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Old Dec 09, 2010, 05:32 PM
SF Valley RC Flyers
Killionaire's Avatar
Joined Feb 2010
284 Posts
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OS Needle valve position, 65ax vs 61fx

I noticed that the 65ax has the needle valve in front of the cylinder and is positioned horizontally and angled backward. The 61fx has the needle valve behind the cylinder and can be positioned vertically or horizontally. On my 61fx it's vertical and I have an allen wrench attatched to it, which protrudes above the cylinder and is very easy and safe to access.

Why the heck did they change the needle position in the 65ax? Isn't this a much worse position? Isn't it much hardler to access? Doesn't it make it more likely that you'll injure your fingers from the propeller or the hot engine? I wouldn't be able to attach an allen wrench to this, because the sides of the airplane's engine compartment would block that.
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Old Dec 09, 2010, 07:39 PM
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Joined Mar 2008
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Quote:
Originally Posted by Killionaire View Post
I noticed that the 65ax has the needle valve in front of the cylinder and is positioned horizontally and angled backward. The 61fx has the needle valve behind the cylinder and can be positioned vertically or horizontally. On my 61fx it's vertical and I have an allen wrench attatched to it, which protrudes above the cylinder and is very easy and safe to access.

Why the heck did they change the needle position in the 65ax? Isn't this a much worse position? Isn't it much hardler to access? Doesn't it make it more likely that you'll injure your fingers from the propeller or the hot engine? I wouldn't be able to attach an allen wrench to this, because the sides of the airplane's engine compartment would block that.
You do realize , that if the rear mounted Needle valve position is important to your particular plane's setup , that theres nothing stopping you from purchasing a remotly mounted needle valve setup like on the FX , and mounting it to your AX

In operational practice , you would run the AX's original needle wide open , and use the remotly mounted needle to acchieve your mixture control . Ive seen this done more than once , and it works like a charm !!!!
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Old Dec 11, 2010, 05:58 PM
TigreJohn
United States, CA, Corona
Joined Aug 2008
996 Posts
I believe the change may be due in part to relieve two problems, one perceived and one occasionally actual.

The perceived complaint deals with the "supposed" lag that occurs when you are setting the needle valve on the ground. Resetting your needle valve does not instantaneously result in a change in the engine RPM. If there is a time/effect lag, I find its not noticeable. Taking possibly a few more seconds, if that exists, to get it right on the ground, big whoop. Your concern is getting it right so you don't cook the engine and/or have a "flame-out" and having to deadstick the airplane.

The other situation a combination of factors. The particular engine (I can't remember which, couldn't find the forum) that had a rear mounted NV generated enough crankcase heat that transfered to the NV that if mounted in a poorly ventilated cowl, the fuel would boil in the NV causing vapor lock. That fix was a firewall mounted NV or opening up the cooling air exhaust in the cowl.

With tongue in cheek, we are all "Master Builders" and would not our could not create such a situation (BS). So the problem MUST be with the engine. I can see OS not wanting negative publicity, so get rid of the rear NV.
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Old Dec 13, 2010, 11:16 AM
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United States, TX, Weatherford
Joined Dec 2006
1,670 Posts
Killionaire, as you can see it does not make much difference where the carb is so long as the proper fuel mixture enters the engine. It is just simpler to put it on the front of the engine from a manufacturer's point of view. And if you think being close to the prop is dangerous, then you can go remote with the needle valve assembly.

I have seen remote needle valves simply suspended by the fuel tubin, mounted on the fire wall and on the back of the engine. Anywhere where it can do its job. Yes, there may be some lag with the distance away from the ingestion point of the fuel into the engine but it all works out end the end.

So location is a matter of convenience not operation.

Chip
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Old Dec 13, 2010, 04:17 PM
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Canada, BC, Langley
Joined Nov 2007
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There have been reports (true or not) that the remote needle is prone to 'vapor lock'. I could understand that the carb mounted needle would be kept cooler by the evaporation of the fuel by the carb. May be some truth to it?
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Old Dec 14, 2010, 05:57 PM
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Joined Jan 2004
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Quote:
I could understand that the carb mounted needle would be kept cooler by the evaporation of the fuel by the carb. May be some truth to it?
at 35F & 85% humidity I've seen ice form on the out side of the carb body.
Jeff
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