So in recent years I let my "good habit" of tracking milage at each fillup lapse.  Just recently I started back up but didn't bother to calculate my mpg from each fillup.  

I sat down last night with my phone calculator and filled in the blanks.  Turns out I'm averaging 25 mpg city driving and 30 mpg highway.

I should be expecting better, right?  Nothing seems off in the sound/exhaust/performance of the bike.  Maybe it's time to send my injectors up to Mr Injector up in N. Idaho?

zaubertuba wrote:Turns out I'm averaging 25 mpg city driving and 30 mpg highway.  I should be expecting better, right? 

I've read through your previous posts and there is nothing that indicates you have cleaned the air filter, replaced the spark plugs, measured the valve clearances and balanced the throttle bodies—the condition of those components have a bearing on fuel economy.

Would you bring us up to date?

I did the filter, plugs, deoxed all the electrical connections I could reach (particularly those in the ignition path), replaced a lot of rubber, and early on did check for leaks in the throttle bodies.

Though, I did a lot of that work when I first got the bike and had everything apart doing the deep drivetrain stuff. The air filter's probably still good but I can check it again, and I probably should check my plugs, as well.

I have *not* checked valve clearances (my shop guy, on hearing the low mileage of the bike said I shouldn't mess with them yet), nor balanced the throttle bodies (I've been thinking a lot about this since it seems to come up a lot in talking about engine vibes).

zaubertuba wrote: I have *not* checked valve clearances (my shop guy, on hearing the low mileage of the bike said I shouldn't mess with them yet), nor balanced the throttle bodies

30K miles is an appropriate stage to check clearance and balance throttle bodies.

It is unnecessary to "mess with" valve clearances in order to check them. All that's necessary is having the appropriate feeler gauges and the ability to remove and replace the cam cover without creating leaks, all of which is well within your ability.

On those mpg figures it should be running like a bag of nails,sooty plugs overfueling and a strong smell of fuel.Either your calculations are wrong,someone is nicking your fuel, if any of the above don't apply, my thoughts are on the injectors
best of luck.I would have my own serviced rather than buy on  ebay if possible

zaubertuba wrote:I did the filter, plugs, deoxed all the electrical connections I could reach (particularly those in the ignition path), replaced a lot of rubber, and early on did check for leaks in the throttle bodies.

Though, I did a lot of that work when I first got the bike and had everything apart doing the deep drivetrain stuff. The air filter's probably still good but I can check it again, and I probably should check my plugs, as well.

I have *not* checked valve clearances (my shop guy, on hearing the low mileage of the bike said I shouldn't mess with them yet), nor balanced the throttle bodies (I've been thinking a lot about this since it seems to come up a lot in talking about engine vibes).

After everything else is checked and confirmed, you don't wanna be mucking round with attempting to 'balance' the TBs for road running as it is done with incremental turns of the air screws for engine idle only.

I suspect your calculations are off.  If you were using that much fuel, after using one tank the plugs would be so fouled that there is no way the engine will start.

How are you measuring the miles?  I assume it's the tripmeter.  Does it skip or run slow?

Point-Seven-five wrote:I suspect your calculations are off.  If you were using that much fuel, after using one tank the plugs would be so fouled that there is no way the engine will start.

How are you measuring the miles?  I assume it's the tripmeter.  Does it skip or run slow?

My tripmeter doesn't work so I'm using odometer mileage. I'm calculating using: (CurrentMileage-PreviousMileage)/AmountOfFuelAtPump.  I double-checked my math.

Gotta get ready for interviews so I didn't have time to check anything today.  I'll pull the plugs tomorrow night and have a look.  It *has* been harder starting, I noticed, this past week.

Hard to start it may be time to check the temp sensor. If it is faulty the ECU can think it is cold so makes the mixture rich and maybe lean at start because it may be giving the same reading hot or cold. There will be a write up Bert did about checking it.

Vacuum caps and vacuum pipes to the fuel pressure regulator if faulty can do this. So can the Z pipe crank case vent.

