Running lean, getting awful mileage; not sure how to fix. 1994 K75

I just recently completed a 9000+ mile cross country trip and prior to leaving did all the normal maintenance items (air/fuel filter, plugs, oil change, spline lube, etc). In the beginning of the trip the bike was running fantastic. Once I hit higher elevations (5000ft+), however, the bike would stall at idle even with the elevation plug installed.

Back closer to sea-level in California the bike again ran great with a noticeable increase in power. All the way across the lower states the bike seemed fine. Performance was good and so was fuel mileage.

At the Tail of the Dragon I had the bike dynoed (for ****s and giggles) and also had an Air/Fuel analysis done. This was a WOT run, but the graph showed that I was running very lean toward the top RPM range (eg ~80mph A/F = 16).

Coincidentally (maybe?) that night we ended up doing the last 600 miles as we were eager to get home. On my last tank I went only 130miles and was so low on gas I didn't think I would make it; my fuel mileage had plummeted. Environmental conditions were similar to other areas of the trip and weight had not changed at all. Tire air pressure was the at the same level it was through out the trip. This seems to contradict the fact that I was told I was running lean.

Once I got home I checked the normal items. The air filter was just fine. The spark plugs all look great and were consistent with one another. Turned out that the fuel filter was almost completely clogged. I could barely blow air through it!! That was swapped out for a brand new OEM BMW filter. I was certain that the clogged filter was impeding fuel flow, lowering pressure, thus causing my lean condition.

Well, I have ridden the bike three times since the fuel filter change and I am still running lean. I know this by doing the starter button test. After the bike is warm I put the choke lever on the first position. This causes the bike to hold a steady engine speed of ~1,300rpms. Pushing the starter button causes the engine speed to increase to just over 2000rpms.

In addition my bike is still stalling, or getting ready to stall, when I come to a stop. Where do I go from here? I just recently had my stock injectors cleaned and plan on installing them in place of the Ford injectors I have in there now. Some may say that they are the problem but why? I have had those injectors in for thousands of miles before the trip with no issues.

The only thing I can think of is that the fuel filter was too clogged for too long and wore the pump down and it has become weak because of the abuse. Is that a possibility? I do not have a fuel pressure gauge to determine if it is putting out adequate pressure. Judging by sound alone the fuel pump is performing no different.

Thanks for the help.

Thought -

Fuel pumps on these bikes are sometimes damaged by the pump damper dissolving in the fuel we have here. My 85's pump damper had dissolved into black goo and chunks of rubber - usually when this happens the fuel pump sock falls off and this junk gets sucked into the pump - and if small enough into the filter.

Which may have been what happened to your bike.

Might be worth pulling the fuel line going into the filter and activating the pump to visually check the fuel flow. While you are there check for damaged fuel lines.

Always check the easy stuff first.

Todd - I'd suggest first that you include the year/model of your bike in the thread title. People who know something about that year/model might be more tempted to open it and look at your problem. I'm adding it for you.

As far as your problem? What caused the rapid plugging up of the new filter? Chances are good whatever it is - is still in your tank and has plugged up the new replacement you just put in. Unless you drain and clean out the tank after getting a bad tank of fuel, you'll be spending a bunch of money on filters trying to get the crap out.

That's thought 1..

Other random thoughts:

1. The idle test - dunno where you heard how to do it - but the test Rob Lentini used to discuss involved running the bike at normal idle speed (on a K75 that's right around 1,000 RPM) and press the starter button. Idle speed should increase a few hundred RPM. Try doing that and see what happens.

2. If the bike is idling at 700RPM, it's idling too slow, and that's why it feels like it's going to stall out. The idle speed is adjusted with the idle-screw on the throttle body assembly (believe it's between 2-3 TBs). DO NOT ADJUST any of the inter-throttle-body screws (the ones that connect the TB shafts together.) These are adjusted at the factory that assembles the TB assembly, and they're not a user serviceable item.

3. Have you looked at the big-brass-screws? I'd consider removing them and seeing if they're gunked up. If you haven't done this in a year or 10k miles they likely will be. They also have O rings on them that are known to leak. Most dealers tend to unscrew these puppies for some reason, never screw them in. These are used to balance the TBs at idle - so you need a set of vacuum gauges to adjust them.

