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Old 08-07-2013, 07:44 PM   #1
in-trim OP
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Location: Land of Redwoods and private gardens
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DEcomPRESSED 2003 KTM 640 Adventure

Howdy folks,,, wasn't sure if I should've posted this in the 640A thread or start my own, but most of the decompression threads I've been reading were their own thread. So here goes. Oh yeah,,, I apologize in advance for my rambling posts, guess I should've been a literary major.

About 5 months ago I got a 2003 640A with 24k on it from another inmate. It wasn't the cleanest nor dirtiest bike out there, but it felt like a good runner and had worked on suspension.

Starter started making whirring noises last week and found out about starter clutch issues on this forum (thank you all for your info), so I replaced that. Ran fine for 2 days and now it won't start,,, ie; e start won't push engine past TDC.

Pulled valve covers off and valves are still at 0.15,,, but found out why my manual decomp doesn't work, it's lobe in the valve train doesn't hit the exhaust valve arm. Lever has full range of motion with no binding. Is there an easy fix for this?

Auto Decomp - here's where it might get interesting. This is my third thumper, but my other two were red giant's xr250 and xr400,,, and from my reading and slight riding, the lc4 is obviously different beast. Anyways, I've never really heard what I would think is the auto decomp clack,,, just a loud vibrating heart. Not sure exactly what to listen to/for,,, but I've done the plug out kick and wheel turn to spin engine to listen for it. Maybe I'm deaf and can't hear it, or maybe I'm thinking it should be louder (since so many describe it so often), or more likely mine is quiet because its worn out? Besides the clack sound or doing a compression check,,, is pulling the head and manually inspecting auto decomp on the cam the only other way to verify its functioning properly?

Obviously I'm a newbie and as such have probably forgotten some pertinent info.

Cheers all,,, thanks for any help.
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Old 08-08-2013, 02:51 AM   #2
dnrobertson
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I'm sure more knowledgeable will be along soon but here's my take.

My auto-decomp has not worked for the last 12 months. My 2005 with a new battery also often refuses to turn the motor past TDC (especially when it's cold).

I'm pretty sure that if you are not hearing the clack/clack at low revs (under 1500) then it's not working. The only way to know for sure is to pull the cover. You don't have to pull the head to remove cam/ replace inlet rocker arms/bearings (24K you say, could be time to replace inlet cam rocker bearing).

Find out why the manual decomp is not working and fix it. It's not rocket science. Get the parts manual and look at the assembly. Look around on the web and download a copy of the workshop manual. (Paging bmwktmbill ).

The just use the manual decomp when starting the motor.
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Old 08-08-2013, 10:40 AM   #3
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Thanks David. I've got both the owners manual and repair manual, and I've looked through both as well as on ADV and online parts fiche sites but have yet to find anywhere that shows/describes parts for the manual decomp lever/arm. I haven't taken it out yet, but when I looked at it with the valve cover off, I didn't see any obvious signs of wear on rocker arm or man decomp lobe. The lobe is supposed to be a circle with one half forming a right/90* angle, right?

It was my thought as well,,, fix the manual decomp first, then deal with auto decomp when more service is needed.
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Old 08-08-2013, 05:59 PM   #4
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Guess I'm tearing into on my own

Got on a regular computer and found shaft for manual decomp in the parts fiche,,, but it's not a close up shot so no way to compare to mine easily. Guess I get to open her up and see things up close and personal. Without much free time and not much space to work on her (no garage and g/f would flip if I rolled her into the house), I was hoping for some advice for an easy fix,,, but hey, at least it's all in the same area and I get to learn more about her insides. My local shop hasn't been a great help so far.
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Old 08-12-2013, 11:43 AM   #5
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Now with pics and a vid

Ok,,, maybe some visual aids will drum up helpful insight from folks on this forum with way more LC4 knowledge than I possess.

Does the exh rocker arm usually have a flattened top edge where the manual decomp lobe presses? There isn't a notch or grove where it would hit, just a slight silver rub spot.



Since I can't find any obvious wear or broken parts,,, I'm guessing the shape of this internal manual decomp lobe is normal?



Lastly,,, on the auto decomp cam, am I correct in my assumption that the spring should hold/push the decomp cam up and away from the camshaft, but allow it to get pushed back/down? Following this assumption, mine seems to be weak and maybe that's why my auto decomp isn't working? Yeah ok, so the video kinda sucks,,, but I was trying to show that the auto decomp cam is all loose, and I didn't know if that's normal?


