Forum

[ChrisD]

Gentlemen, I run 1996 535cc twin-plugged Classic. I tried it initially with two 12v coils, but that was cramped on the mudguard and needed two condensors, which was quite a squeeze in the points chamber. So I bought one of our hosts dual coils and all seemed well with the spark at cruising speed, although higher rev misfires have plagued me (>4000+revs or over 100km/hr on the 17T sprocket) and I am now thinking of electronic ignition.

Our hosts show the Boyer Bransden system for twin plugged heads with two separate coils, and I understand from elsewhere they are 6v coils, to be wired in series. I have elsewhere found it said that “dual coils are really only intended to fire one plug under compression at a time. It has been known to work ok on low compression engines in a standard state of tune”.

I had not heard this before, and wonder if this could be causing the high rev misfire I have. What do you think they refer to as low compression? The static compression is ~8.0:1 after making a metal-metal head joint and adding a 2mm plate. Please note the twin-plug BB system from our hosts might cost only GBP171 in UK, but landed here in Cape Town costs over GBP300, so it is too expensive to “take a punt”.
Any thoughts on this coil issue?
Cheers, ChrisD

[tribonnie]

Hello, just a thought but have you retarded the ignition for the twin plug setup. The Hitchcock's scramblers run twin plug heads and we set the fully advanced ignition set at about 24 degrees (compared with single plug heads at 32 degrees). Sorry if teaching what you already know but, there is a far faster fuel burn and hence the need to retard. Any 12 volt twin plug coil should suffice, the new Triumph twin ones seem to be good which you should be able to source locally. I would certainly recommend this type of coil versus a twin coil setup particularly if you are still on points. Then there are all the basics, good plug, ht lead, plug cap, decent earth for the low tension side, etc. Richard

[Thack]

ChrisD - you refer to Hitchcock's "dual coils" but do you mean the one they sell which has two HT leads? That's what I've tended to call a "double-ended" coil in previous times.



Anyway, I agree that these double-ended coils are normally used in "wasted spark" systems, where one of the cylinders is not under pressure and so requires a much lower voltage to make it spark. If such a coil is to be used on a twin-plug single-cylinder engine like yours then it would need to generate twice the voltage of a normal coil.



I obviously don't know for sure, but I'm betting the double-ended coil H sells doesn't generate twice the voltage of a normal coil. I bet it's really intended for a wasted spark system and H has co-opted it for this purpose.



I wouldn't be surprised at all if this is why you are getting high speed misfires. You can confirm this by taking out one of the plugs and tapping the electrodes closed, effectively turning it back into a single-plug system. If the high speed misfire disappears, then you know the problem is insufficient spark voltage to jump two pressurised spark plugs in series.



By the way, I've never heard of incorrect ignition timing causing a misfire. Detonation if too advanced, stinking hot exhaust pipe if too retarded. But a misfire?



About the Boyer Bransden system you are considering. Personally I would contact BB directly and ask them if their twin-plug system is intended for wasted-spark systems, or twin-spark systems like yours (emphasise to them that such a system requires twice the normal HT voltage from the coil). Make sure they understand the question!



I'm sorry I can't be of much help, but honestly I wouldn't trust anyone except BB themselves on this one.

[Norm]

Fitted an old coil of one of my Hondas to a 612 twin plug I built a few years ago and it seemed to work ok with no problems. Bike is now back in the UK so not sure how it is going

[Thack]

Hey, Norm, that's useful to know, because it's real experience instead of just speculative ramblings from me!



I suspect we can summarise by saying: 1/ A dual-spark system requires a higher spark voltage than a wasted-spark system; 2/ "wasted spark" coils might work - and did for Norm - but aren't designed for a twin-spark system.



Oh, and 3/: Find out from BB which their system is designed for.

