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Revs on sidestand '85 K100RT

P

Paswonky

Guest
Hey guys - looking for insight here. When I go to put my 85 K100RT in the garage, I put it on the sidestand, still running, and notice that it tends to rev up a little.

When I straighten it up the idle goes down. Any ideas?
This is not a cable issue - checked that!
Thanks!
 
Last edited by a moderator:
Hi Paswonky (real name?)

Welcome to the K-brick forum. Please read: http://forums.bmwmoa.org/showthread.php?46057 - I'm going to edit your post to add that info to the title, since the '85 K00 was a different bike from the K100 that followed it in many ways.

To answer your question..

That's usually a sign that the rubber mounts to the head that connect the throttle bodies have perished. They can fail in several ways.. they can crack, they can harden up and fail to seal to the head. When the bike is on the sidestand there is a very slight flex in the engine/frame that pushes the intake plenum (above the throttle bodies). If the mounts have leaks - the idle changes.

The test for leaks - and this is how I do it - NOT how I'd suggest doing it..

  • I take the bike OUT OF THE GARAGE and centerstand it.
  • I get some nice FLAMMABLE carb cleaner
  • I get my garage fire extinguisher and put it near the bike
  • I warm up the engine
  • I then spray very small spritz's of carb-cleaner around the throttle body area. If there is any change in idle, you have a leak(s)..
  • I then use the red straw that comes with the carb-cleaner to narrow down where the leak(s) is(are).
  • I spray around the base of each TB.
  • I spray around the injectors
  • I spray around the crankcase vent hose that is behind/right of the TB assembly.

ANY change in RPM indicates a leak to me.

  • I DO NOT spray a lot of cleaner
  • I DO NOT spray any down near the exhaust or into the plug chamber

I have never had to use the fire extinguisher.

Other people use other means to check for leaks (everything from WD40 to propane).. but this has always worked for me, and tends to clean up the area that you're likely to be working in shortly.

The rubber mounts are now almost 30 years old, so it wouldn't be a big surprise to find them leaking. R&R is a PITA - come back and let us know how your leak test works out (however you decide to do it) and I can give you a rough outline of the work required to replace them.
 
Would you please give us the rough outline for replacement anyhow? My bike has the same problem.

My K75 has 18 year old rubber and I figured that I would just replace them all at some point.
 
Hi Lynn,

Rough outline?

Remove everything in the way.. and replace them.

A bit more detailed:

You have to R&R the intake plenum in order to remove the TB's and get to the rubber mounts. This requires disconnecting the big rubber hose from the AFM in the air-filter, then snipping all the one-time-use Oetiker clamps holding the rubber connectors from the top of the throttle-bodies to the bottom of the intake plenum, then snipping the releasing the clamps on on the top of the actual throttle bodies (generally screw clamps.)

I'd strong suggest replacing ALL the rubber you can while it's apart - so thats 3x the rubber intake to plenum hoses, and 3x the rubber throttle body mounts. Those mounts bolt to the top of the head (try not to drop any parts into the openings while it's apart.) - Make that 4X if you have a K100.. :)

Once all the clamps are off - the intake plenum can be rocked toward you out of the frame. It is VERY tight clearances. Then you can remove the TB assembly by removing the bolts holding the mounts to the head. If you were ever thinking of replacing the throttle cable, or choke cable - there would never be a better time. These both connect to the throttle body assembly. Also a great time to replace the hoses going to the fuel pressure regulator (mounted on the rear of the TB assembly.)

Once you have the TB assembly loose - you can remove the rubber mounts and replace them with new ones. If you have the screw clamps there - you can fasten them to the TB's and then mount them and the TB assembly to the head. If Oetiker clamps were used they can be crimped at this point - AFTER making sure the screw or crimped-ear doesn't interfere with the movement of the TB linkage/butterflies.

When I did mine - many years ago - I used a smear of RTV sealant on the bottom of the mounts. I used electrical grade which does not give off acidic fumes. Since you don't have an O2 sensor to worry about poisoning the RTV works well for this application. Tighten them evenly (there is some torque spec somewhere).

