Honda-SOHC

SOHC.co.uk Forums => CB750 => Topic started by: philward on March 25, 2020, 07:19:46 PM

Title: Aftermarket Coils
Post by: philward on March 25, 2020, 07:19:46 PM
Whats caused this! Bought these from M&P and haven't started bike yet. I've been manually timing project K6 and heard a 'sizzling' - took tank off to discover this (coil was to not to touch. Checked connections and aftermarket coil yellow connected to points and black to black. The other coil is connected the same and no issues. Only time I've had ignition on is during testing of rest of electrics - maybe max 10 mins at a time
Is it just a duff coil?
Title: Re: Aftermarket Coils
Post by: Bryanj on March 25, 2020, 07:24:39 PM
I would say yes
Title: Re: Aftermarket Coils
Post by: Rob62 on March 25, 2020, 07:50:09 PM
If you’re only testing the rest of the electrics, leave the kill switch off...
Title: Re: Aftermarket Coils
Post by: philward on March 25, 2020, 07:57:41 PM
If you’re only testing the rest of the electrics, leave the kill switch off...
Never thought of that Rob! Simples!
Title: Re: Aftermarket Coils
Post by: Moorey on March 25, 2020, 08:04:14 PM
Duff coil the other one has been energized just as long and that's fine.
Title: Re: Aftermarket Coils
Post by: Bryanj on March 25, 2020, 08:51:14 PM
Not necessarily maybe only 1 set of points were closed
Title: Re: Aftermarket Coils
Post by: AshimotoK0 on March 25, 2020, 09:18:26 PM
Probably a low resistance coil design. It's all about Amp/Turns .. they probably reduced the number of turns of the coil to decrease resistance in order to save the cost of copper but still get the same magnetic flux. Unfortunately,  that vastly reduces the 'duty-cycle' ... meaning if you leave the coil energised  for a long period it is likely to suffer burnt out windings. Never really seen that happen on a genuine Nippon Denso coil (typical DC resistance of a Denso coil is typically around 4.5 ohms). It's different with a CDi system coil as they can be low resistance and low duty cycle because they can never experience long term continuous current flow in normal use or when the engine is static but the ignition is switched on.
Title: Re: Aftermarket Coils
Post by: Moorey on March 25, 2020, 09:33:51 PM
Not necessarily maybe only 1 set of points were closed

Quite true.
Title: Re: Aftermarket Coils
Post by: philward on March 25, 2020, 09:42:48 PM
All very interesting - I've emailed M&P for a relacement. Will be careful in short term and recon a std set as per James method before I sell the bike
Title: Re: Aftermarket Coils
Post by: gtmdriver on March 26, 2020, 11:27:13 AM
Make sure that the coils have the same primary resistance as the originals. (3 -4 ohm).

Coils designed for use with electronic ignition systems have a very low resistance (1.5 ohm) so when the points are shut twice the current will be flowing through the system. I'm not saying that was the reason for your failure but if you fit the wrong coils the points will burn very quickly and you could damage the condensers.
Title: Re: Aftermarket Coils
Post by: AshimotoK0 on March 26, 2020, 01:20:10 PM
All very interesting - I've emailed M&P for a relacement. Will be careful in short term and recon a std set as per James method before I sell the bike

You could also try the Ashimoto method too Phil.  ;D
Title: Re: Aftermarket Coils
Post by: Johnwebley on March 26, 2020, 01:46:16 PM


  not having the skill to do the Ashimoto fix,and running Boyer ignition  ,after my original packed up because of old age and cracked hard HT leads,I bought Silvers coils,they got hot and failed after about 12 months,
so I spent a little extra,and purchased Boyer compantable coils .

 now several years later,still working great
Title: Re: Aftermarket Coils
Post by: philward on March 26, 2020, 10:19:01 PM
All very interesting - I've emailed M&P for a relacement. Will be careful in short term and recon a std set as per James method before I sell the bike

You could also try the Ashimoto method too Phil.  ;D
I will be going that route Ash - I run the same coils on my std K2 and checked the resistence when I fitted them and they were 3 - 4 ohms.
Title: Re: Aftermarket Coils
Post by: philward on April 13, 2020, 12:39:16 PM
I am currently waiting for mterial to refurb original coils using Ash's method but in the meantime using the time to move the K6 Cafe Racer project on while I have the extra spare time. I am going to get it started hopefully as I only have paint and upholstry to complete bike. (when original coils refurbed, I will change the coils to the originals).
I got the replacement coil from M&P but in the time since I bought the coils, they've changed coil supplier and the new supplied coil is adifferent make to the original (and different physical size too). The wires are different colours and I dont trust either coils male/female connectors (ie, whether they connect to the correct coil polarity by merely plugging in male to femal to corrosponding harness connectors.
So, does does it matter which way around the wires are connected on coil and if so, how can I ascertain which wire on coil connects to the points?
Thanks in anticipation
Title: Re: Aftermarket Coils
Post by: Bryanj on April 13, 2020, 02:38:14 PM
Dont matter with the twin ht coils
Title: Re: Aftermarket Coils
Post by: philward on April 13, 2020, 05:02:44 PM
Thanks Bryan - Just touched button and within 5 seconds we are up and running! Got bike out to test down our cul de sac and no gears!
then I realised that I hadn't yet  fitted the chain! Chain fitted and bike rides ok with a few tweaks. Clutch a little jerky and thats the only engine part I didn't replace (I normally replace on all my builds but on this occasion, decided it looked ok) - I will put a new set of springs, friction and steel plates in over coming weeks.
So, just need to sort painting when all this lock down ends - and make up a seat cussion for my missus to sew up on her machine.
Always good to get the engine going after an engine rebuild!
Title: Re: Aftermarket Coils
Post by: philward on April 15, 2020, 02:42:08 PM
Electrical guru's knowledge needed! As per last post, got bike started straight away with teplacement coil (while on with original Honda  coils refurb). Neutral light on in all gears so whilst doing a continuity test on the switch with ignition on, (few minutes) heard the sizzling sound from under tank - as per original coil problem - and on lifting the tank, the other (remaining coil from original aftermarket coil pair) has a gaping crack as per original post oicture. The replacement coil (a 'Hendler') wired into the same position/circuit as previous defective coil is fine. Bike runs with coil split (although I obviously won't run it from now on).
Is it a bad batch of coils (ie, both original  aftermarket coils) that account for failure or can something on the bike be causing the problem? Need to establish this before fitting the original Honda refurbed coil set in case they are detroyed.
Thanks in advance
Phil
Title: Re: Aftermarket Coils
Post by: AshimotoK0 on April 15, 2020, 03:37:05 PM
The coil should be able to  withstand 12v across it for a considerable time without any damage. I think it's either a defective coil or a coil intended for CDi electronic ignition.
Title: Re: Aftermarket Coils
Post by: philward on April 15, 2020, 03:47:46 PM
It does seem strange that both the original aftermarket coils on different points circuits have failed - as long as its not an issue with the rest of the bikes ignition circuit. They were bought from a reputable firm (M&P) who should know that a CB750 does not have CDI (or maybe not!)
Title: Re: Aftermarket Coils
Post by: Bryanj on April 15, 2020, 06:50:56 PM
I would gues either a  bad batch or wrongly labled batch
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