Topic: wasted spark coil on an A65

[paulages]

hey guys. can't get a straight answer out of the britbike forum i frequent for my 1971 BSA Thunderbolt, so i thought i'd ask here. The A65 has a 360 degree crank, which means that both cylinders rise and fall together, kind of like 1-4 and 2-3 do on our SOHC4s. as you know, our hondas use two dual-fire coils with only two sets of points, which creates a wasted spark on one cylinder when the other fires.

I have the same type coil for my BSA (two 12V posts, two high tension outputs), which i'm told will work, but i'm a little confused about how to wire it. if each set of points connects to its respective condenser (in parallel) and then to the same post on the coil, won't this effectively reduce the dwell of each while the other is open, unless there is a diode between each condenser and the coil?

[paulages]

ok, got an answer on the britbike forum. the points cam prevents both from being closed at once, so only one condenser id even necessary. both tie to the negative post of the coil, as does the condenser.

carry on. 

[bryanj]

The A65 had two seperate coils so no wasted spark, if you really want to use the twin plug coil just use one set of points

[w1sa]

The A65 had two seperate coils so no wasted spark, if you really want to use the twin plug coil just use one set of points

I think he'd have to change the points cam to a 'twin lobe' arrangement.

[paulages]

The A65 had two seperate coils so no wasted spark, if you really want to use the twin plug coil just use one set of points

i know this is the stock setup, and though the cam lobe definitely fires them separately i'm told the dual fire coil will work fine. my worry is about dwell... i'll give it a try as instructed i guess and see if it works. lucas coils are only $10-15 each, i'm just trying to use what i have.

[w1sa]

A typical dual points/ dual coil setup for a 360 twin, is normally accompanied by a points cam/points lobe profile that provides about 90deg of camshaft rotation/dwell time for each coil's firing.

A dual points/ dual coil(dual fire/wasted spark) setup for a inline 4 provides the equivalent 90deg of dwell time for each coil's firing. When comparing your proposed installation (dual points/single coil/dual fire) to the inline 4 setup, there is no difference in the coil firing intervals (per rpm), between the two.
And, this is also achieved at higher average rpm in a '4' , than the A65 would operate at.

Sooo.......you should not have a 'dwell' problem in your suggested configuration.....

[bryanj]

Now that i am on the way out of bed rather than on the way in, i think you will have megga trouble with the one coil as i am fairly certain that when one set of points is open the other  set is closed.

The only way i can think of is with two twin lead coils and twin plug (a misnomer really as there are 2 per pot so 4 total) cylinder head which was a conversion for racing.

The Boyer electronic system uses 2 x 6 volt coils in series and the pickups fire the box twice so yes you have a wasted spark---in that system you could use 1 x 12 volt twin spark coil like on the SOHC4

[w1sa]

To eliminate any doubt regarding your proposed set-up, rotate the engine and observe the points cam movement. You want to see that each set of points only remains closed for a period less than 180deg (it should be closer to 90deg) of points cam rotation.

That being the case the coil will only be saturating during that time. At 90deg of dwell each set of points opening/closing is separated by another 90deg of rotation. At 180deg of dwell there is too much saturation time (wasted energy and additional heat)  and not enough separation to ensure efficient single coil discharge.

If you've got closer to 90deg of dwell, the two points firing will work.

Also consider that with both condensers in the single coil circuit, the rating of the existing condensers will become inappropriate. Two original condensers wired to the one coil will double the 'condenser' performance and may contribute to poor running, especially at higher revs. Too high a rating= poorer high end performance.  Too low a rating= burning the points.
Either, halve the condenser rating used for two condensers or eliminate one original from the circuit. Ideally choose a condenser rating that matches the coil's output performance.

[bryanj]

Dont forget BSA and Triumph point are camshaft rather than crankshaft mountedso dwell numbers are different

[w1sa]

I agree BryanJ. The degrees of rotation I mention are at the 'points housing' and are equivalent to camshaft degrees of rotation. I am talking camshaft rotation, not crankshaft.

[paulages]

you're both a little off, actually.  the points are mounted on the idler pinion that goes between the crank and cam timing gears. i got spark tonight... stupid me: i had a bad spark pug i was testing it with.  the dwell looks like it will be ok- each point is closed a little more than 90 degrees, and they are never closed at the same time. got nice fat spark on a good plug.

on to the next hurdle...

[w1sa]

Fantastic...
Now, If the Hondaman ignition module could be successfully wired in..........?

[paulages]

Fantastic...
Now, If the Hondaman ignition module could be successfully wired in..........?

that would be nice. i haven't been so happy with the sparx electronic ignition in the past.

[bryanj]

Sorry Paul, i was thinking of the Trumpets! Even when they were a current bike most of us old timers (young then) avoided working on them like the plauge---they were just too damn awkward and tempramental

[paulages]

Sorry Paul, i was thinking of the Trumpets! Even when they were a current bike most of us old timers (young then) avoided working on them like the plauge---they were just too damn awkward and tempramental

yeah, the infamous timing side bush failure happened to me. couldn't afford the roller bearing conversion, but all tolerances are nice and it ought to last a little while anyway. at least by '71 they had improved a lot of the problems with the earlier engines. cast iron oil pump, oil holes in the primary side rod, etc.

[bryanj]

One of the biggest problems was the aftermarket bronze bushes that were sold instead of the white metal lined steel ones, Bronze being harder and not as good a bearing metal wore the crank more, Aslo those pushrods were a pain to refit