1980s Japanese Motorcycle electrical question (NGC)

[BSABEN]

My 1980 XS850 has what appears to be a week spark. It is a bear to start but once running sounds very good. I have dhecked all electrical connections and grounds and the kill switch and ignition switch. Carbs have been cleaned, new plugs, good battery, good plug caps and leads, coils tested and good and valve adjustment recently done. compression about 135 on all three.

My question for test purposes is can i run 12 volts direct to the feed on the coil without damaging the cdi? Two wires go to each coil. I assume one is power and one is from the "ignitor" that tells the coil field to collapse and create a spark (like what points would do; create a ground). I am not sure if the wire that feeds power to the coil is a continuous supply of 12v.

I would like to test this to see if there is a power draw somewhere that is causing the weak spark.

[John Croucher]

Tried and true test.  Start the engine, grab the spark plug wires and caps one at a time to find leaks.  Most likely you have bad wires and caps on the old bike. 

And check the coils.

[nc43bsa]

I've worked on exactly one of those bikes, so you'll have to remind me.

Does it have points and condensers?

[Old Jock]

Don't think points as OP metnions CDI

I just had a quick look at a wiring diagram IF IT'S THE CORRECT MODEL

The positive feed to all 3 coils comes from the ignition switch so I'd say yes it would be perfectly feasible to run a flying lead from the battery, for example, to the coils

You would still need to have the switch on though as it appears the CDI also takes a positive supply from the same line (or run another temp supply)

Remember not to leave the supply to the coils on too long. With the supply in place and the engine dead the coils will start to overheat pretty quickly when the engine is not runnning

John

[Kiwi_Roy]

I'm guessing its a Yamaha XS850
http://www.classiccycles.org/media//DIR_1653304/DIR_1792956/7b4ebf4db60cffe5ffff81ecffffe417.pdf
As John says but perhaps measure the R/W wire to see what's on it, might be low for some reason e.g. dirty switch.
 I can see the R/W is plus 12V but I can't see the ground to the ignitor, perhaps its just through the chassis and the negative is assumed to be there, not shown as a wire like on our Guzzi's
A good ground to the ignitor would be essential, its the other half of the circuit, perhaps spruce it up a bit.
Pull apart all the connections clean them up and smear the joints with Vaseline, even the wire to lug crimps can develop Voltage drop if they corrode, if you replace wiring dip the copper in Vaseline before crimping the lugs on, the connection will last many times longer, there's a reason they use tinned copper wire for marine applications.

[berniebee]

 I can confirm that you can connect power to the r/w wire of any coil (they are all connected to r/w for to get 12v.) without damaging the igniter unit. Actually the same r/w wire powers the igniter too. While you're at it, unbolt the igniter and clean the frame underneath it at the bolts, because it is grounded to the frame.

Hard starting is not uncommon with these triples. The igniter and coils don't work reliably unless you have a fresh and fully charged battery. After forty years, every connector drops even more voltage than when it was new. There are several connectors between your battery and the coils/igniter. Go over every connector in the path between the battery and the coils and clean 'em. Fuse box, ignition switch and stop switch connectors.

I've had "brand new" batteries that were questionable.  Hook up your battery with booster cables to a known good battery - ideally a car battery. If your bike starts right up, find yourself the best quality battery money can buy and install that.

Finally, even with a new battery and cleaned connectors, some bikes won't start reliably.  Some people install a relay controlled by the stop switch, that feeds power directly from the battery through a new fuse holder to the coils/igniter.

[moto-uno]

  Quite some time ago when these were still recent bikes , I worked on a few that exhibited poor starting . I simply dropped the plug
gap down to around .022"to .025" and low and behold it worked . No one wanted to pay for anymore troubleshooting . It worked
and cost almost nothing , give it a try  .   Peter

[Kiwi_Roy]

Further to the last two posts, there's another thing that goes wrong with bikes of this vintage the wiring around the headstock has been twisting back and forth thousands of times.
The copper in the wires work hardens where it flexes and the strands snap, it can be completely broken through just making contact in overlapping pieces,
If you pull as hard as you can on each wire often the insulation will stretch proving that it's broken inside.
When you replace the wiring around a headstock don't wrap it up tight. Re-arrange it to flex gently over several inches rather than at one point.
A friends early 70 Kawasaki had 5 broken wires. Really the looms should be replaced at that age, all new components and fresh wire will breathe new life into the old girl.
I say again dipping the lugs in Vaseline will double the lifespan of joints in wet areas keeps the copper shiny instead of going black or corroded.
And I shouldn't need to say vaseline is Gods gift to battery joints.

[Two Checks]

135 psi sounds a bit low to me. Its not a game stopper. Once warmed up is it hard to start? If not that would point to something in the cold start circuit in the carbs. Iirc those have jets in the "choke" circuit. Try covering a carb throat when starting.
I had one of those that was the factory full zoot touring model. What a POS.

[moto-uno]

  In reference to your compression tests , do you perform these with the throttle fully open ?
Kinda hard to compress what's not there  ,  Peter

[Two Checks]

Yes, wide open.

[BSABEN]

Update: ive been cleaning connections, starter button, kill switch, etc. it will start but takes a lot of cranking. Runs great once it starts.

Compression test was done with throttle open. 135 seems on the low end but all cylinders are the same and when it is running it seems strong.

Battery shows 13.1 volt but voltage at wire to coil is 11.3 when cranking.

More tinkering tonight. I will trace voltage from battery through ignition to coil and will check where wiring bends around the steering head

Thank you for all the good advice.

[Kiwi_Roy]

Battery, fuse box, ignition switch then back to the coils.
The Voltage drop may be worse than you are measuring because of inrush current, i'm sure you are on the right track.

[moto-uno]

  And don't be afraid to drop the plug gap , if it makes no diff , you can just regap . Easy and free  . Peter

[bodine99]

My 80 GS-850 same deal set of green Dyna coils fixed it

[Kiwi_Roy]

  And don't be afraid to drop the plug gap , if it makes no diff , you can just regap . Easy and free  . Peter

That too, the spark might look fine out of the cylinder and not able to make it at 150+ psi.
I do think that a weak spark is probably the result of little Voltage drops throughout the system.
My pet peeve is that nasty carbon core HT lead, you should be able to measure the resistance from plug cap to chassis, identical for all 3 cylinders.
If the HT is not grounded at the coil it will be connected to the primary and perhaps the common positive.

[BSABEN]

I went thru all of the wiring and cleaned connections. Disassembled, cleaned and reassembled kill switch, hi/low and blinker switch and ignition switch. Wiring on these bikes seems really unecessarily complicated! Anyhow, a connector of 4 wires coming out of the voltage regulator had a lot of corrossion and a touch of melted plastic. I cleaned these connections and at this point the bike starts a lot easier. Spark is still not the thick blue I would like but at least the bike now starts amd rums good.

[berniebee]

Did you clean the fuse box connector?