Author Topic: '86 K100 Refuses to accelerate above 2500 RPM after warming up  (Read 5631 times)

Offline acyclicsalmon

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'86 K100 Refuses to accelerate above 2500 RPM after warming up
« on: October 05, 2021, 12:33:03 AM »
Greetings from the perpetually lovely California Bay Area, I come to the finest k-brick forum in the land to interrogate some of the brightest german bike wizards to assist in helping me determine what next steps I should undertake to get my bike running.

[About the bike]
  • verified 1986 K100RS
  • 28k miles
  • Very well maintained, always garaged and all that jazz
  • My favorite money pit ('88 Honda Nt650 is a close second, cursed electrical gremlins)

What I'm dealing with, in details

Here is a video illustrating the problem:

[let me paint a picture here] -- On Wednesday I finally took my k100 for its first proper ride in about 1.5 months (She's been recovering from a mild fender bender that destroyed my exhaust, which has since been replaced).  I went out for a short ride, perhaps 15 minutes, to grab a pint with the miss.  She linked up with friends, I went for a longer ride on the highway to ring out the bike.  I filled up with a full tank of premium, and got about 5 miles on the highway before my power cut, bucking me forward, before quickly recovering to about 50% power, which was just enough to allow me to get off the highway.  I rode her gently at about 30 mph, before the same bucking sensation further limited my speed to about 15-20 mph.  From that point on, I was able to slowly make my way home at about 15-20 mph, with my tach struggling to go past 2-2.5k.  I could get to 2.5k only with the gentlest of increase, anything sharper and she'd knock back to 1.5k before eventually getting back to 2k rpm.

[here's what I've checked so far]

My immediate thoughts were 1) overheating, and 2) the throttle position switch, which I had learned kills fuel during deceleration until 2k.  Below is what I've done since the incident:
  • Removed the TPS switch cord.  Let the bike cool, and went on a ride the next day.  The bike rode for maybe 10 minutes without issue before the same thing happened again.
  • Checked to ensure the coolant levels are good and that the thermometer functions.  Coolant is fresh (<2k miles), and thermometer was pulled and functioning perfectly (popped open instantly at 80*C, as expected)
  • Pulled the spark plugs (while bike was hot).  They were black and sooty and the spark plug holes weeped oil.  Even though this sounds suspect, I checked the spark plugs literally 2 days prior and they were perfect, meaning perfectly grey and clean.  My strong hypothesis is the 10 mile drive home after the incident caused them to foul and this is not the primary cause of what we're seeing
  • Checked the fuel tank -- gas was clear and clean

[here's other things I've taken care of earlier this summer, just to provide further color ]
  • Replaced the fuel filter with an OEM unit, fuel pump rubber housing, fuel pump filter + bracket, internal tank fuel line (500 miles ago)
  • Replaced the fuel line to the throttle rail (about 2k miles ago)
  • Replaced the air filter and inspected the MAF sensor -- all clean and all good AFAIK (3k miles ago)
  • Fresh synthetic oil + oil filter (3k miles ago, though at time of incident my oil was a hair low (slightly before the top part of the circle on the eyeglass)
  • Replaced some random rubbers (like the z-tube crank breather hose) and got a fresh odysee battery (3k miles ago)
  • Treated the fuel tank plug with some Deoxit

With all the above checks, my intuition is that the Hall Sensor has gone kaput, but I'm cautious pulling it being aware that she may not live to exit my garage spot.  I'm also electrically challenged, so the LED test described so well on this site leaves me somewhat terrified.  Does all the above indicate to you that this is likely a Hall Sensor problem? I have a local guy selling one for $120, which seems as good as it gets

I should also leave as an addendum, I did accidentally ride my K100 for about 300 miles last year with 0 coolant.  The coolant drain plug dropped somewhere between my coolant change in Seattle and Bend, Oregon, when my bike shut down due to overheating (oops).  My thinking is that the heat could have accelerated the hall effect sensor's doom, but I don't want to make any assumptions since the last 4k miles have been without incident.

Bike Photos

I've included some bike photos, since its always easier to help a patient when you can see who you're working with.  I will say, I checked out 4 different K100's before landing on this one, she was hands down the cleanest and had the longest and most full maintenance record.   I will keep this bike alive, even if she ends up in the hands of our Bay Area's 150$/hr specialty BMW mechanics...

