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Fuel Starvation?

Old 12-09-2008, 08:42 AM
  #1  
EASYJETRIDER
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Default Fuel Starvation?

Help!

I've got a Vmar extreme stick - low wing with a Super Custom (SC) 46 and a 10x6 prop.

The plane flies beautifully, and the engine runs lovely until............ I flick the plane with full elevator travel..... and the engine quits........every time! The engine and the tank are in the positions prescribed by the ARTF instructions, although the tank does seem to be low in relation to the main needle......could this be the problem?

Or have I simply set the rates too high and no matter what the position of the tank - it will always cut with the given rates due to the excessive g-force?

Any suggestions?
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Old 12-09-2008, 10:26 AM
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Default RE: Fuel Starvation?

I assume you are using muffler pressure to the fuel tank. Make certain that the muffler pressure connection at the muffler is open by inserting a needle into it and "feeling" for any stoppage. Do the same at the fuel tank. If there was a stoppage, re-adjust both the high and low speed valves/devises. If there was no stoppage, adjust the high and low valves/devices to be slightly richer. If there is still a problem, replace the plug.
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Old 12-09-2008, 11:22 AM
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Steve Steinbring
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Default RE: Fuel Starvation?

If this is a recent occurrence after a hard landing, I'd start by examining the tank. It could be that the KLUNK in the tank has flipped forward and when you enter positive G maneuver you endup fuel starved. Take the airplane and hold it up vertical then shake it hard towards the tail to get the fuel pickup line with the klunk back into the proper position.


I'd also check your fuel lines as well.
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Old 12-09-2008, 11:43 AM
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Default RE: Fuel Starvation?

Yep, sounds like a clunk stuck at the front of the tank.
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Old 12-09-2008, 12:13 PM
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Default RE: Fuel Starvation?

Thanks guys,

I've checked the clunk and its definitely where it should be. I've also checked the pressure line from the muffler and all appears to be clear. As far as running it slightly richer - Its already slightly rich (not much) and don't think I want to go any richer at the mo.

I've had the problem with it quiting since flying the model for the first time (maybe 15 flights ago). Big fast high g loops are no problem, rolls, inverted flight - no problem - its only when I try a flick roll or apply reasonably abrupt up elevator to enter a spin does it quit - and when it quits, it quits - just stops immediately.
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Old 12-09-2008, 01:14 PM
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Default RE: Fuel Starvation?

During high "G" manouvers the clunk WILL pull on the fuel line causing it to stretch.

This can cause it to lock into the wrong corner on the rear of the tank briefly causing it to be out of the fuel, or to hit the back of the tank which permits a slower flow that leans the engine.

Add another 5mm's of distance between the back of the tank and the clunk.

Also use some mechanism to hold the tubing onto the metal feed through stopper tubes.... e.g. I often use wire ties INSIDE the tank.


Are your fuel lines SHORT? If there is a semicircular loop anywhere, is there any chance it is being pinched off during high G rolls?
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Old 12-09-2008, 03:09 PM
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Default RE: Fuel Starvation?

Sounds like you already know what you are doing but I'm going to mention it. With the plane on the ground and the engine started, what happens if you point the nose skyward and apply full throttle? I understand at full throttle pointed skyward the engine will lean....
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Old 12-09-2008, 03:53 PM
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Default RE: Fuel Starvation?

Hi Warnberg,

When I do the 'nose high check', the engine does pick up in rpm but certainly does not splutter or start to lean cut.

I've double checked the clunk and the tubing and all looks OK, as per the advice given above.

I'm contemplating incorporating a header tank as a possible option?? Alternatively, I might mount the engine on its side (90 degrees from its present position), in order to place the carb more on a level with the tank??

Could I be jumping up the wrong tree? Is this an engine problem and not a fuel feed problem???

Any thoughts on the above???
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Old 12-09-2008, 04:38 PM
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Default RE: Fuel Starvation?

This is something which will not be cured by putting the engine on it's side.

The fact that this occurs only during fast high G maneouvers tends to point to a minor plumbing or pressurization problem.

You MAY inadvertently cure the problem by revamping everything as you've indicated, but you'll be just as likely to do so if you pull out the tank and plumbing and redo it all too.

