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Old 10-31-2005, 10:32 AM   #1
bigtaters OP
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Location: Las Vegas, NV
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99 LC4 Runs only with choke on...

Hey all,

I've got a 99 KTM LC4 640 that, for the past two weeks, will only run with the choke between 1/2 and full. I pulled the carb and checked the float, main jet (157.5), and needle. They seemed to be pretty clean, but I went ahead
and cleaned them anyway. This didn't help the problem... My next thought is that there is an excess of air getting in, especially since I bught the bike with two of the bolts in the air box cover broken off. I'm at work right now and won't have a chance to mess with it until this evening, so I was wondering if anybody had any input before I get back into it.

Take care,
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Old 10-31-2005, 10:47 AM   #2
creeper
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Quote:
Originally Posted by bigtaters
Hey all,

I've got a 99 KTM LC4 640 that, for the past two weeks, will only run with the choke between 1/2 and full. I pulled the carb and checked the float, main jet (157.5), and needle. They seemed to be pretty clean, but I went ahead
and cleaned them anyway. This didn't help the problem... My next thought is that there is an excess of air getting in, especially since I bught the bike with two of the bolts in the air box cover broken off. I'm at work right now and won't have a chance to mess with it until this evening, so I was wondering if anybody had any input before I get back into it.

Take care,
Chances are, one of the following is true:

An air leak between or at the carburetor and cylinder head.
A clogged passage or jet in the low speed fuel circuit.
Exceedingly lean jetted low speed circuit. (doubtful, as the bike was running fine before I assume)
A damaged diaphragm (again, doubtful)

Oh yeah... this is unrelated, but that main jet might be a little on the rich side.
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Old 10-31-2005, 10:54 AM   #3
potatoho
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I let mine sit for about 4 weeks. In that time, something got clogged. I dropped the bowl and sprayed with gumout and everything was fine. It was probably the pilot jet but not sure. That only takes about 5 minutes, less on a BST, if you've replaced the bowl screws with allen heads. On a BST you can actually reach the pilot jet from the main jet plug with a carb cleaner hose, though it is beyond awkward. You'd have to stick the straw in where the notch on the main plug is, and then angle it to find the pilot hole.

If you've sprayed all holes with carb cleaner when you had it out then I guess that's not it.
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Old 10-31-2005, 11:22 AM   #4
eaglej
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You can have a piece of crap in the pilot or main jet that a quick jet of carb cleaner won't blow out.

A nice trick a friend showed me: Pull a few strands out of a copper wire, and use them to poke thru/ spin the jet around. Copper won't scratch or gouge out a brass jet, like using a steel torch tip cleaner will.

J
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Old 10-31-2005, 12:40 PM   #5
bigtaters OP
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Quote:
Originally Posted by eaglej
You can have a piece of crap in the pilot or main jet that a quick jet of carb cleaner won't blow out.

A nice trick a friend showed me: Pull a few strands out of a copper wire, and use them to poke thru/ spin the jet around. Copper won't scratch or gouge out a brass jet, like using a steel torch tip cleaner will.

J
Thanks for the tip about the copper wire - I'll probably add that to my tool box.

When I cleaned the jet, I actually took the carb off of the bike, removed the bowl, and removed the main jet for inspection and cleaning. I did not remove the float mechanism or needle, though. Just checked that they were freely moving and had no visible deposits on them, then hit them with the carb cleaner...
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Old 10-31-2005, 12:44 PM   #6
bigtaters OP
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Quote:
Originally Posted by creeper
Chances are, one of the following is true:

An air leak between or at the carburetor and cylinder head.
A clogged passage or jet in the low speed fuel circuit.
Exceedingly lean jetted low speed circuit. (doubtful, as the bike was running fine before I assume)
A damaged diaphragm (again, doubtful)

Oh yeah... this is unrelated, but that main jet might be a little on the rich side.
Thanks for the suggestions - I'll check them out tonight. It is very possible that I didn't reinstall the rubber boot between the carb and head properly. If that doesn't do it, then I'll dig back into the carb a little deeper...
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Old 12-07-2005, 11:38 PM   #7
Doc_Busa
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Any updates Tater Boy? Sounds like a job for Madam Butterfly...
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Old 12-08-2005, 06:19 AM   #8
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Tried and true

Try the WD40 test (ignore if I'm telling you the obvious). With bike running - with as little choke as possible in your case to keep it turning over, spray WD40 or carb cleaner in one area at a time around boot between carb and head and listen for any change in engine note - if so, that's a leak area.

Older VW beetles, especially FI'd ones, were notorious for air leaks in boots. The rubber dries and cracks with age and it doesn't take much of a crack, often invisible to naked eye (unless boot is flexed) to admit too much air, on the worng side of the mixture circuit, when under vacuum.

Some GOOP, etc on crack, if found should fix until a new boot is delivered - if boot is problem.

Should be simple to track down as either too little fuel to too much air is causing issue.
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Old 12-08-2005, 08:47 AM   #9
bmwktmbill
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Is th slide lifting? Start it up and stick a screwdriver in and lift the slide. If that makes it run check the diaphram or for an air leak between carb and the engine. It has to rev up if it runs and if the mainjet is clear and the slide is up. With twins when a cylinder goes dead but the bike runs this can be a cause and it runs on only one cylinder.
Bill in Tomahawk, WI
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Old 12-08-2005, 09:43 AM   #10
AllEuro
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Quote:
Originally Posted by bmwktmbill
Is th slide lifting? Start it up and stick a screwdriver in and lift the slide. If that makes it run check the diaphram or for an air leak between carb and the engine. It has to rev up if it runs and if the mainjet is clear and the slide is up. With twins when a cylinder goes dead but the bike runs this can be a cause and it runs on only one cylinder.
Bill in Tomahawk, WI
Sounds like you may be on to something My old R100gspd had the above described problem after a bit of sitting while I was on the mainland.
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