Why wont my engine start? - RC Groups
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Jun 10, 2009, 06:18 PM
We all love Cox wont-starts
Jaketownsend36's Avatar

Why wont my engine start?

My engine after priming and holding the battery on for a while will make smoke come out of the exhaust ports and there will be "boiling" fuel coming out of the ports aswell. What am i doing wrong for this to happen?
My engine has started once before, before this kept happening, but only for about five seconds maximum. What is causing this?
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Jun 10, 2009, 07:17 PM
Registered User

What type of engine do you have? I assume it's a Cox not that it matters really which model Cox it is but it helps sometimes. You say you are holding the battery on it for a while...it only should take a nano-second for the glowhead to heat up all you are doing is wasting your battery otherwise. I would dare to say your battery is getting weak. With a good strong battery and the piston at the bottom of it's stroke you should be able to see the plug glowing by looking through the exhaust slits. If the engine is flooding by to much prime or by a needle valve opened to far you would feel the prop "kick back" while winding the spring starter or while turing it over by hand with the the plug lit.
If before it would run but for only 5 seconds then it clearly just it not getting fuel. It's burning off the prime and that's all the fuel it's getting. If this is a tank mounted engine like a Babe Bee or Black Widow then I would say your needle valve is clogged or the small piece of fuel line in the tank is gummed up. I find on Sure Start based engines 3 1/2 turns open of the needle is a good starting point as you maybe aren't opening the needle enugh also. Regardless of the model if it's an older reed valve engine then the reed itself my very well just be glued to it's seat ...I've seen it a hundred times. Sometimes if someone has "rebuilt" an older model engine they will install the circlip which is the small circle spring slip that retains retains the reed backwards keeping the reed closed.
In the end as long as you remember to provide two things for a Cox engine it's going to run and those two things are fire and fuel.

Jun 11, 2009, 03:12 AM
We all love Cox wont-starts
Jaketownsend36's Avatar
The model of engine that i have is a sure start cox. I have the needle-valve open 4 1/2 turns and the engine is lying flat, with the spinner pointing into space, will this stop fuel flow into the engine or will this make fuel flow easier? Also, is 4 1/2 turns too many turns?
Jun 11, 2009, 07:34 AM
Registered User
4 and a 1/2 turns is to much, your mixture is too rich. 2-3 is plenty especially with a prime.
Jun 11, 2009, 07:46 AM
Registered User

What is the setup with your fuel tank? How big is it? How far from the engine? Is it below the engine, on the side of the engine?

Jun 11, 2009, 01:23 PM
We all love Cox wont-starts
Jaketownsend36's Avatar
the tank is below the engine, which is about 1" away from the tank. The tank is a 2oz clunk tank.
Jun 11, 2009, 01:34 PM
Registered User
Is fuel being seen moving up the fuel line to the engine? I would think it's having problems pulling fuel that far uphill.

Jun 11, 2009, 01:55 PM
Registered User
What type of fuel are you using?

Jun 11, 2009, 02:24 PM
Registered User
Citing: This guide is from Bernie/Xenalook

What equipment do you need?

* Small, external fuel tank (unless your engine is equipped with an attached fuel tank)
* Well-charged battery (1.5 volt D cell)
* Glow plug clip
* 20% to 35% nitro model engine fuel (also see our guide Cox 1/2A Fuel )
* Size 5", 5" or 6" propeller, with a pitch of 3" or 4"
* Propeller screw & washer



After-run oil - Cox wrench/tool - Chicken Stick or electric starter

Adding a few drops of after-run oil to the engine after a flying session will help protect and lubricate it.


Also indispensable is the Cox wrench. You can disassemble the whole engine with this and, important for field work, remove the glow head.


If your engine has a spring starter, starting is very easy. If the engine does not have a spring starter, you can either hand-prop by flicking the prop over with your finger, or use a Chicken Stick. You can also use a 1/2a electric starter.


However, be careful when using an electric starter on this small engine because, if you flood it, you run the risk of breaking something. Plus, if your engine is well-maintained, it should start up, no problem, by hand.


The Chicken Stick is a handy tool for preventing your fingers from being wounded by a spinning prop. You can build your own Chicken Stick by wrapping electrical tape around a piece of dowelling.


Cox .049's will run on a wide variety of propellers. For general flying, a 6x3 will run well, and also will a 5x3. If you are going for flat out speed, a 5x4 prop will serve you well.

How do you get the engine running?

1. Fill the tank with clean fuel. This is pretty self explanatory and everyone has their own preferred method.

+ Use an in-line fuel filter, if possible, to keep any dirt particles out of the fuel tank. Keep your fueling equipment CLEAN. Bag it after each use and never set it down in the dirt.

