venting carbs to atmosphere?

redsnake3

redsnake3

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when venting to atmosphere what is the advantages/disadvantages? why would someone want to do this? how much of a change in jetting is there usually? what about the needles?
 

I've never heard of this needing to be done except for on the Vmax4 due to a design flaw so to speak. But on the Vmax 4's application it was said that underhood pressure would build up at high speeds causing pressurization of the vent lines which would cause a rich condition at high speed and reduced top end speed. The "cure" for this was to route the vent tubes to a location of neutral atmospheric pressure such as the handlebars to allow the vent tubes to operate properly.
 
On a SRX I have vented them to the airbox before. I had to richen the mid and top. When they are vented in the belly pan underhood pressure pressurizes the float bowls.The sled is jetted to comprimise this. But what happens if you are going thru the powder wot and their is no underhood pressure? Lean condition! And if you are jetted on the edge already... well you no what happens.
 
Red you are talking on a viper right? You would fatten it up a bunch, due to no preasure on the carbs. You would have to redo all jetting, pilots, mains needles. I'm not going to do it. Maxdlx
 
Max, i was thinkin of doin this to run smaller jets but im not really seeing any reason to do it. what would doing this help?? i dont see any reason to do it if there is nothing positive about it.
 
The reasoning is at high speed on modded motors the carbs get preasurized and can't flow enough fuel. even if you have 170 in the carb it will only flow about the same as a 160. i am thinking of calling aaens tech and talking to them about it. I did some reaserch last night, and am now leaning that way. I am keeping the vari flow, because it will work on a piped viper. Paul
 
Stay vented

I think its the other way around guys. When I changed over to bored carbs and big bore my main jets went down. My mains went from 165/167.5 down to 153.6 today. At the same time I vented to atomsphere. The vacumn line to the airbox makes the carbs run leaner. Red stay vented to airbox unless you bore your carbs. Also notice if you unplug your vent line to your airbox the Viper will run pig rich and won't burn the fuel. You should remember trying to concerve fuel is not the goal with the modded Viper. If your touring and need more fuel get a tour buddy. Everyone who tries jetting back for mileage blows up. Maxdlx be very careful with that Variflow. Leaning back a piped Viper is dangerous. Variflow = Timebomb
 
You get more float bowl preasure on a viper vented to the airbox, because of the forced air induction. So at higher speeds it slows down fuel delivery due to the forced air. It says right in aaens instructions, if you run v force reeds or runn alot of long high speed runs to unhook the airlines for this very reason. I would think they have ran them enough to know what they are talking about. the vary flow on a piped viper is safe IF you know what you are doing, and watch you egt's. Now the tempa flow is a different story. Maxdlx
 
I think the variflow/tempaflow is a VERY GOOD IDEA,, Providing you know EXACTLY what your doing and you have SOME SORT of EXHAUST GAS TEMPERATURE PROBES to let you know when your motor is MELTING DOWN!!!! Fuel is WAY TOO INCONSISTENT to be assuming that a tank from BP will run the same as a tank from MOBIL!!!! For MOST GUYS,,LIKE MAC sAID,,, it's like a timebomb!!!! Heres the scoop on venting to the airbox vs venting to under hood.. The CARBS are designed to work at a certain pressure. (barometric pressure) the FLOAT BOWLS are designed to work at the SAME PRESSURE as the carbs.. if you ADD PRESSURE (by going 100mph) to the float bowls,, THE FUEL WILL RUN THROUGH the main jets FASTER than being vented through the airbox... I DO NOT KNOW if there is a POSITIVE or NEGATIVE pressure created by going 100MPH under the hood.. I WOULD THINK A NEGATIVE PRESSURE because people are saying that you have to jet FATTER!!!! A HOOD SCOOP on a race car works because you are creating a POSITIVE pressure by FORCE FEEDING air through a hood scoop,,, thus creating a LEANER condition!!! You have to fatten up the mainjets with a scoop!!! (in theory)... IN ALL REALITY,,, KEEPING the vents IN THE AIRBOX will keep your air fuel mixture MORE consistent than venting to the handlebars or the engine compartment.. REMEMBER,, UNLESS you have a BUNCH of BAROMETRIC pressure sensors ALL OVER THE PLACE, and a wind tunnel,, you really have to listen to guys who been there and done that!!!
 
