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Old 01-22-2008, 09:37 AM   #1
Hotshot OP
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LC4 rich idle

Ok, I know, I know, It should be somewhere in the LC4 index, but I have not found clear info on what I'm looking for so here goes.
Situation: 01 Lc4 640 with BST40 carb, seems rich on idle. Can be difficult to crank on cold days, and seems to flood easy when cold. I've had the carb apart several times, adjusting float levels, changing main jets, ect. Each time I take the carb apart, it seems to start and run great, then a few days later, it has a fit and won't start. It's getting fire to the plug. Each time, take the carb out and apart, check float, jets, ect., put her back in, charge the battery, a little fuel, wala, runs like a champ. Oh, running a 160 main jet, 1.5 idle jet (I think), T4 can with no pre-silencer.
So, how to lean out the idle mixture. Is that the screw which is located towards the front of the carb, accessed from underneath when the carb is installed? The same screw that Stenhouse makes which is adjustable with your hand?

Next, needle jet clip position. By reading the various tuning guides, I assume moving the clip towards the top (being the end with the clip slots) will provide less fuel when the main jet is active and lowering the clip will provide more fuel when main jet is active. Middle being stock. Confirmation and opinions please.
Ok, that's all the dumb questions for now. Thanks
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Old 01-22-2008, 10:06 AM   #2
NICO
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You have taken the carb apart and adjusted the float, but are you sure it is adjusted correctly? (not tryin' to be a dick here, it can be a PITA) I followed Gaspipe's method in the Index and was able to get it close. What worked well was to blow through the gas inlet line and move/adjust the float until the air stopped flowing. At this point, the float should be parallel to the body of the carb. If not, adjust the tang to suit.

Did you inspect the float "needle" and seat? You need to make sure these are in good working order or the the valve may not close all the way. You didn't mention fuel dribbling outof the overflow, so I doubt this is an issue.

As for the fuel mixture screw, yes this is what Stenhouse sells to make it easier to adjust. I am a cheap bastard and go the old fashioned way - a small screwdriver and a couple burnt knuckles. (that exhaust gets mighty hot). IIRC, the Index notes you should be about 2 to 2-1/2 turns out from seated. Double check, as I may be off on this.

The needle is as you have describe. Middle clip position is stock, moving the clip down allows more fuel throught the main, raising the clip restricts it. The main, however, has little to nothing to do with rich/lean during idle.

Couple basic questions;

Is your air filter nice and clean? If dirty/clogged, you will not get the air needed to run right. If recently oiled, this could also limit the air flow.

Has anything been done to the airbox? While I don't necessarily agree, it has been noted that a hacked/modified airbox is a bad thing with the BST.

When was the last time the valves were adjusted? Hard starting is a common indicator the valves may be out of spec. Read up and check/adjust those out, it isn't that terribly difficult to do.

Good luck, I'm not sure if anything here will help but this is what comes to mind.
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Old 01-22-2008, 11:58 AM   #3
meat popsicle
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Quote:
Originally Posted by Hotshot
... Each time I take the carb apart, it seems to start and run great, then a few days later, it has a fit and won't start. ... Each time, take the carb out and apart, check float, jets, ect., put her back in, charge the battery, a little fuel, wala, runs like a champ. ...
Hmmm,

what could change between when you put the carb back together and it "runs great, and "a few days later it has a fit and won't start"? Something must be changing during those few days...
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Old 01-22-2008, 12:20 PM   #4
herobikes
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Fits

Check your choke action , if its stuck partially on you will have all the problems you described.
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Old 01-22-2008, 01:57 PM   #5
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Quote:
Originally Posted by herobikes
Check your choke action , if its stuck partially on you will have all the problems you described.
That might be it. I'll make sure it is seated where it goes into the carb properly and check the lever as well.

NICO,
I serviced the air filter several months ago, just before I had a catastrophic bottom end failure. It's ok. So, as I mention the re-build, the valves are in spec. as well (well, they were when I put the engine back in a few weeks ago). Less than 200 miles on the engine. No gas pouring out of the bowl overflow. I've been very careful on checking and adjusting the float, but who knows. It just seems to be running a little rich. Still backfires through the exhaust when you let off the throttle, but from what I've read, that's pretty normal. Yes, the airbox has been modified. I think a 2"x4" hole cut in the top (or close to that). The bike started fine before I had the re-build done. Once it's running, she runs like a champ.
Actually, you really have to ease on the throttle from a dead stop. Too much throttle, and she dies. Once you get going, you can throttle until you are looking at the sky. Would de-creasing the idle jet size help the stalling?
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Old 01-23-2008, 10:44 AM   #6
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Quote:
Originally Posted by Hotshot
Actually, you really have to ease on the throttle from a dead stop. Too much throttle, and she dies. Once you get going, you can throttle until you are looking at the sky.

Would de-creasing the idle jet size help the stalling?
Check out the choke circuit, as mentioned. Might be it.

The off idle stumble/stall you mention has been linked to the needle clip being set to the 4th from top (richer) position. Some have found returning the needle clip to the stock 3rd from top position fixed the problem.

This stumble could still be from the float being set improperly. The bowl has a chance to fill at idle and then drains out to a workable level when the revs rise.

As for the idle jet size, I kinda think it is more important to dial in the fuel mixture screw -vs- the actual idle jet size. I think there are only 2 sizes readily used/available on the BST, the stock 45 and the richer 47.5.

I could be waaayyyyy off on this. I am in no way, shape, or form, any sort of carb expert. I have read throught the BST tuning threads quite a bit and am only conveying what I have understood, or tried to understand, from our resident BST guru's.

Hope you get this worked out.
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Old 01-23-2008, 11:00 AM   #7
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All of that makes sense. Thanks for setting me straight. I really think it's the choke circuit, like you mentioned before. I was just messing with it. Yesterday, it ran great in the garage. This morning, wouldn't crank. Flooded. Pulled the spark plug and it was fouled. Replaced, ran ok, but when I pulled it off of the stand to push it outside, it died and wouldn't crank again. It was acting like the choke was engaged. So, I checked my cable routing and it would change the position of the choke cable when I turned the bars. I made some corrections, so I hope that helps. It still acts like it's rich on idle a little.

Thanks everyone for all of the advice.

Quote:
Originally Posted by NICO
Check out the choke circuit, as mentioned. Might be it.

The off idle stumble/stall you mention has been linked to the needle clip being set to the 4th from top (richer) position. Some have found returning the needle clip to the stock 3rd from top position fixed the problem.

This stumble could still be from the float being set improperly. The bowl has a chance to fill at idle and then drains out to a workable level when the revs rise.

As for the idle jet size, I kinda think it is more important to dial in the fuel mixture screw -vs- the actual idle jet size. I think there are only 2 sizes readily used/available on the BST, the stock 45 and the richer 47.5.

I could be waaayyyyy off on this. I am in no way, shape, or form, any sort of carb expert. I have read throught the BST tuning threads quite a bit and am only conveying what I have understood, or tried to understand, from our resident BST guru's.

Hope you get this worked out.
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Old 01-23-2008, 12:28 PM   #8
gunnerbuck
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It could be that your float needle and seat are worn and not properly stopping the fuel causing erratic fuel level in the float bowl....
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