Newly acquired '85 K75S with some problems

[szabgab]

Dear all!

I'm new around here, as I have just bought an 1985 K75 with approx 80.000 miles (130.000) km's on the odo. SO far I have been quiet, only reading, and following, what's going on...  I love this bike of mine so far, although it is giving me some headache, ever since I started poking around fixing things and finding bigger things to fix. I have some experience with Jap bikes (especially early '80s Hondas), but this thing is new on me.

Anyway, the bike starts rather hard, and needs choke even in 35 C degrees heat, reacts to throttle badly, when cold, once warmed up, bike runs great. I assumed a lean condition, I guess air leak somewhere, and surely enough the breather hose was cracked basically head to toe. So I bought a new one and went around replacing it, but the plastic stub on the airbox is broken off too... Great, I thought, I will need to glue it, but nothing seems to be bonding the damn pieces together - I have tried epoxy, super glue, flexi super glue, RTV, even some mechanical bond - basically a hose extender, that fits perfectly in the inside diameter of the nipple and the airbox opening. But even so, the airbox side has got too little material there, and wont hold the thing.  If I hold the hole closed on the airbox with my finger, idle is lot better as is the throttle response. No matter how I try to glue this thing, bike just runs lean. I'm not sure, if it is related (I'm certain it is) my MPG is crap, it was just fine, when I bought the bike (although the PO had put some miles in, I did some, so this could be misleading), but on the second full tank I could barely ride 150 miles till the engine started to starve and the light came back on. Interestingly enough once the light comes on (although this supposed to be the two light model), the bike drives like crap, on tight right turn it will not accelerate and behaves very ill for a minute or two afterwards, clearly is fuel starved. What could be the issue there? Shouldn't the  bike have something like 7 litres of fuel left on first light? (Once the light is solid, I can put in 15-16 litres of petrol in the tank, that does not have the flap anymore).

Also I know, they all do backfire, but mine does it ALL the time, whenever I let of the throttle, engine braking or not. TPS works as it should, so is this another symptom of a lean running engine?

Thank you and sorry for bulldozing you all with a ton of questions :)

Regards,

Gabriel

 

[The Mighty Gryphon]

Regarding the airbox, I think it is made from Polyethylene or Polypropylene two plastic materials that are practically speaking impossible to use adhesives on.  You can try welding the broken part with a soldering iron, but since the material is thin your chance of success, especially since this is your first go at it is pretty low.  Best bet is to put some duct tape over the hole in the air box, connect a piece of rubber hose to the spigot on the engine and direct the vapors away from the rear tire.   Then begin to look for a used air box to replace your damaged one.

Regarding the backfiring, it may indicate an exhaust leak where the headers enter the muffler.  You also may need to get into the habit of closing the throttle completely when coasting.  Slight throttle openings will increase the amount of backfiring.  It took me a couple years to get out of the habit of holding a tiny bit of throttle when coasting.

[Laitch]

You have a used bike with a fair amount of both mileage and neglect in its history. You must find and eliminate any intake leaks in the air box/throttle body manifolds. You must test the coolant temperature sensor in the standpipe behind the radiator to determine if it is capable of send enrichment signals to the injection system when the engine is cold. It might be stuck in its lean range and that's why the engine runs better when it's warmed up.

Are you trying to fix this air box port problem without removing the box? Post some photos of the damaged area.

Forget about the two light fuel warning situation for now. You're lucky to even get it on the road. Right now, think of every procedure that you've done to this moto already, post the complete list here then you'll be able to tackle this in an orderly way. List everything you've done to this moto. As Supershooter can tell you, this could take a while.

Here comes the bulldozer back at you. :giggles

[Andy FitzGibbon]

The fuel level sensors in the earlier bikes, which use a temperature switch, are known to be problematic. Mine (1985 K100) generally comes on 10-20 miles into a ride, regardless of fuel level.

Andy

[szabgab]

Regarding the airbox, I think it is made from Polyethylene or Polypropylene two plastic materials that are practically speaking impossible to use adhesives on.  You can try welding the broken part with a soldering iron, but since the material is thin your chance of success, especially since this is your first go at it is pretty low.  Best bet is to put some duct tape over the hole in the air box, connect a piece of rubber hose to the spigot on the engine and direct the vapors away from the rear tire.   Then begin to look for a used air box to replace your damaged one.

Regarding the backfiring, it may indicate an exhaust leak where the headers enter the muffler.  You also may need to get into the habit of closing the throttle completely when coasting.  Slight throttle openings will increase the amount of backfiring.  It took me a couple years to get out of the habit of holding a tiny bit of throttle when coasting.