Thanks nosboy, ive been reading alot of your posts and was hoping you would chime in. so from what im understanding about what mac is saying is that there is no real gain? other than with the bigger jets you basically cant fine tun e as well because of the lack of jet sizes that big. i have had the variflow on my piped viper since it was piped and have had no problems from it. i have no egts but am steadily checking the piston wash and plugs, i run in real cold temps and have the baseline jetted for about -45 celsius and i just use the variflow to lean out in colder temps by checking and seeing how the engine is reacting in that situation. oh also i have vforce reeds and was wondering if there is any power gains if i do vent to atmosphere. Max if you find a set up for the variflow with venting to atmosphere let me know how.
 
What about if your Scoops are closed off since all the Mountain vipers scoops are sealed off by plastic. I was told TO vent to the engine compartment to try and get rid of the DCS. I was told Also that it was making my engine running Lean from the high speeds. I was thinkin about removing the Vari-flow all togeather, i am still not sure wheather i want to or not. I am what you say a Newb at the thing, i really never dealt with it much just knows I can Lean it out or richen it up from the Weather changes. But it really dont seem to work, Maybe because i have Real fat jetting.
i also think the vari-flow is for Fine tune jetting.

I hope to figure out this DCS problam with my sled, i am going to be calling CPR or Bender to see if they have an idea cuz im SURE im not the only one Asking about this. im sure they have ALOT of people call in about this problam
 
This thread is all mixxed up compairing Apples and Oranges

As I read this thread I can see many points of view. Many correct principals and others contradicting one another. The original question asked was weather or not to vent to the air box or to vent to atmosphere? Lets break this down and decide if you like Yamahas ideas for proper carb setup. First lets start with Yamahas intended reason for the "Forced Air Induction". This was Yamahas answer to create a leaner carb setting at idle thru mid-range then becoming richer at higher speed and ultimately richer jetting with increased speed. It works like this. The float bowls have vacuum inside the bowls caused by negative pressure from inside the air box. That hose connected to the air box is sucking on the carbs. Then as the machine speeds up the Forced air concept come into play. The pressure under the hood begins to rise to become positive pressure negating the effects of the airbox vacuum in the float bowls. We have started with negative vacuum in the float bowls moving slow to becoming positive pressure at higher speed making our jetting system richer. Those Japs are so cleaver. Other points of view are Variflow and Temp a blow two systems that lean fuel out our fuel supplies. I personally don't believe they belong on our piped vipers. But who am I to argue I won't debate this topic. Finally you must decide weather you believe the sled needs to be leaned out at idle and slower speeds then richened up at high speed. I personally like the idea of jetted to atmosphere with no fuel compensating. Did Yamaha make a mistake with all this vacuum and pressure theory? No of coarse not they didnt know what we had in store for the Viper.
 
Now this kinda goes with another thread I posted on. Had to do with heat under the hood and pulling that heat into your airbox. Well I asked what if you blocked off the intake vents to your airbox that are under the hood to elminate the hot air intake, and install a cold air intake system so your pulling in COLD FRESH AIR from the top of the sled. BTV answered back by saying you can do that BUT besure to route carb vent lines to the OUTSIDE. He said it would eaiser to get your jetting correct, but never said anything about leaning out on all the jets. But I didnt ask and I'm not taking any chances so I'm keeping my jetting where its at.
 
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Ive done this on a 780 piped viper (venting to atmosphere) and its recommended by hauck to prevent a lean condition in the high RPM/MPH. Its good for about 4 jet sizes example 165's to 155's I dont think its the forced air induction as much as the neg pressure in the air box due to the carbs flowing air at high RPMs
 
So by venting to the atmosphere you can POSSIBLY lean out on your main????? And nothing else right???? I'm not saying I'm gunna do this but once I get mine tuned in I may try blocking the vents to the airbox and pulling all my air from the top of the dash, cause I aleady have cold air system installed.

ExtremeViper: You actually drop 4 jets sizes by doing this on your 780 Vip????
 
Mac in post number 15 said it ALL!!

You also will gain NO fuel mileage doing this, 90 percent of your gas mileage comes from the inherrant large pilot jet and rich needle settings, the PIPE DESIGN of the aftermarket pipes require the large pilot jet and when your trail riding this is where your spending most of your time, not wide open on the main jet only. Its the on and off the gas thru the trails, into the turns etc. thats pumping the fuel thru the motor.Your only running in the 1/4-5/8 throttle area 90 percent of the time trail riding.

Clutching and gas mpg go hand in hand but no matter how good of a clutch tuner you are with a piped viper, you will not see much better then 9-10mpg having everything perfect for the conditions, if you wanna play your gonna pay!!
 
Yes droped 4 jet sizes by unplugging the vent lines went from a 165 to 155 main. This is on a ported 780 hauck w/reeds, vemon heads, pipes, stock air box, the works. remember all builders are different. if you unplug the lines from the air box it will be richer.
 
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