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Old 04-19-2015, 06:12 PM   #1
lineaway OP
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Keihin Carb

Always had issues with stalling on the Raga with a Keihin carb. Last week before the Texas round I thought I would try and cure it. Since I was re-jetting the bike anyhow. All keihin`s seem to only have one vent on the right, even though there is one made in the body on the left. So take a 2.0 to 2.5 mm bit and drill it carefully out. You can FEEL it when it goes through. It is surprisingly like 6-8mm. So far no downhill type stalls that I previously had. Bene?

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Old 04-20-2015, 04:56 PM   #2
slicktop
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Not Bene,
but it sounds like you are thinking outside the box, as if an engineer had already thought it out and management said no.
Sadly I'm not a carb mechanic and that's why I promote Lectron smart carbs, but that's an impressive run around.
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Old 04-21-2015, 06:21 AM   #3
Jonny042
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Quote:
Originally Posted by lineaway View Post
Always had issues with stalling on the Raga with a Keihin carb. Last week before the Texas round I thought I would try and cure it. Since I was re-jetting the bike anyhow. All keihin`s seem to only have one vent on the right, even though there is one made in the body on the left. So take a 2.0 to 2.5 mm bit and drill it carefully out. You can FEEL it when it goes through. It is surprisingly like 6-8mm. So far no downhill type stalls that I previously had. Bene?

I've always attributed the downhill stalling to the fact that the carb is mounted pointing down when it was designed to be horizontal. I have fixed this on my two bikes with Keihins by lowering the float level a bit.

I always assumed the unused vent on the left would serve the same purpose as the one on the right?

So I guess what I'm asking - did you change anything else while you had the carb apart?
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Old 04-21-2015, 01:13 PM   #4
lineaway OP
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Just the main as I was headed to Texas. But I have not ridden it enough yet to form a real opinion.
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Old 04-22-2015, 11:08 AM   #5
motobene
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Two vents versus one? That made me think about what is the purpose of float bowl venting?

- Venting fuel vaporization pressure, so that pressure does not resist the resupply of fuel into the float bowl

I can think of no other purpose. Typically the exits high up on carb bodies are true vents. That is, simply holes into the top of the float bowl cavity. It's one cavity, so only one vent is needed. The third 'vent' coming out the flow bowl itself is often a high fuel level overflow.

Most of the other things about venting are negatives:

- Atmospheric exchange, allowing the fuel to go 'stale', in which case more venting is not better

- Pissing fuel out when you lay a bike over

- Siphoning fuel out (some designs and some conditions only)

Another vent I would think does not help regarding a sloshing/enriching response when the bike is suddenly put on a steep angle.

Every carb I've known will go a bit richer on steep drop-offs... some worse than others. A high float level can make the problem go from a minor annoyance to over enriching such that the motor will die.

A helpful trick is to open or blip the throttle when suddenly pointing steeply down, to quickly process the excess fuel from slosh. I have generally had little enriching problems from my Keihin in these conditions. My dying problems in the past were from too lean on bottom, to advanced timing, and not enough flywheel weight.
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Old 04-22-2015, 02:34 PM   #6
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The funny thing Bene, is the PWK`s on the moto X bikes use 4 vents due to fuel glogging the vents and stopping flow. So after I get more time riding, we will see.
http://www.keihin-na.com/aftermarket/pwk/
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Old 04-23-2015, 05:13 AM   #7
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Quote:
Originally Posted by lineaway View Post
The funny thing Bene, is the PWK`s on the moto X bikes use 4 vents due to fuel glogging the vents and stopping flow. So after I get more time riding, we will see.
http://www.keihin-na.com/aftermarket/pwk/
I have a Freeride with that carb, it has 2 vents out the top and a 3rd on the nipple that comes off the bowel. I (and a lot of other Freeride owners) have all types of weird stalling/jetting problems. I believe they are vent related. My other bikes have 5 vents total.
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Old 04-23-2015, 08:37 AM   #8
motobene
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Quote:
Originally Posted by lineaway View Post
The funny thing Bene, is the PWK`s on the moto X bikes use 4 vents due to fuel glogging the vents and stopping flow. So after I get more time riding, we will see.
http://www.keihin-na.com/aftermarket/pwk/
Interesting! Quad Vents to prevent post-jump bogging from fuel- plugged vents!

Two possibilities: The two added vents aren't just more of the same, but rather may go down into the float bowl area via tubes or some other trick (note their angle).

The two added vents are more of the same They tap into the same vent passage, but their up-angle provides a counter to siphon effect or blockage.

If it's the latter you are doing some good adding that second vent.
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Old 04-29-2015, 08:17 PM   #9
lineaway OP
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Been another week. So far no stalls at all. I give it the . No cost upgrade besides a few inches of vent hose.
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Old 05-01-2015, 07:40 PM   #10
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Sorry to tell you but this old news. Same thing on shercos.
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Old 05-01-2015, 07:42 PM   #11
lineaway OP
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Yeah, this has been around awhile but nobody does it. So have you done it on the Beta?
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Old 05-01-2015, 08:00 PM   #12
lineaway OP
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Kind of like jetting. Evert time I ask someone what jets they are running, I get the same response. I dunno. Simple, mark the jets with a permanent marker on the float bowl.
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Old 05-02-2015, 04:35 AM   #13
trailer Rails
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Quote:
Originally Posted by lineaway View Post
Kind of like jetting. Evert time I ask someone what jets they are running, I get the same response. I dunno. Simple, mark the jets with a permanent marker on the float bowl.
I have a big dry erase board in the garage that I track all that stuff on.


I was comparing my PWK for my Freeride vs the one on my KX100, the venting is much different. The KX carb is also jetted much more rich on the main.
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Old 05-03-2015, 06:55 PM   #14
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After two days of riding hard, not a single stall. My son had three fives today and none bike related. Well maybe his last one. Section eight he cleaned all day till his last ride. Under cut ledge, and he did not quite get enough lift. The rear shock blew during the event and is now a pogo stick.
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Old 05-04-2015, 09:28 AM   #15
motobene
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Quote:
Originally Posted by lineaway View Post
After two days of riding hard, not a single stall. My son had three fives today and none bike related. Well maybe his last one. Section eight he cleaned all day till his last ride. Under cut ledge, and he did not quite get enough lift. The rear shock blew during the event and is now a pogo stick.
What kind of shock?
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