K75 Misfire/Gratuitous Flames/Terrible Partial Throttle

[dowdogg]

Hello fellow motobrickers,

Today I bring you tidings of another (partially) broken K-bike.

The Problem:

When around 20%-50% throttle at any RPM, the bike misfires terribly and sounds like a two-stroke. Other side-effects include constant flames (awesome, but sadly detrimental) during the misfire and of course gratuitous pops and bangs. When cruising on the freeway, I notice that the header collector glows red hot. The effects seem to be exacerbated with less fuel in the tank and during the first 5-10 minutes of riding (warm-up). No problems at idle and WOT feels great. Other than the misfire, the bike has never felt better or faster. I purchased the bike in Spring of 2017, never had this problem until a few months ago.

Here is a video for reference:



check around the 19 second mark for a small blue flame (cool huh). Its like that but 10x on the freeway cruising and makes for some very scared and inquisitive on-lookers at night-time.

The Bike and its details:

• 1990 BMW K75 Standard
• Unknown mileage (purchased for a hefty sum of $1100)
• Completely removed instrument cluster (tach, speedo did not work, actually, nothing worked except the neutral light)

Parts replaced/fixed (ALL REPLACED WITHIN 5000 OR LESS MILES)

• New battery
• New 4 hole injectors
• New NGK plugs
• New Crankcase Breather Hose
• New Fuel Filter
• New OEM Throttle body Vacuum caps
• New OEM Throttle body rubber bushings
• New OEM Crankcase Gasket
• New Fuel lines (all around) and OEM hose clamps
• New vacuum lines all around
• New FI relay
• New Starter relay
• Fan would not start ever, so I mounted a manual fan switch via a relay to the Temp Switch Relay
• Cleaned air filter
• Cleaned and checked fuel pressure regulator
• Checked and tested current fuel pump (aftermarket mostplus brand)
• Rewired fuel pump connections
• Almost every electronic/connector cleaned with DEOXIT or Electrical connection cleaner.
• Almost every ground (chassis, etc) sanded, cleaned and checked.
• Throttle bodies synced
• L-Jetronic K bike fuel injection adjusted and set

Other things have been changed but mostly non-functional/irrelevant. I beg the community to save my K with their infinite and endless knowledge. I am reaching wits end at this point sadly after much debate and strife (has not prevented me from riding the bike 50+ miles on a daily basis, duh).

Thanks guys, lets hear some hopefully game-changing comments about what the problem is and how to fix this piece.
Cheers!

[wally.fisher]

What causes most backfiring on these K bikes is a badly adjusted TPS but as you've done so much to the bike it's difficult.
How did you adjust the Jetronic fuel injection?


Sent from my iPad using Tapatalk

[Laitch]

What were the results of the coolant temperature sensor test? Were all the values within range from hot to cold?
When you removed the instrument cluster was it replaced by something else?
What was done to this bike a few months ago before this started happening?

[billday]

Did you replace all that stuff because there was a problem you were chasing?

[rbm]

+1 to what Laitch asked. 

In your list of things you did, you mentioned: Fan would not start ever, so I mounted a manual fan switch via a relay to the Temp Switch Relay
This possibly points back to a failing temp sensor to which Laitch alludes.  By installing a switch, all you've done is bypassed the symptom without diagnosing the problem.

[The Mighty Gryphon]

+1 to what Laitch asked. 

In your list of things you did, you mentioned: Fan would not start ever, so I mounted a manual fan switch via a relay to the Temp Switch Relay
This possibly points back to a failing temp sensor to which Laitch alludes.  By installing a switch, all you've done is bypassed the symptom without diagnosing the problem.

Rob, thanks for saving me from having to post that. 

Misfiring/backfiring sounds like over rich mixture.  Over rich is tolerated at idle and WOT, but not at small throttle openings.  First culprit for that mixture is the emgine temperture sensor.  The fact you didn't have a functional fan before adding a manual switch points at it.

[Chaos]

rich mixture missing is usually more rhythmic, and the glowing headers would point to a timing issue or lean mixture, wouldn't it?  Could be exacerbated by the free flowing exhaust.  Also did you ever check the valves, could be tight with unknown history.  Jut barking up the tree here, have no idea what's going on.

