HT Leads on coils

[royhall]

Discovered yesterday that after the initial superb results with the 350F that I now have a 3.5 cylinder bike.

When I put the coils on (originals) I noticed that one HT lead was turning in the coil and was quite loose. But decided to go along with them anyway.

Not sure at this point if its on the bad running cylinder (Christmas duties prevent vital garage time, Bah Humbug) but from memory I think it may be.

Anyone else had this problem.

Cheers and Happy Christmas.

[hairygit]

It is very likely to be your problem. H.T. circuits don't like loose connections, they can make and break with the vibrations from the engine, or road bumps etc, at least it's an easy fix!

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[gtmdriver]

If you replace the coil or coils take care that you get them with the same primary coil resistance.

The Honda coils on my CB350F had a primary resistance of 4 ohms but the replacements from David Silver only had a 2 ohm resistance which would have drawn far too much current through my Boyer ignition unit. I had to fit a pair of 1.6 ohm ballast resistors in series with the 12v supply.

[AshimotoK0]

VERY common problem with those coils and Cb twins coils. I am supposed to be writing up how to fix this step by step with pics.  Will do it next week as I have two sets to do and I now have all of the materials . BUT I can't find a sourceof the thin rubber sleeving on the outside of the HT lead to make it look original.

Ash

[Gixxer-18]

Hi Ash, try googling this stuff, it's not cheap, and it's branded, but that branding should be removable with MEK. It's about 1mm wall when heated and shrunk. Use the 12/6 size 12mm to 6mm. I've got it on some of the boats wiring in the engine room.
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It might not be of any use but it's a lead!
Regards
Nigel.

[royhall]

Not checked the resistance on the original coils yet, but the pair of Silvers replacements read 3.4 and 3.1 ohms. I am assuming this will be okay as I am running points. Is this correct. Cheers.

[gtmdriver]

Not checked the resistance on the original coils yet, but the pair of Silvers replacements read 3.4 and 3.1 ohms. I am assuming this will be okay as I am running points. Is this correct. Cheers.

You should be OK with these. The points may burn a little quicker but you should be OK.

[AshimotoK0]

Hi Roy. The various four-cylinder SOHC Honda  manuals are scant on resistance details referring you to the Honda dedicated electrical  tester. The '60's twins manuals were much better on this. Those coils sound Ok to me, I think the cold stall current of the coils should be 3-4 Amps so if you take 12 volts and Ohms low this equates to  12/4 -> 12/3 Ohms =  3-> 4 Ohms (R=V/I).

You have to be careful with cheopo multimeters as the low Ohms range is often not very accurate. The best way of measuring resistance with a cheopo one is to put a known resistor in series with the coil and measure the current through the coil and the voltage drop across the coil. The resistor wants to be about 15 Ohms. You then work out the resistance of the coil using Ohms law R=V/I . Doing it this way you are only passing a lowish current through the coil and minimizing the self-heating, as the coil resistance increases as it self heats. Obviously, if you have an expensive meter (Fluke or TTi) it's not necessary.

Attached is the  type of info in the sixties Honda gave.

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[royhall]

Thanks for that Ash. Cant say I understood it all though.   Take your point about the cheap multimeter. My garage one is a cheapo but I have a fluke on the work van (couldn't be arsed getting it out), so will test them again with that. Will be interesting to see if the readings are different. Will post the new readings later. Cheers.

[royhall]

There's interesting, didn't expect them both to be the same. They are both 2.7 ohms on the Fluke, retested on the cheapo and got 3 & 3.3 this time. Think I will bin the cheapo, it was only £3.99p with 10 litres of BP. Does that change them being okay to use on points. Cheers.

[gtmdriver]

The lower the resistance the more current will flow through the points and the condenser so their working life will be affected but points are not so critical as electronic ignition.

My Boyer system, for example, specifies a minimum coil resistance of 4 ohms.

[AshimotoK0]

Thanks for that Ash. Cant say I understood it all though.   Take your point about the cheap multimeter. My garage one is a cheapo but I have a fluke on the work van (couldn't be arsed getting it out), so will test them again with that. Will be interesting to see if the readings are different. Will post the new readings later. Cheers.

Roy ...if in doubt ask ..please don't say you don't understand ... I can always elaborate 

Just measured two CB750 coils TEC FL703-12V coils with a high accuracy bench DMM at work and one reads 4.60 Ohms and the other 4.65 Ohms .temperature 19°C.

Secondaries measured 14,540 and 14,930 Ohms ( 14.54k Ohms and 14.93k Ohms). if they read open circuit it is almost certainly the HT leads that have fatigued at the point they come out of the coil. If the reading is much higher you have a supressor cap with resistor fitted internally. Remove the cap and remeasure, in this case.

Brought all of my coils into work to do the fix on new HT leads ... so watch out on here later this week for details and pics.

[royhall]

Not a problem Ash. I didn't know what ohms law was, its a long time since I last heard that in college. But thanks to the power of Google I am up to speed. Holding off on the job until your coil repair post appears. Cheers Ash.

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[AshimotoK0]

Out of interest what manufacturer code do 400/4  / 350/4  coils have on them?

[royhall]

Will have a look when I get home.

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