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Old 08-14-2013, 04:14 PM   #1
Kyalami OP
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Help with Continued Air/Fuel Problems

Fella's

I'm new to the forums - although I've been lurking for the past month - and just wanted to start off by saying thanks up front for the ton of help I've found on here so far. I just completed rebuilding most of the engine on my '99 LC4 640 - and it would have been a massive challenge without your help. Thank you.

I'm at a point where I finally have run out of ideas, and the search function hasn't led me to answers for a specific problem I'm having. I don't want to make this too long winded - but to give you some background...

I inherited this bike (yes, for free...) from a frustrated buddy who never managed to get it running right, wasn't mechanically inclined enough to work on it, and got tired of throwing money at the dealer who also apparently couldn't get it to run well. After a complete teardown... here are the specs and modifications (that I am aware of because I'm new to KTM's):

1999 LC4 640 SMC
Full Remus Exhaust
Slide hole modification in the carb
Removed air intake snorkel
Main Jet - 155 (and I just tried a 160)
New Jet needle (set on middle clip #3) and new needle jet
New (larger of the two options) idle jet

Here is the problem:

When I fire the bike up, it runs great with the choke on full. Idles well, throttle seems crisp from 1/4 to wide open. When I start to close the choke, the idle jumps noticeably, and it bogs badly when I open the throttle. With the choke off, it wants to stall when I open the throttle 1/4.

All those symptoms point to the bike running lean, and apparently the Remus pipes flow a ton of air. Can I still be lean even with a 160 main jet? I do have a 170 that I'm going to try this weekend, and I might also max out the jet needle adjustment to the #5 slot at the same time. Is it possible that I'm simply flowing too much air and need to restrict something, or do I need to mess with the fuel adjustments until I get it right...?

My apologies for the simple question.... I realize this topic has been covered a lot on here - but despite the reading I've done, I still can't figure this thing out...

Thanks in advance - especially to the older posts by Creeper, Warewolf and Laramie LC4 - you made my life a shitload easier the past month...

Cheers

Jason
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Old 08-14-2013, 05:35 PM   #2
davesupreme
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low speed is pilot jet, not main... if the slide is drilled, i would think you have too much air for the pilot circuit, not enough pilot jet?.... what kinda carb?.... keihin?... does it have an accelerator pump?....
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Old 08-14-2013, 05:45 PM   #3
_cy_
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problems like this could be caused by mechanical, electrical or fuel.

on the fuel side sounds like the low speed circuit is clogged.

best way to clean up a carb is not with a soaking or compressed air.

use a can of berryman chemtool with a noozle. place tip of nozzle in each and every orfice. spray with safety glasses.. if spray comes out other side, you've got a proof positive sign that circuit is clear.

repeat for every circuit inside carb .. don't fudge and skip any.. do them all. if you don't get a response from your low speed screw .. it's clogged or butterfly is open too far. or you got some other problems.
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Old 08-14-2013, 05:53 PM   #4
wrk2surf
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Make sure the boot isn't cracked
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Old 08-14-2013, 08:14 PM   #5
Kyalami OP
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Thanks guys. This is on a BST40 carb. I cleaned the daylights out of it... so I know it's not that. All pathways are completely clean.

I'll play with the pilot jet. It is (I'm told) the larger of the two options for this carb, and is new so I think I'm okay there.

I also think it's time to ditch the Remus pipes. I think they are part of the problem and are absurdly loud anyway which really pisses off the neighbors...

Jason
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Old 08-14-2013, 08:24 PM   #6
_cy_
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Quote:
Originally Posted by Kyalami View Post
Thanks guys. This is on a BST40 carb. I cleaned the daylights out of it... so I know it's not that. All pathways are completely clean.
if you've not took a can of carb spray clean with a nozzle, then sprayed each circuit and watched spray come out other end. you really don't know all circuits are clear.

LOTS of folks have soaked their carb in carb dip, looked great and still have clogged circuits. worst yet high pressure air can embed a piece of dirt further in.

it's not how clean carburetor looks outside. but are all internal circuits confirmed clear?
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Old 08-15-2013, 05:48 AM   #7
davesupreme
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yeah, +1 on that....

i dunno about those carbs.... the pilot circuit/screw on Keihin FCR's and CV's feeds gas, and the screw itself is only one hole.... there's other real small holes in the venturi that are fed by it also, and they can get clogged.... the pilot screw is just the last step tuner....

keihin's have about a million pilot sizes available, are you sure you got only 2?... sounds crazy.... what sizes do you have, and what sizes does the manual say?....

cracked air boot is a definite possibility, won't run for shit down low with one....
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Old 08-17-2013, 12:45 PM   #8
Kyalami OP
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Thanks fellas. I did spray the carb out with a nozzle... but the original pilot jet was completely clogged when I replaced it, so I'll triple check that circuit to make sure I didn't miss anything.