[rbm]

I think there would be two possible failure modes for the NTC sensor - fails open or fails short.  If it fails open, it tell the temp module that the water temp is stone cold and the ECU will enrichen the mixture.  If it fails short, it tells the temp module that the water temp is boiling hot, and the temp module turns on the fan at the same time the ECU should be providing a leaner mixture.  Since the fan never turned on, I suspect the temp sensor has failed open.  This supports Gryph's theory (unless enrichening only happens at idle speeds and there is a leaner map at mid throttle; don't know).

[The Mighty Gryphon]

My thinking is that at idle a rich mixture will run, and the only thing you might notice is sooty plugs.  If the mixture is lean, the engine will also idle, but comig off of idle it will have a tendency to stumble and die.

A lean mixture will also detonate on acceleration, especially at full throttle where the mass of air entering the cylinders under compression can get hot enough to preignite the lean fuel mix.

At wide open throttle engine performance with a rich mixture will probably be okay since the injectors are probably sized to match the air flow with the throttles aide open.  They can't flow enough fuel to substantially upset the mixture. 

I'm only guessing here, but I suspect that a rich mixture will take longer to complete the burn, and will still be burning as the exhaust valves open, allowing the flame to travel to the headers. 

What kind of gas mileage are you getting with all this going on?  I would expect that it is down, maybe as far as less than 30 mpg.

[dowdogg]

What causes most backfiring on these K bikes is a badly adjusted TPS but as you've done so much to the bike it's difficult.
How did you adjust the Jetronic fuel injection?


Sent from my iPad using Tapatalk

I followed this guide:

http://skylands.ibmwr.org/tom/tech/co-setting.html

What were the results of the coolant temperature sensor test? Were all the values within range from hot to cold?
When you removed the instrument cluster was it replaced by something else?
What was done to this bike a few months ago before this started happening?

1.
1. I'm assuming you are speaking about this test (http://www.ibmwr.org/ktech/fan-diagnosis.shtml)

If so, the temp probe worked within the ohm range described.

2.

I attempted to do so, but after finding the increasing complexity to get the aftermarket dash I purchased to work, I gave up and scraped the whole idea completely.

3.

I will give a timeline of the aforementioned repairs:

REPAIRS DONE WHEN PURCHASED (APRIL):

• New 4 hole injectors
• New Crankcase Breather Hose
• New Fuel Filter
• New OEM Throttle body
• Vacuum caps
• New OEM Throttle body rubber bushings
• New OEM Crankcase Gasket
• New Fuel lines (all around) and OEM hose clamps
• New vacuum lines all around
• Cleaned and checked fuel pressure regulator
• Checked and tested current fuel pump (aftermarket mostplus brand)
• Rewired fuel pump connections
• Almost every electronic/connector cleaned with DEOXIT or Electrical connection cleaner.
• Almost every ground (chassis, etc) sanded, cleaned and checked.
• Throttle bodies synced
• New FI relay

Side note, was running a shorty muffler (essentially a straight pipe) for the majority of this time period. All these repairs were done not necessarily out of necessity but i just wanted a mechanically sound machine.


4 MONTHS AGO:

Hot day, noticed after a 45 minute to an hour ride that the bike was overheating (water pouring out of the overflow). Thought it was a fluke. Realized the situation kept occurring on long rides. Proceeded to do this test (http://www.ibmwr.org/ktech/fan-diagnosis.shtml) because the fan would not run after 10+ minutes of idling. Had never had a overheating or fan problem since purchase. Did all the relay tests and everything seemed fine, when jumped the fan operated normally. Decided to circumvent the situation and installed my own relay (onto the temp relay) with a manual fan switch. Has not overheated since.


2 MONTHS AGO:

Starter relay dies, replaced with new battery. misfire plateaus and doesnt get any worse but continues. I ride the bike to work 50-60 miles a day with no problems other than the misfire.




