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What is the proper way to cold start an engine on your back?


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I've got an EOS 100 Booster with a pull start and I'm trying to get a handle on how you're supposed to cold start an engine on your back.

Because you cannot reach the choke lever with the motor on your back it appears that you have to take the motor on and off twice just to cold start and engine.

- press on the diaphragm of the carb and press the primer until fuel just reaches the inlet on the carb

- release the diaphragm and give the primer a few more presses to increase pressure in the fuel line

- press the diaphragm for half a second to squirt fuel into the intake

- engage the choke lever on the carb

 *** Is this where you put the paramotor on your back? ***

- with no throttle, pull start the engine while it's choked

*** With the choke engaged, is the engine supposed to be able to turn over and continue idling on its own, or is it simply supposed to sputter for half a second and then die? The whole idea of the choke is to get the engine running for a little while so it can warm up so the fuel can properly vaporize and burn, right? With the choke engaged, is there still the threat of a prop strike on your hands and limbs? ***

- take the paramotor off your back so you can then disengage the choke

- put the paramotor *back* on your back and pull start the engine while giving it a little bit of throttle to really start it


Is this correct?

For people who start their engines on their backs, do you put on the choke, put the motor on your back, pull start, take the motor off your back, release the choke, put the motor back on your back, and then start it up?

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  • 3 weeks later...
On 11/10/2019 at 01:24, cas_whitmore said:

Forget choke , get fuel up to carb with diaphragm pressed and one more squeeze for luck , put on your back and start pulling with a touch of throttle .


Doesn't work for me on my EOS 100 Booster. 

On a cold start without the choke first I will be pulling until my pull start rope breaks, and this is with a bit of throttle. 

Pulling with the choke doesn't actually seem to start the engine either. 

Without choke I can pull 50 times without it starting.

With choke I pull it 5 times without it starting, but then disengage the choke and within 5 more pulls it'll start.

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What Cas says is my exact routine and just to double check, I just went out to start my Polini Thor from cold and its a bit nippy up here in Scotland this late afternoon...

The motor has not been started in about a week or so and I pressed diaphragm, squeezed primer bulb (and one for luck) exactly as Cas said -- mine started first pull. Now,  mine is not always first pull -just so happens it was today - but its certainly within a couple or three..

I did have a situation a while back where it was giving me a bit of hassle with multiple pulls to get it started and occasionally flooding but that went away when I changed the plug to a new one. Immediately cured it actually ...

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Also -just to add, I have never needed to use the choke -  my motor is not new -(not sure who you were asking) its a very low hours used Thor...My previous motor -a PAP Ros 125 -absolute nightmare to start. Very high compression, even with the decompression valve pushed in. I could only start that motor on the ground. Tried a kick strap, bungee chord -I just got exhausted trying - -got so engrossed in trying to figure out how to start the motor on my back that I wasnt progressing in any other way...hence moving to a flash start motor...one arm starting over my shoulder....whats your motor Cas?

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Hi , it’s a eos 100 booster .

only time my booster motor gave me mind altering frustration with starting, was due to slack head stud , I noticed a hint of black residue at the head joint ,that was after the 3rd time I’d nipped them up as per the torque figure , this time they won’t relax loose anymore .


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On 28/10/2019 at 02:54, cas_whitmore said:

Had a flight today same start routine , good as gold just add that mine has no choke . 

Is yours a brand new motor.

I'm confused. I have the EOS 100 Booster as well but it has a choke and it only has 7 hours on it.

I depress the diaphragm on the carb and squeeze the bulb to get fuel just to the entrance of the carb.

I then release the diaphragm on the carb and squeeze the bulb until it's firm and then I leave the bulb alone.

I then press in on the diaphragm for half a second and the pressure built up in the primer bulb squirts fuel into the intake.

I put the motor on my back and then proceed to pull 50 times with a touch of throttle with no luck. 

Should I let fuel squirt into the intake for longer? I'm concerned I'll flood the engine.

Edited by fuzzybabybunny
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21 hours ago, cas_whitmore said:

Yes make sure it gets in to the motor , if you flood it don’t worry about it , motor is horizontal it’ll blow the excess out off the exhaust, eventually ! Can you check the head studs,  don’t assume there tight only take 10 mins , also what frame is it in .


I can check the head studs... But it's a new motor with only 7 hours. It should be tight, right?

It's on a SkyMax Expedition frame (not their newer Star frame).

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Did reply to this , seems to have vanished? 

Check the studs mine was loose at 10 hrs , they didn’t wait for ten hrs , the symptoms you are describing are the same as I had prior to finding a loose head stud causing a compression leak , motor run fine once you got it started ,hot or cold pig to start .a small leak will also stop the easy start spring winding tight and flicking your motor over , interesting stuff , hope this all helps .

cas . 

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