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Fuel evaporation rate and vented tanks?

Discussion in 'The Hokey Ass Message Board' started by Mike51Merc, Sep 6, 2012.

  1. lostforawhile
    Joined: Mar 23, 2008
    Posts: 4,160


    with the airport there,it's probably popular with the guys who have STCs to run mogas in their planes, they can't use corn gas
  2. lostforawhile
    Joined: Mar 23, 2008
    Posts: 4,160


    it's very easy, get a simple one from the eighties, one connection goes to the tank, where it absorbs fuel vapors, you still need the modern tank set up for that, if you use one from a fuel injected car, you won't have the connection to the carb, it's hard to vent the carbs anyway, you are mainly venting the tank, you will have a purge diaphragm which usually Ts into your PCV valve, it will usually say PCV, and controls the purge through that vacuum signal, and you will have a line that says purge and goes to the intake manifold, no electronics needed.
  3. CutawayAl
    Joined: Aug 3, 2009
    Posts: 2,144

    from MI

    I don't remember, but I have seen a lot of them. Even motorcycles have them. Most recently I found that my Mazda Miata V-8 engine swap project car has a cap like that. This design is so common that in some states part of the emissions test is checking the cap with a pressure tester to confirm the cap isn't leaking and the valve functions properly. I haven't been able to fine a chart of what pressure the cap valves are supposed to open. Manufacturers have used these kinds of caps on carbureted cars. If you have a good needle and seat the tank pressure wouldn't be a problem. If you did have a leaky carb needle, some fuel pumps can't hold back any pressure, while others require a little pressure to open the check valve(s).
  4. Dapostman
    Joined: Apr 24, 2011
    Posts: 294


    The vapor pressure of gasoline is about 3 psi, almost any tank should be able to take that.
  5. squirrel
    Joined: Sep 23, 2004
    Posts: 46,111


    Be careful about pressurizing older tanks...for exmaple, the tank on a 55-57 chevy has the fill tube sealed to the tank with an O ring, and no secure mechanical connection. If the tank pressure goes too high, it could possibly pop the filler neck out...that would be bad news

    Just make sure the tank system can support the pressure.
  6. Keep
    Joined: May 10, 2008
    Posts: 662


    Couple of things to consider.

    Newer motorcycle canisters are small and would be easily fit anywhere on a hotrod, might even be able to pull the rest of the system for use.

    If you seal the tank with a non-vented cap and do not have any sort of pressure release, the fuel will just push past the carb and into the intake until the pressure releases. This usually only happens on very hot days in my bucket, but having an exposed black gas tank has shown me that when it gets hot, the pressure build pretty quick.

    As for sealing the carb, I would only consider doing this for storage, but some rubber vacuum caps would fit over the vents pretty nice. Should keep things sealed up for the winter.
  7. lostforawhile
    Joined: Mar 23, 2008
    Posts: 4,160


    most modern fuel caps will also release pressure if it goes too high it's an emergency vent, as far as the O ring, lots of Ford products have the big rubber ring sealing the filler neck to the tank, it works fine. if the pressure gets too high the excess pressure would bleed off until it could seal again
  8. lostforawhile
    Joined: Mar 23, 2008
    Posts: 4,160


    old jap car canisters for FI , three hoses,very simple, you might need to install a pcv system,but thats a good idea anyway as it makes a wold of difference in preventing sludge
  9. 48ford
    Joined: Dec 15, 2001
    Posts: 431


    I also have a boat,
    and on thw water the gas is all gas not a mixture with alcohol.
    The tanks all vented outside of the boat .
    but It looks like the gas stays on full,slow evaperation.
    so I think that the mixture of pump gas is a lot of the problem.
    but now you guys got me thinking,this looks like a good thing if we can make it work
  10. What happened , get scared ?
  11. Bad Banana
    Joined: Jun 20, 2008
    Posts: 794

    Bad Banana
    from Texas

    Who you talking to? :confused:

    It is winter here in Michigan (or damn close) and I thought I said earlier in the thread I probably would not be doing any real work on this until next spring. Maybe one of the guys that live where it is eternal hot rodding weather year round would try some stuff. My Olds is in storage and I made a temporary gas cap seal for the winter. :cool:

    I have other irons in the fire right now but I am still looking at various vehicles to possibly use parts from in my travels over the winter. Just not high on the priority list right now and I assume it is the same with most everyone else who was interested in this thread.

    I will probably get back on it in late winter/early spring.
  12. pitman
    Joined: May 14, 2006
    Posts: 4,918


    C'mon Vicky, maybe he "met with misfortune..."
    Static spark at the boat dock! :eek:
    Not my favorite way to start the day.
  13. tbill
    Joined: Oct 21, 2007
    Posts: 303

    from central ny

    how fast does it evaporate? new car fuel tanks are still vented, thru the evap system, [vent valve normally open, vent valve hooked to charcoal can, which in turn is hooked to tank at top of sending unit/pump assm.], so do your newer daily drivers lose as much fuel? granted, float bowl evaporation is going to be way more noticeable,[from a driveability aspect] but it would seem loss from the fuel tank should be equal no matter what year vehicle we talk about as they are vented in one way or another. as mentioned, only thing killing the smell is the evap can.

    or am i overlooking something on this?
  14. garcoal
    Joined: Nov 15, 2006
    Posts: 277


    well im going to have to eat crow on this one, i know i threw the bullshit flag and everything. this fall i is was getting two small motorcycles for a ride ready. neither would start good or run right. after lengthy tuneups,valve adjustments you name it. i checked the gas tanks. i know i NEVER leave a gas tank empty. you know no matter how knive edge your tuning skills are they still need gas. after a quick splash of gas they were both starting, idling and running happily. so gentleman i admit we seem to have a evaporation issue here

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