3 MONTHS AGO:
YESTERDAY:


More troubleshooting. Set the FI according to this guide (http://skylands.ibmwr.org/tom/tech/co-setting.html) assuming that my 4 hole injectors were ruining the AFR. Misfire still present but I managed to make the bike idle nice and low, maybe too low? Side note: throttle got stuck at WOT for around a second last night after this adjustment, happened the morning as well, so I'm staying off WOT. Only seems to happen at 100% throttle, not 99%. Might adjust the AFR again to previous setting for safety :hehehe



See the new timeline

[dowdogg]

Did you replace all that stuff because there was a problem you were chasing?

See the timeline. Some of it was, some of it wasn't, I just wanted a mechanically sound bike.

[dowdogg]

+1 to what Laitch asked. 

In your list of things you did, you mentioned: Fan would not start ever, so I mounted a manual fan switch via a relay to the Temp Switch Relay
This possibly points back to a failing temp sensor to which Laitch alludes.  By installing a switch, all you've done is bypassed the symptom without diagnosing the problem.

I did diagnose and test the temp sensor when diagnosing the fan and found no error within the ohm ranges at the time. That may have changed now, not sure. side note: i love your build, it was part of my inspiration to get a K!

[dowdogg]

rich mixture missing is usually more rhythmic, and the glowing headers would point to a timing issue or lean mixture, wouldn't it?  Could be exacerbated by the free flowing exhaust.  Also did you ever check the valves, could be tight with unknown history.  Jut barking up the tree here, have no idea what's going on.

Have not checked the valves yet even though I know I should have :P please dont burn me at the stake.

[dowdogg]

My thinking is that at idle a rich mixture will run, and the only thing you might notice is sooty plugs.  If the mixture is lean, the engine will also idle, but comig off of idle it will have a tendency to stumble and die.

A lean mixture will also detonate on acceleration, especially at full throttle where the mass of air entering the cylinders under compression can get hot enough to preignite the lean fuel mix.

At wide open throttle engine performance with a rich mixture will probably be okay since the injectors are probably sized to match the air flow with the throttles aide open.  They can't flow enough fuel to substantially upset the mixture. 

I'm only guessing here, but I suspect that a rich mixture will take longer to complete the burn, and will still be burning as the exhaust valves open, allowing the flame to travel to the headers. 

What kind of gas mileage are you getting with all this going on?  I would expect that it is down, maybe as far as less than 30 mpg.

My commute to work is around 50-60 miles a day. I get around 2-3 days worth of riding on 4 gallons. My assumption is around 35-40 MPG, surprising, I know.

[rbm]

I did diagnose and test the temp sensor when diagnosing the fan and found no error within the ohm ranges at the time. That may have changed now, not sure. side note: i love your build, it was part of my inspiration to get a K!

Thanks for the compliment!


If the temp sensor was working properly, the fan should come on when the coolant temperature reaches 217*F.  Yours used to come on normally you said and now it doesn't so that's either a bad sensor, bad temp module or bad wiring.  The fan diags pass so that should indicate a good temp module and good wiring but doesn't exercise the temp sensor.


You don't mention whether the bike is difficult or easy to start from cold.  Is it easy to start?  If not, does the engine struggle to keep idling until the bike is warmed up (about 1-2 minutes)?  If the bike is hard to start cold and idles poorly until warm, that indicates a bad temp sensor usually.


Your new exhaust system is very open and short. The K75 is very prone to afterfire if the back pressure is low or there is a leak in the first 20" of exhaust.  Yours might just be acting the way it should given the components you've selected.  Is it possible to swap back the standard K75 exhaust and ride with that for a test?  If so, and if the afterfiring symptoms disappear, the diagnosis could point to the new short exhaust as being the culprit.  If not, further tests would be required to isolate the failing component.


Your mileage is pretty normal which is pointing away from an enrichened or enleaned mixture IMO.

[Laitch]

1.
Have not checked the valves yet even though I know I should have :P please dont burn me at the stake.

I wouldn't consider it. You'd probably have a tendency to backfire :giggles

Your mileage is pretty normal . . ..