So a couple things:

Rick does this corroborate what you're thinking:  It was cold this morning. Bike would not start and after two tries wouldn't even turn over.  Took two more charges with my smart charger before I could get it to fire up (and stay running). In the afternoon when it was sunny and warm, and I had no problems whatsoever.

Possibly related: I forgot a while ago, I'd unplugged the TPS in attempting to troubleshoot a surging/stuttering issue.  I haven't had the issue since and never plugged the TPS back in.  

Battery and Alternator:  Voltage at rest is 12.25 V. On idle it drops to around 11.8 V (no wonder that pesky batt light comes on at idle, lol).  Revving up it comes up to around 13.5 V, so seems the alternator is doing what it's supposed to do.

Spark plug condition--at least here's #1:

That doesn't look like an overrich running condition to me. Gap is at .034

I also let it idle this evening and sprayed some WD40 on the throttle bodies/rubber hoses and that didn't seem to effect a change of any sort, as far as looking for leaks (before I start yanking stuff apart, anyway).

Still in a quandary....

That spark plug makes it very hard to imagine how you are using that much fuel.

At 12.25v, the battery voltage is very low.  That can create issues with the injection system.

The hidden curse of the K. Battery must be well on top of the job,also 13.5v from alternator should be better,14v+well worth a look at the brushes etc .I have been 
caught out with a bad battery in the past easy missed.

Temp sensor on coolant 'T' into engine block ≅ 2.5KΩ @ 20°C

test pins 10 and 5 on the ECU connector plug under the seat. Flat blade screwdriver, 'lever forward' in the hole of the tooltray to loosen ECU connector plug

check the wires from Pin 10 & 5 of the fuel injection control unit plug for open or shorts

Pin 10 - violet/green
Pin 5- brown

So the resistance on the temperature sensor reads around 2.25K at 75-degrees F, which, looks to be right in-line with the graph for that thermistor.

Rest of the plugs look as good as the first.

But, I just pulled the tank, siphoned about 2.25 gallons off to do it, and for kicks and giggles recalculated my mileage.  I've gone 86 miles on half a tank which puts me around 38 MPG.

The only change for this last stretch I made was re-plugging in the TPS.  I think that was it--most of my driving has been around town so a lot of starting/stopping so it makes sense that not letting the motronic cut the fuel off on all the slowing down would burn a lot more fuel.

Your bike has an L-Jetronic ECU, not the Motronic.  If it did, it wouldn't run above idle with the throttle sensor disconnected, that is, if it would run at all. 

It's important to know which ECU you have because the operating principles and troubleshooting information are vastly different between them.

As far as fuel efficiency, 38mpg is pretty bad, but not totally unusual.  I will get that low when I'm doing a steady 85+mph on the Interstate or the engine is spending a lot of time idling.  Are you sure that you are getting good spark under load in all four cylinders?  A cylinder that is not firing may be hard to feel and will waste a lot of fuel.  A weak plug or coil that has a hard time sparking at high cylinder pressures can do that.  It's hard to spot because if you check for spark with the plug out of the engine the plug will look okay.  I just had an almost brand new plug do that to me last winter.

Last, are you sure of your assumed starting volume of fuel?  I am curious as to how you know there was exactly 4.5 gallons of fuel in the tank at the start.

Laser temperatures guns are now pretty cheap and they are a very useful tool for various temperature related problem diagnosis. They can be used to find a miss firing cylinder by checking the individual exhaust header temperatures. This needs to be done as close to the cylinder head as practical. A faulty cylinder will show a lot lower temperature reading.
Regards Martin.

Point-Seven-five wrote:Your bike has an L-Jetronic ECU, not the Motronic.  If it did, it wouldn't run above idle with the throttle sensor disconnected, that is, if it would run at all. 

It's important to know which ECU you have because the operating principles and troubleshooting information are vastly different between them.

