How do you prime your engine? - RC Groups
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Feb 29, 2004, 02:13 PM
Calm winds and CAVU skies
globemaster3c17's Avatar

How do you prime your engine?

I always have a difficult time getting my engine to start the first time of the day. I don't have my engine manual with me, but from what I remember from it you put your thumb over the carb and turn the engine over several times. I never see the fuel come up through the fuel line (I use a Great Planes easy fueler, so I don't fill it up through the carb line). There is a bubble of fuel that is always visible in the fuel line, but it never moves during this whole time. I always have to crank for a while, and then give it a few tries with my electric starter before it finally fires up. It always seems to go from no fuel in the carb, to flooded because it'll crank for a while without firing, then it locks up from being flooded and I just burn a groove in my spinner. Any suggestions would be appreciated.
Last edited by globemaster3c17; Mar 02, 2004 at 02:14 PM.
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Feb 29, 2004, 03:54 PM
Registered User
If you use muffler pressure, turn it over a few times (or spin it with the starter) with your finger over the exhaust exit. This is safer for you and less likely to flood the engine than choking the carb. You should then have enough fuel in the crankcase to get it going.
Feb 29, 2004, 10:02 PM
Registered User
downunder's Avatar
All engines have their own peculiarities when it comes to priming them just right and you have to learn what they need and how best to do it by trial and error. The prime can change drastically between hot and cold as well.

The biggest mistake is overpriming (flooding) especially with an inverted engine. The first start of the day is always the trickiest particularly if the day is very cold because then the old oil inside gives a lot of drag. This is where using an electric starter all the time is the worst thing you can do because then you never get a feel for what the engine wants.

If you flick over a cold engine that hasn't been primed the prop will come to a sudden stop from the oil drag. What's needed is to get just the right amount of fuel up into the top of the engine to free it up. Prime with a few turns with your finger over the carb (at full throttle) then flick quickly a few times to try to spray that fuel up into the cylinder. What you're looking for is, when you flick, the prop will start to oscillate a couple of times as it bounces backwards and forwards as the piston hits against compression in either direction. Now the engine has freed up and should be just about ready to start (don't forget to close the throttle ). By trial and error you might find it needs a little more prime. On a cold start I always like to crack the throttle open a bit beyond idle as well.

You'll know it's ready to start if you connect the glow starter and feel a bump when you turn the prop over by hand.

On the second start of the day you may not even need any prime (other than making sure the fuel has reached the carb) especially if you've last shut down with throttle trim because there'll still be some fuel in the crankcase. Just give it a few flicks to help spread that fuel up into the cylinder and it should be ready to go. Once again, trial and error will soon teach you what it needs.

Done properly you'll soon be wondering why you ever got an electric starter
Mar 01, 2004, 02:53 AM
Registered User
I totally agree with downunder, I have an electric starter and I hardly ever use it, only when I can't seem to get the needle right on a new engine. I do my startup procedure almost exactly as downunder described.
Mar 01, 2004, 04:00 AM
Registered User
That sounds about right. It sounds like what Mr 3c17 is missing is the "full throttle". If the fuel doesn't move in the line you may have forgotten about opening the throttle.

Mar 01, 2004, 02:15 PM
Calm winds and CAVU skies
globemaster3c17's Avatar
The odd thing (to me at least) is that the fuel bubble doesn't move, (unless I pinch it) but fuel is actually flowing through the line. And I do use full-open throttle when I am priming. One thing that is not so good about my situation is that I haven't had a knowledable person with me most of the time. So I've never watched someone start an engine until after I've done so several times. I had a lot of problems getting it to start the very first time, mainly due to how hard it was to turn the engine since it had never ran before. And I had only the manual and my own ideas to help me out on that. Everyone, thanks for your replies.

Calm winds and CAVU skies!
Mar 02, 2004, 11:20 AM
Registered User
I always prime my engines at with the throttle wide open. Many times, its the quickest way to get fuel to the carb. I, of course, make sure I prime it to the point where the air is bled out of the line and the fuel just reaches the point of the carb inlet.

Globemaster, ask if you need an engine tuning for "dummies" guide. Tuning is easier than most people make it out to be and not being scared to touch the needles to fine tune your engine will help you in the long run.
Mar 02, 2004, 02:17 PM
Calm winds and CAVU skies
globemaster3c17's Avatar
Homebrewer, I understand how to adjust the mixture. I asked about priming, not adjusting the mixture. Yes, I occassionally need another opinion on a technique problem, but I don't need a dummies book. My problem has been that I never see the fuel start to move through the line. Since I don't see the fuel in the line, I often end up over-priming and have a flooded engine. So I asked about priming technique to see if I could get another look at it.
Mar 02, 2004, 02:22 PM
Registered User
Are you so insecure that you have to be defensive about every posting I make in reply to your queries? Again, engine tuning, engine priming, engine warmup are all interelated and not discrete topics by themselves. To ask a quesiton about priming clearly indicates your inexperience in engine starting/operation/tuning and as a fellow full scale pilot, you can agree with me that it can be foolish (and sometimes dangerous) to make assumptions and take things for granted.