Not where I come from. My bike gets 48 to 50 US mpg running gasohol. If it were getting that little, I'd swap it for a '98 Valkyrie.  :hehehe

[rbm]

Not where I come from. My bike gets 48 to 50 US mpg running gasohol. If it were getting that little, I'd swap it for a '98 Valkyrie.  :hehehe

Well, I guess my MPG calculator (i.e. brain and pencil) was not working right.  I am getting 50.6 MPG average when I look it up in Fuelly.  That's 4.7 L/100km for us Metric folks.

[dowdogg]

Thanks for the compliment!


If the temp sensor was working properly, the fan should come on when the coolant temperature reaches 217*F.  Yours used to come on normally you said and now it doesn't so that's either a bad sensor, bad temp module or bad wiring.  The fan diags pass so that should indicate a good temp module and good wiring but doesn't exercise the temp sensor.


You don't mention whether the bike is difficult or easy to start from cold.  Is it easy to start?  If not, does the engine struggle to keep idling until the bike is warmed up (about 1-2 minutes)?  If the bike is hard to start cold and idles poorly until warm, that indicates a bad temp sensor usually.


Your new exhaust system is very open and short. The K75 is very prone to afterfire if the back pressure is low or there is a leak in the first 20" of exhaust.  Yours might just be acting the way it should given the components you've selected.  Is it possible to swap back the standard K75 exhaust and ride with that for a test?  If so, and if the afterfiring symptoms disappear, the diagnosis could point to the new short exhaust as being the culprit.  If not, further tests would be required to isolate the failing component.


Your mileage is pretty normal which is pointing away from an enrichened or enleaned mixture IMO.

Thanks for the reply! The bike has absolutely no problem starting under any situation. Hot or cold, One click of the starter and it fires right up and idles. Starts up faster than my car (2011 BMW 335i) which I find hilarious. As I mentioned before, for the majority of the time before I was running a shorty muffler which is literally a 4 inch pipe with little to no baffling connected to the header collector. Essentially open header.  I never had any misfire problems with that set up. My current set up is a 18” muffler with around 3” inch of piping = 21” of exhaust. It was during the more restrictive and longer exhaust set up days that the misfire started. So I’m not sure if the culprit is backpressure. When I get home I will put on the stock system to see if it makes a difference.

[Laitch]

I am getting 50.6 MPG average when I look it up in Fuelly.

Suddenly, envy has taken control. I've gotta shed weight—battery covers, tank bag, handlebar grips, tool kit—maybe even get a gastric bypass.

[dowdogg]

Suddenly, envy has taken control. I've gotta shed weight—battery covers, tank bag, handlebar grips, tool kit—maybe even get a gastric bypass.

I don’t know how you guys get 50 mpg. My cruising speed is 85 or so and I feel the need to keep the bike heathly by redlining it at least once a day. That probably doesn’t help. At 70mph maybe? I only weigh 135. That probably compensates a little.

[Chaos]

mine gets about 50mpg at 60mph, but every 10mph faster get a corresponding 5mpg drop in mileage. So 80 mph and 35 mpg is about par.

[The Mighty Gryphon]

Assuming 50-60 miles round trip, and 4 gallons every 2-3 days your mileage is somewhere between 25mpg(100 miles and 4 gallons every 2 days) to 45mpg(180 miles and 4 gallons every 3 days). 

I'm going to figure the actual is somewhere about 2/3 of the way from the lowest to the highest.  That puts you around 38-39mpg.  Compared to my mileage on my commute which is similar(55 miles at 60mph average speed), you are using about 11-12%  more fuel than my full fairing K75RT with my 200 pound butt in the saddle.  The only time that my K75 gets that low on mpg is when I am doing 80 mph average speed on the interstate with a fully loaded touring kit. 

Sure sounds rich to me.  Double check your engine temperature sensor.

[Laitch]

I feel the need to keep the bike heathly by redlining it at least once a day.

Judging by the title of this thread, redline running doesn't seem to keep your bike healthy. In fact, it sounds like it gives it a case of the jitters. :giggles   

[dowdogg]

So it seems that general consensus is that the water temp sensor needs to be replaced. Everybody on par with that? Want to call my local dealer today if possible to place the order. Thanks guys!

[rbm]

Compare pricing at the dealership to EuroMoto Electrics:  http://www.euromotoelectrics.com/TEMPSENS-K197-p/tempsens-k197.htm