I've had my 1987 K100 for a few tank fulls of petrol and I'm disappointed in the fuel consumption. First tank full was 45mpg, second 37mpg. The bike is running very well otherwise. What could be causing such poor figures?

I'm not mechanically gifted but would like a few ideas to try before I take it in to my mechanic. Thanks in advance.

I'd first consider the easy and obvious possibilities: tire pressure and dragging brakes.  
If that doesn't give you some answers, what does the end of the silencer look like?  Sooty, as in running too rich?

Like brickrider has said, your way of riding.
For Some of us this is normal autobahn, breaking, Adjust the valve clearance to fit within service.
Air filter is dirty. Check the compression!

My consumption is around 51 mpg UK, 61 mpg US and 4.6 kml
But I'm picking flowers on the way :-)
Yours is 45 mpg UK, 37.5 mpg US and 6.3 kml

It could be, besides some other things you fuel injectors, they could be dripping instead of spraying.
The pistol rings could be done.
There could be wrong timing.
Do you have black smoke coming out the pips?

If you need some new good and tested four hole injectors instead of one hole old injectors, I have got some, could ship to UK, price shouldn't be bad.

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Cheers

Thanks.  Brakes ok,  exhaust clean. Riding style is relaxed, not in a hurry to get anywhere nowadays!!!  I'm going to have a go at changing the air filter and spark plugs.  The injectors and valves are beyond my capabilities.  I'm getting a Mahle air fi!ter, but what plugs: Champion or Bosch?

Swapping fuel injectors is pretty easy. (and a good idea)

Just be sure to blow some compressed air or vacuum around their bases before removing them from the engine.  Sometimes there is dirt or tiny rocks around the bases of them which you don't want falling into the cylinder head when you remove the injector.

Realy easy to remove, 2 screws and pull, sometimes you have to use some oil and a grip tong.

Remove injectors
http://www.motobrick.com/index.php?topic=3781.0

Video showing the spray picture with one hole


Cheers

More info on fuel injectors:
http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/fis/fis.htm

I have the four hole injectors in several of my Ks. They work fine but I'm not convinced they're much better than a good set of original ones. I've experienced no noticeable change in fuel mileage or power.

Just my 2 cents worth but I feel you're better sticking with the injectors you have. They are highly reliable and last for around 300 to 400 thousand kilometers. If your bike has had a history of having been sitting around unused for an extended period then it could be they need a clean (see Youtube or get them done professionally). A leaking injector will add to your fuel consumption but may not make any appreciable difference to your tailpipe. Check each sparkplug for deposits over a few weeks instead. You could also pull the injectors and rail out and observe for leaks under presure.
Read this: 1 hole Vs 4 hole injectors

Sparkplugs will definately affect consumption and need changing regularly. I find mine last no more than 10K klm. You'll get a slight uneveness at start and a bit more vibration than normal thru the bars. Use a DMM set to 20 ohms range. They should read less 0.5ohm or less from connector to electrode. Keep any good ones as spares and replace with new SPs. If you have OEM resistor leads then you'll need NKG D7EA plugs. Don't use DR7EA plugs with resistor leads.
Make sure your plugs have the 'terminal nuts' screwed on and they're tight. you won't get a proper lead/plug connection without them and this will dramatically affect the quality of the spark and combustion.
One thing that is often forgotten is to nip up the SPs after a hundred klm or so as the compression washer will squash out a bit. Gummy SP thread and stains around the ceramic base is evidence of combustion gases blowing past.

If your consumption is high and eratic, its a sure bet your valve shims need adjusting. Its not hard to do. Chris Harris on Youtube explains and demonstrates the process well... don't mind his language, he knows his sh!t.
There's a special tool to get your shims out and its pretty cheap. Try searching this forum to find the link.
Apart from other symptoms, I had very high and eratic consumption from 6 to 8 Lit/100 km. Valve shim adjustment brought that down to around 5.3 ±0.3

Engine oil can play a part in fuel consumption also and I've found that a good full synthetic oil can reduce your consumption by up to 10%. Fuel savings can more than pay for the cost of the oil. I'm currently using Nulon full synthetic which has a moly content. There is a down-side. If your seals and gaskets are in poor condition you can get oil weeping. Synthetic oil molecules are much smaller than in mineral oils.

Currently my consumption varies between 4.4 and 5.4 lit. Its become eratic again over the last 6 months or more. I get a lot of noise from the timing chain so I'm suspecting the valve timing may be changing slightly under different conditions. Have just recieved a new kit from Motorworks and will be fitting that next week so I'll let you know how that goes.

Thanks guys. Redex, air filter and spark plugs ordered.