To be honest, I hardly ever prime my 2 strokes on purpose. I just attach my ignitor and spin the engine with a starter until it starts. It usually takes about 3 seconds of spinning for it to get started. If I do prime it, it may take a quick blip on the starter to get the engine running. Since you are obviously overpriming your engine, just use a starter and ignitor and not worry about it.

However, to answer your question here are 4 common methods to prime your engine.

1. WOT, rotate engine with finger over exhaust.

2. WOT, rotate engine with finger over carb intake.

3. Blow into fuel pressure line until fuel reaches carb intake.

4. For some very hard to start/prime engines, 2 drops of glow fuel into the carb or into the glow plug hole usually helps.

Each one of these procedures can cause you to overprime and engine and it will take practice/experience w/ your particular engine to determine just how many prop rotations it will take to prime your engine on the first start of the day.
Last edited by Homebrewer; Mar 02, 2004 at 02:34 PM.
Mar 02, 2004, 02:55 PM
Registered User
Oh, just noticed you mentioned Great Planes Easy Fuelers. I used to use these many years ago and soon realized that they often fail by failing to close completely. As a result, air is often introduced into the fuel line, fuel flow is often restrictive and as a result engines may be hard to start and tune.

I almost exclusively use a third line to fuel my tank. Its easier and less likely to fail.
Mar 02, 2004, 04:07 PM
Calm winds and CAVU skies
globemaster3c17's Avatar
So far, I've primed it only by covering the carb, open throttle, and cranking. And yes, I don't have a lot of practice starting my engine when it is cold, maybe 10 times or so. ( I do have a decent amount more practice starting it while it is warm (but that's really easy), as I have tipped my plane over many times while learning to taxi because it is always windy when I have time for my plane). Next time, I'll try out another way of priming that you have listed, and I won't try to prime as much.
Mar 02, 2004, 05:40 PM
Registered User
Tony Oliver's Avatar
Here's my way! Better with smaller models. I've just replied to another post with details :

Mar 03, 2004, 04:42 PM
Registered User


You way ensures just the right amount of prime to get her running for a small engine.
I however practice and believe in positive habit transfer and personally would never use or recommend this procedure because an individual who is accustomed to this procedure could inadvertantly use it on a larger 2 stroke engine. I saw an individual almost have his hand cut off at the wrist by rotating a large Supertigre engine with the glow driver on it. It wasn't a pretty sight, luckily the ambulance arrived quickly. I use a chicken stick on all of my engines that I "hand start" or use an electric starter.
Mar 03, 2004, 06:01 PM
Registered User
Tony Oliver's Avatar
As you suggest, I would not use that method to start larger engines (over 61 size?) but still use it to get baulky engines going. Often there is so much needle twiddling done that all settings are lost. The only option is to take the plug out, spin it up/empty the engine and start again. 'Feel' is important when you do that, but risks are not acceptable.

I started my model engine interest in the 1950s when there was no such thing as an electric starter. You learned very quickly to treat engines with respect and how to operate them safely. I don't remember any instances of people losing fingers as seems often the case nowadays.

I was part of a small group who flew c/l speed. It was treated as a regular thing to start a McCoy 60 on a 9x12 prop by hand using 30% nitro fuel. We later progressed to a hand turned flywheel starter but remained with hand starting for FAI models and up to .29 class B racers.

Individuals must take responsibility for what they do and not try something they are unsure of or don't fully understand. Practical considerations - like keeping all your fingers - far outweigh the supposed 'stigma' which some feel about using assorted devices to get an engine going. You do what you need to and feel happy with.
Mar 03, 2004, 06:10 PM
EDF Head
Haldor's Avatar
Here is what I did:

Remove glow plug, squirt some raw fuel into combustion chamber and run it with your starter untill fuel is present in the fuel line while throttle is open. Then check plug for correct glow which should be bright orange/yellow - not white or dark orange.

Then reinsert glowplug, and flip the prop over twice without glow, the light it and flip once hard against compression - not over - and it would flip back and start on first go.

This was a inverted OS.46 SF with std muffler/pressure (2stroke BB'd engine)
Last edited by Haldor; Mar 03, 2004 at 06:16 PM.