4. It's possible you have intake air leaks. Any leak downstream of the air-flow-meter (located in the air filter box) will be "unmetered" air - and will cause a lean condition. What condition is your crankcase vent hose (the Z shaped hose to the right of the TBs)? Have you tested your throttle-bodies, injector seals and the brass-screw O rings for leaks?

5. Have you adjusted the CO bypass screw in the air-flow-meter? (Which is what the Lentini mixture test is great for..)

Just some thoughts.. hopefully helpful.

Hey Don.

I was actually told about the starter button test by one of Bob's BMW mechanics. He said that hitting the starter button while the bike is running adds approximately 10% more fuel. If the RPMs increase it indicates a lean mixture; a drop means there is too much fuel. I was surprised to see my tach increase almost 1,000rpms.

I didn't even think about the new fuel filter being clogged so soon. I will check it tonight to see what condition it is in. I am anxious to cut open the old one to see what's inside but needed to wait for all the fumes to vacate before using the Dremel.

The bike was idling fine in the beginning of the trip and with no problems til we hit higher elevations. When the symptoms returned at lower elevations I assumed that it had to be because of a failure rather than incorrect idle. I will readjust the idle when i get home. A thought I had on that was since I replaced the throttle cable before the trip i'm thinking that the cable may have been a tiny bit too tight preventing the throttle bodies from returning to the true idle position. And once the cable has stretched a bit, introducing so play, the throttle bodies are now able to come to rest and the position is not correct.

Before the trip I had the screws adjusted at Bob's and when I was getting the splines lubed the crankcase vent hose was replaced. That's not to say that it hasn't gone bad again as I did not check it (I know that they have a high failure rate.

I have never messed with the CO bypass screw. That's the screw on the top, right side, of the air box right?

Hi Todd,

It's possible the cable was off. It shouldn't be used to set idle speed - the screw near the sector it connects to is for that.

The CO adjustment is the one on the top of the airbox. It's basically an air bypass (controlled leak) around the air-flow-meter. The adjustment can be done via the Lentini test - which is:

Idling, fully warmed up (1,100 RPM or so)
Press the starter button.
- If it speeds up and stays faster - the mixture is lean
- If it speeds up and then returns to the initial speed - the mixture is correct
- If it slows down and stays slow - the mixture is rich

Screwing the CO screw in (clockwise) closes the air passage and richens the mixture.

The test is assuming that there are no air leaks. If there are air leaks all bets are off.

As far as the big-brass-screws (BBS's) - you might carefully see where they're adjusted to. Slowly turn them in until you just feel them touch. Normal is 1 to 1.5 turns (factory is 1.5 turns out from bottom.) I've seen lots of them that are WAY out, it seems dealers like to do that. If they are way out, I'd suggest:

- Check your valve clearances. If the valves are out of adjustment, you'll never get the bike to run right.

- beg/steal/borrow a set of vacuum gauges. A TwinMax can be used, but it's a bit of a PITA on a triple since you have to do a lot of hose swapping.

- Preset all the BBS's to 1.5 turns out from bottom.

- Adjust the idle speed screw to give 1,100 RPM idle

- Check the balance between the TB's at idle. Very minor tweaking may be required using the BBS's. The screws shouldn't be more then 1/2 turn different from each other, if they are - there is an air leak somewhere or a valve not adjusted correctly.

- Check for air leaks - this is what I'd do - not what I'm suggesting you do.. since you can burn your house down (or poke your eye out..) Bike outside, fire extinguisher handy. With the bike warmed up and idling - take some flammable carb cleaner - with the little red straw - and make tiny sprays of it at the base of the TBs (where the rubber bits fasten to the head) and around the top where the hoses connect them to the intake plenum, around each of the BBS's, and around the base of each injector. ANY - and I mean ANY - change in idle speed means you have a leak. You can also spritz a bit around the crankcase vent hose. You might also spritz a bit around the big rubber elbow where it fastens to the intake plenum, and where it comes out of the airbox. If you find leaks (idle speed will increase) - fix them.

Good luck, and let us know how it works out!

quick question in regards to the BBS:

turning the screws which way richens or leans the mix at idle? Riding around today the bike only seems to be running lean (and I even thought I heard some detonation- knock) at idle.

What do the plugs look like? Do all three look the same?

Ted said:

What do the plugs look like? Do all three look the same?

Click to expand...

Yes. They all look normal (tanish color). That was the first thing I checked.

toddlamp said:

quick question in regards to the BBS:

turning the screws which way richens or leans the mix at idle? Riding around today the bike only seems to be running lean (and I even thought I heard some detonation- knock) at idle.

