Major Stumble, Misfire. Troubleshooting Help

[Janky59]

Hello guys, my 1992 907ie recently (a week ago) developed a significant engine stumble. I’ve owned it 2.5 years, it is usually a reliable starter and steady runner.
I’ve read many posts here for troubleshooting, have done some tinkering but I’m getting nowhere.
It started normally last week on the high idle lever, normal crank over and no throttle necessary. While I was putting on my gloves it dropped in rpm a lot, then stalled. I restarted it, but it needed some throttle and didn’t sound like usual. I continued out on my ride, but knew right away something was off. I was hoping riding it and getting it hot would sort it, but no luck. The temps were 60’s-70’s, quite routine. After a few miles the engine temp gauge was reading normal.
I ran thru the gears hard, no change. I did that again with steady throttle for a bit at 6K-7K rpm, no change.
So what is going on is: the engine is stumbling badly, I think 1 cylinder is dropping in and out randomly. It will run/pull normally for 2-3-4-5 seconds, then there will be a split to maybe full second of reduced power. It’s not totally dieing, it stops pulling hard and sounds different, then it picks back up for 5-6-10 seconds then stumbles again.
Starting it since this glitch, it needs the throttle opened for it to light off.

I have an audio clip saved, but it doesn’t look like the site will let me upload it. And the video clip is too big to upload.

Particulars on the bike: 907ie, mostly stock. 20,650 miles. Belts and valve clearances set before I bought it at 16K or so miles. It does have a pair of Termi slip ons, big hole in the top of the airbox with an K&N filter, and a Feracci ECU chip. Earth X battery.

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So far I’ve:
Unplugged/reseated any FI related connections I can get to.
Looked over the grounds.
Unplugged/reseated the side stand switch.
Unplugged/reseated the ECU connector.
Unplugged/reseated the 3 coolant temp connectors on the coolant manifold.
Installed fresh spark plugs.
Removed the air temp sensor and verified it responds to warm and cool airflow.
Measured both coil primary and secondary resistance values, 0.5-0.6 ohm and 4.5 to 4.7 k ohm.

All help appreciated…

[higgy]

The first thing I would do is replace the in tank fuel filter. After which I would make sure the fuel pump is working.
After verifying the filter and pump time to do some voltage drop tests
start with the fuel pump relay. both sides
and then the coils

see where that leaves you

[Janky59]

Here’s a link to a YouTube video that my daughter helped put together. Part video clip and part audio clip.
Take a look and a listen.

https://youtu.be/LJqv_ei3y_U?si=jOJShZr_Z0oG7oPO

[higgy]

I have moved my filter for easier access as well. Still I would start there. Running on one cylinder for sure. Did you check for water in the fuel?
easiest way to check for that is to drain the tank. Other possible issues could be the spark plug wires leaking somewhere along their way to the plugs.
all I can think of at the moment. Lets us know what you find

[Janky59]

Higgy, I didn't check for water. But with it apart some, draining the tank will be easy to do.
I've thought about the plug wires for sure, and coils.
Something I did yesterday was to start the bike, ran it for maybe 5-6 seconds with the idea to touch the horizontal and vertical exhaust pipes right out of the head and see if there was a notable temperature difference.
Both got hot to the touch, but the vertical seemed a little cooler.
I based this on how long I could keep my fingertips in contact before I had to pull away.
The horizontal had more "ouch" to it than the vertical. A little subjective, but I think it's an indicator.
Maybe I'll buy one of those infrared temperature guns or call some friends.
Anyway. this will all have to wait a bit as Mrs. Janky59 and I are headed on a road trip to Fl this morning.
Updates to come, regards.

[Janky59]

Update: I sourced a new coil from Gotham Cycles, installed it for the vertical cylinder, no improvement. Then I ordered a set of plug leads from Magnecor, initially 1 wire was too short, so I got that sorted with them.
The replacement wire from Magnecor arrived yesterday, things reconnected etc. and no improvement.
I did get ahold of a non contact thermometer. The vertical cylinder is the issue. Its exhaust pipe temperature close to the head was 80-90 deg F lower than the horizontal one. I going to injector cleaning next.

[higgy]

What are the specs on the new coil ?