[see next post for photos]


  • Oakland, CA
  • 1986 K100RS

Offline acyclicsalmon

  • Motobrick Curious
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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #1 on: October 05, 2021, 12:34:32 AM »






  • Oakland, CA
  • 1986 K100RS

Offline Scott_

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #2 on: October 05, 2021, 05:06:34 AM »
I'm guessing that all 4 plugs where black and sooty?
I'd suggest you check your water temperature sensor and associated wiring as it does affect the fuel/air mixture.
Verify your air flow meter is not jammed/sticking.

Re the oil level, the ideal level is dead center of the glass. The ring around the glass defines the maximum and minimum levels.
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Offline frankenduck

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #3 on: October 05, 2021, 06:33:22 AM »
Typical HES failure symptom is that the bike just dies suddenly and completely when it heats up and then restarts fine when cooled down.

Crappy running when the bike heats up can be caused by coils going bad.

See page 21 here:

https://bmwmotorcycletech.info/K100%20troubleshooting%20Starting.pdf





Once I had a Collie pup. Dug a hole and covered him up. Now I sit there by the hour. Waiting for a Collie-flower.
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Offline daveson

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #4 on: October 05, 2021, 07:44:12 AM »
A long shot but a quick check. If the problem disappears after lifting the fuel cap, the tank vent might be partially blocked, probably worse after filling. The fuel filter might be partially blocked from swelling if it's drawn some condensation from the fuel.
  • Victoria, Australia
  • Current;'85 K100RT~100,000km; four other bricks. Past; 1500 Vulcan, V Star 650, KLX 250(dirt bike) TT250(dirt bike)

Offline The Mighty Gryphon

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #5 on: October 05, 2021, 07:48:11 AM »
I am thinking the engine temperature sender or the coils.  Both are known weak points, especially the early model coils. 

The sender(or it's wiring) can cause the mixture to get way too rich to run properly.  It will cause all 4 plugs to be fouled.  On the other hand, a weak coil will cause misfiring, especially at wider throttle openings, and will only affect the 2 cylinders it fires.  Testing with an ohmmeter doesn't always spot the problem.  Some coil problems only surface when they get hot.  Do your coils have black or orange insulators where the plug wires connect?  Early models had black and the later ones were orange.   

Also, Scott is correct about the oil level.  Ideally, it should be on the dot in the middle of the circle.  A high level may make the plugs dirty, especially if you park on the side stand for long periods.
  • In my garage in Marilla, NY
  • '91K100RS White/Blue
Current:
'91 K100RS16V "Moby Brick Too"

Past:
'94 K75RT "Ilsa, She Wolf of the SS"
'92 K100RS16V "Moby Brick" (RIP, deceased in a vehicular assault)
'94 K75S Special Edition Dakar Yellow "Cheetos"
'89 K100RS Special Edition "Special Ed"

Offline Laitch

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #6 on: October 05, 2021, 09:04:26 AM »
My strong hypothesis is the 10 mile drive home after the incident caused them to foul and this is not the primary cause of what we're seeing
The "incident" caused, and continued, the fouling.
  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles
I wept because I had no radials until I met a man who had no splines.
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Offline frankenduck

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #7 on: October 05, 2021, 10:24:33 AM »
Do your coils have black or orange insulators where the plug wires connect?  Early models had black and the later ones were orange.   

Also, Scott is correct about the oil level.  Ideally, it should be on the dot in the middle of the circle.  A high level may make the plugs dirty, especially if you park on the side stand for long periods.

The top and bottom of the sight glass ring are the same as MAX and MIN on a car oil dipstick. Any level inside the red circle is fine. Be sure to check the sight glass with bike on level ground on the center stand with engine cold or been sitting for a couple of minutes.

I always top off to about here:



Early and late coils also have slightly different terminal locations.