Click to expand...

Neither way. They are an air-bypass around the throttle-body throttle-plate. They are using metered air. They don't change the mixture at all. They only change the idle balance.

1987 k75s

Interesting thread. As often happens while reading these forums you learn something new. I had never heard of pressing the start button while the bike is idling to determine if the fuel mixture is right. I had always assumed that pressing the starter button of a running bike would give the same result as turning the ignition key in a running car, a loud crunch. Anyrate after reading this I tried it with mine & nothing happens, doesn't speed up, doesn't decrease, nothing happens. Does that mean anything?

deilenberger said:

Neither way. They are an air-bypass around the throttle-body throttle-plate. They are using metered air. They don't change the mixture at all. They only change the idle balance.

Click to expand...

Ok thanks.

Interesting new info. After riding it a few times and going through a couple tanks of fuel the bike feels great at all throttle position EXCEPT idle. I have adjusted the idle up a bit and when putzing around town at stop lights, after warming up, I can usually maintain 1,000rpms.

HOWEVER...anytime that I am traveling at highway speeds, and pull the clutch (eg preparing for an off ramp) release the throttle, and allow the rpms to drop, the bike almost always stalls. This seems to be relational to how fast I close the throttle.

In addition, when I am warming up the bike (and most times when the bike is already at operating temp) the choke, even at full position, rarely increase the rpms above 2k. I know this is not correct. Before, with the choke at position #2 and when warm, the engine should be at 4k at least.

I have pretty much isolated my issue to be at idle. And when I can get it to idle, and hit the starter button, the rpms always jump up ~1,000rpms and stay there til I release the button.

If it hasn't already been mentioned and emphasized, step one is to check/adjust the valve clearances.

And set the warm (after at least 10 miles of riding) idle to 1100 rpm.

If then the cold idle is too low you can chase that issue, but it probably won't be.

So since it increases speed when enrichened with the starter button it is too lean. Clean injectors, new filter, check regulated pressure.

PGlaves said:

So since it increases speed when enrichened with the starter button it is too lean. Clean injectors, new filter, check regulated pressure.

Click to expand...

Newly overhauled (Mrinjector.com) OEM injectors were installed the other day. New OEM fuel filter was installed last Friday'ish. Valves were checked and adjusted shortly before the trip (~10k miles ago). Could they already need adjusting again?

How do I check the the pressure? I am also thinking that my fuel pump is failing. Is there anyway to determine that ahead of time?

What sensors come into play during idling? I know that the TPS is there to tell the FI system to shut off; would that have anything to do with this? I have checked that as well and there is a audible click as soon as I barely increase throttle. Is idle 100% mechanically controlled?

How exactly does the choke work? Does it work the same as if I manually turn the throttle? I turned the throttle the perceived amount that I feel equals what the choke should be doing when fully engaged and the rpms feel much different (much higher when using the throttle as opposed to choke)

So after a 45min ride home from work I have determined that, in my unprofessional opinion, the bike is running fantastic in all areas except when idling.

Sounds to me like a textbook unmetered air leak somewhere. I recently found an awesome doc on chasing down unmetered air leaks in classic K bikes, only problem is that it is in German. Go here and click on "Falschluft" - it is a zip file with a PDF inside. Fortunately you can copy and paste from it and plug it in to Google Translate. If someone were to find a fully translated version of that document I would buy you many beers at the next rally

Evey time you change injectors, you will affect the CO content (fuel mixture).

When Bob's checked your CO, was it with fresh oil? As a motor runs, hydrocarbons make their way into the crankcase and the when the motor runs, rise through the crankcase vent and into the intake stream. If the oil has more than a few miles on it, you can not get a good reading with an Exhaust Gas Analyzer.

Checking for air leaks requires nothing more than a can of Carb/Throttle Body cleaner. Look the intake over for obvious leaks-where the throttle bodies mount, vaccum caps popped off, hoses that have slipped off, and the o-ring on the oil filler. Then spritz any suspicious spot with the cleaner. Air leaks will show up via an increase in idle speed. Wear safety glasses and have a fire extinguisher nearby, and know how to use it.

lostboy said:

Evey time you change injectors, you will affect the CO content (fuel mixture).

