K75 lousy mileage

[35634]

My trusty 87 K75S 's mileage has dropped from 45-49 to about 35-40 in the last
year or so. Can't pinpoint when because the odo was dead until recently. It has a
lufty pipe the airbox was replaced with a K&N conical filter kit, but this was done back
in 1988. Ran some Techron through it to clean the injectors. No loose or cracked
hoses I can find. Runs & pulls great idle through redline, starts fine, just crappy mileage.
180k on her, bought new and regularly maintained mostly be myself.

Any suggestions welcome, only have access to low tech shop and no dealers around here.

[Gilly]

Sounds like we have been following each other through life. I bought my 87 K75S new also, you definitely "lapped me" on mileage though. I got 93,000 on mine and sold it last summer, bought a new bike, a new 07 GT. I also had the conical Lufty muffler and the air box eliminator like you have (and the big valve kit too). What color is yours?
Here is a pic of the old and new bike (the present owner replaced the stock windshield):
http://www.bmwmoa.org/forum/showpost.php?p=453840&postcount=13
Nice thing about the GT; It DOESN'T really need any performance upgrades.....
I did a lot of the maintenance myself too. Not all of it.
My thoughts are to check the plugs, I usually did plugs every year or two.
Check throttle body synch, but almost have to have a manometer for multiple cylinder, a bank of 4 is what I normally see (I think made for 4 cylinder bikes).
When was the last time the K&N was cleaned?
Gilly

[zoridog]

That happened on my airhead. Turned out to be a front brake pad that did not retract. Does the front wheel spin freely?

[PGlaves]

Start with a complete tune up and diagnostic session. New plugs, adjust the balance using the brass air bleed screws, check valve adjustment. Carefully look at the condition and color of the old plugs after a fully warmed up highway run - turning the engine off at speed and coasting to a no-idle stop. Compare to the color charts in almost every Haynes or Clymer manual ever published.

Then - in the dark spray mist on the plug wires to check for deterioration and arcing.
Do a compression test - followed by a leakdown test if the compression is low.

Do a fuel pressure test - "T" connected into the line to the front of the fuel rail.

I suspect that somewhere along this path the cause of the mileage drop will become apparent.

[35634]

Thanks everyone, I checked most of the items (except balance & compression) in the
last year, guess I ought to do them all at once. Gilly, this original litter bike started
dark metallic red (lava red), faded unevenly to orange, painted it black, (had a green
Flexit sidecar for a while) had it painted marrikesh red (pretty), hit deer in '05 and
trashed fairing. Now it has a 1980's GPZ550 bikini fairing, C radiator shroud, and is
mostly still marrikish red or close. A colorful history, isn't it? I'd love to update to a 1300GT but wary of not having a dealer nearby.

[Gilly]

But the trips to the dealer will take SO MUCH less time!!:thumb
Gilly

[98lee]

Thanks everyone, I checked most of the items (except balance & compression)


You checked valve adjustment?



:dance:dance:dance

[PHMarvin]

Could the change to "ethanol enhanced" gas have anything to do with your reduced mileage?

[deilenberger]

My trusty 87 K75S 's mileage has dropped from 45-49 to about 35-40 in the last year or so. Any change in your riding habits during the same time period?

I rarely ever got better than 42MPG on my K75S.. and for around town and commuting (lots of local traffic/lights in NJ) 35MPG was actually pretty good. The bike might get up near the upper 40's if I spent all day on the highway at the speed limit, but as soon as I pushed it up over the speed limit mileage started dropping back down into mid 40's.

Truly a case of YMMV..

[knary]

Thermostat?

[Kayseventyfive]

'round town and commuting (lots of local traffic/lights in NJ) 35MPG was actually pretty good. The bike might get up near the upper 40's if I spent all day on the highway at the speed limit, but as soon as I pushed it up over the speed limit mileage started dropping back down into mid 40's.

Truly a case of YMMV..

Thank you for posting that.

My mileage has been about the same, and I have always wondered if I could do anything. Last cleanup, I noticed some oil fuzz around the rear left side of the engine, so I suppose I am due for a new vent hose.

Maybe that will help.

Hope springs eternal. :)

[35634]

My riding is a mix of highway & city, used to get a solid 45. Thanks knary for suggesting
the thermostat, it's original and could be stuck open. Valves are good. I should check
the odometer with mile markers on the highway. I suspect it may be working sporadically. That could mess up the mileage:banghead

[PGlaves]

My K75T got 42 miles per gallon. If it didn't, it needed something. It got that tank after tank after tank, winter, summer, year after year. I couldn't tune it to do better, and it almost never did worse. High speeds into headwinds at altitude was the exception - but a mix of 90% highway, 10% town stayed at 42 for the better part of 19 years.

[Gilly]

My old one regularly got 45 to 48. I was determined once to find out how good the mileage could get and measured it at 55 mpg, but man that was one boring ride.
Towards the end of my ownership I wasn't getting quite 45 anymore, about like what Paul said he got.
I also suspect this was due to the corn squeezin's in the gas:hungover

Gilly

[chuck_donahue]

my 85 k100rs gets 150 miles to a tank in the warmer weather, when the fuel suppliers start mixing the gas for winter I only get 125 miles out of a tank. My k75 gets 200 miles but it only gets 1 tank of the winter stuff.
I live in northern Illinois. Winter gas sux.

I think to drop 5 mpg over 180,000 is nothing to worry about. If it runs good, I would first check to make sure your wheels spin nice and free, no stiff berrings, and the brakes arn't still slightly on.