Once I had a Collie pup. Dug a hole and covered him up. Now I sit there by the hour. Waiting for a Collie-flower.
New to K bikes? Click here.
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Offline acyclicsalmon

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #8 on: October 05, 2021, 12:13:59 PM »
Wow ! thank you to everyone who contributed to helping me diagnose this problem.  Really a one of a kind community here.  I've lurked for over a year but this is my first time actively posting, glad to do so.  Some responses below:

  • The youtube link was included, I hope others were able to see it.  If not, here is the link again, but I left a space in the middle in case its the auto video load thats breaking it: https://www.youtube .com/watch?v=aWsNbFavYVc
  • I'll bust out my ohm meter to check the coils, I've never peeled the cover back so its my first time seeing them.   I'll report which type of coil I have as well.
  • good to know about oil, i swear i had someone tell me you should always be at top of red circle.  Currently I'm at exactly the yellow line, maybe a hair above.
  • I absolutely could see the temperature sender being a problem, I do seem to remember the fan coming on in a strange fashion recently, where it never full ran, just seemed to speed up and slow down in an oscillating fashion.  I chalked it up to simply lower bay area temperatures..... 
  • I will try the fuel tank vent suggestion next time this happens

Thank you again, this community is absolutely wonderful and a trove of information that I hope is preserved for years to come.
  • Oakland, CA
  • 1986 K100RS

Offline acyclicsalmon

  • Motobrick Curious
  • Posts: 28
Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #9 on: October 05, 2021, 10:51:29 PM »
Fun updates! Was able to peel back the coil covers for the first time, dang they look cool.  I couldn't find my multimeter so I didn't test them, but they did look pretty darn clean and in good shape.  The connectors and plugs were all clean and I hit them with a touch of deoxit just in case.



While seeking out the temperature sensor unit (which I now see is a pain to pull), I noticed an uncovered nipple, as well as a hose with a check valve on it coming from what looked like the tank.  I assume that hose should probably connect to this nipple, but wanted to double check.

Can someone enlighten me to what the hose and nipple are for? I assume thats the tank vent line, but why would it connect to the engine? I tried digging through my Clymer's manual but it was hard to find what I couldn't identify.



On a final note, I think I'm going to go ahead and buy a new temperature sensor unit anyway -- I forgot to mention a pretty big point earlier, namely that my bike has the worst fuel economy I've heard of on a brick, something to the tune of 20-24mpg.....  Given that I dont see any hose leaks, this seems like a likely very likely culprit, and the new sensor is only about 25 bucks.  Though this ultimately seems unlikely to be the root cause of my current issue.  Thoughts appreciated, and thank you once again.
 
  • Oakland, CA
  • 1986 K100RS

Offline frankenduck

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #10 on: October 05, 2021, 10:58:56 PM »
The nipple at the front middle of the engine is supposed to hook up to the tank vent pipe up under the right rear corner of the tank. It's there to meet CA emissions standards because ten molecules of gas escaping into the air will destroy the planet. Find a plastic or rubber  cap to put over it and forget about it.
Once I had a Collie pup. Dug a hole and covered him up. Now I sit there by the hour. Waiting for a Collie-flower.
New to K bikes? Click here.
K Bike Maintenance & Mods: Click here.
Buy parts here.

Offline Laitch

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #11 on: October 05, 2021, 11:24:37 PM »
The idea was fumes would vent into the crankcase, be drawn through the z-hose along with blow-by gases from combustion, and be recycled into the fuel/air mixture to be burned off in combustion. There were problems with this. The attached document explains the situation.
  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles
I wept because I had no radials until I met a man who had no splines.
https://tinyurl.com/RillRider

Offline The Mighty Gryphon

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #12 on: October 05, 2021, 11:53:32 PM »
At 24mpg the problem is an incredibly over rich mixture.  I just had a friend who cooked the temperature sender on his boat's diesel when the engine overheated.  I suspect you have done the same to yours.

You can check the temperature sensor by measuring the resistance between pins #10 and #13 on the big ECU plug.  Should be about 2.5K ohms at 70F and about 250-300 ohms when the engine is warmed up. Not only can the sensor be bad, but there can be corrosion on the connector that makes the ECU think the engine is cold.
  • In my garage in Marilla, NY
  • '91K100RS White/Blue
Current:
'91 K100RS16V "Moby Brick Too"

Past:
'94 K75RT "Ilsa, She Wolf of the SS"
'92 K100RS16V "Moby Brick" (RIP, deceased in a vehicular assault)
'94 K75S Special Edition Dakar Yellow "Cheetos"
'89 K100RS Special Edition "Special Ed"

Offline acyclicsalmon

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  • Posts: 28
Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #13 on: October 06, 2021, 02:21:59 AM »
Thanks Gyphon, that certainty inspires confidence that this is the issue.

