1988 K75S Sticking Throttle

[smarmit]

Hi. I'm not only a new BMW owner, I'm a new rider. I used the word 'new' twice, however I'm not very new (late 40's) and my bike is 1988 K75S. So new and old combined - seems good!

I just went for my first ever motorcycle inspection here in Massachusetts and got a big fat fail. Un-noticed by me (see "new" rider qualification above) the throttle is sticking in place and not snapping back. While I have nearly zero motorcycle repair experience, I'm pretty handy and glad to give it a go. My question for you kind folks - is that something I should/could tackle? The tech at the shop thought it may just need some lubing? After a bit of googling, I'm even questioning if the inspector is right about this. Should it be snapping back like it would on a Harley for instance? I only mention Harley because this inspection station is a Harley-exclusive shop. Just happens to be very close to home and does inspections.

Thanks for any thoughts and please be gentle, I'm a noob after all.

-Scott

 

[AKsuited]

When I bought an '86 K75, it had the same problem - the throttle would not return to idle by itself. That definitely needed to be corrected, and I had an independent BMW mechanic sort that out as well as replace the clutch cable which was hanging on by a thread (because the previous owner did not keep the round ends lubed, which is of course critical with any type of cable activated clutch).

If you aren't comfortable with correcting the throttle, have someone with lots of experience do it for you. No shame in hiring an expert.

 

[98lee]

The FIRST thing to check is see if it has the optional factory friction screw installed. Alot of people add these to their early Kbikes. Look underneath the throttle perch between the right grip and the front master cylinder. Look from the front of the bike. If you see a silvery screw head about the size of a nickel, that's it. Just unscrew it (or remove it) until the throttle returns properly. Alot of people use these (myself included) so they can rest or flex their right hand while riding. You just set it so the throttle does not return (it's teflon tipped and puts slight drag on the throttle) and you leave it that way until it wears a little then you snug it up a little. It doesn't affect normal riding with it tightened other than not returning and since you don't LET GO to slow down, there's no issue.

The second thing to check is see if the right grip is pushed too far on to the handlebar so that the inner edge rubs on the perch causeing drag.

The third thing is a possible bad throttle cable.

The fourth thing is a problem with the throttle body. Open the throttle by hand at the throttle body and see if it snaps back. If it does, the problem is not the throttle body. If it doesn't, there is a problem at the throttle body.

If the bike responds immediately to throttle inputs while rolling off the throttle while riding, it is more likely #1 or #2. If it doesn't immediately respond to a rapid chopping of the throttle, it is more likely #3 or #4.

There is also the possibility of something being gummed up or miss assembled under the cover between the grip and the master. That is where the rotation of the grip is converted to a pull on the cable through a gear and chain assembly.


Check #1,2, and 4 and get back to us. We'll be able to help you further once we know the results of those checks.

 

[smarmit]

When I bought an '86 K75, it had the same problem - the throttle would not return to idle by itself. That definitely needed to be corrected, and I had an independent BMW mechanic sort that out as well as replace the clutch cable which was hanging on by a thread (because the previous owner did not keep the round ends lubed, which is of course critical with any type of cable activated clutch).

If you aren't comfortable with correcting the throttle, have someone with lots of experience do it for you. No shame in hiring an expert.

Thank you! I'm going to check the items mentioned in the other reply and then do exactly this if it seems too daunting.

 

[smarmit]

... <snip>

Check #1,2, and 4 and get back to us. We'll be able to help you further once we know the results of those checks.

Thank you so much for all this detail. It feels very responsive to me when rolling off the throttle, though my experience being limited I can't be too confident of that. I will check all of this and reply back.

 

[smarmit]

Friction screw for the win

@98lee has it exactly right! I do have that throttle friction screw. Loosened it and now the throttle returns correctly. I’m a bit surprised a bike inspector would not know to look for this, but then I realized the bike is older than the inspector! Ha.

Thanks again folks.

Scott

 

[98lee]

Scott,

As far as I know, the only bikes that had the provision for (threaded hole in the throttle perch) the screw were late 80s airheads, all K75s 85-95, and 2v K100s 83-90. They eliminated the provision after that. The screw didn't come with the bike, you had to special order it.

Usually the screw interferes with the nut on the bottom of the perch mounted mirrors. That is why it is only in the parts fiche on bikes that had fairing mounted mirrors likes RSs or RTs. Bikes with aftermarket barend mirrors or people who threaded the perch holes and use aftermarket mirrors also avoid that interference.

Anyway, I'm glad it was an easy (and cheap) fix.

 

[tinytrains]

Glad you found it, but what the heck is a "Bike Inspector"? In 43 years of riding, no one has ever "inspected" my motorcycles.

Scott (also).

 

[pauls1150]

That's a DMV-Certified shop + person for verifying that the vehicle has operating lights, horn, mirrors, tires, exhaust, etc...
California doesn't have bike inspections, while some other states do; and the car "inspections" are typically only when the officer who pulled you over wants more things to add to your ticket. Result: so many cars are not road-worthy, but are out there anyway, grrr...

By the way, the under-slung friction screw was available as a separate modification (tapped lower throttle housing & screw) for a while, but I think some states call out "friction-type" cruise controls as illegal, tho I've never heard of anybody getting written up for it.