Jerky throttle, intermittently won't idle cold

[k75novice]

Hi all,


I recently picked up a 1994 K75 with 62,006mi. Since buying it, I have ridden about 100 miles in total.


It has had an issue where (only when cold, as in the first 5-10 minutes of riding, the throttle is very jerky (e.g. holding it at WOT might give slow acceleration for 5 seconds before suddenly actually behaving like WOT)


Also, I believe this same issue is making the bike fail to hold an idle when cold (I have to hold the revs at 3-4KRPM at lights to prevent it dying. When it dies, it always takes ~10-20 seconds to start up (compared to instantaneous starting usually).


Intuitively, this feels to me like an electronic issue, the intermittent throttle feels flaky like a wire is making a loose connection.


I have set the throttle position sensor (TPS) so it clicks when just off idle, and I tested it with a multimeter for continuity. It failed, so I installed a new TPS yesterday. After one lap around the block, the bike seemed to still have the issues. I disconnected and reconnected the ECU connector and it appeared fixed, and I rode it for about 60 miles (two 30 mile trips) without the issue. This morning, on my commute, the problem was back again, same as always. Did not have time to unplug and replug the connector, will try this PM.


My plan of attack (in order of increasing cost):


-Confirm with multimeter that new TPS is working
-Clean the pins on the ECU connector
-Clean grounds (which ones would be implicated, and where are they?)
-Replace my slightly smaller battery with an OEM spec one


In addition to technical advice, I would appreciate search term suggestions. I have tried searching for "won't idle cold", "jerky throttle", "inconsistent throttle" and wasn't able to find someone with this exact problem.


PS- The following information about the bike may be irrelevant, but are all the things noteworthy about this bike's behavior:


-VERY slow (<1mph/second) acceleration at 80mph in fifth gear (suspect clogged fuel filter)
-Occasional knocking noise at low RPM, goes away when revving over about 2kRPM (suspect cam chain tensioner, plan to assess with stethescope, reshim valves, synch throttle bodies. I don't think this is rod knock, and don't think it is fatal.)
-Battery is smaller than OEM (~75% of the size). Bought the bike with a dead battery, swapped in a brand new one for a Yamaha XS750.
-Throttle is jerky coming off of idle. I'm trying to remember if my L-Jetronic BMW 633CSi was like this, but it doesn't feel "normal."
-Bike was dropped on the right side. Hand grips were replaced with terrible chrome ones. There is a considerable amount of motion in the throttle grip before actually tensioning the cable.


PPS- Some novice questions I have about the bike


-Does this bike (built October 1993) have L-Jetronic or Motronic? What I've read is inconsistent.
-Does this bike have a catalytic converter, or is that just California models? It does not appear to have a cat, but I don't know what a motorcycle cat is supposed to look like.

[K1300S]

all K75's had L-jetronic.
no cat on K75's

first order of biz before chasing problems with parts is do to a complete tune up.  new air/fuel filters, fluid flush, injector clean, tb synch, mixture adjust, throttle cable check, fast idle check/adj, valve adjust, etc.  follow the book.

this provides a known baseline....THEN start chasing the issues.

[k75novice]

[size=78%]mlytle,[/size]
[/size]
[/size][size=78%]Appreciate the advice. I'll admit I'm rushing to ride this thing as the weather is getting good, but you're right in that a tune up is necessary.[/size]

[Laitch]

Welcome.

Descriptions and illustrations of a K-bike's operating system—including location of grounds—are embedded within this troubleshooting guide. Introductions to maintenance and to K-bikes in general can be found in the Technical Library Index.

[The Mighty Gryphon]

As part of the tune up, go through and check the condition of all the rubber parts, specifically the rubbers above and below the throttle bodies and the z hose that connects the crankcase breather to the airbox.  These are well known problem areas on these bikes and will affect driveability, often with symptoms like you are seeing.