When Bob's checked your CO, was it with fresh oil? As a motor runs, hydrocarbons make their way into the crankcase and the when the motor runs, rise through the crankcase vent and into the intake stream. If the oil has more than a few miles on it, you can not get a good reading with an Exhaust Gas Analyzer.

Checking for air leaks requires nothing more than a can of Carb/Throttle Body cleaner. Look the intake over for obvious leaks-where the throttle bodies mount, vaccum caps popped off, hoses that have slipped off, and the o-ring on the oil filler. Then spritz any suspicious spot with the cleaner. Air leaks will show up via an increase in idle speed. Wear safety glasses and have a fire extinguisher nearby, and know how to use it.

Click to expand...

Thanks, I will definitely give that a try. Tonight I gave the bike a fairly thorough look and didn't not see anything in the intake track that was obviously out of spec. Crankcase hose looks to be in perfect condition.

Todd -

It sounds like a classic flailing approach.. ie - replace parts and hope the part fixes the problem. If the last part didn't - what's the next part that "might" be a problem?

I outlined a number of diagnostic steps. Other people have suggested the exact same diagnostic steps. Have you done them? If so - what are the results?

So far you have come back with more guesses on what part you think might be failing, with no data to support it. A hint - fuel pumps, especially positive displacement fuel pumps - generally work or don't. If one somehow managed to fail by producing low fuel pressure - that would be more of a problem at high speed engine operation then low. It is much more likely if there is a fuel pressure problem to be a plugged filter or a bad fuel-pressure-regulator (or the vacuum hose going to it, which would be found with the intake leak test.)

Without thinking through the problem, and trying to track down the cause, it gets real expensive real fast fixing motorcycles by throwing parts at them.. the last part you replace will be the problem but how many parts were replaced before the last part. In this case - parts may not do a thing since we don't know if your basic adjustments and settings are correct.

The questions you need to provide answers to:

- are the valves adjusted correctly (having them done 10k miles ago doesn't mean they're still correct.) - they need to be measured.

- what is the position of the BBS's (and are they clean with good O rings on them)?

- are there any intake leaks? These are most noticeable at idle (hint!)

And another hint - a brand new fuel filter can be instantly plugged if you have a bad tank of fuel, and didn't drain and replace it with good fuel. Being new doesn't mean it's good. It just means it's new.

deilenberger said:

Todd -

It sounds like a classic flailing approach.. ie - replace parts and hope the part fixes the problem. If the last part didn't - what's the next part that "might" be a problem?

I outlined a number of diagnostic steps. Other people have suggested the exact same diagnostic steps. Have you done them? If so - what are the results?

So far you have come back with more guesses on what part you think might be failing, with no data to support it. A hint - fuel pumps, especially positive displacement fuel pumps - generally work or don't. If one somehow managed to fail by producing low fuel pressure - that would be more of a problem at high speed engine operation then low. It is much more likely if there is a fuel pressure problem to be a plugged filter or a bad fuel-pressure-regulator (or the vacuum hose going to it, which would be found with the intake leak test.)

Without thinking through the problem, and trying to track down the cause, it gets real expensive real fast fixing motorcycles by throwing parts at them.. the last part you replace will be the problem but how many parts were replaced before the last part. In this case - parts may not do a thing since we don't know if your basic adjustments and settings are correct.

The questions you need to provide answers to:

- are the valves adjusted correctly (having them done 10k miles ago doesn't mean they're still correct.) - they need to be measured.

- what is the position of the BBS's (and are they clean with good O rings on them)?

- are there any intake leaks? These are most noticeable at idle (hint!)

And another hint - a brand new fuel filter can be instantly plugged if you have a bad tank of fuel, and didn't drain and replace it with good fuel. Being new doesn't mean it's good. It just means it's new.

Click to expand...

I meant to try the starter spray trick last night but didn't have time.

I am not just throwing parts at it. I have completed most of the suggestions given. I was using the Ford injectors so I wanted to get the bike back to stock but also hoping that would solve the problem.

I would really like to know what is going on at idle so that I know what parts to focus on. I have already done most of what was suggested.

So far I have:
Check spark plugs
Check air filter
Replaced fuel filter
Installed refreshed injectors
Checked BBS (#1 - 1 1/4 turns, #2 - 1 - 1/2 turns, #3 - 1 1/4 turns)
Adjusted idle
Eyeballed the typical intake leak points (crankcase vent hose, throttle body plugs, etc)

Next I will:
Use starter fluid for intake leaks
check valves
Empty tank and replace fuel filter