1991 ST1100 very weak spark

About last fall I noticed a hesitation. This summer I put on new coils and the problem went away, long enough to do a BBG. Then was out in a rain storm and seemed to lose a cylinder. Made it home running on three and after a couple of weeks of good cylinder/bad cylinder found out that the right bank would get weak spark intermittently. I then put one of the old coils back in and it ran great - for a few miles.

I began to see a trend in that every time I messed around in the coil area the bike would get better. With this in mind I removed the small wire harness that connects to the coils and soldered each connector, then verified good and stable connectivity with an ohmmeter. Put the harness back on and now almost non-existent spark. I then bypassed the 3P connector for the coils and soldered the wires. No change.

My troubleshooting reveals battery voltage during crank on the Bl/W wire that is common to the coils. I disconnected the Spark Unit and verified I have good ground to frame on the G wire, good connectivity as well as battery voltage on the Bl/W wire with key on, good connectivity on the Bu/Y wire that goes to the left bank coil, verified the side stand switch works on the G/W wire, verified the neutral switch works on the Lg wire, and have about 450 ohms between the Y and W/Y wires that are the pickup coil for the Pulse Generator.

At this point I suspect the Spark Unit, but really have nothing to go on. I did put my meter on the Bl/Y wire for the left bank coil and can get glimpses of battery voltage that seems to indicate the Spark Unit may be working. I don't have an oscilloscope to verify.

Any help is appreciated.

Dave

I had a bit of this trouble with my ST when I got it roadworthy after a 10 year park-up. I had some general corrosion in the red connector feeding power to/from the right switch pod, and that gave low voltage at the ECU connector. The red connector lives behind the steering head in front of the airfilter/carbs.

I also had some dodgy connections between the coil and plug cap that gave erratic resistance readings. I trimmed the spark leads back 4mm or so at each end to get new mating surfaces, and cleaned out the resistor in the plug cap for good measure.

I'm with Terry on his suggestion that you look into the plug wires and caps. The fact that you see changes while messing with the coils makes that seem like a good place to look.

Thanks TerryS and dwalby for quick responses. I've been in that red connector as well as having it apart several times with this episode and it seems clean. I will clean the kill switch when I work on it tomorrow. I did measure the plug caps and they were in spec; the plug wires were new with the coils.

Just to double-check, measure the resistance from cap-to-cap on each side. You should read about 25K Ohms. 5K for each cap and 15K for the coil secondary.

Might be worth while to remove some plastic to get to the spark unit and check the connector contacts for corrosion.

sterjll said:

I did measure the plug caps and they were in spec;

Click to expand...

with high voltages weird things can happen. your plug caps could measure fine in a static test, but intermittently fail in their real application. Not saying they're bad, but keep them in mind, they're 26 years old and have been in a hot environment that entire time.

Jim Van, both sides measure 25K. I had the spark unit off earlier and it was clean.

dwalby, I replaced one cap in June for an unrelated issue. I'll keep your idea in mind. What's really frustrating is that I started out with the right bank intermittently misfiring, and now an extremely weak spark on both banks.

I made a new coil harness that goes from the 3P connector to the coils. No change.

Sounds like a really interesting problem. And it seems like you have checked all the obvious causes.

Any chance the primary connections have been switched around by mistake ?

And, how "weak" is weak ?

Might be time to try another module , to rule out the module. I bought a spare off Ebay for about $25. I carry it in my top case. That way, I will never need it.

BTW, I bought some yellow silicone 7mm spark plug wire by the foot at my local auto parts store for my ST1100. Didn't trust the original wires. I may replace again in a few years, just to be on the safe side ( Did you hear that, Mr. Bush ? ).

Jim Van said:

I carry it in my top case. That way, I will never need it.

BTW, I bought some yellow silicone 7mm spark plug wire by the foot at my local auto parts store for my ST1100. Didn't trust the original wires. I may replace again in a few years, just to be on the safe side ( Did you hear that, Mr. Bush ? ).

Click to expand...

Jim thanks for posting about the plug wires; is there any specification I should look for other than 7mm?

