Carb vent mod?

[XS1100_OEM4ME]

I was thinking (I get in more trouble that way) The older bikes (mid 70's and before) had carb overflow lines that went "strait down" and avoid the "carb fuel motor dump" alltoghther by running off any excess fuel strait to the ground instead of "up" to the top of the air box causing increased pressure / fuel overflow inside the cards, right into the motor I am sure the EPA headed off "global warming" by mandating the mod to keep gas from ever trikeling onto the earth So I am thinking, why not plug the air box holes and run the lines strait down with the battery overflow hose Wouldn't that negate even the possabilty of the "fuel in the motor" thing ever again Please don't sick the EPA on me

[bikerphil]

Not for sure, but I think the fuel comes out the air jet in the inlet bell of the carb and flows into the air box. The vent hose is a bit higher than all this and wouldn't come into play with a high bowl level. Maybe someone can confirm this?

[natemoen]

It actually comes out the main jet area first.

[XS1100_OEM4ME]

Vent hoeses

When I had my problems, I took the vent hoeses off of the air box and left them up besides the air box nipples, as per someones recomendation on here, and when the carbs flooded, gas came "up" and poored out of those hoeses

Not for sure, but I think the fuel comes out the air jet in the inlet bell of the carb and flows into the air box. The vent hose is a bit higher than all this and wouldn't come into play with a high bowl level. Maybe someone can confirm this?

[cywelchjr]

Gas may come out of those vents, but those vents don't vent to the crankcase directly, but will run out onto the ground. The gas that gets into the engine does so through the cylinders, by going FORWARD and into the combustion chamber and then running down through the rings into the crankcase. It comes up through the mains and idle jets, and of course it comes out the vents as well, but it's the mains and idle jets that get the gas into the engine, the vents actually do the job of getting it away from the engine, so that part isn't the problem. Even though with pods and the vents hanging free get gas in the crankcase if the carbs overflow, cause it's not the vents that cause it (other than clogged vents can cause them to overflow).

[XS1100_OEM4ME]

Vents

I do realize that the fuel that flows into the cyls does it from the front (intake) part of the carb. My point is, if there "were" a low level overflow vent, like on the older 70 & 60's carbs (the OEM may be too high up) the fuel would never get enough over the float controled hight to flood anything but your driveway

Gas may come out of those vents, but those vents don't vent to the crankcase directly, but will run out onto the ground. The gas that gets into the engine does so through the cylinders, by going FORWARD and into the combustion chamber and then running down through the rings into the crankcase. It comes up through the mains and idle jets, and of course it comes out the vents as well, but it's the mains and idle jets that get the gas into the engine, the vents actually do the job of getting it away from the engine, so that part isn't the problem. Even though with pods and the vents hanging free get gas in the crankcase if the carbs overflow, cause it's not the vents that cause it (other than clogged vents can cause them to overflow).

[cywelchjr]

I do realize that the fuel that flows into the cyls does it from the front (intake) part of the carb. My point is, if there "were" a low level overflow vent, like on the older 70 & 60's carbs (the OEM may be too high up) the fuel would never get enough over the float controled hight to flood anything but your driveway

The vents have NEVER been that low. They have to be higher than the jets, cause otherwise the gas would flow out the vents and the engine wouldn't run. The vents aren't for overflow, they are to vent fumes. Frankly the problem is because the carbs are tilted towards the engine rather than back away from it. If they were tilted away, then the gas would flow away rather than towards the engine. The fact that even where the vents are, they can have gas come out of them, says them being lower would make no difference. But, I think part of the reason the bike being on the side stand vs the center stand can make a difference between it going on the ground vs in the engine says something, cause at least on mine the carbs tilt a LOT more forward when the bike is on the center stand then when it's on the side stand, which is another reason to normally keep it on the side stand.

[XS1100_OEM4ME]

I am guessing!

I am guseeing my "point" is being overlooked On my 72 CB750 the bowls had vents tubes "just above the fuel level, which would not effect the "jets, or fuel flow of "any kind", unless, the fuel went "above" all of the carbs normal functions. Then, and only then, did the low set fuel vent "kick in" and save the motor They worked great (never changed my oil with gas in it then, never needed to)

The vents have NEVER been that low. They have to be higher than the jets, cause otherwise the gas would flow out the vents and the engine wouldn't run. The vents aren't for overflow, they are to vent fumes. Frankly the problem is because the carbs are tilted towards the engine rather than back away from it. If they were tilted away, then the gas would flow away rather than towards the engine. The fact that even where the vents are, they can have gas come out of them, says them being lower would make no difference. But, I think part of the reason the bike being on the side stand vs the center stand can make a difference between it going on the ground vs in the engine says something, cause at least on mine the carbs tilt a LOT more forward when the bike is on the center stand then when it's on the side stand, which is another reason to normally keep it on the side stand.

[GLoweVA]

radical thought

I just had an idea,
what if we could flip the carb boots over so that the vac ports (ones used for syncing) are on the bottom,
then hook up some simple ball inline check valves.
That way if the carbs did overflow, the gas wouldn't get past the carb boots because it would drain out the check valves. With the engine running, the check valves will suck in and seat.

You'd have to take the check valves out for syncing of course.

Just a radical idea.

[jwhughes3]

Seems like alot of trouble

When if its clean and adjusted it doesn't leak. We just have to keep them that way


Now I have had gummed carbs and overflow, but if I had been riding enough it would have been fine.

Maybe its more an issue up north cuz of winter.

John

[XS1100_OEM4ME]

I agree!

I agree, when everything is perfect, no problem. But, I have read on here so many times where everything wasn't working perfect and seen everything from , "darn, I have to change my oil again" to "damm, I need a new motor" It never happened on the older bikes I am talking about, because of the overflow safegard of a fuel bleed off system if the fuel went even slightly above the float controll hight Any here ever hear of a spun bearing on those early CB750's or any other early bike with that safegaurd because of gas in the oil

When if its clean and adjusted it doesn't leak. We just have to keep them that way


Now I have had gummed carbs and overflow, but if I had been riding enough it would have been fine.

Maybe its more an issue up north cuz of winter.

John

[bikerphil]

Bro, don't use the centerstand and if you do, block the front wheel up so the gas would run towards the air box and not the motor. Park it on the sidestand, problem solved. Works for me.

[XS1100_OEM4ME]

Side stand

Mine was on the side stand the 1st time I poored 1/2 a quart of gas into my motor Had just rebuilt the carbs (a while back) and turned on prim, went inside to get have a smoke and when I came back out (short time later) the thank was half empty and air box and motor were full of gas not to mention I could start a bonfire with the gas on the yard

Bro, don't use the centerstand and if you do, block the front wheel up so the gas would run towards the air box and not the motor. Park it on the sidestand, problem solved. Works for me.

[cywelchjr]

Mine was on the side stand the 1st time I poored 1/2 a quart of gas into my motor Had just rebuilt the carbs (a while back) and turned on prim, went inside to get have a smoke and when I came back out (short time later) the thank was half empty and air box and motor were full of gas not to mention I could start a bonfire with the gas on the yard

That's interesting. Mine the carbs tilt toward the back of the bike on the side stand and toward the front of the bike on the center stand. My XS400 exhibits the same behavior, and when I was having trouble with the carbs showed the same issues and behaviors.

[bikerphil]

Mine was on the side stand the 1st time I poored 1/2 a quart of gas into my motor Had just rebuilt the carbs (a while back) and turned on prim, went inside to get have a smoke and when I came back out (short time later) the thank was half empty and air box and motor were full of gas not to mention I could start a bonfire with the gas on the yard

Bet you won't do that again.