I'm hoping someone out there can help with some thoughts and/or guidance,,, cause my local KTM shop isn't much help and I live 7 hours north of Scuderia West AND I'd much rather work on this bike myself, so I know what's been done to it... Cheers.
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Old 08-12-2013, 02:44 PM   #6
Boon Booni
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So, is the manual decompression lobe not contacting the exhaust rocker arm?
Without something being broken, I don't see how the manual decomp could fail to work. How about a pic of it installed?


I don't think it's unusual that the auto-decomp doesn't work so well. There's a spring and pin in there that wears over time and decreases the effectiveness of the auto-decomp.
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Old 08-12-2013, 04:53 PM   #7
clintnz
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Is it possible to install the manual decomp shaft in upside down? Maybe that's why it won't engage on the rocker?

With the rocker cover off it's not too much more work to pull out the cam & inspect the auto decomp assembly as long as you are confident resetting the cam timing.

The smaller of the 2 pins under the decomp cam seems to be the one that wears, this was mine at 95k km:



It needed a pull of the manual decomp sometimes to get spinning. I actually just thought it was that the battery was quite old, but after the rebuild with a new cam fitted it began spinning very noticeably easier on the starter. It also produced a much louder clacking sound when the idle was lowered under where it should be.

Cheers
Clint

PS if that's 24K miles it's done you should replace the cam follower roller bearings while you are in there. Those are specials from KTM but the cam bearings themselves are cheap at any bearing shop so do them too. Rebuild the water pump too.
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Old 08-12-2013, 07:42 PM   #8
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My 640 had low idle clicking issues due to the spring under the cam being worn out (1$ spring) solved my issues. my cam moved like that with the spring worn out and was pretty snappy with the new spring. Never noticed the rubbing though on my lifters.
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Old 08-12-2013, 08:04 PM   #9
Boon Booni
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Quote:
Originally Posted by straight_8 View Post
My 640 had low idle clicking issues due to the spring under the cam being worn out (1$ spring) solved my issues. my cam moved like that with the spring worn out and was pretty snappy with the new spring. Never noticed the rubbing though on my lifters.
Where did you find a replacement?
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Old 08-12-2013, 10:15 PM   #10
bmwktmbill
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In-trim you are the current Ex-pert sad to say.

It's a centrifugal device?

You had slack in your decomp cable?

That decomp mech is mysterious.

bill
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Old 08-13-2013, 08:31 AM   #11
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Ordered the spring from my KTM dealer on a hunch that it was the culprit. Pulled the cam out and took off the floating cam and replaced the tiny spring under it. Idle clicking (exhaust valve opening) problem solved for me.
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Old 08-13-2013, 08:34 AM   #12
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It basically did the klunk on shutdown sporadically when low idle when on a trip, problem got worse and would act up around 2K RPM.

It would be centrifugally activated according to RPM and a tired spring would only require more speed to keep the lobe from opening one exhaust valve.
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Old 08-13-2013, 08:40 AM   #13
Boon Booni
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Quote:
Originally Posted by straight_8 View Post
Ordered the spring from my KTM dealer on a hunch that it was the culprit. Pulled the cam out and took off the floating cam and replaced the tiny spring under it. Idle clicking (exhaust valve opening) problem solved for me.
Huh, never saw that before..




.. and at 55c, it couldn't hurt to try it v. replacing the whole cam to fix the issue for $150.
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Old 08-13-2013, 11:00 AM   #14
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My 2 wheeled world has been righted

Thanks for the replies folks. 2 things were the culprit on this. Yeah, she's got 22k miles, so bearings are on order and will be replaced when I get them.

First thing was that my manual decomp lobe (end of shaft that engages on exh rocker arm) was slightly, and I mean slightly (.14mm difference on calipers) smaller than a stocker. I have a friend who keeps EVERYTHING, and he had a LC4 or two years ago,,, so I spent a couple hours digging through his dirty garage and lo and behold he had a new manual decomp shaft.

Second thing was my valves were slightly (1/8 turn or so) larger gap than .15,,, so I readjusted them till the feeler felt like I was pulling it through putty,,, still moves but doesn't slip in and out as easy as before... Kinda makes me think of an analogy,,, somewhere between trying to suck a fresh milkshake thru a straw, to well, ummm,,, lubed up portions of an adult educational film (well, maybe it didn't slide that easy)...

I've got an '87 Toyota 4wd van that reminds me of my 640A,,, coolest vehicle when running, but man o man,,, makes me want to throw tools and make up curse words when they have issues that don't seem possible. Ok, ok,,, you got me, my diagnostic skills are sometimes lacking.

Thanks again. Cheers.
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