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

[98lee]

This is the second valve job i've had done in the 29,000 miles that i've owned it.

But, this is the second valve job (adjustment) in the past 9,000 miles!!!!


But that was because BOTH of them were done improperly.


Checking every 15-20K should be good enough IF they are done to the specs that I suggested each time.


If you set the exhausts on the tight side, you'll probably end up having problems well before that. That's what Bob's did the FIRST time.


The higher the milage, the more often you want to check them because, once exhaust valve recession starts, it's rate increases.

 

[toddlamp]

If it isn't one thing it's another.

Now when I am at idle my oil pressure light comes on. I am due for an oil change so I just hope that I'm a bit low.

Im also getting some backfiring but that's most likely just cause by a leaky gasket.

 

[deilenberger]

If it isn't one thing it's another.

I'm sorta famous for that saying around our house..

Now when I am at idle my oil pressure light comes on. I am due for an oil change so I just hope that I'm a bit low.

Not good. What weight oil are you running? And what's your idle speed now? Oil level is certainly simple enough to check (centerstand, stop engine, wait 3 minutes, look at sight-glass) so I would do that immediately.

Im also getting some backfiring but that's most likely just cause by a leaky gasket.

One hopes - but which gasket? The K75 isn't prone to loose exhaust header bolts.

Under what conditions are you getting the "backfiring" (which is actually "after-fire" usually on K75's - caused by excess unburned fuel in the exhaust stream combining with heat and oxygen...)

Just curious - have you done another compression test? A burned exhaust valve can also cause this.

 

[98lee]

If it isn't one thing it's another.

Now when I am at idle my oil pressure light comes on. I am due for an oil change so I just hope that I'm a bit low.

Im also getting some backfiring but that's most likely just cause by a leaky gasket.

NOT GOOD!!!

Ah, there's this round plastic window on the side of the motor that will tell you if it's low.

The backfiring is more than likely a misadjusted throttle position switch.

Once you get the valves adjusted CORRECTLY, THEN get the throttle bodies balanced and the idle set correctly, THEN get the throttle position switch set.

 

[PGlaves]

The mica gasket between the muffler and the header can aggravate the tendency to pop off in the exhaust. And, since they last at least down to the corner and back they are often suspect.

The other thing is to check the throttle position switch.

Re: Oil light - make sure hot idle is at least 1050; 1100 works better.

 

[toddlamp]

I'm sorta famous for that saying around our house..Not good. What weight oil are you running? And what's your idle speed now? Oil level is certainly simple enough to check (centerstand, stop engine, wait 3 minutes, look at sight-glass) so I would do that immediately.One hopes - but which gasket? The K75 isn't prone to loose exhaust header bolts.

Under what conditions are you getting the "backfiring" (which is actually "after-fire" usually on K75's - caused by excess unburned fuel in the exhaust stream combining with heat and oxygen...)

Just curious - have you done another compression test? A burned exhaust valve can also cause this.

I am definitely aware of the sight window but never knew what the proper level is. The oil level is still visible but toward the bottom of the sight window. I will top off asap. I'm using BMW dino 10w40.

Backfires are occurring during deceleration but not all the time

I will double check the compression as soon as I have a chance. Do I need to let the bike cool off before checking compression?

I asked the service guy if the tech thought the valve could be burnt and he said no.

The backfiring is more than likely a misadjusted throttle position switch.

Once you get the valves adjusted CORRECTLY, THEN get the throttle bodies balanced and the idle set correctly, THEN get the throttle position switch set.

Throttle position switch seems to be adjusted just fine; clicks exactly when its supposed to but I will double triple check it once I get the valves redone.

The mica gasket between the muffler and the header can aggravate the tendency to pop off in the exhaust. And, since they last at least down to the corner and back they are often suspect.

The other thing is to check the throttle position switch.

The backfiring does seem to be similar to how it was last time I needed to replace the gasket where the headers and muffler meet.

Re: Oil light - make sure hot idle is at least 1050; 1100 works better.

My idle is a little low then. I went off the OEM sticker that say idle is to be 950 rpms. The light flickers once the tach hit 1000rpms and goes solid for rpms under 950rpms. Once I get on the throttle even a tiny bit the light goes out.

 

[deilenberger]

I am definitely aware of the sight window but never knew what the proper level is. The oil level is still visible but toward the bottom of the sight window. I will top off asap. I'm using BMW dino 10w30.

Backfires are occurring during deceleration but not all the time

I will double check the compression as soon as I have a chance. Do I need to let the bike cool off before checking compression?

No. You'll get a more accurate reading with a warm engine.