[Janky59]

Sorry for the late reply Higgy.
The new coil measures 0.5-0.6 K ohm on the primary side. (My meter only has a K ohm setting) The secondary is 6.92 K ohm.
From another post, these numbers are appropriate.

[Janky59]

Update: not a good news update.

I sent the injectors to Injector Repair LLC, Glenville, NY, and spoke to owner “Jim”. I was hoping he would say the vertical cylinder injector was wonky, but he said they were working perfectly. They did their standard cleaning, new o-rings, new screens and before/flow tests. For $40 it was quite reasonable, and Jim was engaged with me trying to help troubleshoot my problem.

But no fairy tale ending.
I reassembled yesterday, (new NGK DCPR8E plugs, new plug wires, new coil for vertical cylinder) did a short test ride around the neighborhood today. No improvement; still major stumbling, intermittent running on 1 cylinder (I still believe the vertical one is the issue), lack of idle, temperamental starting, etc.

Another temp reading of each exhaust pipe near the port after running 15 seconds has the horizontal at 175 F and vertical at 140 F.

I guess I’ll read up on how the vertical cylinder gets its firing signal.

[higgy]

Just in passing, stick to the champion spark plugs. I know but they really do work best.

questions Are you using a non stock coils ? 0.5~0.6 shouldn't cause problems

The wrong impedance can cause misfiring.
Air temp and water temp.
Valve clearances
Plug gaps champion RA4HC 0.7-0.9mm 0.0275~0.035 inch

Weak or No Spark:
A larger gap means the spark has a longer distance to travel to jump between the electrodes. This can make it more difficult to produce a strong, consistent spark, especially at high engine speeds or when the engine is cold.

Misfires:
If the spark is weak or inconsistent, the fuel-air mixture won't ignite properly, causing misfires. Misfires can lead to reduced power, poor fuel economy, and increased emissions.
Poor Fuel Economy:
Incomplete combustion due to misfires means more fuel is wasted and not fully burned, leading to lower fuel efficiency.
Hard Starting and Rough Idle:
A weak spark can make it harder for the engine to start and can cause a rough or uneven idle

[Janky59]

The coils are stock. The horizontal coil from its appearance and markings is the original one. The vertical coil is a replacement that I installed during this troubleshooting process. It is the OE type, mentioned previously being sourced from Gotham Cycles, and the resistance numbers above were for that coil. See the attached pic, it’s on the left.

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I was looking at the wiring schematic this morning. What are the 2 items labeled “MAE” ? They aren’t mentioned in the legend page.
As a wire from the vertical coil goes to one of them, I want to go down that rabbit hole. What do they physically look like ? Are they also referred to as “power modules” ? Maybe I have a loose connection there.
IDK, just trying stuff.
FWIW, this misfire isn’t a subtle one, it’s where the bike decelerates in a split second, takes up the chain slack, then refires, accelerates a second, then stumbles again, randomly, continuously, on and on.

[Janky59]

Update: Success !!
I tracked down MAE thing, yeah, I think it’s AKA the power module. It is located on the plate holding the V regulator and the relays, sitting below the regulator. I unplugged/reseated each connector going into it, and loosened/tightened the nearby ground. Everything looked ok, nothing suspicious.
Then…
Higgy, you are the master of Paso’s
After trying all the stuff I’ve mentioned, I acted on your suggestion to change the fuel filter. The initial start up after the swap wasn’t promising, but I thought I needed to ride it before making the call. Riding out of the neighborhood there was a handful of minor sputters, not as strong as previously, then as I rode further it improved. After getting some gas I did a 30 mile ride; man it runs good now, starts right away, idles perfect. Wahoo!

Then for kicks I put the original coil for the vertical cylinder back on it, of course it still runs perfectly.

I had it in my head I was chasing an electrical problem but it seems not. Yeah I spent some extra money and it took some time, but I learned more about this bike and did a couple of things I hadn’t done before. Thanks for the help along the way.

[higgy]

I have found the fuel filter to be the main culprit with a lot of issues on the 907. As the filter becomes clogged it makes the pump work all that much harder shortening the life of the stator due to the increased electrical loading. Once the stator weakens every thing works that much less efficiently.

The units you refer to are amplifiers for the ignition signals.

Glad you got it sorted,I know how frustrating it can be at times