+ If you are using an engine that requires an external tank, you should make sure there is fuel in the fuel line before trying to start the engine or you will likely run out of primer before the fuel has had a chance to flow.

+ You can do this in a number of ways. One method is to attach a fuel line to the tank vent and blow lightly into the line until you see fuel flowing into the fuel line. Alternately, you can pinch or block the overflow, just as the first bit of extra fuel comes out while filling the tank. This will pressure the fuel line.

2. Turn the needle clockwise until it stops. Do not force it. Then, turn the needle 3 full turns counter-clockwise. Once running, you can lean the mixture by turning the needle clockwise.

3. Squirt a few drops of fuel into either the top of the air intake, the carburetor or into the cylinder exhaust port. Close the opening with your finger and flip the prop over a few times.

4. Connect the glow clip to the glow head with the fresh battery attached. A one or two-cell battery box soldered to the clip leads works great. Or use our handy starter kit.

+ It is important that you attach the battery after you prime the engine. Priming with the battery attached can cause the fuel to ignite and the prop to kick, potentially leaving your fingers wounded.

+ DO NOT use a battery over 1.5 volts as you will burn out the glow plug.

5. Now, using your chosen method of starting, begin turning the engine over. If your engine has a starter spring, simply wind the spring one turn clockwise and release.

+ If your engine does not have a starter spring, simply flicking the prop over vigorously (from right to left when facing the prop) works as well as anything else.

+ If the engine fires but does not start, and there is not an excess of raw fuel flowing out of the exhaust ports, enrich the mixture by turning the needle a quarter turn counter-clockwise. Repeat the starting procedure.

+ If the engine runs but slowly bogs down, stopping with amounts of fuel appearing around the exhaust ports, the mixture is too rich. Lean it out by turning the needle a quarter turn clockwise. Repeat the starting procedure.

6. Once the engine is running continuously, remove the glow clip. Play with the needle to adjust the setting to optimal RPM.

+ Turning the needle counter-clockwise allows more fuel to pass through, and the engine will start to run rough or lumpy. You will see copious amounts of fuel and oil flowing out of the exhaust ports.

+ Turning the needle clockwise will lean out the mixture, causing the RPM to increase. However, the RPM will only increase to a point, after which there is not enough fuel in relation to air being absorbed; and the engine quits.

+ It is a good idea to run the engine a bit on the rich side, as this will promote longer engine life. Also, you don't want to lean out the mixture fully on the ground because, once in the air, the engine tends to lean itself out and may stall if the setting was already very lean on the ground. Turning the needle counter-clockwise 1/2 turn from full RPM is a good compromise.

+ You may also notice that, when your engine starts, the propeller is pushing wind forward instead of backwards! Sometimes, when starting by hand, the engine backfires and continues to run in the reverse direction. Reed valve engines will run equally well forward or backwards. Just pinch the fuel line and the engine will stop running. Repeat starting procedure.

+ A brand new engine should be broken in. Simply run the engine for one full fuel tank in a very rich mixture setting. Let the engine cool off and repeat with a slightly leaner mixture (higher RPM). Repeat once more at almost full RPM and you are set to go.
Jun 11, 2009, 05:17 PM
We all love Cox wont-starts
Jaketownsend36's Avatar
Thank youu for the information. i tried starting it again, but still, there was no success.
Jun 11, 2009, 06:26 PM
Registered User

1) Are your batteries fresh?
2) Do you see your plug glowing through the exhaust slits?
3) I believe you said in another thread you don't have a set of cylinder/head wrenches to be able to bemove the head correct?
4) What is the reason for the engine to be mounted the way you have it...pointed upward with tank below? On the Cox heli's with the engine mounted like this the fuel tank is small and plugs right into the needle valve spray bar.
5) Is fuel moving up the fuel line?
6) Is the engine running off the prime at least?

The answers to these questions will help us get your engine running.

Jun 11, 2009, 06:37 PM
We all love Cox wont-starts
Jaketownsend36's Avatar
1) My batteries are fresh.
2) I do not see the filament glowing, but there is some smoke coming out of the exhaust ports when i run it.
3) I a getting some spanners for the engine, they are in the post at the moment.
4) The engine at the moment is screwed to a length of thick pine with the engine directly below the engine, but it is laying flat with the spinner pointing upwards (I will get pictures)
5) The fuel is moving up the line
6) It has run off the prime on its first run, but wont now.

Jun 11, 2009, 07:06 PM
Registered User
Try a new set of batteries....sounds like they are just heating the plug and not getting it glowing orange.

Also what size prop are you using on it?

Jun 11, 2009, 10:05 PM
Slow 'n Steady...Grease 'r In
kpriddle's Avatar

Jun 12, 2009, 10:59 AM
We all love Cox wont-starts
Jaketownsend36's Avatar
Okaii thanks
i will try the new battery and see if there is an improvement

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