Look for cracks in the rubber at the clamps.  There are descriptions of using propane or starting fluid to find leaks.   Do a search on "propane throttle body leaks".

The Z hose is located at the top rear of the engine block behind the throttle position sensor.  A new one is about $10 and even if it's good it's an excellent idea to change it.

[johnny]

greetings...

after new fuel filter and fresh no corn fuel... needs 100 bottles of techron in back to back to back tanks of no corn ethyl...

j o

[milq]

greetings...

after new fuel filter and fresh no corn fuel... needs 2 bottles of techron in back to back tanks of no corn ethyl...

j o

I've never been much of a believer in additives, but I used some Techron today and my views have changed. After 10 minutes of riding in town the idle speed is about where I expect it and smooth. Baffling that it helps that much.

[stokester]

I've never been much of a believer in additives, but I used some Techron today and my views have changed. After 10 minutes of riding in town the idle speed is about where I expect it and smooth. Baffling that it helps that much.

Ditto.


I've never believed in the "Mechanic in a Bottle" but do regard the Techron additive as helpful in keeping the injectors clean in the K75 as well as my four-wheelers.  The AC-Delco and other OEM treatments come in very similar containers.   :dunno2:

[wally.fisher]

Check your tps, if installed incorrectly you can experience similar problems.


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[k75novice]

It was the Z hose! God I love hate how sensitive to vac leaks L-Jetronic is. This bike truly reminding me of my last BMW, an L-jet equipped 633CSi. Both blue, drop dead gorgeous, but slow and temperamental 


I put a $3.00 bottle of STP injector cleaner ("treats 21 gallons") in when I filled up on the way to work. Marketing tells me I need Techron and will do that going forward.


After work, when experiencing the issue, I took a look at the Z hose (PCV hose) and saw that it's only cracked at the crankcase side due to HEAT DEATH. I pinched the supple hose in the middle and the problem went away. Went back to the office and grabbed a binder clip for a get-me-home fix. New OEM Z hose has been ordered.

[The Mighty Gryphon]

Thanks for letting us know what you found.   I am guessing that you are about the 4,739th person to post with a problem with that Z hose.

[Elipten]

The Z hose should be replaced on a routine schedule and replaced when buying an older bike from clueless owners


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[k75novice]

In the interest of having a complete story documented in this thread, I need to add that new vacuum hoses have not fully cured my cold running problems. Symptoms go away when fully warmed up, though the throttle has an annoying 1 second hesitation before actually revving. It's almost certainly not related, but there is noise from the timing chain at lower RPMs probably due to broken chain guides.





After checking for vacuum leaks above and below the TBs, and at every other vac fitting with starter fluid and carb cleaner, on multiple occasions, I have replaced the elbow hose from the airflow meter to the intake plenum, the "Z hose" crankcase breather, and the vacuum hose from throttle body 3 to the fuel pressure regulator. The fuel tank vent was detached from the tank and plugged with a screw by a previous owner; it does not leak. I have also visually checked all rubber pieces from every possible angle and cannot see any cracks.


After wasting my time with a leaky fuel pressure tester (this one - DON'T BUY) , I replaced the fuel pressure regulator anyway (it probably wasn't bad).


I also synchronized the throttle bodies using this kit, which worked well.


I have put more Techron fuel injector cleaner through two tanks of fuel.


I replaced the battery with an Odyssey PC680 and verified I have good voltage (13V IIRC), and that the alternator is supplying good voltage (14V IIRC)


I replaced the choke cable thinking it had stretched (I couldn't adjust it to lift the throttle stop to the 3mm specification, and it wouldn't click into the second detent). With the new cable in, I realized that I just needed to mess with a choke cable stop that limited its travel, keeping it from going to the second detent (why is this stop even here??)


I cleaned all of the under tank electrical connections, including every pin in the fuel injection computer, ignition computer, all the relays in the relay box, and the main under-tank ground using sandpaper, scotch brite and a flathead screwdriver. I cleaned the battery negative to frame connection.


