Misfire

[sthomag]

I've got a problem with my 79 F that is driving me nuts.

It has started misfiring at low RPMs, only on one or sometimes two cylinders. I've gone through the ignition with a fine-tooth comb: the pickup wires are in excellent shape, I've switched out the ignition module with no effect, and I've checked and cleaned every connector. I switched out the spark plugs, which look clean. Still, when I put a timing light on No. 2 cylinder, I get an intermittent interruption in the spark.

This only happens at low RPMs, up to around 3,500 or 4,000, lower if the engine is completely warmed up. Above that, it clears its throat and runs well.

The bike has Dyna coils, with maybe 10,000 miles on them, and plug wires and caps replaced the same time.

Could this possibly be a fueling issue? The carbs were cleaned less than 10,000 miles ago. I did fill the tank right before this started happening.

[aquamantx]

I am not an expert by any means but I don't think fuel will be the culprit. You say you lose spark on the #2 plug wire. I don't think fuel interupts the coils firing on bikes this old. I would check wires again. I believe each coil fires 2 plugs. I would expect the miss to be on the 2 plugs off the same coil.

[hamjam]

check the resistance to the plug cap if ok
cut a half inch off the plug wire and reinstall the cap
if not replace the cap

[MaximPhil]

Hi sthomag,
I would check the pick up coil wires and the operation of the vacuum advance mechanism all under the left engine cover.

Phil

[vedgar]

pinch off the vacuum advance hose to kill it

See if the misfire is gone when the vacuum advance is at steady state.

[motoman]

My first suspect with this scenario is low voltage TO the coils at the lower rpm range. The low voltage suspect area is FIRST the handlebar kill/run switch. This is the common spot for a voltage drop. The kill/run switch DOES complete the running circuit. Easy check for voltage drop: key switch on, check voltage AT battery terminals. THEN check voltages TO each coil. They should be the same. If voltage TO coils is less than at batt. terminals with key on, then voltage drop is due to corroded brass contacts in kill/run switch. If this be the case, remove and carefully disassemble switch, preferably with switch inside a sandwich bag so the teeny internal spring doesn't get lost. Drop all pieces into a shot-glass full of evapor-rust overnite. Then remove and wash pieces off with water, blow dry, re-assemble and install.

[TopCatGr58]

Hey there,

Okay, I have to ask, you say it's a 79 with Dyna coils, but are they the 3 ohm type, or the earlier 1.5 ohm type that can be gotten also, and what are listed in the OLD Dyna coils tech tip!?!?? And if they are the 1.5 ohm, then you should still be running the Ballast Resistor. BUT if they are the 3.0 ohm type, did you remember to remove/bypass the ballast resistor since the TCI will see the 3 ohms from the coils themselves, and the BR is not needed, and WILL cause a reduced coil voltage since the TCI routes the power thru the BR while running, so the voltage will be ~9 volts instead of 12, which is what the 3 ohm coils are designed to work with(12V), and that reduced voltage could be contributing to the lost of spark on 1 of the plugs. It's a rarer event, but a few folks have reported the loss of spark on 1 plug which will occur with reduced voltage....coil generates just enough power to send spark thru the 1st plug directly off the coil, but not enough juice to push it thru the engine/head and then back up thru the other plug, so you can drop 1 cylinder on a coil pair, and it IS an electrical issue, not a fuel issue!

T.C.

[Bonz]

Does the Kill switch (how it grounds to the bar) complete the running circuit on 78 and 79? I was thinking it was only on 80 and later XS1100's?

[3Phase]

Does the Kill switch (how it grounds to the bar) complete the running circuit on 78 and 79? I was thinking it was only on 80 and later XS1100's?

All of the XS11s and the XJ11 Stop/Run 'kill' switches open or close the 12V Ignition circuit with the Red/White wire. No circuit, no current, no ignition.

The Tip-Over/Emergency Stop switch does ground the Black/White wire from the TCI but it's not used for the Stop/Run switch.

[hamjam]

Hi have you had any results with this yet ????
Thanks
Jim

[sthomag]

Hi have you had any results with this yet ????
Thanks
Jim

Sorry to take so long to get back to you. I've been abroad for three weeks; just got back yesterday. I really do appreciate everybody's help on this.

The Dyna coils are 1.5 ohm (red), and they're running with the ballast resistor.

I measured voltage at the battery (13.5 volts) and at the coils. Both coils measured 12.5 volts. I assume the 1-volt drop is due to the ballast resistor.

