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Old 04-05-2008, 08:52 PM   #1
GSteve OP
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640 A carb issue

Let me start off by saying that I am a noob to carbs and I have searched the Thread index, etc. for the answer to these issues with no joy.

I have an '06 640 Adventure that came to me in Virginia (altitude) via Colorado and Utah (high altitude). The canister had been removed somewhere along the line. The snorkel and a portion of the top of the airbox has also been misplaced. The float level was very high and had a 157.5 main jet. The needle was in the fourth slot. The engine ran OK but had an off idle stumble to the point that it would die.

I reset the float level which leaned out the everything quite a bit. However, the off idle stumble is still there if the bike is allowed to idle for a few minutes, such as at a red light. I've adjust the mixture quite a bit and have not been able to get rid of the stumble.

Next problem, which may be related to the stumble. While letting the bike idle and trying various adjustment of the idle mixture, I noticed gas dripping occasionally from the small tube in front of the rear wheel. I assume that this is an overflow for the float bowl, right? I am an admitted noob when it comes to working on carbs.

Thinking that the float needle valve was bad and allowing fuel to overfill the float bowl, I replaced the valve. That did not fix the problem. This only occurs after the motor is warmed up. No drips and no stumble while still cold or warming up. Both only occur after complete warm up.

Thoughts? Suggestions?
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Old 04-05-2008, 09:29 PM   #2
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Steve, it sounds like there is still a float issue. The seam on the goat needs to be level with the carb body. If all looks good you could have a problem with the float itself. It could have a hole in it. Or it could be getting hung up on something.
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Old 04-05-2008, 09:49 PM   #3
gunnerbuck
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Moving the jet needle clip to the 3rd position should get rid of that off idle stumble... It's not uncommon for the BST to weep out the overlow tube on occasion but it shouldn't be a common thing like you describe.... Like Zerodog suggested recheck your float height and you could suspend your float in fluid to see if you have a sinker.....
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Old 04-05-2008, 10:00 PM   #4
GSteve OP
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Rob, I checked that again today and it still looks to be right, 5mm all the way across. Correct me if my guess is wrong, but if there were a hole in the float(s) then I would have an overflow all the time, plus there would be fuel in the float itself, correct?

I keep thinking that the float bowl is getting pressurized somehow. I replaced the valve, so how else could it get pressure? I just went out to the garage to check on the float bowl vent line and found that someone had installed one of those clear in line fuel filters in the end of the vent line. I pulled it off and the vent line seems to be clear and not pinched. But, the fuel filter didn't flow very well at all. Very tough to blow through! It's too late tonight to check it out, but I sure will tomorrow. If that doesn't work, then I'll pull the carb one more time and check the float again.

I'm open to any and all other ideas.
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Old 04-05-2008, 10:35 PM   #5
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??????Fuel filter on the overflow tube.

Ok, a couple of questions.

Where is this filter and tube located.
Is it a filter on the overflow coming out of the bottom of the the float bowl?
If so ok it doesnt need to be there, but if not.
There is a air large vent on the right rear, upper portion of the carb body.
When stock it has a black plastic "filter" on the end of a short 90 deg rubber hose about 3/8" ID.

That hose has to have a filter on it as it is a pressure eqaulization vent for the carb and has air flow in and out of the carb, going through it.

There is also a smaller 1/4" id overflow hose on the right side of the carb that usually routed to the back past the starter relay and out to under the seat.

If you are holding the the carb body upside down and level and trying to set your float height that way, it doesnt work, youll always set it high.
There is a very tiny spring in the top of the needle valve that will compress under the weight of the floats

If you hold the carb on it side in your left hand,notice how the floats swing if you tilt the carb up and down slightly.
You want to rotate it to the left till the tab on the float contacts the needle vavle and stops.
At that point your seam in the float should be parallel with the bowl face.
It took me a while to wrap my head around the apperant voodo required to set the float level on a bst. But once you figure it out its easy.

And your sure your getting the float needle to hang correctly on its little tab.



What size pilot does it have in it?
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Old 04-06-2008, 05:26 AM   #6
GSteve OP
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Quote:
Originally Posted by cevquit
??????Fuel filter on the overflow tube.

Ok, a couple of questions.

Where is this filter and tube located.
Is it a filter on the overflow coming out of the bottom of the the float bowl?
If so ok it doesnt need to be there, but if not.
There is a air large vent on the right rear, upper portion of the carb body.
When stock it has a black plastic "filter" on the end of a short 90 deg rubber hose about 3/8" ID.

That hose has to have a filter on it as it is a pressure eqaulization vent for the carb and has air flow in and out of the carb, going through it.

There is also a smaller 1/4" id overflow hose on the right side of the carb that usually routed to the back past the starter relay and out to under the seat.

The fuel filter was on this vent hose.


If you are holding the the carb body upside down and level and trying to set your float height that way, it doesnt work, youll always set it high.
There is a very tiny spring in the top of the needle valve that will compress under the weight of the floats


Nope, I set the float height with the carb on it's side as outlined in http://www.advrider.com/forums/showt...ighlight=float. The seam is at about 5mm all the way across (level).


If you hold the carb on it side in your left hand,notice how the floats swing if you tilt the carb up and down slightly.
You want to rotate it to the left till the tab on the float contacts the needle vavle and stops.
At that point your seam in the float should be parallel with the bowl face.
It took me a while to wrap my head around the apperant voodo required to set the float level on a bst. But once you figure it out its easy.

And your sure your getting the float needle to hang correctly on its little tab.


Seems to be centered and moves freely.

The stumble (and the drip at the overflow) only happen after the bike is warmed up and sits at idle for a few minutes, such as at a red light. When cold and warming up, throttle response is great even after turning off the choke. But once warmed up and after and extended idle the problem starts.