Spark plugs were replaced with OEM (Bosch?) plugs (exact plug listed in the owners manual, gapped to spec) by the previous owner the day I picked up the bike.


I have checked the resistance of both resistors on the coolant temp sensor: ~1.5KOhm after running the motor for a minute and letting it sit for long enough (~30 min) that the whole engine felt cold. Spec is 2.5KOhm @ 20ºC, 1.25KOhm@40ºC, I think my coolant was in between those temperatures at the time. Fully warmed up (right after the fan cycled on) they both measured about 220Ohms whereas the spec is 190Ohms at 100C. Resistance specs were obtained here. So I have concluded that the coolant temp sensor is OK.


My current hypothesis (being unable to check either the ignition or fuel injection computers for unlikely faults) is that one or more valves are tight. I cannot find a straight answer as to what symptoms are usually experienced when this happens, but I hope that this is my issue. I have ordered valve shim replacement tools from polepenhollow@yahoo.com, seen here.
[/size]-Does anyone know if the BMW valve shims are labeled with their thickness, or will I need a vernier caliper to measure them if I need replacements?
[/size]
[/size]In other news, I passed my road test on this bike (when it's warm it behaves mostly OK) and am finally fully legal after 2 years of riding with a permit.

[k75novice]

How can I edit my above post to increase the text size for the last few lines?

[bocutter Ed]

How can I edit ...

You'll know when you can ...

[kmev]

A symptom of too-tight exhaust valves is cold bloodedness.

[Motorhobo]

-Bike was dropped on the right side. Hand grips were replaced with terrible chrome ones. There is a considerable amount of motion in the throttle grip before actually tensioning the cable.

[/size]
[/size]Did you remove the throttle cam cover to make sure the alignment Mark on the throttle tube lines up with the mark on the cam? If it doesn't then you will have too much slack in the cable, ergo too much freeplay in the throttle...you'll have to turn the throttle grip 3/8 of an inch or so to get any response at all. It's a common mistake...[size=78%]

[k75novice]

Did you remove the throttle cam cover to make sure the alignment Mark on the throttle tube lines up with the mark on the cam? If it doesn't then you will have too much slack in the cable, ergo too much freeplay in the throttle...you'll have to turn the throttle grip 3/8 of an inch or so to get any response at all. It's a common mistake...

Just did this. What a beautiful, easy fix for the throttle slop. Thanks for the tip!

[Laitch]

-Does anyone know if the BMW valve shims are labeled with their thickness, or will I need a vernier caliper to measure them if I need replacements?

How can I edit my above post to increase the text size for the last few lines?

You can use a vernier caliper or a micrometer. The micrometer will be more precise. New shims are numbered but the numbers become difficult to read as wear occurs, so measure them. Once you set the valve clearances, balance the throttle bodies if the air filter is in good condition. Did you inspect the condition of the air filter? It's about the only part of your bike not found by me in your post—that and the fuel filter. How old is that?

Visual checking of a rubber throttle body manifold isn't usually sufficient to determine its condition. When the bike is running, if you carefully spray propane or carb cleaner around each manifold and it causes a change in revs, that is an indication of a leak.

You don't need to mess with the font size of your post, even if you could. Your message is legible. Whether it is comprehensible is up to you and the reader. Font size won't help that.

[k75novice]

Thanks, I've ordered a mitutoyo caliper.


Air filter was changed by PO just before sale, looks good. Fuel filter I changed a few weeks ago. I've been all over the throttle body rubbers checking with carb cleaner on both sides, didn't notice it causing any change in idle.


Font size on the last few lines of that one post is about size 1, at least on my machine. If everyone else can see it fine, so be it.

[thecableguy]

k75novice,  You should be able to edit posts once you hit somewhere around 10 posts...  And, yes I also see your text at 1pt.  This seems to happen occasionally to some people here.  Don't know why.  I copy it into Notepad to read. PITA...


