Gas tank pressure

Discussion in 'Whizzer Motorized Bicycles' started by RdKryton, Mar 24, 2008.

  1. RdKryton

    RdKryton Active Member

    First off I did search for an answer but came up blank. I want to know if a Whizzer gas cap is vented. My tank builds a lot of pressure. Is this normal? I wonder if this is why I had a problem with another carb. Maybe it could not hold back the pressure and always flooded the engine. Any ideas?


  2. cory

    cory Guest

    Good question

    Thats a good question. My tank is from a new Whizzer kit and it does the same thing but havnt had a problem with the carb.:???:
  3. carb

    My stock 18 mm carb did that too. Changed to 26 mm no more prob. Don't know why??? maybe because they are restricted from factory & can't breath after modify????
  4. bill green

    bill green Member

    Hi all the whizzer fuel cap is eco happy ,It is Aone way vent valve it only sucks in so no vapers get out . So it does build pressure . About the 26mm carb has an external vent line that could be the difference. Hope this helps Bill Green Vancouver Whizzer
  5. billspirit

    billspirit Guest

    Last year in the heat of summer 90/100 deg, Both of my whizzers 06 /07 NE5's were having running problems like zapor lock, come back from a ride park them in the garage & they had a lot pressure built up both making squeeling noise from the cap, releve the pressure & ride again in the hot sun & they would start running bad, ride with cap loose & they ran OK. I removed the valves from the caps & drilled a 1/16 hole all the way through, never had another problem. I do not fill the tank all the way to the top, so I don't have leakage through hole. Worked for me! Bill B
  6. peter nap

    peter nap Member

    I've noticed the same thing with my 07.
  7. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    Yes to Billspirit's comments!
    The cap can cause the tank to develope a vacuum. Drill out valve on the bottom of cap, but be sure to remove all particles with an air hose. The reason some tanks don't vent correctly is due to the paint [or lack of] around the filler opening. Some tanks will leak if filled to the top for the same reason. At Portland , IN in 2005 my 1999 was only turning 65 MPH on the dyno until I loosened the cap and it shot up to 68 MPH.
    Have fun,
  8. HoughMade

    HoughMade Guest

    I am a tad confused- do y'all mean build a vacuum?

    I cannot understand why a fuel tank (with a carb, at least) would ever build pressure, that is, increase the pressure inside.

    I do understand why it would build a vacuum as the fuel is used and there is no air let in to equalize pressure.
  9. peter nap

    peter nap Member

    Actually, it can and does do both. Gas will vaporize inside the tank when sitting. Put some in a gas can, cover the vent and shake it. when you uncover the vent, you have pressure.

    I haven't seen that as a problem. The problem is when the gas won't flow enough to operate the bike at a given speed.
  10. sparky

    sparky Active Member

    I don't know anyone with a Whizzer, but I am curious about this building gas pressure concept.

    My gas cap apparently needs to have a hole or two in it. But my friend's GoPed is completely sealed, and when I take the cap off I can hear the pressure that built up. I thought this was due to the Walbro carb, but my friend claims that it's the tank that builds the pressure.

    Anybody know for sure what builds the pressure & why he doesn't need holes in his lid but I do?? Sorry if ya'll think this is a threadjack.
  11. HoughMade

    HoughMade Guest

    That, of course, makes sense- I was assuming they meant building pressure when operating.
  12. MoonKS

    MoonKS Member

    I've only had my Whizzer for a few days now - but I notice that when I open the gas cap it is like opening a soda bottle - I get a hiss of positive pressure and small burst of air. I like the idea of drilling a 1/16 hole - I will do that if I run into issues.

    I love this forum - I am learning so much! Thanks for the great info, all!



    Wichita, KS
  13. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    Hi, It is possible to have a vacumm, or the tank under pressure on a Whizzer. If the carburetor wants large quanities of gas [like my modified motors] then the tank can be in a vacumm [not good], and if it is a hot day and the tank is "shook" it can become pressurized. Simply alter the vent in the cap to correct either condition.
    Have fun,
  14. sparky

    sparky Active Member

  15. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    I would guess the sealed tank would work unless the demand for fuel is greater than the tank's ability to vent. Many gas caps are designed to allow air to flow in only one direction, however the holes are so small they often get "plugged up". My personal Whizzer test motors [using a 26MM carburetor] have required more gas than the cap was designed to allow, and on a Dyno run several years ago it made a difference of 3 MPH [65MPH to 68MPH]when the cap was loosened. The simple fix is to enlarge the vent hole in the cap or discard it.
    Have fun,
  16. HoughMade

    HoughMade Guest

    All I can say is that in a gravity fed system, probably any fuel system, the tank has to be vented in some way or a vacuum will be built up as the fuel is used. The vent does not have to be in the cap. but there has to be a vent somehow.
  17. sparky

    sparky Active Member

    The thing is that my friend's GoPed is completely sealed w/ no holes and it runs just fine... My friend claims that it doesn't need to be gravity fed because of this special patent-pending tank, but I thought it had to be the Walbro carb. Just trying to find out if I pick up one of these tanks, will I now be able to turn my 2-stroke upside down.

    Perhaps this would be a better question for a GoPed forum or the General Discussion.
  18. HoughMade

    HoughMade Guest

    The only thing I could think is if the tank it flexible or has a flexible bladder so it can deform as the fuel level goes down....but I just pulled that idead out of my,....I mean, the air.
  19. bill green

    bill green Member

    Hi Sparky Take a gas can put some gas in it and seal it up, now put it in the sun about for 15 minutes, then crack open the cap, This is what your tank is doing.The eco cap only sucks in (one way) but it meets gov. standards .on the other hand the float and fuel valve have a operating presure it works well within.presure numbers unknown to me .I have had to crack open cap on long rides or hot days.My best results were to drill the cap with very small drillbit on inner plastic only to make into A two way vented cap as per Quentions suggestion hope this helps Bill Green Vancouver Whizzer
  20. lobsterboyx

    lobsterboyx Guest

    i hate to necromance this thread, but i got a call from my girlfriend today saying that the whizzer was spewing gas all over the garage and she was worried about the water heater.

    the gas was dripping out of the carb drain, it was a hot day here in so cal, so i went directly to release the pressure from the cap and it seems to be holding tough now, i simply loosened the gas cap so it wouldent drain anymore.

    can anyone tell me exactly where to drill so i dont blow up my house?

    i dont want to mess up the pretty chrome cap either..