[Norm]

Thack what makes it more interesting is I had pulled the coils off my Honda a few years earlier because I was having coil problems. Coils probably original, so 30 years old and no misfire problems with it. Chris should be able to find an old twin coil in SA. The other interesting thing is I think the Honda has CDI ignition not the Kettering system the points ignition uses

[Norm]

Just one more thing, at the end of the day I'm not sure if it improved the performance but people did notice the twin plugs

[ChrisD]

Thanks for your thoughts, guys.
Yes, I have retarded the ignition, although not as far as Tribonnie suggests – about 4 degrees so far. Since the misfire is instant on opening the throttle, it cannot be ignition-related as the revs don’t change.
Yes, the dual coil is indeed our hosts twin-lead coil (PN 92075) which is sold as “If you have modified your Bullet to twin spark plugs, this is the coil for you”. But I will consider the Triumph or even Honda twin coils.
I believe the twin-lead coil works OK because if, at high revs (~3000rpm) on the stand (not the road) I pull off one plug lead, the revs immediately drop a few huundred and the engine is 'rougher'. If the twin-lead coil couldn’t handle two plugs firing at that pressure, then I’d expect the revs to rise (I should say it was no better with two separate coils, just very difficult to set up the twin condensors in that cramped space). I should also point out that the points are no more eroded now after ~2000km with the twin-lead coil than they were with the single plug and single coil.
I run Iridium plugs, so no way could I afford to tap the contacts closed!
I have asked both BB and Pazon if their systems work with a twin-plugged single and both say yes – with either twin lead coil or dual 6V coils.

In the meantime, I have re-measured the cam lifts with a dial gauge (both the OEM cams and our hosts Performance cams) at my usual 15degrees spacing (should have done 10degree steps but couldn’t face the extra work). This was after I had set the cams up correctly on-the-dots and it didn’t run, then I set the cams up to match the Redditch cams and it shows reversion at 1/3 throttle or more (charge spewing backwards from the carb throat- very dangerous). See my comments in my post on “Concentric Mk2 Air Jet” in late November. As a result, it occurred to me that, notwithstanding my ‘expert’ bike engineer’s advice after fitting my new roller bigend and new mains a few months ago that “it all went back together perfectly with no offset”, the timing side crankshaft must be offset. And guess what, so it is, by ~20 degrees (head-banging on wall)!
So, one step forward, two steps back – I should have checked, I know.
ChrisD

[Tim NZ]

First off, you did not need two condensers; you only have one set of points to prevent for burning. But you would have been better off with two small 6v coils in series, over 2 x 12v parallel.

Coils for points use whether single or double ended, have a different resistance to a coil intended for CDI or TCI. Run a coil intended for a High Energy EI on points and eventually it will melt! Make sure you have the right coil for the job. DONT buy a double ended coil intended for a Late model Trihard, it WILL NOT be right for your bike!

A 'wasted' spark coil simply fires one spark over a idle plug, total voltage requirement to initiate combustion remains the same. One spark is positive the other negative. Most every bike fitted with a double ended coil fires a wasted spark!
(The previous comment on 'double ended' coils in a previous reply were seriously misinformed)

A coil from a Points Honda4, (or similar) or older Hardley Dangerous will be OK.

IF one plug is breaking down, then power will be reduced and could lead to overheating (misfire) on the other (now grossly retarded) plug.
Check both plugs for any sign of overheating...
Replace both if in doubt. (I prefer projected tip plugs)

Otherwise, and most likely, is that your misfire is a result of too-lean; check your float height? Next fit a main jet 2 sizes larger and try again.
Or it is possible that there is too much circuit resistance, especially in the HT side?
I have seen people unwittingly run resistor plug, resistor cap, and resistor lead. One only, your choice.
Any oxidized terminals in the ignition circuit?
Have you tried a much smaller plug gap? .5mm

Along with the need for less total advance with twin-plugs, is the need to increase the to total advance range. So that the static ignition point is not at or after TDC, I restrict the AA range down to 9 degrees (18 total) by fitting a shim into the slot in the back of the AA unit, and reduce the weight of the Bob-weights (at the ends) to raise advance rpm.


