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Old 05-12-2014, 12:56 AM   #1
2001KTM640Lc4E OP
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Eek What is this Mikuni BST 40 pipe (KTM)!?

Hey hey hey!

I bought a 2001 KTM 640 LC4-E in Virginia about 3 years ago, rode it 12,500 miles around America and brought it back home to New Zealand on a ship (first bike I ever rode). I still have it now, but about 6 months ago it stopped performing as well - lurching while riding at constant speed and difficult to start. I had done everything that I could to try and remedy the issue, including cleaning the carb and replacing some worn seals/o-rings in carb etc, but just couldn't get it to perform well again. I suspected that the head gasket was blown as when it had been sitting for 4 weeks or so, it would be very hard to start and would blow white smoke out the back, so took it to a KTM mechanic and asked him to check to see if the head gasket was blown and if not then just do what he could to get it running better.

3 hours and $300 later, the mechanic had got the bike running at about 85% of what it should be by adjusting the needle height and blocking off a pipe that came off of my carb (Mikuni BST 40). Turns out that I wasted my money because the head gasket was in fact blown, as I found out 6 months later when I finally (rather embarrassingly) mixed coolant with my radiator water and noticed green liquid coming out of the union between by exhaust headers and tailpipe (showing that water was getting into my cyclinder). I am now proceeding to replace the head gasket myself, and sure enough, there was green coolant sitting on one of my valves once I got the head off. I am of course concerned by the fact that the mechanic didn't pick this up and am now wondering why he blocked up this pipe on my carb. Can anyone comment on what this pipe does and whether or not it should be blocked please



2001KTM640Lc4E screwed with this post 05-12-2014 at 01:48 AM
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Old 05-12-2014, 02:15 AM   #2
2001KTM640Lc4E OP
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Eh? What used to be in place of the plug

Before the mechanic put that plug in place, this plastic elbow was in place. This plastic elbow has a sort of mesh screen/filter inside of it which I can blow through. Again, unsure why he removed the elbow filter and replaced it with a plug and to be honest he didn't seem to explain it much either.

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Old 05-12-2014, 05:42 AM   #3
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That is a vent to the underside of the diaphragm. It needs to be open to atmosphere and it ideally should have that filter/screen put back in. When the diaphragm lifts, air enters the underside chamber to make up the difference in volume. If you plug that port, the slide can't operate properly.

It would probably be a good idea at this point to remove the top cover and inspect the condition of the diaphragm and slide.
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Old 05-12-2014, 05:51 AM   #4
nevermind
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Look at the parts fiche on one of the online sites and that should give you more info.

IMHO, this is where the hose from your crankcase vent feeds back into the intake tract and the black thing is a PCV valve. Looks like the original hose was cut and plugged so you need to find the crankcase vent nipple and see what he did there.
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Old 05-12-2014, 08:30 AM   #5
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Quote:
Originally Posted by nevermind View Post
Look at the parts fiche on one of the online sites and that should give you more info.

IMHO, this is where the hose from your crankcase vent feeds back into the intake tract and the black thing is a PCV valve. Looks like the original hose was cut and plugged so you need to find the crankcase vent nipple and see what he did there.
Sorry, but that is completely incorrect.

Gonerydin is correct. It is a vent for the carb diaphram and is a necessary piece for the operation of the carburetor. It can not be left open or else you allow unfiltered dusty air into the carb. Put either the stock unit back on, or some other quality filter. If you go to the BST40 thread, there is a ton of great information about how this carb works and how to tune it properly.
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Old 05-12-2014, 10:53 AM   #6
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Remove the plug and install a Uni Filter Breather http://www.bikebandit.com/uni-crankcase-breather-filter
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Old 05-12-2014, 11:36 PM   #7
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While the head is off drop the piston to BDC and check the bore for scratches, if there are some noticable ones suspect a broken top piston ring.

If it was me, after everything is back together I'd run some cheap oil without changing filters until full temp is reached and then drain it, add beter dino oil and run it 500 miles, drain it and change filters and add the oil of your choice.

Rebuild the waterpump, replace the cam followers and all the o=rings on the shafts?

bill
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Old 05-17-2014, 03:55 PM   #8
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Quote:
Originally Posted by gonerydin View Post
That is a vent to the underside of the diaphragm. It needs to be open to atmosphere and it ideally should have that filter/screen put back in. When the diaphragm lifts, air enters the underside chamber to make up the difference in volume. If you plug that port, the slide can't operate properly.

It would probably be a good idea at this point to remove the top cover and inspect the condition of the diaphragm and slide.
Hey, thanks gonerydin! Good to know that the mechanic was wrong on this one. So by blocking the diaphragm vent, the mechanic could have caused damage to my carb (diaphragm and slide)!? That's concerning. I might head back into the store and see if they'll sort me out a deal on the new head gasket and other parts I need, because it sounds like they mucked up?
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Old 05-17-2014, 03:59 PM   #9
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Remove the plug and install a Uni Filter Breather http://www.bikebandit.com/uni-crankcase-breather-filter
Awesome, thanks for that. So what about this filter makes it better than the factory elbow joint filter that came with the carb (as shown in photo earlier)? Is it just better at allowing clean air to flow back into the underside of the diaphragm, with less dust than the elbow joint?

Also, I'm guessing that it is fine to use for this application even though it's called a crankcase breather filter? Just noting that earlier post by Kawidad about the crankcase PCV valve.
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Old 05-17-2014, 04:14 PM   #10
2001KTM640Lc4E OP
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Quote:
Originally Posted by bmwktmbill View Post
While the head is off drop the piston to BDC and check the bore for scratches, if there are some noticable ones suspect a broken top piston ring.

If it was me, after everything is back together I'd run some cheap oil without changing filters until full temp is reached and then drain it, add beter dino oil and run it 500 miles, drain it and change filters and add the oil of your choice.

Rebuild the waterpump, replace the cam followers and all the o=rings on the shafts?

bill
Hey Bill, cheers for that info. Will do that. There's obviously no issue in dropping the piston to BDC as I can easily put it back to TDC when putting the camshaft gear back on just by locating the correct spot with my crankcase locking bolt right? There seems to be a lot of posts about people mucking this up, but it should be pretty straightforward so long as I put it back into place right?

Thanks for the oil advice. That's a good idea.

Based on the manual, I should replace the two O-rings for the waterpump sealing flange regardless of kms. The only other things it refers to is replacing the two grooved ball bearings and lubricating them and replacing the waterpump cover gasket. Is that what a rebuild entails or are there more items involved?
The bike has only done 11,000 miles. Do you think it is necessary to rebuild the waterpump?

Same question as for the waterpump - is it still necessary to replace my cam followers and cam follower shaft O-rings even though it's only done 11,000km?
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Old 05-18-2014, 04:02 PM   #11
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Quote:
Originally Posted by 2001KTM640Lc4E View Post
Hey, thanks gonerydin! Good to know that the mechanic was wrong on this one. So by blocking the diaphragm vent, the mechanic could have caused damage to my carb (diaphragm and slide)!? That's concerning. I might head back into the store and see if they'll sort me out a deal on the new head gasket and other parts I need, because it sounds like they mucked up?
I wouldn't say that your mechanic necessarily broke or damaged anything, but he didn't fix it either. At any rate, you should inspect your diaphragms any time you are have carb issues. That fact that he blocked that tube is evidence that you need a different mechanic altogether.
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