I asked the service guy if the tech thought the valve could be burnt and he said no.

Same tech who misadjusted them a 2nd time?

Throttle position switch seems to be adjusted just fine; clicks exactly when its supposed to but I will double triple check it once I get the valves redone.

"Sounds" OK - never had one fail, but anything is possible I guess as these bikes get older.

The backfiring does seem to be similar to how it was last time I needed to replace the gasket where the headers and muffler meet.

Gives you a hint where to look right?

My idle is a little low then. I went off the OEM sticker that say idle is to be 950 rpms. The light flickers once the tach hit 1000rpms and goes solid for rpms under 950rpms. Once I get on the throttle even a tiny bit the light goes out.

As Paul said - idle on the K75 is supposed to be 1,050-1,100 RPM. Dunno what sticker you're reading. It will also tend to stall on occassion coming down from speed if the idle is set below 1,000 RPM. The 4 cylinder can idle at 950 OK..

 

[98lee]

Todd,

You REALLY need to get a shop manual and read it if you are going to try and maintain your own bike.

Oil is full at the top of the circle (with some air showing between the top of the circle and the top of the glass). Try keeping the level between the dot and the top of the circle.


According to the riders manual, SAE 10w30 is only good for between 20 BELOW and 35 degrees F. I can't believe that is your outside temp when you are riding. It's WAY too thin for a K75.

I believe you should be using SAE 15w50 which the manual recommends for between 5 BELOW to 85 degrees F. Or SAE 20w50 which the manual recommends for 15 to 105 degrees F. I can't believe you are out riding in weather below 15 degrees. 20w50 would handle all your riding very well. There's no reason to use anything else.

 

[PGlaves]

An idle of 950 and 10W30 oil will cause the light to come on, especially if the bike is warmed up. I ride year around - doesn't mean much in south Texas - but I ride in temperatures down into the 30s here and subfreezing on occasion in the mountains, or the Yukon, when riding in the summer. I use 20W50 oil in K75s all of the time and wouldn't consider using 10W30. If it is cold enough to need 10W30 it is colder than I want to ride in.

Even when we lived in Iowa and Kansas the bikes had 20W50. The garage stole a little heat through the wall from the house to which it was attached so it was almost always at least 40 in the garage on a cold winter riding day. And once out and about it was never parked long enough to cool down to where 10W30 might have been needed. I don't like even 10W40 in K75s. That is only good up to ambient 68 in theory, and the water cooled engine doesn't really care if it is that cool or not except on startup.

 

[toddlamp]

Dunno what sticker you're reading. It will also tend to stall on occassion coming down from speed if the idle is set below 1,000 RPM. The 4 cylinder can idle at 950 OK..

The sticker underneath the seat on the bike

 

[toddlamp]

Todd,

You REALLY need to get a shop manual and read it if you are going to try and maintain your own bike.

Oil is full at the top of the circle (with some air showing between the top of the circle and the top of the glass). Try keeping the level between the dot and the top of the circle.


According to the riders manual, SAE 10w30 is only good for between 20 BELOW and 35 degrees F. I can't believe that is your outside temp when you are riding. It's WAY too thin for a K75

I have a shop manual. I was just never sure if the bike was actually filled when the oil level was at the red dot or as you say toward the top of the glass.

I believe you should be using SAE 15w50 which the manual recommends for between 5 BELOW to 85 degrees F. Or SAE 20w50 which the manual recommends for 15 to 105 degrees F. I can't believe you are out riding in weather below 15 degrees. 20w50 would handle all your riding very well. There's no reason to use anything else.

I ride all year round here. A day hasn't gone by this fall/winter that I haven't ridden so yes, believe it. Last year I was out riding in single digit temps and expect to do it again this year. As cold as it can get here is as cold as I have ridden in.

I don't have a quart with me but I think it might be 10w40. That is what I have been using for over 2 years with the bike with no issues. I think that being due for a change and being a bit low is the reason my pressure light is flickering.

 

[toddlamp]

An idle of 950 and 10W30 oil will cause the light to come on, especially if the bike is warmed up. I ride year around - doesn't mean much in south Texas - but I ride in temperatures down into the 30s here and subfreezing on occasion in the mountains, or the Yukon, when riding in the summer. I use 20W50 oil in K75s all of the time and wouldn't consider using 10W30. If it is cold enough to need 10W30 it is colder than I want to ride in.

Even when we lived in Iowa and Kansas the bikes had 20W50. The garage stole a little heat through the wall from the house to which it was attached so it was almost always at least 40 in the garage on a cold winter riding day. And once out and about it was never parked long enough to cool down to where 10W30 might have been needed. I don't like even 10W40 in K75s. That is only good up to ambient 68 in theory, and the water cooled engine doesn't really care if it is that cool or not except on startup.