Try making these assumptions and work with them to cure the problem dealing with one or all may overcome the problem:

- The carb barrel may be moving sideways in the carb, changing the needle setting, particularly at mid to low throttle.
- The weight of the fuel in the lines during high G grows enough to overcome the pressure differential causing fuel starvation.
- Something may be pinching off the fuel flow, either in the tank or outside of it.

I've had the first one happen to me, where the spring in the carb barrel body was so weak that the fuel flow was cut off whenver the plane rolled quickly or if I flipped the plane on one side such as in a knife edge.

Stretching the spring a bit to put more tension on the barrel cured the problem.

BTW: What engine is it?
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Old 12-10-2008, 11:47 AM
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Default RE: Fuel Starvation?

Thanks opjose,

The engine is a SC (Super Custom) 46, 2 stroke.

I dont think its the spring in the carb, the aircraft flies on its side and performs fast axial rolls without hickup.

My thoughts are definitely towards your other two points. I've inspected the tank and tubing and honestly cannot see where a restiction (pinch) might occur during high 'G'. So the only other possibility is the weight of fuel issue during high 'G'. I've inspected the muffler and pressure line and can't see any problem there either.

Could it be the engine in any way as opposed to the fuel issue?
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Old 12-10-2008, 12:44 PM
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Default RE: Fuel Starvation?

Re: Engine

In a way, yes... The muffler itself may not be developing enough pressure to hold the fuel in place as it's thrown back by centripetal forces.

Try shortening the fuel lines as much as possible.

Otherwise the engine doesn't "care" about orientation or normal "G" forces as long as the fuel is flowing into the carb.


If you have an in-line filter you may also want to try to eliminate it in favor of an in-tank filter, such as the dubro brass filter clunk.

Also make sure that the nipples are tight ( but don't break the threads! ), and you may want to be sure that you are using medium as opposed to small fuel tubing throughout.


Finally also try a long plug, just in case what is happening is that the fuel is drowning the element at high "G".
This will also permit you to richen the engine slightly more which may help hold speed if it is a starvation issue.

What prop are you using?

If the engine is new I find that the APC props additional slight weight help to keep the engine running, where it may cut out with other lighter props. After the engine has several gallons of fuel through it this is not required though.


BTW:

The last time something like this happened to me, it turned out that I had a hairline crack on one of the fuel fittings at the threads.

High "G" manouvers were pulling on the tubing just enough to open the crack and let air in.

I went to tighten the nipples and discovered that it broke off without any effort or force, leaving the broken tip in the socket.

After I removed this and installed a new one, the problem went away.


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Old 12-10-2008, 01:47 PM
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Default RE: Fuel Starvation?

The easy way to rule out the G-force question is to try the same maneuver, only inverted. Rather than apply up elevator, from the inverted postion apply down elevator. This will cause the G-forces to act exactly opposite.

You never mention throttle setting when you are performing the maneuver. Are you at full throttle? Does the engine exhibit the same behavior from a lower throttle setting (i.e. half throttle). Is there a threshhold throttle setting above which this behavior occurs?

I ask because it could be related to throttle linkage. If you are cutting the throttle at the entrance to the maneuver, there could be flex in the engine mount or the connecting rod that is causing the throttle to be pulled further closed, causing the engine to starve.

Brad
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Old 12-10-2008, 01:50 PM
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Default RE: Fuel Starvation?


ORIGINAL: bkdavy

I ask because it could be related to throttle linkage. If you are cutting the throttle at the entrance to the maneuver, there could be flex in the engine mount or the connecting rod that is causing the throttle to be pulled further closed, causing the engine to starve.
Excellent point and another strong possibility.
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Old 12-10-2008, 03:38 PM
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Default RE: Fuel Starvation?

Thanks for the feedback guys - excellent as usual.

I like the idea of trying the same maneuver from inverted - brilliant thinking - I'll try this next time i fly.

As far as throttle settings are concerned - I'm not sure, but I think it has happened across the range from idle to full.

Opjose,

Can you please tell me what you mean by a 'long plug'? I'm slowly coming round to the idea that it might be the opposite of fuel starvation - a rich cut!!!

Cheers
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Old 12-10-2008, 04:01 PM
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Default RE: Fuel Starvation?

Re: Long plug

Most .40 and larger 2 stroke engines run well if not better ( especially when mounted inverted ) if you use a longer nosed plug on them.

Long nose plugs are used on 4C engines, so grab yourself an O.S. type "F" plug and give it a shot.
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