Jim

[Laitch]

Font size on the last few lines of that one post is about size 1, at least on my machine. If everyone else can see it fine, so be it.

What I do is Preview my posts and remove all the stray coding then I preview them again for subject and predicate agreement then I preview them again for topographic orientation then preview them again to make sense of them and when that doesn't work, I post them.

[wally.fisher]

Check those so called Bosch spark plugs again, most K run better with NGK plugs,.


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[k75novice]

I checked the valve clearances using the instructions provided with the polepenhollow@yahoo.com kit, which says intake valves should be at .15-.20mm, exhausts at .25-.30mm. Here are the results:


All three exhaust valves (and one intake valve) were tighter than 0.006in (or approx. 0.15mm - my smallest feeler gauge). I'm guessing in the bike's 62,000 miles this has never been checked. Thanks P.O.! So I have no idea whether the shim is slightly too big or way too big, and will have to work my way up in increments of 0.05mm to get back into spec. I've ordered a 2.45mm shim for intake #1, and 2.15mm shims for exhausts 1,2,&3. None of the shims I removed had the sizes marked on them, and were measured with a digital caliper.


The polepenhollow@yahoo.com valve shim removal tools did the job, but are apparently made of mild steel rather than hardened tool steel, so the "J" piece got mangled after I got it stuck in the space to the left of the cam lobe for exhaust valve #1 when trying to reinsert the shim. The tool dropped from the rim of the empty bucket to inside the bucket, wedged in place by the valve spring against the camshaft and sharp inner bucket edge, and it was so difficult to get out that in the end it bent about ten degrees to the side and a few MM of metal came off of the apex. I'd recommend using the "J" tool on whichever side of the cam lobe has more space, to avoid this problem; for exhaust #1, use the J tool on the right edge of the bucket and the holding tool on the left.


Because this information was hard to find, I'll add that the shims are 29mm, a diameter common to Kawasaki, Yamaha and BMW. I don't know of a shop that does shim exchanges near me, so I found the best price on shims at http://newmotorcycleparts.net/motor_parts/valve_shims.html.


For those who may try: I attempted to take off the clearance needed by sanding down one of the shims on a glass plate with wet 220 grit; after 30 minutes I maybe took off 0.01mm (the borrowed calipers weren't very repeatable). I'd say this is a waste of time unless you're trying to get a size in between what's available.


I am happy that I finally seem to have found the smoking gun after hundreds of dollars spent on unrelated bullshit. I can't wait to experience how much more power this bike makes when its valves can close all the way. Right now, it seems to top out at 85-90mph, so I hope there's more power to be had for the sake of being able to pass on the highway. Of course, this should also fix the bike's problem of not idling cold, and having a hanging throttle after revving up in neutral.


Here's a pic of the cams I took for my bike's records:

[Laitch]

The polepenhollow@yahoo.com valve shim removal tools did the job, but are apparently made of mild steel rather than hardened tool steel, so the "J" piece got mangled after I got it stuck in the space to the left of the cam lobe for exhaust valve #1 when trying to reinsert the shim.I'd recommend using the "J" tool on whichever side of the cam lobe has more space, to avoid this problem; for exhaust #1, use the J tool on the right edge of the bucket and the holding tool on the left.

Well, the tools are not foolproof that's for sure.  I don't go along with your recommendation; furthermore, I recommend that you recheck the shims you reinstalled to be sure that they are fully seated.

The long-handled J tool shouldn't be in place when the shim is being inserted. The shim is nested into its bucket recess by using a pick, needlenose plier or a magnet while only the holding tool is in place. Once the shim is nested, the J tool is carefully inserted to lower the shim/bucket assembly allowing removal of the holding tool. Then the "J" tool is extracted. The Clymer manual has clear illustrations of the procedure. Here's a video.


As you indicate, you are a novice. I don't recommend that novices follow the recommendations of novices.