So both coils are getting 12.5 volts, but only the one powering plugs 2 and 3 seems to be not doing the job. Could it be a bad coil? I guess the next step is switch them around and see if the misfire moves also. Is there a way to measure impedance or something to test the coil?

I also took apart the kill switch. It looked fine, but I dressed the contacts with a diamond file anyway. Voltage through it is also 12.5 volts.

I didn't start the bike because it is really cold out there right now, and I didn't want to open the garage door.

I'm thinking one possibility is a failing voltage regulator. Would that cause low voltage at low RPMs, leading to a faulty spark? Otherwise, I'm kinda out of ideas.

Again, sorry for the delay getting back to you.

[motoman]

Sorry to take so long to get back to you. I've been abroad for three weeks; just got back yesterday. I really do appreciate everybody's help on this.

The Dyna coils are 1.5 ohm (red), and they're running with the ballast resistor.

I measured voltage at the battery (13.5 volts) and at the coils. Both coils measured 12.5 volts. I assume the 1-volt drop is due to the ballast resistor.

So both coils are getting 12.5 volts, but only the one powering plugs 2 and 3 seems to be not doing the job. Could it be a bad coil? I guess the next step is switch them around and see if the misfire moves also. Is there a way to measure impedance or something to test the coil?

I also took apart the kill switch. It looked fine, but I dressed the contacts with a diamond file anyway. Voltage through it is also 12.5 volts.

I didn't start the bike because it is really cold out there right now, and I didn't want to open the garage door. Also, voltage at battery should be checked with key ON......then, with key still on, check voltage plug-in to each coil. Should be same-same, unless if there is a voltage drop THEN, definitely a poor connection either at STOP/RUN switch or at the wire plug-in for that switch on top right side of frame under fuel tank.

I'm thinking one possibility is a failing voltage regulator. Would that cause low voltage at low RPMs, leading to a faulty spark? Otherwise, I'm kinda out of ideas.

Again, sorry for the delay getting back to you.

For checking ohms resistance of coils, set your meter to OHMS, test leads across both plug-ins of coil. Should read 1.5ohms. If reading lower than that, would suspect coil has internal resistance failing.

[sthomag]

For checking ohms resistance of coils, set your meter to OHMS, test leads across both plug-ins of coil. Should read 1.5ohms. If reading lower than that, would suspect coil has internal resistance failing.

Went out and bought a decent multimeter today, instead of the cheap Radio Shack thing I've had for 30 years. This is what I found:

Both coils primary resistance: 1.6 ohms.

Secondary resistance on both (between the two plug cable sockets) was about 12.46 K ohms.

Resistance on all the plug caps with plug cables attached is between 5.26 and 5.5 K ohms.

Battery voltage with ignition on, but not running, is 12.25 volts. Voltage at the coil positive terminals is 11.75 volts, again with the ignition on but the engine not running.

Anybody have any ideas?

[motoman]

That voltage drop is highly likely fault of STOP/RUN switch. Best option for cleaning those contacts: Remove switch, take apart(careful of teeny internal spring), Drop ALL pieces, including switch into a shot-glass full of Evapo-Rust overnite. Then remove and rinse pieces off, blow-dry, re-assemble and install. Bettin' that'll give voltage TO coils (key on) equal to voltage at battery(key on). Key switch internal contacts are another common location for voltage drop.

[TopCatGr58]

Hey Thom,

Okay, glad you checked the manual to check the ohms on the coils.

A few points. The voltage you checked at the coils was with the key on, but the engine not running. The TCI bypasses the B.R. during starting to send full battery voltage to coils for easier starting, but once it's running, the TCI then routes power THRU the B.R, and that's when you'll see/measure any voltage drop at the coils. So...you can put a jumper wire onto each coils (+) terminals so you can then check them with the engine running to find the running voltage, would be concerned if below 9 volts.

Other voltage loss points is the MAIN KEY ignition switch contacts, can be taken apart and cleaned. Also, check the TCI to harness contacts, clean as well. Best to clean EVERY harness contact point, but you can use the wiring diagram to follow the voltage path back from the coils to try to find corroded connectors.

Also, have you checked and cleaned the ALT connectors BEHIND the fuseblock, they also corrode and cause voltage loss, as well as melt the connectors! The ALT doesn't put out much voltage, barely 12 as idle/low rpms, but should be close to 14.5 at 2500 rpm....best to check that out AFTER cleaning the ALT harness connectors. Also, clean the ground wires/contacts BELOW the Reg/Rect with the frame, poor grounds also can cause voltage drops.

Also, check the connections at the B.R. for corrosion which could also cause excessive voltage drop since the power has to go thru it before it gets to the coils.

T.C.