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The EvaporATive Emissions System Exposed!!!

The EvaporATive Emissions System Exposed!!!

Welcome to this week’s tech talk at Venom Motor Sports!

In this edition of “Tim’s Tech Corner” we are going to chat about Evaporative Emission Systems and how they function. Please take a minute to also watch the “youtube” video below on the x22 EVAP system. The video will help with your understanding of how the x22 EVAP system operates and what to check for if you are having problems with engine stall at idle.

Ok, onto the EVAP system.

Tim...I drive my bike for about 15 minutes, the bike works perfectly. Then I come to a stop sign and the bike stalls out. What’s worse is it will not start up again! Then I push the bike all the way home and try it again in my driveway and bamm! it starts up right away. What the heck is going on here?

Well for starters I have seen this problem a few times on bikes with a defective gas tank cap. Back in the day all of the caps had holes in them to vent the tank directly to atmosphere. This meant that as the fuel level in the tank dropped air would rush in the void and always ensure that full atmospheric pressure was available to act on the fuel and ensure positive fuel flow to the carburetor. If the vent was plugged the tank would draw down into a vacuum and reduce fuel flow and produce a stall. The cure was really simply...open the gas tank cap...get the bike home and by a new cap.

The other important function of the vent cap was to release excess pressure that would build up in the gas tank sitting out in the hot sun. The excessive pressure from gas evaporating in the sun would be simply vented again out the hole in the gas cap.

So the humble gas cap was used for years and years and is still in use in smaller displacement bikes. However, new EPA standards came about to develop a system to catch and deal with these hydrocarbon gas vapors instead of simply releasing them to the environment.

This is why EVAP systems for your bike were developed. They are very similar to EVAP systems found on automobiles.

Tim...How does an EVAP system work?

The above diagram give us a good idea of what the main components in an EVAP system look like. EVAP is the short form for “Evaporative Emission Control System”.

The system consists of three lines. One line goes to the gas vent on the gas tank. The second is a breather line that will vent liquid fuel to atmosphere should the tank be overfilled and also allows air under atmospheric pressure to enter into the canister and mix with the hydrocarbons in the charcoal filter. The third line goes to the purge valve which operates as a check valve allowing flow in one direction only...and then the line proceeds to the vacuum nipples on the carburetor body.

Under normal operation a vacuum is produced from the engine and this vacuum creates a “suction“ which acts on the purge “check valve” and causes the hydrocarbon vapours in the charcoal canister to be “sucked” into the carburetor and mixed with the air/fuel mixture being fed into the engine.

So the EVAP system prevents hydrocarbons from being discharged into the environment. It also solves two problems for us. The EVAP system regulates the pressure of the gas vapours inside the fuel tank so it does not get to low or two high.

The system is designed to keep the fuel tank under a very slight negative pressure. This means that when you open the gas cap you should hear a slight whistle sound as air rushes into the tank. That is normal. What is not normal is when the negative pressure is too high and reduces fuel flow.

Tim...What do you do to cure this funny stalling out problem?

Ok, so when the pressure in the fuel tank is too far below atmospheric pressure there is a problem in the EVAP system. Typically it is a very simple problem to cure.

You need to check on all the lines going to the black Charcoal Filter. Ensure that none of them are bent over, kinked over or....closed shut because of a super tight “zip tie”!

The over tightened “zip tie” is the often the problem here. When your bike was being built a factory technician used black or white plastic “zip ties” to hold the wiring and various lines in place against the frame. Usually wrapping the “zip tie” around the frame then around various components. Once in place the “zip tie” is pulled snug and the wiring and lines are held fast in place. The problem is if you pull on the “zip tie” to hard it will collapse a fuel or vacuum line. So what is the fix, check all of the fuel/vacuum lines for tight “zip ties”. Remove the tight zip tie and replace it with a new one that is just snug, not tight.

This will ensure that there is no line collapse and your fuel and vacuum lines will be fine.

Full unrestricted fuel and vacuum line will ensure proper flow and pressure in your fuel system and stop the embarrassing stalling at street corners.

If you have any tech questions about our vehicles, please feel free to email them to...


Check out our blog next week as we take an in depth look at the x21 EXPOSED!

Till then, Enjoy the ride!


Technical Support Specialist

Venom Motorsports Canada


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