venting carbs to the atmosphere?

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mcviper

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Hey just have a question about venting carbs to the atmosphere.I have seen this on a few threads now and don't understand how it works. Is it as easy as pulling the tube out of the airbox and pointing it down? Also how does this enable you to run smaller jets when using triple pipes? thanks
 
the problem is that the vent lines are hooked to the carb fuel bowls. And with pipes there is to much neg. presure in the air box. Which pulls fuel away from the jets so nomatter how big the jets are and the more load on the engine there is more vac. in the air box and the leaner the engine runs and thats when they go BOOM. bender set up calls for something like 162 jets and the things still go lean and are hard on fuel.
 
Thank you for the explanation. I do have benders, they also recommend opening up the air box. do you think is this required if the vent lines are vented to the atmosphere? We only had it out once (friday) and it was pretty mild so didn't ride to long or hard. They are on a 02 viper by the way. thanks
 
so should the vent lines be pulled off the airbox then and well this be good enough to fix the problem? i just piped my viper and i dont want to have a huge repair bill cus of this venting problem
 
venting to atmosphere does nothing other than being able to use smaller jets, it will still be the same on fuel so there is no real advantage.
 
The original question asked was whether or not to vent to the air box or to vent to atmosphere? Let’s break this down and decide if you like Yamahas ideas for proper carb setup. First let’s 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 carb bowls. 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 air box vacuum in the float bowls. Remember we have started with negative vacuum in the float bowls when the sled is moving slowly to becoming positive pressure when the sled is moving faster speed making our jetting system progressively richer. 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 whether 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. I control all jetting circuits on my sled. ---mac---
 
Hey Mac what kind of jetting are you running on your modded viper?? I presently have an 835 BB with Bender pipes, bored carbs and stingers removed from pipes.. Sled has added SRX rear heat exchanger.. I am looking into modifying the cooling system further as I still find the sled runs very warm. I have 170 mains in it now and still find it runs lean on very cold days.. Thinking of porting the cases and running 38's on it.. Sled is very fast but I'm a bit concerned on long WOT runs.
 
jabber:

if you haven't done so already take out the thermostat.
170's for that motor w/ bored carbs & pipes is border line. if you add tunnel heat exchangers you can run the leaner jetting - if not go up 1 or 2 sizes on the mains.

bob
 
Bob - If you remove the thermostat the coolant will pump to fast and not remove enough heat. Second problem will be that the sled won't warm up for a long time. The thermostat sends coolant the short cycle under the engine until the thermostat opens. Then the coolant takes the long way around through the coolers. Jabber I battled the temp issue since I modded my sled in 2003. I run a digital temp gauge after the head and now run 135-145 degrees no matter hard I beat this sled. Here is how I solved the temp issue.

http://www.totallyamaha.net/forums/showthread.php?t=37264&page=1&pp=10
 
mac:

i have done a ton of research & testing w/ cooling systems.
below is a direct quote from dynotech on this subject. you can read the complete article @ dynotech - blog - under mon. feb. 5th 2007. jim has since tested & proven this theory to be valid in subsequent testing.

"I subscribe to Cycle World magazine primarily to read Kevin Cameron's monthly one page TDC column, and to try to absorb detailed technical articles he writes that appear in about every other issue.

In the most recent issue (March?) Kevin addresses coolant velocity in cylinder heads. According to KC, turbulent high velocity coolant is vastly more effective in removing heat from combustion chamber "domes" than lazy slow moving coolant. Worse yet, is any area of stagnant coolant that rests against the domes' coolant side surfaces with slow or no movement which can create dangerous detonation causing hot-spots. "

the 835's had a heat removal issue @ times w/ stock cooling system & thermostats ( the h.p. exceeds the cooling system capacity for long hard pulls ). the problem was reduced by removing the tstat. i took it one step further & plumbed in tunnel heat exchangers to the existing system. my 835 usually runs around 90 - 100* F max. this has allowed me to run leaner jetting & lower BFSC on the dyno & make more h.p..
yes, you do have to let the motor warm up longer - but not a lot. sx's don't have thermostats & either did polaris's for a lot of years. they're not necessary in a 2 stroke.

bob
 
I have already added the SRX rear exchanger and removed the T stat.. I was thinking of blocking off the bypass coolant cct. under the motor and forcing all coolant through the rear exchangers.. I'm thinking I should also add a bit more cooling!! We do some long hard pulls and I'm pretty worried about this heat issue.. Add case porting and 38's and I'm betting things get even worse....
 
Bob - sounds good. I know in vehicles like cars and trucks the thermostat is essential. You can't run without it. Can't say I have tried running without a thermostat in a snowmobile. I'm going to test this idea myself and report my findings. With the digital gauge I should see instant results positive or negative. Anyone else thinking of testing this should remember you will also need to plug the short loop under the motor to prevent the coolant from taking the short cycle.
 
Mac you did bring up lots of good points and you are right . I don't want to debate this topic ether. But the thing is that the presures don't change fast enough and the engine goes lean before it gets a chance to rich on top end. you need to be going at least 100MPH for this to work. Thanks for explaining better for everyone.
 
I'm finding these numbers somewhat unbelievable. If you read the artical Bill Davis reports coolant temperatures of 100-110 maximum degrees. Other machines have a maximum temp of 80 degrees. I wash my hands in warmer water. As soon as we get some snow I will do some simple testing to confirm the removal of the thermostat on my 780. Before my radiator install I was 155-165 and never any cooler. After the radiator install 135-140 max. Keep in mind the thermostat was regulating this temp because the thermostat closes if the water becomes any cooler. Who knows maybe we can achieve less temp without this thermostat. For anyone thinking of bypassing the short loop I used to use a piece of wood from a broom handle about 3/4 of an inch long tapered and sanded down to make a plug. With the thermostat removed I could tap it into place with a mallet. Also don't forget to drill the hole. I used a 1/8 bypass hole worked perfect. A big thanks to the Buffalo ---mac---
 
So on the viper remove the t stat and plug the bypass loop under the engine, drill relief hole in plug to eliminate cavitation? Where is the best place to plug this line without having to pull everything apart??
 
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