‘87 K75S Failed compression test - burned valve

[fuego]

I think so too. I have never heard of a burnt valve on any early K-bike, especially the K75. If you do proper maintenance on them those motors will easily go over 500 hundred thousand miles without having to to do anything major. Those motors were overbuilt and thus, under-stressed.

Well, I will find a decent machine shop around here and get that head rebuild. The rest of the bike is so clean, it's worthed! And I will ill keep a serious eye on the valve ajustement .

Nice to know that with proper care I won't be in there again !

Ignace

 

[k1200lt00]

I would check if you have and intake leak or exhaust gasket leak, also a injector not working 100% making it run lean.

 

[98lee]

I would check if you have and intake leak or exhaust gasket leak, also a injector not working 100% making it run lean.

+1

 

[fuego]

There WAS a major leak at the little hose getting the oil vaps in the intake. This little hose was brittle and perforated. Another one was at the big tube going from the airbox to the intake plenum.

both of these will be renewed of course !

I only did 1000 miles last fall, just bought the bike ! By then it rode quite correctly.

I'm in a groud up restoration, it will be like new when finished .

Thanks all for your invaluable help !

Ignace

 

[98lee]

There WAS a major leak at the little hose getting the oil vaps in the intake. This little hose was brittle and perforated. Another one was at the big tube going from the airbox to the intake plenum.
Ignace

Ignace,

That would do it! Both of those air leaks are AFTER the air flow sensor, so the computer would have no idea that all that extra air was being sucked in. It would make the bike run extremely lean. Since there is no oxygen sensor to alert the computer of a lean condition, burned valves would be the result.

The crankcase vent hose is a known weak spot. It is an extremely thin hose located in one of the hottest spots on the engine. It should be inspected for cracks just below the top clamp and just above the bottom clamp at least twice a year. Don't expect it to last more that 3 years (depending on mileage).

The large hose between the air filter box and the plenum usually gets damaged only by mechanics who are unaware of the need to not damage it. It rarely needs to be disturbed and with care, it is rare to need to replace it, but if torn, it results in a huge air leak that sometimes can close at idle, but open up as rpm increases, making troubleshooting frustrating.

Glad you found the cause! Fixing a problem is not just fixing the symptom. You’ve got to fix the CAUSE.

 

[fuego]

Tanks Lee !

You're right about the big hose between the air box and the air intake. Some unaware mechanics must have felt the need to remove it for some unclear reasons... It was thorn at the lip going in the air intake !!!

The breather hose was shot though and was so for probably a long time. I've ordered two of them for each bike. I pretty well figured out by their sorry state that the would be a tune-up item

Here is a picture of the cleaned block. I was happy to see these clean and unalterated pistons, given the bad shape of the valve !

Ignace

 

[gregfeeler]

Tanks Lee !

You're right about the big hose between the air box and the air intake. Some unaware mechanics must have felt the need to remove it for some unclear reasons... It was thorn at the lip going in the air intake !!!

The breather hose was shot though and was so for probably a long time. I've ordered two of them for each bike. I pretty well figured out by their sorry state that the would be a tune-up item

Here is a picture of the cleaned block. I was happy to see these clean and unalterated pistons, given the bad shape of the valve !

Ignace

 

[robsryder]

I have attached some images taken from the Max BMW parts fiche for the K75S. This shows the engine block and to be clear, I'd like to make certain that the hose in question is that shown as item #10. Is that correct?

 

[robsryder]

Perhaps we are also talking about the item(s) shown below as items #1 and/or #4?

 

[robsryder]

Below is the air cleaner lower howsing image. There seems to be little here, save for the air cleaner, that needs attention.

 

[robsryder]

Next shown is the top air cleaner housing. Perhaps the hose, shown as item #1, would need some attention?

 

[98lee]

The OP was referring to #10, in the first fiche, AND #1, in the last fiche, being torn. A perfect storm for burning exhaust valves! The only thing worse would be if the #1 and #4 manifolds, from the second fiche, were also cracked.

 

[robsryder]

Just a FYI for those interested - BeemerBoneYard has the vent hose (#10 in first fiche above) available at a significant savings -

https://www.beemerboneyard.com/11151460480.html

 

[98lee]

Unless they have improved their manufacture of them (they have them reproduced), they are actually WEAKER (along the mold line) than the Factory BMW part. I bought 20 of them when they first started offering them, but returned them after inspecting them. I didn’t think saving a couple of bucks was worth the risk of having it fail SOONER. YMMV

 

[robsryder]

Another less expensive (than OEM) alternative for the vent hose is EME. One wonders if similar, less robust, manufacturing occurs with these items -

https://www.euromotoelectrics.com/product-p/venthose480.htm

 

[jagarra]

Sometimes it is best to stick with an OEM supplied part, in this case you know there is engineering to back the specifications that the product was built upon. In many cases with third party suppliers, they make what appears to fit an application, but the materials and construction don't meet the original OEM specs, fit and durability suffer. As long as BMW is willing to supply an original item that is the best choice, they see demand, they continue to supply.

 

[88bmwjeff]

Sometimes it is best to stick with an OEM supplied part, in this case you know there is engineering to back the specifications that the product was built upon. In many cases with third party suppliers, they make what appears to fit an application, but the materials and construction don't meet the original OEM specs, fit and durability suffer. As long as BMW is willing to supply an original item that is the best choice, they see demand, they continue to supply.

I had an older 3-series and I have my 88 R100 RT. I've noticed that some BMW parts were significantly inferior to the previous ones. I had too many BMW parts fail on my 3-series, which was one of the main reasons I sold the car. I felt sort of bad for my independent mechanic. Since he warrantied the work, he was doing certain repairs two or three times. While it didn't cost me extra money, it was inconvenient. For parts on the RT, I've noticed a lower quality that the part they replaced. So, just because it's a BMW part doesn't mean that it's as good as it was back in the day. Your experience may differ.

 

[98lee]

Another less expensive (than OEM) alternative for the vent hose is EME. One wonders if similar, less robust, manufacturing occurs with these items -

https://www.euromotoelectrics.com/product-p/venthose480.htm

View attachment 87298

When I inquired about these, I was told by the EME owner: “If you’re that concerned about these, maybe you should just buy the factory ones”.

I followed his advice.

 

[robsryder]

Well I've got an OEM vent hose and I ordered a couple from each BeemerBoneYard and EME. I'll examine the vent hoses from each and report back on what I find.

 

[98lee]

Well I've got an OEM vent hose and I ordered a couple from each BeemerBoneYard and EME. I'll examine the vent hoses from each and report back on what I find.

Look closely at the mold line that runs length wise along the hose (at least on the beemerboneyard ones). It was weaker (thinner) than the rest of the hose.

It really would be nice if someone made one that was BETTER than the factory one. They crack way too easily.