viper carb venting?

kart74

kart74

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OK i have heard conflicting stories on how to vent the carbs. I am running aeen pipes and they told me to vent the carbs to the atmosphere and not the airbox. They said that if they are vented to the airbox the vacume presure would not allow you to get rich enough. Now i was just talking to bender about some things and they said to vent them to the airbox, and i will have no problems. Any one have any thoughts on this? If i vent outside the airbox my main jet will be 150-152ish. If i vent to the airbox they will be around 165...
 

I would do what YOUR pipe manufacturer reccomends Aaen
there should be lots of post if you do a search?
 
venting to the airbox provides better low speed running as the pressure is controlled by then engine, it is true that you will need a larger main jet but its of no real step backwards. Venting outside the box will make the sled richer on topend, lower bowl pressure but will also lower the throttle /low speed response, you can use a smaller main jet but it wont make any real differance in mileage. The fuel mileage comes from the pilot/needle settings, thats where you spend 90% of the time trail riding, not on your main jets. Clutching alos plays a big role in fuel mpg, a slipping,heat building clutch set up requires more throttle(more pilot/needle) to maintain the same cruising/trail speed then a clutch set up that has less belt slip.
 
So do you think i should just vent to the airbox and run the 165 mains? I am going to run a 50 pilot and was going to leave the needles stock.
 
i would suggest you call Aaen and get his specs for each way, he built the pipes and has the specs to start you off correctly. Then try it out to see which one you like the best(vented vs nonvented). I doubt you can run stock needles on a viper pipe but i could be wrong, check with Aaen before trying that.
 
carb vent

In 2002 when I was a mechanic for a snocross racer running a viper with aaen pipes we changed to athmosphere venting, because after a jump with a hard landing we had a hesitation in throttle response. This because we get the vent lines full of gasoline and the venting didn´t work until it was sucked back to the carbs. We get this tip from an other yamaha racer, don´t now if he figured out this himself or get it somewhere. After a while we was suprised to hear about some piped viper burndowns, and our sled was running great no engine problems at all. In snowtech magazine december 2003 page 36 "Service & Tuning Notes" We found the answer article: Fuel flow on triple piped vipers.
In short:"Extra coolers help but the vacuum in the airbox make the fuel bowl pressure to low that larger main jets couldn´t flow enough fuel. This was info from Pat Hauck & Aaen performance, athmosphere venting removed this pressure limitation. Main jets dropped from around 160s to around 145s because fuel was no longer in short supply".
After that we had piped viper both for racing and trail riding and had very few burndowns, not more problems than a average snowmobiler.
 
Yep thats what Aaen told me. I think i wil stick with them vent outside the airbox. Now i just have to get the jetting right. I ran 145's in the summer with the lines vented outside the airbox. I think i am going to start at a 150 and go from there.
 
I know my hauck pipes are 147.5 pto and center while the mag side is 150. You could try this setup. Oh yeah, my carbs are no longer vented to the airbox.
 
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so is it poss that if you run air box vented and do nothing except unplug the vent it will act as if you went up in maine jet size?
 
:jack:

There have been a lot of post about this recently and I was going to start my own, but since the two of the options have already been discussed here I thought I would add the third.

On my 97 Polaris XLT the carbs vents run to fillings on the air box, but the fittings are actually plugs. I've even unscrewed them and you can not blow through them and these are no visible holes.

Just wondering if a carb expert could explain the difference between the three: Vent to air box, vent underhood, vent plugged???
 
I don't know of a situation where plugged vent holes would be good or wanted, but vent lines that are unplugged and just hanging there makes the float bowls vented to the atmosphere. When vent lines are vented to the airbox, it creates a vaccum on the float bowls, which holds back some of the fuel flow, which in turn leans the engine out a bit.
 
ttt, anyone have a comment on plugging the vent line? Anyone heard of it, if not I'll take pictures to prove.
 
there would have been other venting for that polaris as you can not plug the vent to a carb. if there is no venting, there is no atmosphere pushing on the gas in the bowl.
this is. if there is no pressure there, the gas wont come out of the jets.
Keep this idea in mind when dealing with any liquid or air( nothing is ever really sucked, it is only pushed to a place of less pressure)

if you have your viper jetted correctly at say, 155mains, unplug the venting, you have just changed your mains to about 10numbers too rich on the top.

also, carb venting to the box only effects from about 1/2 throttle up. if you unplugged the vent( with temp guages) then tried to ride the sled, you dont see any difference at idle and at 1/4 throttle, as you near the half mark the temps start falling and the sled starts running rich. at wot, the sled is a dog, and the temps are way low because your now dumping raw fuel in the exhaust. its that big a difference.

vent either way is fine, just pick one
 
^^^ Sweet. That's exactly the description I wanted. I know exactly what you mean about running the sled after disconnecting the vents from the air box. I've forgotten to reconnect the lines on the Polaris a few times. I'd come out the garage standing up so I could see if there are any cars coming. Hit the ditch on the other side of the road and crack the throttle open. Just about did a head stand on the hood from the deceleration as it fell flat on it's face from too much fuel.

Actually I used to use that to my advantage. I jetted to run best between 0C and -15C. On colder days (say -25) I'd just disconnect the vent line and I could run wide open to my hearts content. Disconnecting the vents changed the jetting so much it would actually have a rich bog just past 3/4 throttle, but it would recover immediately.

Thanks for the info. Appreciate it.

Actually one last question, how much richer does turning the carb heaters on make it? Was thinking about using that on the Viper for some added insurance on the colder days. Would it be equal to a size or two on the main?
 
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it does nothing. that is there to keep slides from icing up in extreem cold and dusty conditions. running the heaters only warms a small section of the carbs on the back side. it is connected to the carb body by a small edge on each side. there is so much air passing through this area that the heat conducted to the bowl area is near nothing.
if its cold, run them. if your jetting your sled, block both sides off. less mess. i dont know why they bothered to make them be able to turn off.
 
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