'87 K75s Rough Idle & Bogging at low RPM's

[kentuvman]

Wondering what the symptoms are of a potentially clogged fuel filter - just returned from the Billings Rally and temps were hot! Had a tank bag covering the fuel cap on top of tank - thought that may have interfered with fuel venting?

I'd ridden through high elevations, had used my altitude switch but turned off when reaching lower elevations - symptoms were rough idle, stalling at idle, rough acceleration at lower rpms but smooth as silk at cruise. Seems to be intermittent. Had tank off before leaving and if I recall, the rear vent tube connects to fuel overflow hose and forward tube to the hose with the small little air pollution filter - is that right?

Checked MPG and it was @ 35 but was riding in wind about 75 -80 so that may be normal.

The hoses that attach to the fuel rail have been replaced as well as the black rubber caps.

Seems to be intermittent.

Looks like last fuel filter change in 2009 and about 18,000 miles ago.

Thank You,

Ken

 

[98lee]

Fuel cap has nothing to do with venting.

Front nipple at right rear of tank is the tank vent and connects to the hose that leads forward to the nipple on top of the block under the fan (vents to crankcase). Rear nipple is the rain drain for the area around the cap and connects to the hose that drains to the road behind your right pegplate.

If the tank vent hose is not connected at the tank end at all, you have created a large vacuum leak (unmeterd air that goes to the intake plenum that the computer doesn't know to add fuel for). This will cause poor running, especially at lower RPMs.

If the tank vent hose is connected, but gets kinked, your tank doesn't vent and running at any rpm can be rough or stop.

The problems with connecting these hoses properly without kinking every time the tank is replaced, plus the feeling that venting gas fumes or raw gas to your engine oil might not be the most desireable method, is why many convert to the Euro spec "Cup" or as BMW calls it the "air accumulator". It is a cup that clips to the frame right under the nipples and drains to the peg plate area. Capping the nipple on top of the block is a MUST with this modification.

There are dozens of other things that could also cause or contribute to your symptoms including:

Kinked tank vent hose
Cracked crankcase vent hose
Cracked or missing vacuum caps on vacuum ports onthrottle body
Plugged fuel filter. Has NO relation to mileage. Caused by fueling up at station with bad tanks.
Water in fuel
Valve adjustment. Tight exhaust valves
Small crack in hose between air filter box and intake plenum
Loose oil fill cap or bad o-ring on cap

 

[kentuvman]

Thanks, Lee

Looks like I should've changed Fuel Filter awhile ago - will try that as my first step.

Thanks again!

 

[98lee]

You can check your fuel filter by removing it, draining it, and letting it dry out for several hours. Lightly blow through it in the direction of the arrow. There should be no or very little resistance, similar to blowing through a large diameter straw like a McDonald milkshake straw. The higher resistance, the more plugged it is.

As a side note, I always scratch the install date on the side of new filters with a scribe and turn it so it is visable through the filler neck. I work on so many different bikes that my or the owners memory or records are not always reliable. But again, time really is not what plugs the filter. Poor fuel and tank maintenance at out of the way gas stations is.

 

[mneblett]

It's been a while since I had apart the '85 K100LT I owned for a short period, but I recall a number of problems I had to address from dried-out vacuum hoses. Your symptoms make me wonder whether you have a vacuum leak. The smooth running at cruising speed could be due to the larger volume of air being sucked into the engine (i.e., a vacuum leak becomes less significant) and the O2 sensor telling the computer to richen up things a bit to cover the relatively small amount of air coming in via the leak. The rough running at idle would be consistent with a vacuum leak being a much greater portion of the air entering the engine, perhaps enough more that the computer can't add enough fuel to compensate.

Just a thought -- it may be completely off, but thought I'd share it. FWIW, I ended up removing the entire air box to be able to eye-ball (and where necessary, replace) all of the hoses behind the throttle bodies, including the crankcase breather hose (a known leak source).

 

[98lee]

No oxygen sensor on early Kbikes.

The only thing to tell the computer how much fuel to add is the airflow meter. That's why vacuum leaks (air that gets to the engine without going through the airflow meter) are a major source of drivability problems.

If it had an oxygen sensor, it would have a means of compensating for some leaks.

 

[mneblett]

No oxygen sensor on early Kbikes.

Thanks, Lee -- learn something new every day!

 

[QUICKDRAW]

Mark

I had a similar experience on a trip to a rally last spring. Bike would miss, run rough and then be OK. After draining the tank and replacing the fuel filter still no luck. I checked the plugs and sure enough one was loose. Replaced all and problems went away.

 

[kentuvman]

Thanks for your suggestions - just washed bike and noticed the hose that attaches to fuel rail from tank was not oriented properly and clamp was not tight either - re-oriented hose and snugged clamp. Will check spark plugs and replace fuel filter - fuel filter is likely problem - good suggestion to blow through it - never would've thought of that. Went through my records and looks like I've not replaced FF since purchasing bike in 2008 and 20k miles later.

I will say we did stop for fuel at some non top-tier stations in SD, WY and MT - anyway, believe I'm headed in the right direction and grateful she ran good at higher rpms and got me home safe! One friend in our group came in a car because his Final Drive on his GSA 1200 failed in the middle of no place in Pie Town NM. Another friend in our group on an Airhead had ignition trouble and had to rent a U-Haul truck to get to the rally so it definitely could have been a lot worse! Keeping the anonymous book handy is always a good idea when far from home! Also good to know your ride!

 

[montec]

My most recent adventure with similar behavior was being caused by a corroded/failing spark plug wire.

The end that plugs into the coil was intermittently failing and causing a miss that I interpreted as a low/rough idle.

The other suggestions are much more likely causes of the issue, but it's something else to consider.

Monte
85 K100RT

 

[kentuvman]

Problem Resolved

A new OEM fuel filter did the trick!

Took Lee's suggestion and scratched month/year on side of filter.

Filter removed is 2008! Very restricted when I blew through it!

Moving forward will carry a spare!