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airbox fresh air mod?

Old 08-20-2010, 06:43 PM
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airbox fresh air mod?

im sure this has been done by someone, but ive never seen it mentioned here so ill ask. while doing some carb work today, we started to talk about the vents that come from the motor front and back and vent into the air box. essentially feeding hot air into the air box. is there any drawback to removing the junk from the air box and having them vent to atmosphere? or even drilling a small hole in the side of the air box and shoving a pipe in there and into that little expansion chamber and having it vent out through there?
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Old 08-20-2010, 07:55 PM
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It's an engineered, tested and calibrated system.

Everything there, is there for a reason. Anytihng you do to change it is a guess, without sufficient information to make an educated one. If you want to change it and test it, go ahead. If you want your bike to run right without headaches, spend your time and money on suspension.

Air vents through the front of the fairing and provides a constant supply of cool air to the intake.
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Old 08-20-2010, 08:33 PM
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Originally Posted by RCVTR View Post
It's an engineered, tested and calibrated system.
Everything there, is there for a reason.

To a point, you're correct.
It's against the law to sell a vehicle with open crankcase venting in nearly all countries, I would imagine.

The crankcase venting can be done to atmosphere with no ill effects (other than maybe some oily residue accumulating near the outlets).

Run a couple of hoses to the rear of the bike, with some sort of filter on the ends (remember, the air flow pulses in and out with piston movement).


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Old 08-20-2010, 10:39 PM
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There are also some advantages to be gained by positive crankcase ventilation. The biggest one is that due to the slight decrease in pressure in the crankcase the rings seal better.

So all in all I believe that there real is nothing to be gained, except an oily bike, by running these lines outside of the airbox. But then again YMMV.
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Old 08-21-2010, 12:09 AM
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As long as you put a filter on them, try it... I doubt thee are any measurable gains though...
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Old 08-21-2010, 08:07 AM
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I've seen an RC51 race engine that had crankcase vents on both valve covers, routed to the airbox. This was presumably done to drop the crankcase pressure lowoer, for decreased windage.

As I understand it, the most important thing about the cool air feed through the front of the fairing, the plastic box below the carbs and location of the intake snorkel is to be sure that the float bowls and the intake see the same perssure, regardless of vehicle speed. The metering of fuel is calibrated, depending on the pressure difference between the carb venturi and the float. If you modify this system, the calibration will change and you will likely lose the independance on vehicle speed.
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Old 08-22-2010, 07:23 PM
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yeah seems like its more trouble than it may be worth, ill just leave it alone. thanks for the info guys
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Old 08-22-2010, 07:49 PM
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run the lines to a reservoir, than a hose to a venturi in the open air stream. HA, PCV overdrive.
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Old 08-22-2010, 08:45 PM
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Originally Posted by autoteach View Post
run the lines to a reservoir, than a hose to a venturi in the open air stream. HA, PCV overdrive.
my original plan was just to open a small hole in the side of the air box and instead of venting that chamber into the air box i was gonna vent it to atmosphere
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Old 08-22-2010, 09:22 PM
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Or like some drag racers and street rodders, run the crankcase vent to the exhaust...with a check valve, of course. Don't want your engine to go boom.

Just kidding.
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Old 08-22-2010, 09:28 PM
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Seriously, you have to have vacuum or some kind of air flow to pull the blow-by gases out of the crankcase. Otherwise the pistons have to pump it out on their downstroke, like a 1941 Chevy with a road draft tube. And of course the incoming air has to be filtered to prevent contaminating the oil.

I don't really see an upside to messing with the stock system.
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Old 08-22-2010, 09:33 PM
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there isnt a vacuum on most cars, they just have a valve that takes the positive pulses and shut on the negative pulses of the crankcase.
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Old 08-22-2010, 10:17 PM
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Originally Posted by autoteach View Post
there isnt a vacuum on most cars, they just have a valve that takes the positive pulses and shut on the negative pulses of the crankcase.
The Type 4 PCV system, which has been used on most cars in the US since 1964, has a PCV valve controlled by intake manifold vacuum, and a fresh air tube which is hooked up to the air intake tube on most PFI cars, or to the air cleaner housing on most TBI or carbureted engines.

What our bikes have is a Type 3 system = fresh air tube from crankcase to air cleaner (air box). Same as the old air cooled VW's.
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Old 08-23-2010, 08:26 AM
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Here's something that works, without messing with your PCV.

Route the float vent lines downward, instead of to the sides. It prevents the float pressure from dropping at high speed, due to the air velocity around the motorcycle.

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Old 08-23-2010, 08:28 AM
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^thanks for the tip!
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Old 08-23-2010, 08:30 AM
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Originally Posted by RCVTR View Post
Here's something that works, without messing with your PCV.

Route the float vent lines downward, instead of to the sides. It prevents the float pressure from dropping at high speed, due to the air velocity around the motorcycle.

It also keeps you from having that embarrassing red light stall out when the lines are clogged with waste fuel... So it's a good idea on many levels...
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Old 08-23-2010, 08:53 AM
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Originally Posted by RCVTR View Post
RCVTR, Do you use the yogurt as a gasket sealant or as a lubricant?
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Old 08-23-2010, 09:12 AM
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It prevents saddle sores.
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Old 08-25-2010, 05:07 PM
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i like that vent idea, when i dig into the carbs again on friday ill have to try that out. thanks
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