I went ahead and pulled the injectors as well, courtesy of this telling thread on BMWMOA: https://forums.bmwmoa.org/showthread.php?48214-Power-loss-91-K75/page3 [Similar symptoms, guy changed near everything but the injectors are what finally did it]

I'll be sending those over to Mr. Injector.   His 2 week lead time works quite well in my situation, as I'll be crossing the pond to both London and Frankfurt for the next 2 weeks anyway. 

I also ordered a new temperature sender unit.  Will post a response to this thread in probably 3 weeks at this rate.

Thank you all very much for the help, its greatly appreciated.
  • Oakland, CA
  • 1986 K100RS

Offline frankenduck

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #14 on: October 06, 2021, 02:41:43 AM »
I'll be sending those over to Mr. Injector.   His 2 week lead time . . .

Two weeks!? Dang. When I started with him many moons ago the turnaround was one or two days.  He must be busy. I guess that's what  happens when you do a great job for a reasonable price. Go figger.

Here's some info on reinstalling the FIs:
http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/fis/fis.htm

Once I had a Collie pup. Dug a hole and covered him up. Now I sit there by the hour. Waiting for a Collie-flower.
New to K bikes? Click here.
K Bike Maintenance & Mods: Click here.
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Offline TriSpark

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #15 on: October 09, 2021, 10:39:06 AM »
I had a similar issue with my 1984 K100 when I lived in London and it would randomly cut onto 2 coils and lose power. Turned out to be corroded connectors and a bad coil in the end. Good luck sorting yours out and keep us posted on the outcome.
  • Chattanooga TN
  • Red 1987 K75s

Offline acyclicsalmon

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #16 on: October 19, 2021, 01:47:26 PM »
Just got my injectors serviced.  Only 5-10% flow loss across injectors so the problem is definitely not in the injectors.  Will install next week in addition to the temp gauge and report back on the fix. 

Will likely look at coils next. 
  • Oakland, CA
  • 1986 K100RS

Offline acyclicsalmon

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #17 on: November 01, 2021, 11:32:23 PM »
Wanted to give this crew an update:

  • I received my injectors back from Mr. Injector.  He did a fantastic job and the experience was quick and painless.  Like I mentioned, there was little flow loss so the benefit was mostly in cleaning them up and installing new o-rings.  I oiled them up and in they went
  • I replaced the spark plugs with D8EA's (the D7's were not in stock) and properly gapped them as per the Haynes
  • I installed the new coolant temp sensor, did a coolant flush, and cleaned up the coolant inlets

At this point I'm feeling eager and keen so I crank her.  One burp then nada.  Hold the throttle open all the way and keep cranking.  Nothing still.  One or two more oily burps later, she's dead from 1-2 minutes of total cranking (I know this is not good but I was anxious to see life)

During one of the cranks I didn't hear my fuel pump come on, so cue panic that I fried a relay or fused something shut from the amperage.

  • Deoxit everything I haven't hit before (under tank ecu), major frame ground points, fuel tank plug, relays, etc. etc
  • Swap horn + fuel pump relay (Does everyone else have 2 orange relays and a black one in their control box?) The black one had different pins than the others but thats what I swapped
  • Checked plugs, looking good and wet from gasoline
  • Pulled injectors, they're squirting perfectly
  • Tested plugs, they're firing nice and blue and perfectly uniform (also shocked myself)
  • charged battery overnight

Still couldn't get her to fire, so I resorted to starter fluid in the airbox.  That was the ticket.  She smoked and billowed and barely held an idle, but after 2 minutes of coughing and sputtering she idled first only on choke, then after 5 minutes just on idle.   Still not 100% satisfied, but this could have been simply what was needed to get the cobwebs out.

Ill test to ensure the temp sensor is doing proper readings on the ECU (as well as the other tests in the diagnosis guide) and take her for a ride probably on wednesday. 

Thanks for everyone's help on this, hope to share more good news soon.



As for the intermittent issue of not revving past 2000
  • Oakland, CA
  • 1986 K100RS

Offline frankenduck

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #18 on: November 02, 2021, 02:40:49 PM »
Bad fuel pressure regulator? (FPR)

Is there fuel in the vacuum hose from the throttle bodies to to FPR?

Do you have a fuel pressure gauge for testing?
Once I had a Collie pup. Dug a hole and covered him up. Now I sit there by the hour. Waiting for a Collie-flower.
New to K bikes? Click here.
K Bike Maintenance & Mods: Click here.
Buy parts here.