Tim

[Thack]

I agree all the stuff Tim wrote about the importance of using the correct design of coil. Coils for capacitor-discharge and inductive systems are different; and amongst inductive systems, points systems and electronically-controlled systems again require different types of coils. It's all more complicated than one might first think.



However, he then says:

Most every bike fitted with a double ended coil fires a wasted spark!




Surely that should be *every* bike, apart from those using twin spark plugs.




This bit worries me more:

(The previous comment on 'double ended' coils in a previous reply were seriously misinformed)




If he is referring to my comments, then it's Tim who is misinformed, as becomes clear from the following:

A 'wasted' spark coil simply fires one spark over a idle plug, total voltage requirement to initiate combustion remains the same.




That sentence is wrong. The total voltage from a double-ended coil (as measured across the two HT terminals) needs to be higher than the voltage from a single-ended coil (measured from the HT terminal to earth). For a double-ended coil, the current in the secondary winding flows down one HT lead, across a spark plug gap, through the cylinder head, across the other spark plug gap, and back up the other HT lead. That is, it must jump *two* spark plug gaps.



Now, the voltage required to jump a spark plug increases a great deal when that cylinder is under pressure. 10kV might be needed to jump a typical spark plug at atmospheric pressure, but on the compression stroke that voltage can at least double - 20kV, maybe more.



With a tradition wasted spark system, you have one plug at atmospheric (near enough - near the top of the exhaust stroke), and the other plug under pressure (near the top of the compression stroke). Thus the total voltage required to spark those two plugs will be (say) 10kV for the low pressure one, and (say) 20kV for the high pressure one, giving us a total of 30kV required from the coil (i.e. between the two HT terminals).



Note: this is more than the 20kV required from a single-coil single-plug system.



HOWEVER, when both plugs are under pressure (i.e. twin plugs in a single cylinder) then each plug requires (say) 20kV to spark, thus requiring a total of 20 + 20 = 40kV from the coil. This is higher than the 30kV a wasted-spark coil normally "expects" to develop.



Thus I maintain that it is entirely possible for a coil intended for wasted-spark use won't work properly in a twin-spark system. The total voltage requirement is higher.



I've uploaded a diagram which might help: Single- and double-ended coils



NOTE: I am not saying this is the cause of ChrisD's problem. Just that it could be, in theory. As Tim NZ says, it could also be a mixture problem.



Tim then says:

IF one plug is breaking down, then power will be reduced and could lead to overheating (misfire) on the other (now grossly retarded) plug.




This makes no sense at all. There is absolutely no reason why one plug "breaking down" should "grossly" affect the ignition timing on the other! There might be a tiny difference in the voltage rise-time, but that would be measured in microseconds, if it exists at all.



"Breaking down" can mean two things: going towards a short circuit, or going towards an open circuit. If the faulty plug is going low resistance or short circuit, then the HT current will follow the short circuit and not spark that plug. It will then continue through the cylinder head and spark the other plug in the normal way. Indeed, it increases the available voltage to the other plug because the short circuit in the first plug means it isn't dropping several kilovolts across it.



If the faulty plug is going open circuit, then it needs a HIGHER voltage to spark it, and it reduces the available voltage to the other plug. This might get so bad that neither plug is able to spark and the engine will misfire or cut out.



ChrisD's symptoms are typical of insufficient HT voltage. Remember, a large throttle opening causes a big increase in the sparking voltage at each plug. Thus a coil that sparks both plugs successfully at small throttle openings might well fail to spark the plugs at large throttle openings.



Tim is exactly right to say that other things might reduce the available spark voltage: using resistor-leads with resistor-caps and resistor-plugs is a great example.



But don't forget, ChrisD's symptoms could also be caused by a carb fault causing weak mixture at large throttle openings. My gut says it's an ignition problem, though.