XS1100 ignition misfire, jumping tach and possible vacuum advance issues

[GMGalambos]

I bought an 80 XS1100 Special not running. I was told it ran fine a year prior. The first thing I did was rebuild the carburetors and synch them, change spark plugs, and change oil/filter. After that, it ran ok but was obviously misfiring.

I then found 3 breaks in the ignition pickup wires. I soldered those back together. It ran better but still not well. Resistance does not change at all now when pulling on the wires.

I then checked valve clearance. Most valves were too tight so I adjusted those and re-synched the carbs. That made no difference.

I checked timing next. Timing was pretty close but I noticed the vacuum advance was not acting right. It rocks back and forth as you open the throttle. When the RPM's finally stumble up to >4-5K, the vacuum advance pulls to full and is stable. I put a vacuum gauge inline with the vacuum advance actuator. The needle will shake around 2-4" of vacuum until the RPM'S increase. Then it rises with engine RPM. I hooked the vacuum gauge directly to the vacuum port on #2 carb. Vacuum fluctuates wildly.

I believe I have weak spark. Using the timing light, I don't get a flash on every revolution. As the engine revs up, the timing light flashes less until it goes away completely. When the engine comes back down to idle, the flashes come back. I put a voltmeter on the primary side of the coils. Voltage starts at 11V and drops to around 6V as RPM's increase. I tried a different set of coils and got the same result. I tried a different timing light and got the same result.

Battery voltage is 12V off. 2K RPM's = 14.15V. Full field = 15.8V

Tach jumps around 1-2K RPM's when revved over about 2K RPM's.

I'm thinking I have a bad voltage regulator, TCI module or both. I'm at the point where I think I need to start swapping parts.

I'd appreciate any thoughts or suggestions. Also, if anyone in the Atlanta GA area has a similar bike, I'd like to take some voltage and vacuum readings off your bike.

I have searched extensively on this forum and with Google about my issues. The jumping tach suggests a voltage regultor issue. The weak/missing spark I cannot find anything on.

Thanks for your help!

[natemoen]

I think the first thing you should do is go through and check the resistance on the ignition coils.

Unplug the small wires going into the coils from the TCI and check ohms across them. Should be close to 1.5 ohms. Along with this there should be a ballast resistor (maybe I am wrong but I dont think they got rid of it till '81 but I have been wrong before) along the left upper rail just behind the ignition coil, check ohms on that as well should again be about 1.5 ohms

remove the spark plug caps and check the ohms from that end. Should be about 15000 ohm. Then check each spark plug cap, shouldn't be more than 5000 ohms.

So this first then report back.

[bikerphil]

When you have the plug caps off, trim 1/4" off each wire. They can corrode at the end and give you a flaky spark.

[SFerinTEXAS]

I must first give you a BIG SALUTE. Its very rare lately to see someone with a 1ST post on here that has shown the initiative you have .Refreshing.

Now some random thoughts from me.

Jumping tach can just be a jumping tach. Charging system seems ok.

Have you done a compression test? Burnt valve may be possible.

Recheck p/u coil wires, fixed may not stay fixed. Manually rotate the advance when doing the ohm continuity test at the tci plug.Also check the vac hose for leaks.

Do the header pipes all get hot after a few minutes at idle? Sling some spit om them or use a thermal temp gun.

[motoman]

Vacuum advace assembly the diaphram attaches to is cruded up, not rotating EASILY. Should just flip-flop. Apparently with the mechanical and springs located there, gets that brown rusty lookin stuff. That assembly gots to be floppy-loose clean and lubed! May SEEM alright, but will bite you in the butt chasing other possible causes to poor off idle and mid-range smoothness if not working properly.......ALWAYS check proper operation of primary and secondary ignition sources before screwin' with carbs!

[TopCatGr58]

I bought an 80 XS1100 Special not running. I believe I have weak spark. Using the timing light, I don't get a flash on every revolution. As the engine revs up, the timing light flashes less until it goes away completely. When the engine comes back down to idle, the flashes come back. I put a voltmeter on the primary side of the coils. Voltage starts at 11V and drops to around 6V as RPM's increase. I tried a different set of coils and got the same result. I tried a different timing light and got the same result.
Thanks for your help!

Hey there,

The bolded text above stuck out to me...as was mentioned the 80SG uses the 1.5 ohm coils AND the ballast resistor in the circuit. During Starting mode, the TCI sends FULL 12 volt power to the coils, once the engine has started, it redirects the current THRU the ballast resistor and so the coils then only should get ~9 volts...to keep from Frying them!

Yours shows 11 volts during running...which is too high, then drops to 6 volts which should be too low! The PO may have bypassed the ballast resistor by just joining the wires on the harness together and disconnecting the B.R. from it...thus sending 12 volts to the coils all the time. This is a NO NO with the 1.5 ohms coils..will burn them up, and can also fry the TCI because of too much current draw, not enough resistance on that line/output.