Yes, I think it is PP plastic, damn hard to repair. There is a special two parts glue, that is supposed to glue it pretty well, one part is some sort of a surface wetter, the other is a cyanoacrylate (superglue, I guess). Anyway, that would be good for a while, although superglue is brittle and not completely oil resistant, so that would void the joint pretty quickly. Other option is to screw in a brass nipple, but the joint is weak to hold a screwed in connector for too long. If I could go inside the box, I could put a locking nut on the other end, but it seems to be a sealed unit. Anyway, a replacement would be probably the best solution, but the most complicated too, as I would have to cut the Oetiker clamps, remove the box, replace the clamps, source a tool for it and so on. Also I'm sure, once I start to poke around the old rubber pieces, they would start to leak like a sieve. 

Exhaust leak is a possibility, when I left the bike on the side stand and turned it into a smoke machine two hours later, the joint was leaking smoke too. Is this a case of removing the muffler, replacing the seal and tighten it up again? Closing the throttle fully is an old habit anyway, so that will not be the problem (although clening the old crap from the throttle gear and using lighter oil, as recommended here did help to smooth the action)

[szabgab]

You have a used bike with a fair amount of both mileage and neglect in its history. You must find and eliminate any intake leaks in the air box/throttle body manifolds. You must test the coolant temperature sensor in the standpipe behind the radiator to determine if it is capable of send enrichment signals to the injection system when the engine is cold. It might be stuck in its lean range and that's why the engine runs better when it's warmed up.

Are you trying to fix this air box port problem without removing the box? Post some photos of the damaged area.

Forget about the two light fuel warning situation for now. You're lucky to even get it on the road. Right now, think of every procedure that you've done to this moto already, post the complete list here then you'll be able to tackle this in an orderly way. List everything you've done to this moto. As Supershooter can tell you, this could take a while.

Here comes the bulldozer back at you. :giggles

I have not done much to the engine, this is just a beginning of a (hopefully) beautiful friendship. All I did was to replace the crankcase breather, the other stuff - fluids, etc are next. The PO has taken the bike prior to selling it to THE K bike professional of the country, if not the region, an old guy, who was a bike repairman of the police forces, who ran K75s for many years. He is supposed to be the number one authority on these tools (he certainly charges accordingly). Anyway, the only bill, I have gotten with the bike was the repair bill issued a week before me buying the bike (2nd of August), and the reapir maestro replaced all the plugs, brake pads, brake lines, replaced brake fluid, checked the valves and adjusted the number three exhaust valve, replaced the timing chain guides. Oil was changed 200 kilomteres ago (although I will do it again, just to be on the safe side)

[szabgab]

Also I'm not sure,  if this is anything to go by,  but the fan kicks in in city traffic,  so the thermostat does some. sort of a work...  Hoe to test the other end of the operation (cold engine).  Oh and BTW,  if I press the starter button with thw engine running nothing is happening,  maybe a 50rpm rise,  but I might be imagining things.

Oh,  yeah,  and the other thing,  I have done is spraying brake cleaner around aNything rubbery,  and there was no change in idle speed,  no matter where I pointed the nozzle (injectors,  boots,  vacuum hose,  etc)

Thanks!

[The Mighty Gryphon]

I wouldn't think it would be all that difficult to replace the airbox.  They are available on eBay for fairly reasonable prices and often include the boots that connect to the throttle bodies.  Here's a nice one:

https://www.ebay.com/itm/1992-BMW-K75-RT-S349-7-air-box-intake-expansion-chamber/232724610032?hash=item362f77bbf0:g:P9MAAOSwQKBavSqQ&vxp=mtr

You do not need to use the clamps with the special tool.  There are Suzuki clamps available that have screw clamps for about 4 USD. 

https://www.partzilla.com/product/suzuki/09402-48208

Other than connecting the large hose to the MAF chamber the installation of the new airbox is a fairly easy job.  Many K75 owners have done this job as part of replacing the throttle body boots.

It's a job that, unfortunately for you, must be done.  The good news is that the parts should cost about 50 USD and take an afternoon to install.

[Laitch]

Also I'm not sure,  if this is anything to go by,  but the fan kicks in in city traffic,  so the thermostat does some. sort of a work...  Hoe to test the other end of the operation (cold engine).  Oh and BTW,  if I press the starter button with thw engine running nothing is happening,  maybe a 50rpm rise,  but I might be imagining things.

In the States there are brake cleaner formulas that are not volatile so verify that what you used isn't of that type. You want to use something that will explode if misused.  cool shades

Action of the thermostat is governed by coolant temperature. It helps the engine warm to operating temperature sooner because it allows coolant to bypass the radiator, then opens to allow the coolant temperature to be moderated by passing through the radiator

The coolant temperature sensor is a different animal. It senses the temperature of the coolant and sends a signal to govern the fuel mixture. A cold engine receives richer fuel than a warm engine. If the sensor is faulty and its signal indicates that the engine is warm when actually the engine is cold, starting and running will be difficult until the engine has run a while because the sensor will be calling for a leaner mixture. The sensor adjust according to an arc of temperatures—not just warm or cold. There are two tests described on this site for assessing the coolant temperature sensor. A faulty sensor is a possibility if intake air leaks aren't the source of the problem.

