1994 K75 fuel/throttle issues (hunting for opinions whilst I research)

[Doogwhan]

Pardon the novella, but I hope to be thorough:

Okay- My '94 K75 was running like a champ, what was left of the splines were lubed, the fluids were topped off, the tires were good, so it was time for an adventure!!!

Hoo boy I got one...

100 miles into a 400 mile jaunt, the bike occasionally starts "coughing" and losing power as I go uphill or move to pass someone. As a precaution, I added some injector cleaner at the next fill up, and decide to monitor things further.

It gets worse.

On flat land, if I kept the throttle in *juuuust* one position, it would run fine. However as I get into Northern Alabama from the relativity flat Gulf Coast it got really bad. Whenever the load on the motor would change, the power would cut out and feel like it dropped completely to idle (the motor never actually died). A rapid closing and reopening of the throttle was the only way to get the motor to come alive again. when trying it to go uphill it would backfire like crazy while trying to accelerate!

The only "happy place" between fuel needs and power output seemed to be between 75-80 mph in top gear (which doesn't feel right on a short-geared K75). Anything else was almost useless. and riding in traffic on surface streets was hell. It would not respond to throttle, only to surge like a bastard when it felt like it...

That said, she got me to where I was going in one piece before she ever actually stalled. so my anger is limited. Having it finally conk out immediately on arrival was fitting. It started right back up, but idles like hell.

TL;DR Now I have a bike that barely idles, surges, and backfires under load and when decelerating. As a side note, my K wasn't one to backfire at all until now.

My suspects are fuel pressure, a failing throttle position switch, or maybe even bad plugs?

Any thoughts are truly appreciated!

*edit* When powered up (not running), I hear no clicks from the TPS whatsoever. Is this normal?

[TimTyler]

Could be a simple air leak at the intake manifold / throttle bodies.

Check or just replace the crankcase breather hose too. Note the product description there: "This is the hose that always cracks and leaks, causing hesitation."

[Doogwhan]

So plugs and the breather hose are a good start to my shopping list. No bank breakers yet. Yet...

[johnny]

greetings doogwhan...

fuel filter and fuel pump power connector have been known to cause what you describe...


j o

[cmichael]

My vote: Breather hose FTW

[Doogwhan]

Wouldn't a cracked breather hose lead to a lean condition by letting extra air in after whatever flow sensor BMW is using?

This bike is running so rich it backfires both on and off the throttle!

(Please don't take my tone as argumentative, I am replacing the darned hose anyway just in case)

[cmichael]

Cracked hose= lean=intake back fire. exhaust back fire is when it is running rich.  The cracks in your hose are allowing non-controlled air in thus changing between rich and lean conditions constantly.

Check the hose and report back; all symptoms point to the hose imho.   

[Doogwhan]

Looks  like this kit hits all the likely culprits, and includes the hose. http://www.beemerboneyard.com/k75maintkit.html

I shall order, install, and report back next time I get up to the bike.

[pallum]

greetings doogwhan...

fuel filter and fuel pump power connector have been known to cause what you describe...


j o

+1

[K75RT Keith]

Yup, start with the filter and fuel sock and while your in there, replace the hoses with internal tank compatible material and replace any other rubber bits in the tank.   

[Doogwhan]

Hokay- I ordered the following "likely to make you K run like crap if they aren't right parts" today:

• Air filter
• fuel filter
• rebreather hose
• spark plugs

What size hoses would I need if I was going to replace the hoses in the fuel tank? (will research on my own as well)

Anything else I should be considering here?

[JamesInCA]

The main in-tank hoses are also 8mm (or 5/16"). To my surprise, the best price I've found is from BMW (16 12 1 180 040), which sells a meter for about the same price NAPA sells a foot of submersible hose.

There's also a shorter 6mm hose that goes from a pipe at the top of the tank down to the return port on the fuel filter screen (16 13 1 455 350).

If your hoses are old and crusty, you'll eventually want to replace the return hoses attached to the FPR as well, which is more of a pain as they run behind the air box. Doing it again, I'd spring for BMW's pre-shaped hose from the FPR to the tank (13 31 1 461 011), as the path is tricky and a regular hose might get kinked. And while you're back there, replace the vacuum line from the rear throttle body to the FPR (3/16" vacuum hose - $0.99/ft at NAPA).

[Doogwhan]

Okay- first time back at the bike since she crapped out... Turns out the breather hose was split at the top and bottom and was responsible for the (potentially one of the) mystery leak(s) on the left side of the engine. Only issue is that the guys at beemerboneyard sent me a different service kit the had four plugs (different models as well I think) instead of three and did not include the darned breather hose. They responded almost immediately (and on a weekend) will send out the right parts on Monday. Props to them for good service, in a world of internet nightmares, they dun' did good.

I replaced the fuel filter which was from 2010, and found it was plumbed with OEM BMW rubber hose that looked great, so I let it be. I checked the spark plugs and they seemed fine. I capped the hole in the breather box with sandwich bag plastic and a zip tie just to get the mixture right.

At this point she seems to be running normally. After I get the breather hose, I will take her for a longer trip beyond the lap 'round the block I took today. I will also check the pump wiring as well.

Also, I am still in search of a budget rear shock replacement, and thanks to you guys that have all helped so far. I really appreciate it!

[wmax351]

The main in-tank hoses are also 8mm (or 5/16"). To my surprise, the best price I've found is from BMW (16 12 1 180 040), which sells a meter for about the same price NAPA sells a foot of submersible hose.

There's also a shorter 6mm hose that goes from a pipe at the top of the tank down to the return port on the fuel filter screen (16 13 1 455 350).

If your hoses are old and crusty, you'll eventually want to replace the return hoses attached to the FPR as well, which is more of a pain as they run behind the air box. Doing it again, I'd spring for BMW's pre-shaped hose from the FPR to the tank (13 31 1 461 011), as the path is tricky and a regular hose might get kinked. And while you're back there, replace the vacuum line from the rear throttle body to the FPR (3/16" vacuum hose - $0.99/ft at NAPA).

For some of these type of things, BMW car dealers have them, and the parts guys are usually pretty helpful.

[Doogwhan]

So months have passed, and I finally had a chance to tinker a bit...

With a new crankcase breather hose and fuel filter, she seems to run flawlessly over the two hours I was able to get out and ride. I wanted to thank you guys for your input and advice. Your support is appreciated! :2thumbup:

Hiding from a summer shower at Scott's Orchard on the Alabama Tennessee border:


I was out looking for peaches, but they weren't ripe yet. Such a shame I had to cruise the countryside instead...

[Scott_]

Also, I am still in search of a budget rear shock replacement, and thanks to you guys that have all helped so far. I really appreciate it!

Here's my plug for a good quality for the price shock. Ram.
http://www.realmengineering.com/page15.html

[cmichael]

Plus one on the ram shock.

[Chaos]

you can get a Progressive 412 for about $220 on amazon, got on a couple years ago, good bang for the buck.