Not strictly true. All e.i. are/should be set physically at full advance, 68 onwards using the marks on alternator rotor and primary cover, the electronics
retard the spark between idle and full advance rpm.
Your bike's system has two coils or one?
While
@solomon could be right about the Boyer being "cooked", there are a number of things I would check/do before condemning it.
As you are in the US, Bransden Electronics (the actual maker of all "Boyer" e.i.) has authorised
Coventry Spares to test and repair/replace as required, although you have to send the Transistor Box, coil(s), (Boyer) stator and rotor to them.
I wonder if it is just coincidence that the rpm where it "stumbles and breaks up" is also about full advance rpm:-
. Is the bike fitted with
one of either resistor spark plugs, resistor plug caps, resistive HT leads? If plugs or leads, have you used an Ohmmeter or multimeter to check for 5,000 Ohm resistance, no more, no less?
. Have you checked the physical (full advance) setting; i.e. the pistons are just BTDC, an alternator rotor line is lined up with the fixed line behind the primary chaincase plug (assuming standard '79 parts)?
. Have you checked with a strobe that the ignition is retarded between tickover and "stumble" rpm? Standard '79 primary cover, regrettably it is not easy to see the ignition advancing (rotor line moving closer to the chaincase line) with increasing rpm because the chaincase timing aperture is so small;
nevertheless, you should at least be able to see the lines apart at rpm a little below 4000.
In what way? "Neg earth" means the harness Black wires connected to battery -ve are the supply to electrical components. The original Lucas Rita e.i. had a Black wire supplying it; if you can also identify this particular wire at the battery -ve terminal, the Boyer Transistor Box White should be connected to it. Or your "second ground wire" should be the only one connecting battery -ve to the Transistor Box White wire.
Be aware all aftermarket e.i. fitting instructions (including Boyer's) are entirely unsuitable for any 79 onwards twin, following them slavishly results in wholly unnecessary mutilation of perfectly suitable standard wiring to fit poorer quality but more expensive e.i. than the original Rita
In addition to possibly using the original Black wire from battery -ve to connect the Transistor Box White wire:-
. harness White/Black wire to connect the Transistor Box Black wire to (first?) coil -ve (do not confuse with the
Black/White wire at the Transistor Box and Boyer Stator);
. if two separate coils, harness White/Pink wires between first coil +ve and second coil -ve;
. harness White/Yellow wire to connect the Transistor Box Red wire and (second) coil +ve to handlebar kill switch?
Irrespective of the above, have you tried temporarily connecting the Transistor Box Red wire and (second) coil +ve directly to the battery +ve terminal? This will eliminate the kill switch, ignition switch and standard fuse contacts as possible causes of the stumble/break up. If you try this, put a fuse in the wire between coil +ve and battery +ve and make the wire long enough so you can poke the fuse out from under the seat, because kill switch and ignition switch bypassed, the fuse will be the only way of stopping the engine once it is started.
Have the harness White/Purple and White/Orange wires been used to connect the Black/White and Black/Yellow wires at Transistor Box and Boyer Stator? If yes:-
. Are the Black/White and Black/Yellow wires connected to the same White/Purple or White/Orange wire at both ends? This is important as 'swapping' the connections between Stator and Transistor Box confuses the signals to the Box timing electronics.
. The White/Purple and White/Orange wires have 'break' connections near the frame steering head; find these, pull them apart, make sure there is no corrosion on the bullets or snap connector internal steel sleeves. Any doubt about them remake the connections with "Japanese bullets", sockets and their insulators, if necessary from
Cycle Terminal (the Japanese bullet components make much more weatherproof connections;
if you use the smaller "3.5 mm" terminals, they are also a good replacement for the red insulated hardware store terminals all the e.i. makers fit to the Stator wires).