Since I am due for a change, and it's just going to get colder, I will go with 15w50 as Lee suggested

 

[deilenberger]

The sticker underneath the seat on the bike

What does the sticker say on "Displacement" (just to the right of your crop..)?

Wonder if BMW had a bunch of K100 stickers left over..

 

[LOSTBOY]

Since it specs ignition timing at 4 degrees BTDC, it is a K75 sticker.

Toddlamp, are you seeing the oil light or generator light flickering?

 

[mneblett]

If you have been using 10W40, I believe the oil pressure issue should be further investigated.

I'm ~20 mi. from you, ride year-around as well, currently have 10W40 in the crankcase of my 162kmi. FY K100RT (which idles at 950) and I've *never* seen an oil light, except between the time the key is turned and the engine is started.

My first Qs:

What's the status/condition of the oil filter? Brand (OEM or aftermarket)?

Have you checked the oil pressure switch and wiring for the usual suspects (leakage at the switch, broken wire making intermittant contact (short to ground)?

If nothing unusual in the above, I'd want to plumb in an oil pressure gauge to see what is the actual pressure in the rails.

I don't want to be crying "the sky is falling" here -- I just want you to consider that if this is a real problem, masking it by going to 20W50 isn't the best way to go: at best you'll still have low oil pressure, only slightly above the switch's trigger point, instead of slightly below. I'd suggest you need to do a bit more investigating before deciding to ignore what may be a legitimate indication. As we used to teach the sailors: believe your indications, at least until you have a solid reason to not trust them.

 

[toddlamp]

What does the sticker say on "Displacement" (just to the right of your crop..)?

Wonder if BMW had a bunch of K100 stickers left over..

740. Here's a better picture

 

[toddlamp]

Since it specs ignition timing at 4 degrees BTDC, it is a K75 sticker.

Toddlamp, are you seeing the oil light or generator light flickering?

Definitely oil pressure light. I just started it and let it idle, cold, at well under 1k rpms and the light did not come on which leads me to believe it is more of a viscosity issue, but I am still also low so I'm assuming both those factors are working against me.

 

[toddlamp]

If you have been using 10W40, I believe the oil pressure issue should be further investigated.

I'm ~20 mi. from you, ride year-around as well, currently have 10W40 in the crankcase of my 162kmi. FY K100RT (which idles at 950) and I've *never* seen an oil light, except between the time the key is turned and the engine is started.

My first Qs:

What's the status/condition of the oil filter? Brand (OEM or aftermarket)?

Have you checked the oil pressure switch and wiring for the usual suspects (leakage at the switch, broken wire making intermittant contact (short to ground)?

If nothing unusual in the above, I'd want to plumb in an oil pressure gauge to see what is the actual pressure in the rails.

I don't want to be crying "the sky is falling" here -- I just want you to consider that if this is a real problem, masking it by going to 20W50 isn't the best way to go: at best you'll still have low oil pressure, only slightly above the switch's trigger point, instead of slightly below. I'd suggest you need to do a bit more investigating before deciding to ignore what may be a legitimate indication. As we used to teach the sailors: believe your indications, at least until you have a solid reason to not trust them.

Oil is due for a change, if not over due.

Oil pressure switch was changed about a year ago.

How low would the oil need to be before it affects the oil pressure causing the light to illuminate? As I posted above, I am thinking it's just a thinning of the oil combined with a low level causing my issue.

I will be taking it in for the valves to be redone and having an oil change performed at the same time.

 

[mneblett]

How low would the oil need to be before it affects the oil pressure causing the light to illuminate?

If you have enough to show at the bottom of the sight glass, you still have enough to cover the oil pick-up; i.e., if you can see oil, the light shouldn't be flickering.

If the oil is overdue for change, I have to wonder whether the oil filter is excessively dirty, and the built-in bypass (assuming it's not a crap FRAM filter) isn't working well.

 

[PGlaves]

If you have enough to show at the bottom of the sight glass, you still have enough to cover the oil pick-up; i.e., if you can see oil, the light shouldn't be flickering.

If the oil is overdue for change, I have to wonder whether the oil filter is excessively dirty, and the built-in bypass (assuming it's not a crap FRAM filter) isn't working well.

Well, at the bottom of the sight glass you still have over 3 qt of oil in the sump. It would not be a pickup issue if any oil is visible. But three quarts will get hotter than 3.5 qt so it could contribute a little to thinning I suppose.