This topic interests me because I might have issues with my bike's fuel consumption.
My 1984 k100 was taken off the road due to the gearbox grub screw issue.
I fixed that problem then decided to overhaul the water pump.
Of course the threaded section of the shaft broke when I was removing it from the pump.
I visited a Ktm dealership in hoppers crossing and to my amazement, the owner placed the shaft in a metal lathe and replaced the threaded section.
I had purchased three of the original design water seals two years ago from Motobins and was determined to use them.
There was no issue installing the seals. I used Reinzosil to seal the joint and the overhaul was a success.
No leaks whatsoever.
My issue now is the fuel consumption.
I haven't measured it, but the end of my exhaust is sooty and don't know how to adjust the fuel mixture.
Thank you for your assistance.

Using unleaded fuel will always give you a black sooty exhaust pipe.
The mixture can be adjusted but I would not recommend doing it unless things get drastic. One common cause with EFI engines is a failing temperature sensor that makes the ECU think the engine is still cold so it keeps the mixture rich. There are quite a number of threads about it here and there is a graph showing what resistance the sensor should read at various temperatures in the downloads section.
Even putting new plugs in can improve things quite a bit but one thing that doesn't have any effect on fuel consumption on an EFI engine is the condition of the air filter except if it is totally blocked. The air is measured after the filter and the filters on most bikes and cars will flow a heap more air than they are usually required to pass.

a good multimeter is a must  for any of the electrical issues we see with k bikes

the easiest way to test the temp sensor is at the under seat computer  pins 10 and 13

have a read of the posts in the forum about this before you try anything

good luck

Just a thought after you've checked out the suggestions above. If the sump is over filled you can get get a thick sooty deposit on the tailpipe. The exhaust will also smell heavily. My PO had over filled by nearly a litre of oil. Took me a while to work out what was causing the soot... but that was before I found K100-forum.com

That brick of yours was a poor pathetic thing when you got it Nigel and you have transformed its life and made it happy again, like it should be for all bricks

  Ha ha! With much help and enthusiasm from fellow bricksters.
The sweet thing got a new timing chain today. All went well except for a stripped head on one of the casing bolts. Covers back on tomorrow and hopefully she'll be purring instead of rattling her old bones bits. I'm also hoping I'll see an end to eratic fuel consumtion.
Cheers

So Holister, did the timing chain make a difference to the consumption? I just got 250km out of a tank and need some answers lol

MarcS wrote: I just got 250km out of a tank and need some answers lol

I've scanned through all your posts since before you bought the bike. In them, you've discussed wheels, brakes, brake discs, seats, 'nad comfort, master cylinders, ECU chips, dog weight, kangaroo danger, envy, and steering—among other things but nowhere did I notice a tuneup being mentioned. Have you checked for intake manifold and crankcase ventilation leaks? In absence of them, have you checked and set valve clearances then balanced the throttle bodies and set the idle with the benefit of new plugs and a clean air filter? If you've done that, then compression and leak-down tests might be in order, or maybe just upshifting sooner.

I have neglected to mention most of the obvious stuff that I have done , as in plugs, air cleaner and crank case breather pipe.I do have a very minor vacume leak below the throttle bodies. That is my next step. I am guilty of not checking the valve shims and that may be a contributor to my problems. I would also like to check the timing but I believe I need a timing light for that. I have no vacume gauges to do the throttle bodies either but it is on my list.

I will start with my small vacume leak and valve clearances, working from there.

I believe upshifting sooner is a bad idea as it might lessen my volume of smiles per kilometer even if it does improve ecconomy. Today I did 250km between 100 and 130kmh with only a couple of small towns to slow down for. I got home on fumes by leaning the bike to the left for the last 3km.

MarcS wrote: I would also like to check the timing but I believe I need a timing light for that. I have no vacume gauges to do the throttle bodies either but it is on my list.

Timing on these bikes usually isn't a problem—line up the sensor plate notches—but throttle body balancing can't be ignored if fuel efficiency is a goal. Make your own. Here's a homemade set with isopropyl alcohol and food coloring as its fluid.

Looks great but I have no experience balancing carbs or throttle bodies. I spoke to the local bike shop mechanic who seems to have as much idea as i do and seems to specialize in lawn mowers and chain saws. Once I have the vacume and valves sorted I will check the notches and start worrying about balancing the TB.

MarcS wrote:Looks great but I have no experience balancing carbs or throttle bodies... start worrying

There are clear instructions on this site concerning the balancing procedure so you shouldn't have a problem with that. As far as worrying goes, get back in the saddle and go. Carry extra fuel and lean to the right sometimes to keep the tires wearing uniformly.

Gotta admit, 250Km seems a bit short of the plot even at those speeds. LFB usually returns around 320Km to the tankful before I start panicking. She starts burping at around 340Km and then I know I'm in trouble... last time it was all downhill to the nearest filling station

I have had poor fuel consumption on my K100RT, one was only 28MPG. I was running the bike in the hot summer and could always smell fuel, Every few miles it would cutout. I have checked everything and found I have water and debris in the bottom of the tank, I was also using the 95 octane fuel.  I have changed the fuel to the higher octane fuel and I have had one tank at 43MPG and one tank at 58.99MPG (UK). the cost per fill is higher, but the cost per mile is lower.