recommendations for pressing engine primer bulb

Discussion in 'The Projects Forum' started by killit2, Nov 8, 2013.

  1. killit2

    Thread Starter New Member

    May 2, 2012
    Working on starting / stopping a small engine with an electric starter from MCU. Everything is falling into place but I can't think of a good way to prime the engine. I measured the pressure required @3lbs max over about an inch but I would like to overrate that by 2x just to be sure and prevent overheating / etc. I thought about servos but that would require a goofy linkage, push solenoids don't seem to have enough strength at 12v (at least not ecenomicly), and linear actuators are just too large to be hanging 6" or more off the side of the engine.

    Am I missing something? Any ideas? I don't want to permanently modify the carb or bowl BTW.

  2. Sensacell

    Well-Known Member

    Jun 19, 2012
    There are some really beefy RC servos out there, look for sail winch type for model boats, for example.
  3. Experimentonomen


    Feb 16, 2011
    Why make a lever system to push the primer bulb when you could find a small diaphragm pump to use for priming.
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    solenoid with some levers/mechanical advantage..
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
    This is an electric start engine, The primer bulb is generally just an aid when pull starting. The electric starter turns the engine over longer and faster than a pull cord, so the primer is not really necessary. The primer puts a small amount of raw gas into the intake track to start faster. The choke is what helps when starting with a electric start.

    The primer bulb is a very recent addition to small engines, added to help pull start engines. I'd be willing to bet the engine also has a pull starter for when the battery is dead. How many riding mowers do you see with a primer bulb?
  6. BReeves


    Nov 24, 2012
    How about using starting fluid. Rig up something on a starter fluid can to squirt a small amount in the intake. I have a riding mower that will sometimes loose prime to the fuel pump. Use to blow air into the fuel tank to prime it now, I just give it a shot of starting fluid.
  7. wayneh


    Sep 9, 2010
    I'm with shortbus - I don't think you need the primer.

    But if I was to rig one up, I think I'd use an automotive fuel injector, which is nothing more than a little 12V solenoid valve. It is not a pumping solenoid, and relies on a supply pressure. It wouldn't take much head pressure from gravity to dribble in a little gas, as long as the gas tank level is above the carburetor, which it almost certainly is.
  8. killit2

    Thread Starter New Member

    May 2, 2012
    Unfortunately this does need to be primed.. Wondering if there was any truth to what you said about the electric aspect of it, I went out and tried using the electric start for quite a bit longer than I normally would and she wouldn't kick without hitting the primer bulb.

    I like the idea of the automotive injector - I may have to do some poking around and see what I can find. Although I really don't want to modify the carb (which includes the primer on its side) directly - still want it all there so I can do it manually as well.

    Question for you all though - they rate servors in kg or N and such, which is fine, but how accurate is this? Is this like best case scenario, max, or minimum? I was thinking perhaps just using a servo with an L shaped arm on it that would rotate and press the button but I need to calculate the required torque over that arm (easy enough) and ensure I get a servo overrated for this.

    Do they have any which provide the output from the pot for external reading?

    Lastly, how do hobby type servos tend to do over the long term in high vibration environments (mounted directly to the side of the engine)?

  9. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    - choke
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
    OK, if your say it needs the primer(never seen it myself), instead of the primer bulb, run a small tube, like the one from the primer bulb, to the crankcase. Well above the oil level. This will give a pulse with every stroke of the piston, a small solenoid valve in the line could be used to shut the pulses off after it starts.

    In the early days of diaphragm carbs this crankcase connection was how they worked. Then they figured out how to do it with a small weight on the diaphragm and the engines vibration, using a primer bulb for starting.