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Thread: Primer plus/minus?

  1. #1
    smcnutt's Avatar
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    Default Primer plus/minus?

    My brother and I have differing thoughts on using the primer before starting and I am looking for the wisdom from the group. Our primer feeds 3 cylinders with the 4th having the MAP guage connected. Both mags have the impulse coupler installed - it came this way. Never had any starting problems unless the battery wouldn't turn the prop. Prior to installing the Tempest plugs I've had issues with fouled plugs after taxi/during run up. I now lean a little bit before taxiing and with the change of plugs, that's no longer an issue.

    On cold starts I always do 3 pumps on the primer prior to cranking. If it's already warm I will typically do 1-2 pumps. Never noticed any downside and it starts with very little cranking - 1-2 blades.

    My brother almost never uses the primer. For some reason he doesn't like it. If the engine is hesitant to start he will pump the throttle while cranking.

    A) is there any downside to using the primer?
    B) what is the downside to pumping the throttle?

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  2. #2

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    Default Re: Primer plus/minus?

    I like the primer, gets the gas closer to the suck and always works well for me. If I could just figure out the recent new problem of why I need half throttle to start I would be happy LOL once it starts you back it down quick to idle, but will not start unless its 50% or more opened, once its running its normal and runs fine at 1/8 inch open, but you can crank it forever at 1/8 - 1/4 inch open and it will never start, it used to, but now it don't.... I wish planes could talk and tell me what was wrong
    Last edited by SMO22; 06-27-2018 at 08:59 AM.

  3. #3
    rocket's Avatar
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    Default Re: Primer plus/minus?

    Up here in the cold you need a primer. Around zero F it's 7 strokes and wait a good 45 seconds before pushing the starter button.

    Stroking the throttle gets raw fuel in there but you unload the idle circuit/emulsion tube every time the throttle is opened which defeats it's purpose of dileverying atomized fuel. You end up with raw fuel exploding in your exhaust: boom boom! It's a bigger problem in round engines with long induction systems.

    Rocket

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    JPerkins's Avatar
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    Default Re: Primer plus/minus?

    Also pumping the throttle doesn't really get the fuel into the engine. These are updraft carburetors and when you pump the throttle when the engine is not running, all that fuel runs into the airbox. It's the perfect setup for an air filter fire.


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  5. #5
    59pacer's Avatar
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    Default Re: Primer plus/minus?

    The last time that I used the primer pump was as a junior instructor in 1972. The CFI asked me to move his prized B model Aztec, so I hopped in, primed the left engine, and hit the starter. The engine kicked back and broke the starter housing. He was far from amused. His first words were "you used the primer, didn't you?". His next words were "you never use the primer. It will always kick back. Always pump the throttle". I've been emotionally crippled ever since, and pump the throttle to prime. There's always the chance of a 'backfire' lighting the fuel in the inlet, I realise, but I'm careful not to use more than a stroke or two.
    Our climate here generally isn't too demanding, so much priming isn't required anyway. I always felt that the primer introduces a stream of fuel which has to evaporate to be much use, whereas throttle pumping produces a spray which is far more reliable. I'll be interested in comments on this assumption.
    A few years back I had to use an extinguisher to put out an intake fire in a neighbour's O-200. He had pumped and pumped the throttle trying to get a start. All he got was a fire, a melted air filter, and a big burst of CO2 from my old extinguisher. That worked, so I didn't need to use foam or powder and ruin his whole day!

  6. #6
    Administrator Steve Pierce's Avatar
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    Default Re: Primer plus/minus?

    I am lazy, usually 2 to 3 half strokes of the throttle and hit the starter. I do use the primer when cold.

  7. #7
    Jim Hann's Avatar
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    Default Re: Primer plus/minus?

    I was under the assumption that the -32 carb does not have an accelerator pump, so pumping the throttle will do no good. The O-235 on the Piper Tomahawk was the same way, no accelerator pump, so it wouldn’t start easily. Renters would run the battery dead trying to start them without prime.

    Of the three short wings I fly, two of them will not start without four shots of prime when cold. If it’s really cold then it’s 5-6 shots. Short stroke primer, now if you want to talk the long stroke primer in a lot of Cessnas, that is a different story.


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    Administrator Steve Pierce's Avatar
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    Default Re: Primer plus/minus?

    The only carburetor I am aware of without the accelerator pump is the one on the O-235. O-290 and O-320s have the accelerator pump.

  9. #9
    smcnutt's Avatar
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    Default Re: Primer plus/minus?

    Sounds like their are more potential downsides to throttle pumping (airbox fire, fuel not direct into cylinders, etc) so other than being "lazy" ;-) why wouldn't you use the primer if it's there? I guess someone did mention an airplane that ALWAYS backfired and busted the starter with primer use.
    Maybe it's just one of those some do and some don't individual choices. Like slipping with flaps.

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    “Common sense is in spite of, not the result of, education.”
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  10. #10
    Frank Green's Avatar
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    Default Re: Primer plus/minus?

    cold engine, 2 shots primer, throttle in 1/4", both mags, usually 2 blades and 1,000 rpm instantly, really cold 3 shots, warm engine 1/4", both mags, hit starter.
    -Super Stub-

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