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Old 12-07-2014, 09:35 AM   #1
Shooter1 OP
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KTM Lc4 640 jetting question

Last summer I bought a '01 Lc4640. The bike had a Dellorto on it and it ran really good but I wanted the smoother throttle response of the BST so I got a clean BST off a DR650 Suz. The DR jetting as 140 MJ and a 42.50 PJ and the needle was non-adjustable.

I previously had a '98 RXCe620 and had put a BST on it. It had the stock mid pipe and muffler and I ran no discs on it. I removed the snorkel and that's all. I ran a 152.5 main with the stock 45 PJ and the pilot screw was backed out 2.5 turns. The needle was set in the middle #3 position. The bike ran perfect from top to bottom.

I installed the BST on the 640 with the 152.50 main, and went up to a 45 PJ, and currentl have the air screw backed out 1.5 turns. The bike has the straight thru mid pipe and an SXC can on it with a spark arrestor installed. The stock needle has been shimmed up anywhere from about .060" to where is is now at .038" <-- this seems best. The throttle response is just not quite where the 620 was in the low transitioning into the mid range, once into the mid range it's strong. The bike will throttle wheelie in 2nd gear with the stock gearing so the power is there.

I was thinking about trying a 47.5 PJ but you can cold start the bike in hot or cold weather and kick the choke off about 15 seconds after it's started and it will idle fine which tells me the pilot circut is OK or maybe even a bit too rich. I live at about 1000 feet elevation. The airbox top was totally removed when I got it so I fashioned a lid out of cardboard to experiment and now have an opening about 1" wide across the back of the airbox, not alot bigger than if the snorkel was just removed.

I can't help but think the PJ is OK since it idles well quickly after a cold start but it my be in the needle taper. I know it's not the motor in any way as with the Dellorto, it had throttle response rivaling my 300 2T and pulled HARD everywhere.

I'd be interested in anyones thoughts here or if anyone had the same issue and what they did to it.

Thanks.
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Old 12-07-2014, 10:44 AM   #2
docwyte
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Is the motor stock? Or do you have a pipe, open airbox cover and removed snorkel? You're going to want a larger main jet for sure, I know Motolab suggests a 162.5 for the high flow head bikes with all the above I listed
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Old 12-07-2014, 06:56 PM   #3
motolab
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Quote:
Originally Posted by Shooter1 View Post
Last summer I bought a '01 Lc4640. The bike had a Dellorto on it and it ran really good but I wanted the smoother throttle response of the BST so I got a clean BST off a DR650 Suz.
How many miles are there on the carburetor?
Quote:
I previously had a '98 RXCe620 and had put a BST on it. It had the stock mid pipe and muffler and I ran no discs on it. I removed the snorkel and that's all. I ran a 152.5 main with the stock 45 PJ and the pilot screw was backed out 2.5 turns. The needle was set in the middle #3 position. The bike ran perfect from top to bottom.
Was it a KTM or a Suzuki carburetor?
Quote:
I installed the BST on the 640 with the 152.50 main, and went up to a 45 PJ, and currentl have the air screw backed out 1.5 turns.
BST40 carburetors have fuel screws rather than air screws.
Quote:
The stock needle has been shimmed up anywhere from about .060" to where is is now at .038" <-- this seems best.
Be aware that shimming the needle clip will indeed richen the mixture, but (except when the slide is against the stop) as the shim increases the preload on the slide spring, it will do so by lowering the slide rather than by raising the needle.
Quote:
The throttle response is just not quite where the 620 was in the low transitioning into the mid range, once into the mid range it's strong.
Please refer to specific throttle openings, as that will tell us which circuit(s) is (are) responsible.
Quote:
I was thinking about trying a 47.5 PJ but you can cold start the bike in hot or cold weather and kick the choke off about 15 seconds after it's started and it will idle fine which tells me the pilot circut is OK or maybe even a bit too rich.
BST40 carburetors have cold start enrichment circuits instead of chokes. A choke is closed when an engine is cold and opened when it is warm, whereas a fuel enrichment circuit is opened when the engine is cold and closed when it is warm. A choke works by restricting the air flow and by lowering the pressure inside the venturi, which causes the existing jets to meter more fuel, whereas an enrichment circuit adds extra fuel without restricting the air flow. A choke requires a fast idle cam or something akin to it (or you have to hold the throttle open manually), whereas an enrichment circuit adds a little extra air simultaneously with the extra fuel, so the fast idle is already built in.

That said, 15 seconds is a bit quick to be able to shut the cold start enrichment curcuit off from cold, especially with the fuel screw only open 1-1/2 turns. I therefore agree with your assessment that the idle mixture is already too rich. There may however be another cause for the additional fuel, such as a clogged/incorrectly sized pilot air bleed jet, incorrect fuel level, cold start enrichment slide not closing all the way, or perhaps an in incorrectly sized/significantly worn emulsion tube.
Quote:
I live at about 1000 feet elevation.
I listed the previous possibilities because at sea level a 45 pilot jet is typically the correct size and requires about a 2-1/4 fuel screw opening.
Quote:
The airbox top was totally removed when I got it so I fashioned a lid out of cardboard to experiment and now have an opening about 1" wide across the back of the airbox, not alot bigger than if the snorkel was just removed.
Considering the many reports of detonation damage in conjunction with my own experience with dynamometer testing, I would not do anything to increase cylinder filling on a pre-high flow head model unless significant measures had been undertaken to improve detonation resistance. I therefore recommend the reinstallation of an unmolested airbox with snorkel.
Quote:
I'd be interested in anyones thoughts here or if anyone had the same issue and what they did to it.
The DR650 carburetor uses a different jet needle profile, slide spring, slide, float assembly, float seat, cold start enrichment slide and diaphragm cover; a larger emulsion tube, a smaller pilot air bleed jet and main jet and does not use the metal shim under the white jet needle spacer as the KTM does. The carburetor should be converted to KTM specification and it should be verified that it's in fundamentally sound condition (no witness line on float needle tip, float seat o-ring not shrunken/cracked/deformed/hardened/otherwise damaged, slide guide not worn out, etc.) before discussing jetting.

Regards,

Derek

motolab screwed with this post 12-07-2014 at 07:06 PM
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