79 SF ignition problem

[BigalG]

last year i rebuilt a 79 special and one of the big mysterys was why i kept loosing spark, bike starts bucking , cuts right out, comes back backfires.... after a lot of advice i got new coils, not the issue, and i replaced the fuse box with blades. Yes that solved it and i ran the piss out of it last summer. This spring i took it out and it ran fine for a while then the old symptoms started up again. I nursed it home as if i pulled in the clutch and revved the engine it would run again for a bit. I came home and let it sit a week. Today i took it for a short ride and the symptoms are still there, runs great , then bucks, cuts out compleetly , starts back up goes a bit, syptoms starts again. I came home and tied the ignition wires blade fuse into another slot and put in a new fuse. No change. same symptoms Any thoughts are appreciated ?

[cajun31]

last year i rebuilt a 79 special and one of the big mysterys was why i kept loosing spark, bike starts bucking , cuts right out, comes back backfires.... after a lot of advice i got new coils, not the issue, and i replaced the fuse box with blades. Yes that solved it and i ran the piss out of it last summer. This spring i took it out and it ran fine for a while then the old symptoms started up again. I nursed it home as if i pulled in the clutch and revved the engine it would run again for a bit. I came home and let it sit a week. Today i took it for a short ride and the symptoms are still there, runs great , then bucks, cuts out compleetly , starts back up goes a bit, syptoms starts again. I came home and tied the ignition wires blade fuse into another slot and put in a new fuse. No change. same symptoms Any thoughts are appreciated ?

check the wires to your timing pickup under the left crankcase cover. They are always suspect if any are broken will cause the problems you are describing. Pull on each one to see if they are broken internally.

[Radioguylogs]

You symptoms follow the problem Cajun described exactly.

[Bax]

BigalG This is a very common issue with the XS bikes. The pickup wires copper strands break inside of the silicon rubber insulation due to the flexing imparted by the vacuum advance assembly roll.

https://xs11.club/forum/repairs/batt...il-wire-repair

---Bax

[BigalG]

ok thanks guys, i will check them out, i did repair two of them last year as well, did not think to go back there. Much appreciated

[BigalG]

i checked the wires and the continuity at the cdi box of the wires.all seemed good. Does the bike need to be running in gear for the pickup wires to show symptoms? excuse my ignorance..

[Bax]

The position of the vacuum advance assembly would be the variable. Some positions there may be continuity and it open circuits in another.

Bax

[BigalG]

so what do you recomend ?

[bikerphil]

The wires usually break where they are clamped to the P/U coil plate. I would undo the wires from the clamps and stretch them, if there is a break the wire insulation will stretch.

[BigalG]

not finding any breaks i ended up swapping my fuse box again and cleaning every wiring connection with contact cleaner that was in the area behind the fuse box. after re assembly it ran for about 10 miles flawlesly. I hope to get a longer ride to test it further this week. Thanks for the help

[jwhughes3]

The breaks are inside the insulation and hard to detect. You need to move along each wire, gently pulling apart to look for “stretch” in the insulation. Once found, clip at the break, splice in a 1” flexible wire. The very flexible braided wires found in voltmeter test leads work well.

[scott0757]

Has anyone located a source to buy some of this type of silicone sheathed flexible wire? I would hate to cannibalize a good set of test leads for that. I just had to do another repair.

[Radioguylogs]

https://www.amazon.com/BNTECHGO-Sili...=AITZO53LNBA1K








You may decide to use a smaller wire, but 20 gauge was good for me.

[DEEBS11]

I just did this repair on my 78. Would stumble slightly just off idle when the motor was hot and the outside temp was 93°. I ran the bike at idle with the left cover off and used a small dental hook to pull on individual wires. The second I touched a wire, the bike shut off. I repaired the wire with voltmeter lead wire and small segments of shrink wrap with internal adhesive. I also modified those pinch points so the are not so rough on the wires. About 800 miles so far with no issues.