That the fan runs intermittently in traffic during warm weather is not unusual. If it runs intermittently in moderate weather and light traffic, it is likely there is not enough coolant in the system or the coolant that is there is too old.

From what you write, it appears that you have done well in having the engine and the moto itself assessed.

Try to focus and keep from imagining things though, unless they are pleasant things.

[The Mighty Gryphon]

With 130,000km I wouldn't be surprised if there is a fair amount of dirt and dead insects in the radiator blocking the airflow.  I had a radiator with enough junk in the radiator to turn on the fan cruising at speed on a hot day.  The cooling system doesn't have a lot of reserve capacity and when a lot of airflow is blocked the temperature will rise. 

Another cooling problem may be mineral deposits in the system caused by using well water.  If it is really bad it may be necessary to do a system flush with vinegar to restore full cooling capacity. 

[szabgab]

In the States there are brake cleaner formulas that a not volatile so verify that what you used isn't of that type. You want to use something that will explode if misused.  cool shades

Action of the thermostat is governed by coolant temperature. It helps the engine warm to operating temperature sooner because it allows coolant to bypass the radiator, then opens to allow the coolant temperature to be moderated by passing through the radiator

The coolant temperature sensor is a different animal. It senses the temperature of the coolant and sends a signal to govern the fuel mixture. A cold engine receives richer fuel than a warm engine. If the sensor is faulty and its signal indicates that the engine is warm when actually the engine is cold, starting and running will be difficult until the engine has run a while because the sensor will be calling for a leaner mixture. The sensor adjust according to an arc of temperatures—not just warm or cold. There are two tests described on this site for assessing the coolant temperature sensor. A faulty sensor is a possibility if intake air leaks aren't the source of the problem.

That the fan runs intermittently in traffic during warm weather is not unusual. If it runs intermittently in moderate weather and light traffic, it is likely there is not enough coolant in the system or the coolant that is there is too old.

From what you write, it appears that you have done well in having the engine and the moto itself assessed.  :clap:

Try to focus and keep from imagining things though, unless they are pleasant things. :giggles

Good point on the brake cleaner,  I'll check the label to be sure,  it burns :)  You are absolutely right about the thermostat,  my bad mixing up the two things,  I wanted to write temp sensor,  but obviously I didn't :)  I have found a great graph on the Ohm readings at different temperatures,  I will dig the sensor out and check it's operation.  Mentioning the fan coming on was just to say,  that the sensor does some sort of a work by switching on the fan on a warm day,  it looks to me operating correctly,  but the coolant is murky in the overflow tank,  so that will be replaced ASAP.  I have a slight oil leak below the pump housing,  so I guess it is seal replacement time coming,  I might just combine the sensor checking,  seal and coolant replacement,  rather than doing stuff my ordinary way,  basically doing everything three times over :D

[szabgab]

With 130,000km I wouldn't be surprised if there is a fair amount of dirt and dead insects in the radiator blocking the airflow.  I had a radiator with enough junk in the radiator to turn on the fan cruising at speed on a hot day.  The cooling system doesn't have a lot of reserve capacity and when a lot of airflow is blocked the temperature will rise. 

Another cooling problem may be mineral deposits in the system caused by using well water.  If it is really bad it may be necessary to do a system flush with vinegar to restore full cooling capacity.

Oh yes, I would not be surprised either, as the coolant is less than pristine in there, so I doubt, the PO has bothered to take off the grille and clear the crap from behind. I will do that as well, as my engine is soooo bloody hot in the crouch area, it is unbearable, Im sure new coolant, cleaning the radiator and putting some heat insulating blanket underneath the tank would go a very long way to make the summer rides a bit more bearable :)

[szabgab]

I wouldn't think it would be all that difficult to replace the airbox.  They are available on eBay for fairly reasonable prices and often include the boots that connect to the throttle bodies.  Here's a nice one:

https://www.ebay.com/itm/1992-BMW-K75-RT-S349-7-air-box-intake-expansion-chamber/232724610032?hash=item362f77bbf0:g:P9MAAOSwQKBavSqQ&vxp=mtr

You do not need to use the clamps with the special tool.  There are Suzuki clamps available that have screw clamps for about 4 USD. 

https://www.partzilla.com/product/suzuki/09402-48208

Other than connecting the large hose to the MAF chamber the installation of the new airbox is a fairly easy job.  Many K75 owners have done this job as part of replacing the throttle body boots.

It's a job that, unfortunately for you, must be done.  The good news is that the parts should cost about 50 USD and take an afternoon to install.