As far as fuel efficiency, 38mpg is pretty bad, but not totally unusual.  I will get that low when I'm doing a steady 85+mph on the Interstate or the engine is spending a lot of time idling.  Are you sure that you are getting good spark under load in all four cylinders?  A cylinder that is not firing may be hard to feel and will waste a lot of fuel.  A weak plug or coil that has a hard time sparking at high cylinder pressures can do that.  It's hard to spot because if you check for spark with the plug out of the engine the plug will look okay.  I just had an almost brand new plug do that to me last winter.

Last, are you sure of your assumed starting volume of fuel?  I am curious as to how you know there was exactly 4.5 gallons of fuel in the tank at the start.

My mistake - L-Jetronic, then.

I actually assumed a full tank of 5.5 gallons to start...aaaand totally botched the math, so I'm back around 25 MPG.

Martin, the only problem with the temperature readings is that if the cylinder only fails to fire under load the temperatures at idle will all look okay. 

I was surprised to see how well a K100 can run on only three cylinders.  I could see how if you were riding around town it would be hard to tell.  All there is to notice is that you need a little more throttle than if you were on all four cylinders.  There might be a little hesitation as the throttle is opened, but not like what you would see if two or more cylinders were missing.

zaubertuba wrote:I actually assumed a full tank of 5.5 gallons to start...aaaand totally botched the math, so I'm back around 25 MPG.

Wow!  That's even worse!  What color are the spark plugs?

Gryph you are right it will only show idle miss firing temperatures unless you take the temperatures quickly straight after a high speed run then the miss firing cylinder will still show a lower temperature.
Regards Martin.

Be certain your odometer is recording accurately, too.

Point-Seven-five wrote:

zaubertuba wrote:I actually assumed a full tank of 5.5 gallons to start...aaaand totally botched the math, so I'm back around 25 MPG.

Wow!  That's even worse!  What color are the spark plugs?

(Plug 1 threads are clean from anti-seize--that was the one I pulled previously and put back.)

Laitch wrote:Be certain your odometer is recording accurately, too.

The previous owner replaced the speedometer cluster. Seems to me I remember from long ago that you had to match the speedo with the final drive otherwise things were off? Or am I thinking of something else?

At 5.5 gallons thats 24.75ltr biggest tank iv heard of most are 18/22 ltr,4.5/5 gallons would be a better calculation.TO VERIFY the clocks use the GPS on a phone.
I fitted clocks from 1992 bike no problems speedo reading spot on.MPH/KPH might be a problem

zaubertuba wrote:

Laitch wrote:Be certain your odometer is recording accurately, too.

The previous owner replaced the speedometer cluster. Seems to me I remember from long ago that you had to match the speedo with the final drive otherwise things were off? Or am I thinking of something else?

K speedos aren't sensitive to different final drive ratios like Airheads are. Different tyre/wheel combos have to be accounted for, however. See Karamba on the forum.

Also, 22 litres is 5.8 US gallons (4.83 Imperial) and that's what an 'early' (1983 thru '85, and later) K100 should be.

The most like mph for miles per gallon would be km/l rather than kpl, or in Australia and other places, it's litres per hundred kilometres averaged, so L/100K.

Better not to fill the tank, you might lose some out the overflow.

What I do; with the bike on the side stand, fill till the level touches the bottom of the filler throat. Next time fill to the exact same point. Do that a few times with the same bowser, then try a different bowser, there will probably be a small difference. Try to do everything the same each time, for example handlebar position, bike angle, fuel level.

zaubertuba wrote:

Laitch wrote:Be certain your odometer is recording accurately, too.

The previous owner replaced the speedometer cluster. Seems to me I remember from long ago that you had to match the speedo with the final drive otherwise things were off? Or am I thinking of something else?

I was referring to the development of faulty gear transmission in the odometer assembly that creates inaccuracy—not a particularly rare occurrence with these Motometers.

Those spark plugs are not showing signs of only getting 25mpg.  If they are the correct heat range, I would say you are getting around 45mpg.  I seriously wish mine looked that good.