resistance across my points. Which I rotated in a clockwise fashion with a 22 mm six sided socket on the hexagon. I made the following observation. As the ATU rotated towards the F mark for both both sets of points. At the point the F mark became inline with the timing mark. My test meter went from confirming continuity across closed points to a brief incident of infinity open circuit. Yet within a couple of mm movement of additional rotation. My test meter continued to show continuity across an open set of points to ground. My question is this: At the fraction of a degree when my F mark aligned with the index mark which gave me an open circuit infinity resistance reading. Is this a normal open circuit length of time for my coil and condenser to discharge. To give me a spark at the plug. Which I don't have at the moment?
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The points should be open for longer duration than that.
Try putting an insulator ( piece of plastic milk bottle perhaps) between the point's contact faces and then meter across them.
If youve still got continuity ( beep circuit test meter) then something there is wrong as it shouldn't track through.
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remove the wires to the points and check each wire to earth for open circuit, then check the points without wires connected.