The CB1100 Community Forum

I was just wondering about this. How did the pre-ECU bikes know when to send spark to a particular cylinder?

Wasted spark / dual fire ignition

Yeah, I'm confused. A 4 cycle engine repeats every 720 degrees so there is a firing every 180 degrees on a 4 cylinder right? But how does the system know where to direct the spark?

If motorcycles didn't need distributors then why did cars have them?

On many ‘70’s Japanese four-cylinder bikes, there were two sets of points, each driven off the crankshaft and each timed to open 360 degrees apart. Basically, that was the “distributor.” Each set of points allowed voltage to be sent to one of two different coils, each coil sending voltage to the spark plugs in two different cylinders (one on the compression stroke and the other on the exhaust stroke, thus a “wasted spark.”) Or something like that.

Thanks, but why didn't cars use this same system?

My 1976 CB750F was dual fire, dual points, dual coil.
Dual points were replaced by a Martek 440 electronic ignition, as maintaining proper dwell with points was a nuisance.

The below link should provide a succinct pictorial explanation.

https://www.crypton.co.za/Tto%20know/Ign...%20sp.html

Prior to EFI (2007 up) Sportster's were primarily dual fire:

Coil Testing
Dual fire coil testing for Ironheads and Rigid EVO, (except for Spοrtster Spοrt 1998-2003)
Coil 31614-83A has two primary screw terminals (+ & -) with twin coil towers.
Primary resistance is measured from + to - terminals, at 2.5 to 3.1 ohms.
Secondary (high voltage) winding resistance is tested from coil tower to coil tower at 10,000 ohms to 12,500 ohms.

Single fire coil testing for Spοrtster Spοrt (1998-2003)
Coil 31646-99 has three connectorized primary terminals (- + -), referenced A (front) B (12 VDC) C (rear) and twin coil towers.
Primary resistance is from + to -, both B to A and B to C at 0.4 to 0.6 ohms. For the 1200S (dual plug heads), measure secondary resistance between both front coil towers and then between both rear coil towers at 11,700 to 12,700 ohms.

Single fire coil testing for carbed (ICM) rubsters, 2004-2006
Coil 31655-99 has three connectorized primary terminals (- + -), referenced 1 (rear) 2 (12 VDC) 3 (front) and twin coil towers.
Primary resistance is from + to -, both 2 to 1 and 2 to 3 at 0.5 to 0.7 ohms.
Secondary (high voltage) winding resistance is tested from coil tower to coil tower at 11,000 ohms to 15,000 ohms.

Single fire coil testing for EFI (ECM) rubsters, 2007 and up: Coil 31656-07 has 4 primary terminals, referenced, left to right, as A (+), B (+), C (rear) & D (front). A or B to D should measure 0.3 to 0.7 ohm and A or B to C should measure 0.3 to 0.7 ohm.
Secondary resistance: 3,000 to 4,800 ohms, tower to tower

Note: If resistance is lower than specified, that winding has partially shorted, bypassing part of the coil.

If resistance is higher, then corrosion or failing internal connections may be adding to the resistance.

(02-02-2020, 11:52 PM)m in sc_imp Wrote: [ -> ]wasted spark is correct. as said above, each coil is fired every 360 degrees of crank rotation. this means it fires near btdc on the combustion stroke, the next time that cyl fires is near top dead center of the exhaust stroke. in effect, the crankshaft itself is the distributor. so, the coil will fire cyl 1 and 4 at same time, and 2 and 3 at same time. however, cyl 1 may be on the power stroke, cyl 4 may be at the exhaust stroke.
the electronic ignition on the rd350 I posted is actually one for a japanese 4cyl.

now, another one forgot is overhead cam twins, such as cb350, 450, xs650, etc. these have points off the cam shaft in the head.

what I think your missing is that theres multiple coils, and that they are simply triggered at the motor, just in a more analog fashion. in lots of ways, more similar to modern car engines with coil on plug setups triggered by a crank sensor. hope this helps

Yes, as did my CB160, CL160, CL350, SL350 and CB360.

Sportsters ran points off the camshaft before a cam position sensor replaced points in the "nosecone".
Some riders got confused with setting timing, since Sportster cams rotate CCW, although the crank is CW.

Thanks! This has been very informative.

Okay, I understand the dual points and wasted spark setup but how is the relationship between the crankshaft position and the points opening established? Maybe it's too simple a question. There's a mark somewhere for TDC and the points plate is swiveled around so the points are closed at that point?

Great capture m_in_sc.

Man, those were the days - maybe some folks still dwell at the point.