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Gas Mileage Problems

Diagnosing K-Bike Fuel Economy Problems

By Don Eilenberger
June 2000

I have a 1985 K100RT with 110K on it. The bike runs OK but the gas mileage has gotten steadily worst since the bike turned about 80K. Compression is within specs and valves are adjusted. I have rebuilt the injection system, with the exception of the injectors themselves as they were clean and had a good spray pattern. O2 level has been set to factory specs. The cam chain was replaced at 60K. The transmission had new seals and a bearing at 80K. The splines are lubed and in good shape. The bike is a bit hard to start, especially when it’s cold. The best mileage I can get is 39, one up and under 5 grand. Over 5 grand and it drops to about 37. When 2 up, 38 is the high end and 34 the low.

Given it’s a finest year K, it sounds like the temperature sensor for coolant temps is going south. The value of it can be measured at the FI computer connector. If the sensor fails high value (most likely failure mode) – the computer thinks the bike is cold when it isn’t – and injects too much fuel.

I am also assuming that since you checked most things – you also checked that the brakes aren’t dragging or anything obvious like that.

BTW – the O2 value setting on K bikes ONLY works at idle. A clue to a rich mixture is the reading above idle – if you read > 2% CO above idle – you’re running rich, which will result in poor milage.

Testing the K bike temperature sensor:

The temperature sensor on the K bike control the mixture of the bike in relationship to the engine coolant temperature. It is a NTC (Negative Temperature Coefficient) sensor, ie, as the temperature rises – the value decreases.

The value of this sensor can be measured without serious disassembly of the bike. Unfortunately – replacement is not quite that simple. It is a dual sensor – one half controls the temperature warning light and the cooling fan, the other half feeds information to the FI computer telling it the amount of fuel to feed (ie – mixture).

To measure the side going to the FI computer, you need a decent Volt-Ohm-Meter (VOM). Radio-Shack carries one which is very handy for bike use – since it is tiny and easily carried on the bike (about $20 or so – Model 22-802)

To check the temp-sensor, you must first remove the connector from the FI computer. Next locate pin #10. This is one side of the sensor. The other side is ground, so any clean frame point will work.

Measure the sensor at several points (suggested by the * below)

0 degrees C 5.5k ohms (freezing)
20 degrees C 2.5k ohms (~ 71F room temperature) *
40 degrees C 1.25k ohms
60 degrees C 600 ohms
80 degrees C 320 ohms
100 degrees C 190 ohms (boiling, or about where the fan turns on) *

The two measurements taken at room temperature and when the fan turns on will give you a good idea if the sensor is OK. If the reading obtained is off by a factor of 2 (or more) it is safe to assume the sensor is bad, or the wiring leading to it has a problem.

When probing the computer connector – do NOT push the probe into the connector, you just want to touch the connector with the tip of the probe (if you deform the connector, you’ll have much bigger problems).

I would suggest measuring the value with the bike at room temp – before starting it, then reconnect the connector, start the bike and let it idle until the fan comes on – then do the second measurement.

Replacing the sensor involves draining the cooling system, the either removing the radiator/fan assembly, or it is also accessable if you remove the filter-airbox assembly Either is a PITA, but the only way to get at it that I know of.

Hope this helps.

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