Next put the rubber hoses to the plenum on the top of the TB's - but do NOT crimp the replacement Oetiker clamps (you need them to be able to move a bit) - but DO slide them over the rubber hoses. Use a small smear of silicone based grease on the spigots coming out of the intake plenum, and try to wiggle it back down and into the correct position. Once everything is in place, you can crimp the ears on the Oetiker clamps using Oetiker clamp pliers (they're used on CV joint boots, so your local NAPA dealer will have the tool in stock.) Again - check their orientation to avoid interference with the throttle linkage.

I'd also suggest replacing the O rings on the brass-air-bypass screws, and cleaning those passages with some carb cleaner before reassembly. The stock setting for them is 1 turn out from GENTLY seated. That's a good starting point. If the leak fix was successful you'll find they all pretty much will be open the same amount when you do the idle TB balance.

DO NOT adjust the linkage between the TBs - except the idle screw. The other screws should be painted GREEN or have green/blue caps over them - which in BMW parlance - means LEAVE THEM ALONE.
​

That's the rough description. Figure on a longish afternoon doing it. No real fancy tools needed, just work carefully. Take some photos along the way and perhaps you can share the job with the forum.

BTW - there is a reason BMW used Oetiker clamps where they did - it's to give a uniform and not overtightened clamping force. If screw clamps were used it's possible to distort the parts being clamped and cause leaks that way.

HTH,
 
deilenberger said:
Hi Lynn,

Rough outline?

Remove everything in the way.. and replace them.
Click to expand...

LOL, Well I guess I ask for that one. I figured that I should replace all the rubber at the same time. I am talking fuel lines and radiator hoses also. I hate to think how much that is going cost. Do you have a SWAG estimate?

Maybe this coming spring would be a good time for a project like this. That way I can pick up a few parts every month and my wife will not have a cardiac arrest about the cost. I figure it will not look quite so bad if the cost is spread out over time. Kinda like taxes.


-Lynn
 
For the intake hoses, clamps, and fuel hoses, about $275 plus tax.
You then must buy, rent, or borrow the clamping tool. Summit Racing has the best price.
http://www.summitracing.com/parts/oes-25326/overview/
Then you have the price of the coolant hoses, about $70 plus tax. Those can be done at a different time.

All these prices came from the latest BMW parts fiche (I just looked them all up).




:dance:dance:dance
 
Last edited:
argent brick said:
LOL, Well I guess I ask for that one. I figured that I should replace all the rubber at the same time. I am talking fuel lines and radiator hoses also. I hate to think how much that is going cost. Do you have a SWAG estimate?

Maybe this coming spring would be a good time for a project like this. That way I can pick up a few parts every month and my wife will not have a cardiac arrest about the cost. I figure it will not look quite so bad if the cost is spread out over time. Kinda like taxes.


-Lynn
Click to expand...
Lynn,

Unless you see signs of failure - BMW coolant hoses from that era must be made of impervium.. they seem to last about forever, with the exception of the hose running from the radiator overflow down to the radiator reservoir.. that hose was made of mush. Luckily it's a standard over the counter size, with no pressure requirements, so your common auto-store 6mm hose should work just fine.

The fuel lines - there are several that are "special" - preformed bends in them. I'd suggest getting them from BMW, it's too easy to end up with a kink in those lines if you use standard fuel-injection hose. I'd also buy the in-tank hose from BMW - it has to be fuel/ethanol resistant on both the inside AND outside (most fuel injection hose isn't meant to be submersed in fuel.. go figure..)

I suspect if you look on Ebay - you'll find the Oetiker tool for the clamps at a reasonable price (less than $20), so it's one worth adding to the special tools drawer if you can't borrow one from a friendly mechanic (can even be a car mechanic since these clamps are used on CV joints on cars..)

Dunno on the parts cost - when I did mine about 18 years or so ago - cost was around $150 for the rubber bits and hoses.. but I imagine they might cost a bit more now.. :)
 
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