I did replace the main jet with a 170 and moved the jet needle location to the #5 (full rich) setting - and I had 6in flames coming out the tail pipes. Other than the massive backfiring, it sounded much better and was so rich that the choke didn't seem to have any effect. So at least now I've found a very lean, and a very rich setting.... and I think I just need to fiddle until I find the right balance.

I'm going to try a 160 main jet with the needle in the #4 position, and see if that is a happy medium.

Dave: I'll check into the pilot jet. I actually don't know what size that is... just that it was recommended to work well with the pipes I have....

Jason
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Old 08-17-2013, 02:06 PM   #9
_cy_
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Quote:
Originally Posted by Kyalami View Post
Thanks fellas. I did spray the carb out with a nozzle... but the original pilot jet was completely clogged when I replaced it, so I'll triple check that circuit to make sure I didn't miss anything.

I did replace the main jet with a 170 and moved the jet needle location to the #5 (full rich) setting - and I had 6in flames coming out the tail pipes. Other than the massive backfiring, it sounded much better and was so rich that the choke didn't seem to have any effect. So at least now I've found a very lean, and a very rich setting.... and I think I just need to fiddle until I find the right balance.

I'm going to try a 160 main jet with the needle in the #4 position, and see if that is a happy medium.

Dave: I'll check into the pilot jet. I actually don't know what size that is... just that it was recommended to work well with the pipes I have....

Jason
get it running without choke first before screwing with main jets. you already stated it was running fine on top end. main jets has nothing to do with low speed circuit.
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Old 08-17-2013, 06:18 PM   #10
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Well,as screwed up as it sounds,I would find stock jetting specs,put the stock muffler and headpipe on it,make it all stock,and see what that does.
As crazy as that sounds they really do run ok from the factory.

Shooting flames out the pipe doesnt sound good,somebody could have messed with the timing,maybe a valve is tight/burned,if you havent checked valve adj Id do that now. Many things affect how it runs besides jetting.

2 strokes are simple,4 strokes are not.
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Old 08-19-2013, 11:30 AM   #11
Kyalami OP
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I'm a few steps away from doing that Dragger.... I just don't have the stock header and muffler right now so it isn't an option. I'd have to buy them first.

CY: does the low speed fuel circuit go fuel intake -> pilot jet -> mixture screw -> needle jet? I took it for a ride and I think I've got it close - but it still only runs well with the choke 1/2 on. I still don't think that it has a blockage problem but I'm going to trace that circuit with carb-cleaner anyway...

The best combination at idle to 1/4 throttle still seems to be with the jet needle set to #5 clip (full rich)... but then it seemed to supply WAY too much gas in the 3/4 - wide open circuit. Logic dictates that if I feed it with more gas at idle and then restrict that gas with a smaller main jet - that should balance it out throughout the range... no?

My pilot jet is a size 50 btw... the old one I replaced was a 45...

I'm still trying to learn the how the fuel flows at different throttle positions so that I can strike the right balance...

Thanks for the help guys.

Jason
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Old 08-19-2013, 12:10 PM   #12
Kyalami OP
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Thanks for the help guys... I think I found the problem. My fuel screw is missing the o-ring, washer and spring which might explain why I can't get it idle consistently...

Jason
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Old 08-19-2013, 03:03 PM   #13
_cy_
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Quote:
Originally Posted by Kyalami View Post
Thanks for the help guys... I think I found the problem. My fuel screw is missing the o-ring, washer and spring which might explain why I can't get it idle consistently...

Jason
outstanding! .. low hanging fruit or easiest to fix items are what needs to be looked at first.
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Old 08-19-2013, 03:20 PM   #14
SATEX
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Quote:
Originally Posted by _cy_ View Post
problems like this could be caused by mechanical, electrical or fuel.

on the fuel side sounds like the low speed circuit is clogged.

best way to clean up a carb is not with a soaking or compressed air.

use a can of berryman chemtool with a noozle. place tip of nozzle in each and every orfice. spray with safety glasses.. if spray comes out other side, you've got a proof positive sign that circuit is clear.

repeat for every circuit inside carb .. don't fudge and skip any.. do them all. if you don't get a response from your low speed screw .. it's clogged or butterfly is open too far. or you got some other problems.
I disagree with the notion that the low speed circuit is clogged, because his idle speeds up when he removes the choke. That said, cleaner is always better.
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Old 08-19-2013, 03:23 PM   #15
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Note to self.. read the whole thread before opening trap. Good news btw... The o-rinf would have been my next suggestion ... honest.
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