Offline frankenduck

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #19 on: November 02, 2021, 03:11:48 PM »
Quote
Swap horn + fuel pump relay (Does everyone else have 2 orange relays and a black one in their control box?) The black one had different pins than the others but thats what I swapped

- Some bikes have 2 orange relays, some have two light blue relays (same relay though, just different color)

- The right colored relay is the horn relay, the middle colored relay is the load shed relay

- The fuel system relay is the one on the left. It is different because it has five pins. Two of them are 87 pins that output to
1 - Fuel injectors and L-Jetronic fuel computer
2 - Fuse 6 to temp sensor relay and fuel pump
Once I had a Collie pup. Dug a hole and covered him up. Now I sit there by the hour. Waiting for a Collie-flower.
New to K bikes? Click here.
K Bike Maintenance & Mods: Click here.
Buy parts here.

Offline daveson

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #20 on: November 03, 2021, 05:33:22 AM »
Maybe I've misunderstood but from your first post it seems that you replaced the muffler which was damaged after a drop. Maybe the replaced muffler is blocked, or became blocked. That could limit the revs. It might have a bit of a hissing sound. If it revs good with the muffler removed, that would be it. A blocked air filter, or inlet, could do the same.

Be sure the relays are back in their right places.
  • Victoria, Australia
  • Current;'85 K100RT~100,000km; four other bricks. Past; 1500 Vulcan, V Star 650, KLX 250(dirt bike) TT250(dirt bike)

Offline Martin

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #21 on: November 03, 2021, 10:27:36 PM »
Non OEM fuel hoses can kink over, especially the supply line where it goes into the tank. Also check the return line
and the in tank lines.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline acyclicsalmon

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #22 on: November 05, 2021, 09:09:27 PM »
Cheers crew, always appreciate the help.  All tubes look great and are unkinked, and I went on a test ride earlier today.  Besides starting rough, rode wonderfully.  However, the bike is absolutely running rich right now.  Sparks right up with the fuel pump fuse removed but quickly floods.

I will check the fuel pressure, I dont have a pressure tester, but I will try and source.  Sparks right up with the fuel pump fuse removed, then quickly floods and has a hard time starting again afterwards. 

Any other ideas why the bike might be running rich at start-up (and probably beyond?) I feel like there was a rattle from the fuel pump around 3k RPM, so will probably check internals though most of those are new. 
  • Oakland, CA
  • 1986 K100RS

Offline daveson

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #23 on: November 05, 2021, 10:11:15 PM »
Two quick checks.

With the fuel cap assembly removed, you should see fuel returning to the tank. If fuel isn't returning, the pressure regulator is prevented from working, and the fuel pressure will be about double what it should be.

With a screwdriver to the injectors you should hear a ticking noise, meaning they are turning on and off as they should. If not ticking, they might be permanently on, also causing overfueling.

  • Victoria, Australia
  • Current;'85 K100RT~100,000km; four other bricks. Past; 1500 Vulcan, V Star 650, KLX 250(dirt bike) TT250(dirt bike)

Offline Laitch

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Re: '86 K100 Refuses to accelerate above 2500 RPM after warming up
« Reply #24 on: November 06, 2021, 12:14:20 AM »
I will check the fuel pressure, I dont have a pressure tester, but I will try and source.  Sparks right up with the fuel pump fuse removed, then quickly floods and has a hard time starting again afterwards. 

Any other ideas why the bike might be running rich at start-up (and probably beyond?)
First, eliminate the idea about the defective fuel pressure regulator. Many of the major auto parts suppliers will loan such tools to people who provide a deposit with a credit card then will refund the entire deposit on return. The tool is loaned without charge. It's use is explained in the troubleshooting guide on site.

Secondly, I read a couple of references from you to a "temp sender" or "temperature sender unit" and that you would check for "proper readings" on the "ECU." Are you referring to the fuel injection control unit and the coolant temperature sensor, and if you are, what were the readings?

Another critical fuel mixture component is the air flow meter and the air temperature sensor mounted in the roof of its intake. Did you check to be sure that the meter's air vane is rotating and rebounding smoothly? Have you inspected the air temperature sensor for fouling. Have you cleaned the meter's electrical connector and verified that it is tight?

  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles
I wept because I had no radials until I met a man who had no splines.
https://tinyurl.com/RillRider

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