I had a look myself, and stuff from the US is prohibitively expensive due to shipment charges, but there is a bloke in Lithuania of all places, who sells K75, K100 parts for a reasonable price and low shipping. After all if I take the airbox off, I might just clean the innards of the throttle bodies too. I had an injected scoot, that was 8 or 9 years old, the throttle body was full of varnish, so this might be completely caked over, if never cleaned before

[szabgab]

The fuel level sensors in the earlier bikes, which use a temperature switch, are known to be problematic. Mine (1985 K100) generally comes on 10-20 miles into a ride, regardless of fuel level.

Andy

Andy, funnily enough the idiot light was working as expected after buying the bike, on second fuel-up it didn't. Obviously an idiot light is just that, but I don't have enough experience with this bike as yet to know, when to fuel up.

[The Mighty Gryphon]

If you have a working tripmeter, that can work very well to tell when you need fuel.  I reset mine to zero every time I fill the tank.  With a full tank I know I can go 300km when driving around town. 

On trips where I am fully loaded and doing up to 130kph that distance will drop to about 250km. 

With these distances I am fueling with about 16 litres with about 4-5 left in the tank as a reserve.

The tripmeter is how I gauge fuel.  The idiot light is just a nuisance, especially when it comes on at night.

[szabgab]

If you have a working tripmeter, that can work very well to tell when you need fuel.  I reset mine to zero every time I fill the tank.  With a full tank I know I can go 300km when driving around town. 

On trips where I am fully loaded and doing up to 130kph that distance will drop to about 250km. 

With these distances I am fueling with about 16 litres with about 4-5 left in the tank as a reserve.

The tripmeter is how I gauge fuel.  The idiot light is just a nuisance, especially when it comes on at night.

Well, yes, 300kms would have been my estimate (and that was the meter reading, when the light came on the first time after I have bought the bike), but now I have dropped that to 250 or so kilometres (with the light coming on at 220 and the bike badly stuttering at 250), which is crap considering I did not do any silly stunts, basically city rides and 60, max 70mph short runs. The PO said, he was getting 4l/100km consumption on interstate roads babying the bike and not going faster, than 60mph, a bit hard to believe it now, when my consumption inches toward SUV category :)

[szabgab]

Laitch -  I digged around this site and others regarding the temp switch,  I see now,  that is basically two switches in one -  one for the fan,  thr other fir the FI temp reading.  Getting it out from behind the radiator might be a bit of an undertaking,  but would checking the FI connector's pin10 and ground give me a good enough reading,  just to figure out,  if I need to go any further,  or not?

[Laitch]

but would checking the FI connector's pin10 and ground give me a good enough reading,  just to figure out,  if I need to go any further,  or not?

That's the main point of just about any diagnostic test, szabgab. I suggest you do it if you understand how to do it and you seem to be a person of action.

[szabgab]

That's the main point of just about any diagnostic test, szabgab. I suggest you do it if you understand how to do it and you seem to be a person of action.

Thank you Laitch, that's flattering :) Anyway, all I wanted to stutter really, if the reading at the computer, rather than the connector of the sensor itself would give me a good enough reading (e.g. the harness will not alter the reading too much)?

[Laitch]

Thank you Laitch, that's flattering :) Anyway, all I wanted to stutter really, if the reading at the computer, rather than the connector of the sensor itself would give me a good enough reading (e.g. the harness will not alter the reading too much)?

It's time for action, szabgab. Report back.

[The Mighty Gryphon]

To answer your question, yes, it would be okay to take readings of the temperature sensor at the big connector on the ECU.  In fact, it is probably better to test functionality there because it takes into consideration the wiring.  I don't know the pins to test, but that information is here, in fact, Laitch is probably posting a link to the pin numbers as I type.

[szabgab]

It's time for action, szabgab. Report back.

Aye aye sir :D So I dug out my multimeter after all instead of blabbering on and tested the bike at the female connector of the computer - red pole on number 10 pin, black on number 13. Temp sensor is great, at around 26-28 celsius it reads 1789 Ohms, at boiling point (fan kicking in) 189-191 Ohms, that looks to be pretty much in the range it should be. Whilst there I have checked the battery too, but that is not good. PO said, he replaced the battery not long time ago, but have had absolutely no paperwork, so he could have been lying. Battery even that flat is being charged at 3500-4000 RPM at 13,5 Volts with the headlight on (but not the high beam), so I hope it is not the charging system, that's kaput. I put the battery on tender, so I will see, what are the readings tomorrow with a fully charged battery.

Also the brake cleaner I was using was not flammable (or at least it did not say so on the label, I did not test it), so I bought an another one, that had a large fire hazard on the side. Air duct number two between the air box and throttle body is shite, spraying it with brake cleaner bike badly stutters and nearly dies. So there is my air leak too. So I guess it is the basic stuff after all, air leak + bad battery?

Thanks!

Gabriel

[szabgab]

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