Once the neighbors are awake, I'm going to give it a test without the clogged fuel filter in the vent line.


What size pilot does it have in it?


I don't remember the pilot fuel jet size and I can't find it in my notes. The pilot air jet is a 1.2.

Thanks for you insights, let's hope that a clogged vent line is the problem? I'll report back later.
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Old 04-06-2008, 09:16 AM   #7
halfcab
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Unless there is a fuel pump on the 640 Adv forget about pressure on the float needle and seat.

With the snorkel and top of air box being gone, your going to have a "BOG" with this CV carb. You can air screw or main jet it to death trying to solve are cover up the problem.

May I suggest to "TEST" solve the "BOG" problem, is Duck Tape the top of the air box and leave a 1" x 3" opening and go for a test ride.
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Old 04-06-2008, 11:55 PM   #8
bmwktmbill
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I ran 15k miles with no snorkel, stock jets, needle raised one notch, and no bog on my '02 Adventure.
How is your fuel mileage?

Are the carb connections to the engine tight?
Check for play in the choke cable.
Remove the air filter and reach in with your finger and raise the slide, does it stumble?
I don't think it should leak fuel. If it does the o-ring on the seat is bad, the rubber tipped needle is bad or the float is too high...IMHO.
b
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Old 04-07-2008, 08:31 AM   #9
ram350
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I had same problem with the stumble off idle. Raise your needle one notch and the problem is solved.
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Old 04-07-2008, 09:10 AM   #10
NICO
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Quote:
Originally Posted by ram350
I had same problem with the stumble off idle. Raise your needle one notch and the problem is solved.
This is contrary to what I've read with some other riders and similar problem. Ram's solution may work for you, but just in case...

Many find relief from the stutter/stall off idle (after idling for a little while, like at a red light) by lowering the needle one notch. Middle notch, #3 of 5, is the stock setting.

YMMV.
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Old 04-07-2008, 08:21 PM   #11
GSteve OP
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Removing the clogged fuel filter from the vent line did the trick regarding the drip from the overflow tube.

The needle is in slot #3.

However, the stumble is still there. Here's a few more details:
If I rev the engine from idle past the "stumble spot" (2000 RPM), then let the engine back to idle and rev it again, no stumble!

Let the engine idle for a minute, then slowly increase engine speed, when the motor reaches 2000 RPM, RPMs drop back to 1500 (idle speed), then increases past 2000 RPM. Then I let the engine back to idle and then slowly increase twist the throttle and engine speed increases slowly in a linear manner, without dropping speed, through the entire rev range.

Let the motor idle for a minute, try the slow rev and the stumble re-appears.

In other words it's not consistent and only occurs after extended idle!

Bill,

Carb connections are tight.
There is slack in the choke cable when the lever is pushed forward.
I'll give the inside slide move a check in the next few days.
There was a slight groove around the rubber portion of the needle on the float valve. I changed the needle and seat and the leak was still there. It seems that the clogged fuel filter in the vent line may have caused the weeping at the overflow tube.

Thanks everyone for the input. The search continues!


Quote:
Originally Posted by bmwktmbill
I ran 15k miles with no snorkel, stock jets, needle raised one notch, and no bog on my '02 Adventure.
How is your fuel mileage?

Are the carb connections to the engine tight?
Check for play in the choke cable.
Remove the air filter and reach in with your finger and raise the slide, does it stumble?
I don't think it should leak fuel. If it does the o-ring on the seat is bad, the rubber tipped needle is bad or the float is too high...IMHO.
b
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Old 04-07-2008, 10:54 PM   #12
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The stumble has been discussed a few times on this forum. Generally the fix seems to be dropping the needle (raising the clip). I would suggest trying the needle position adjustment until you get a result.

It worked for me on my 2006. I also increased the idle speed a bit at the same time.
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Old 04-08-2008, 12:10 AM   #13
bmwktmbill
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Steve,
In my memory there is an o-ring on the entire seat assembly that pushes into the carb body, if it leaks the float level will go too high.

The stumble could also be caused by a plugged pilot jet. I forgot that.

Do you have a good way to adjust the low speed fuel screw, ie. Loaded's fuelscrew replacement?

The reason to lift the slide with your finger is to test the diaphragm for a leak or a tear.
b.
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Cormack McCarthy-The Crossing

The faster it goes the faster it breaks.
And high performance=high maintenance.
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Old 04-08-2008, 03:16 AM   #14
GSteve OP
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The new valve seems to be seated firmly. The o-ring for the float assembly also seems to be in it proper place.

As to the pilot jet, a visual check didn't reveal anything either.

Yep, got the loaded screw and it seems to be working fine as I can vary the rpm at which the dip in RPMs starts, somewhat.

When you mentioned checking the slide from the air box side I thought perhaps the slide might be sticking as the RPM dip occurs just off idle on a warm motor after idling (heat). The throttle will be open slightly, but the slide is remaining closed.

After a few quick revs the slide may not have had the opportunity to firmly seat in the closed position and stick. Let it stay closed for a minute while at operating temp or slightly above, and then it's reluctant to respond to throttle changes at low speed?

In any case, I'll give it a check in the next few days when I can get some garage time. Too much other crap to attend to this week! Thanks again.

Quote:
Originally Posted by bmwktmbill
Steve,
In my memory there is an o-ring on the entire seat assembly that pushes into the carb body, if it leaks the float level will go too high.

The stumble could also be caused by a plugged pilot jet. I forgot that.

Do you have a good way to adjust the low speed fuel screw, ie. Loaded's fuelscrew replacement?

The reason to lift the slide with your finger is to test the diaphragm for a leak or a tear.
b.
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Old 04-08-2008, 08:51 AM   #15
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what pilot jet are you running? Stock is lean even out here. Do you have some popping on decel?
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