Not sure why the voltage would be dropping, unless the coils are heating up from too much current and creating increased resistance to the current flow that could possibly drop the voltage you're seeing...but I'm not a real Electrical GURU..so I could be in error here on this aspect!?

T.C.

[SFerinTEXAS]

I really don't understand how it is possible to get a true voltage reading from a running coil. The faster it runs the more rapid the switching.Thats where I hook up external tach for carb sync.. Not in any of my testing procedures.Try hooking the timing light to each plug wire and see if the light fades on them all.

Voltage starts at 11V and drops to around 6V as RPM's increase.

Could just be the volt meter trying to catch up

[SFerinTEXAS]

SFerinTEXAS I really don't understand how it is possible to get a true voltage reading from a running coil.

MY bad. Got my wires crossed. TCI voltage will be present at the "common" wires, gray if I recall. That being said I recall another experience I had with a low end stumble. weak battery. Would not keep the TCI voltage up at idle where there is almost no charging taking place and would slowly let voltage drop at idle. Could explain the timing light mystery if that is the case. Just saying. Something easy to check and rule out.

[GMGalambos]

Thanks for all the input! I will do some more testing tonight.

I did test resistance of the primary side of the coils, but at the coils and not at the TCI. I will do that. I will also check the ballast resistor and recheck the ignition pickup wires.

I did do a compression test after the valce adjustment. Results on all 4 were 165-170 PSI. Cylinder leakdown on all 4 was 5-10%. Engine was warm for both tests.

I'll also check the vacuum advance unit for crud in it.

I did hook up the timing light to every cyclinder. They all behaved the same. Flashing at idle and no flashing in the upper RPM's.

Battery is new as of a month ago. Vacuum line to advance unit is new too.

I will post my findings tonight or tomorrow.

[natemoen]

Make sure to test the resistance on the spark plug side of the coils as well!

[GMGalambos]

update

Some testing results:

Ballast resistor = 1.9 ohms
Primary side of coils = 1.8, 1.9 ohms
Secondary side of coils = 15K, 15K ohms
Caps = 5.5K, 8.5K, 10K, 10K ohms
Ignition pickups = 75K, 80K ohms, no fluctuation when pulling on wires.

I have a set of spare coils with caps. They tested out about the same as my original set.
Am I looking at stacked tolerances? Too much overall resistance creating a weak spark? I have a pair of used Dynatek green coils (3 ohm) I could put on there. As I get older I'm more a fan of keeping things stock. I would need to aquire a set of ignition wires to put them on too.

[TopCatGr58]

Hey there Gm,

I've found a few caps with 8k ohms and the bike seemed to run okay.
You can take the caps apart...they unscrew from the plug fitting end, and then you can inspect the internal resistor, check for corrosion on it, the spring, clean and recheck resistance on that resistor...then after cleaning if you see the ~5k value, then reassemble and see if you get the same value thru the entire cap. IF the resistance stays way above the 5k range, then you may want to get new caps vs. trying to rig up something inside the caps to take the place of the resistor so that they would be 0 ohms!

The OEM coils are WEAK at best anyways, generating only about 15KV, and the Dynas will get you 30KV or so, much easier starting, better low rpm running when the charging system is not effectively charging and providing the 13+V's to the system...so with OEM's and Ballast Resistor...that voltage can drop below the ~9 volts they would normally see=weaker spark!

T.C.

[SFerinTEXAS]

Ignition pickups = 75K, 80K ohms, no fluctuation when pulling on wires.

720 ohms ± 20% Maybe you meant 750 and 800

[BigRed]

You may want to check your ground strap and the ground connections at your voltage regulator. I was losing some voltage there before. I put on the green coils from Mikes XS, bypassed the ballast resistor, and put in a relay to put full battery power to the coils. The actuating side of the relay uses the bike's stock power wire to the coils.

[GMGalambos]

update

Last night I checked and cleaned my grounds. I took the 4 Spark plug caps that had resistance closest to 5K ohms and cleaned them up. I cut off 1/4" off the wires and resecured the caps. Spark is much more consistent now. I believe my weak spark issue is resolved. However my vacuum advance is not functioning properly. I took it off last night and cleaned it. It seems to move properly. It goes to full advance with 6" of vacuum and holds. When the engine is running, it still rocks back and forth with the fluctuating vacuum off-idle.

Can someone put a vacuum gauge on your #2 carb ported vacuum and tell me what the readings are at idle and off-idle? Mine are so erratic I think I might have a hanging valve on that cylinder. Compression/CLT tested good. Is there something in the carb that smooths out ported vacuum? Something I forgot to put back in when I rebuilt them?