Loved your comment about never needing something that you always carry too.

Not really, just get some 7mm solid core ( not resistive ) spark plug wire. My local auto parts store ( Parts Plus ) didn't stock it, but they had it at the warehouse and I got it the next day. It's relatively cheap - buy an extra foot so you have plenty for the job. I would think it's also available down under. It's generic spark plug wire. But I like the yellow silicone stuff.

Like this : 7mm solid core

Packard 440 solid core is also a high quality spark plug wire.

Much obliged, thank you Sir.

Jim Van, I tried the primary connections both ways and no change. My weak spark test was the screwdriver method. I decided to pull the spark plugs out and see how each looked and -- the spark was good. This was after I had created and installed a new coil wire harness. Put the plugs back in and still could not get the bike to start.

I found an article that references the red connector in front of the carbs and above the coils. There are four connectors there, but only the red and white connectors have an effect on the engine running. I took the male pins out of the red connector and filed them clean (they did not look dirty, but they shined after I filed them). I used a lot of carb cleaner and electronic cleaner on both halves of the red connector, then put it back together. I got the bike to start on the RH bank. Lots of wire wiggling and the LH bank was somewhat alive, just not stable. But, as time and wiggles went on the bike is now running perfect.

I am not sure why, with the exception of the work on the red connector and lots of cleaner. I'll try the bike in the AM and see how it goes.

Thanks for the responses all.

Dave

Glad to hear that you got your bike running Dave.

I finally exorcised my electrical demons by fitting new spark plugs; prior to that I had one cylinder that would stop firing when partly warm, but once up and running gave no trouble for 100's of miles at a stretch. With new plugs I have had no problems at all (quickly touches wood).

Maybe you've got an actual conductor break in a wire somewhere? Might be time for a wire by wire continuity test while wiggling.

Bike will not start this morning, wire wiggling not helping. My next step is to put a timing light on it to verify timing. I have ordered a spark unit that will arrive next week.

As far as the suspect connectors are concerned, try measuring the resistance across the connector of the corresponding pins. You may have a bad crimped connection, or the wires could be broken inside the insulating. The new harness was only half of the wiring, right ?

See : http://koczarski.com/mmartin/Intermittent.htm

Jim, the harness listed in the article is also the one I made.

While working on this thing, I tried pulling the carbs and cleaning them, but no change. Well, there was a change - #4 was dead, no spark. Finally something a little more solid to troubleshoot. I swapped boots - death stayed with #4. I swapped the wires - and the problem followed the wire.

New wires ordered from Partzilla because no one around here carries solid core wire anymore. Should be here late Tuesday barring hurricanes, tornadoes, earthquakes, and nuclear missiles from North Korea....

That's strange, in post #4 you said the wires were new, was one of the new wires bad?

Dalby, yes the wires came with a set of aftermarket coils I bought in early July. I lost confidence in them when a no-spark condition followed a wire to another cylinder.

Here is an update on the bike. Current symptoms are no start when bike is cold. Can hear what might be described as a single cylinder trying to fire. With much coaxing I can get the bike to start, but it is running on only one pair of cylinders. It idles at about 500 rpm, and will either die or stay running with some throttle. I can hear the other pair of cylinders fire occasionally and see the tach increase. As the engine heats up I can give it lots of throttle and get the tach up to 5k on two cylinders. As the bike gets up to operating temperature I'll get all four cylinders firing above 3k or so. With the engine hot I can shut it off and restart it at will, but only on two cylinders. If I let it sit all night and completely cool off I have a dead engine again. A different spark unit has no effect on the symptoms whether the thing is hot or cold.

All of this was done with the fuel tank sitting next to the bike so as to eliminate any effects of heat on the fuel pump.

I have ordered two Honda coils.

Have you replaced the spark plugs yet ? If yes, You may want to consider an exorcism as the next step.

The problem is fixed. The coils I put on this summer were junk. I bought and installed two Honda coils and the bike started and ran like new. A carb synch done, then a test ride, all good.

Thanks for the help and feedback.

Dave