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Old Jan 31, 2015, 11:28 PM
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O.s. 46fx

I attempted to set the low speed needle on my 46FX like I've done numerous times on this and other engines. What I noticed is that turning it in leaned the idle as expected and gave a better transition. However, the low speed needle is now all the way in and it still idles - even a little rich at that. I removed the low speed needle and checked the o-ring, suspecting fuel was seeping by. The o-ring was good, and I confirmed it by blowing forcefully through the fuel end of the barrel with the needle in. I thought if the low speed needle was all the way in it would block the fuel inlet at idle and not even run. So it must be getting fuel at idle, but how? I'm a little stumped on this one.
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Old Feb 01, 2015, 12:08 AM
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Hercules,


It doesn't work like that...

There is only one passage feeding the engine with fuel; and it is through the nozzle, controlled by the high-speed needle.
The low-speed needle in this O.S. carburettor only meters the amount of fuel that passes through the nozzle, by changing the eventual orifice size, as the throttle position changes.

This is the reason the low-speed needle is in the throttle barrel; and it in turn moves laterally as the throttle is worked - this needle moves closer and into the nozzle, the further the throttle is closed; and in the opposite direction as the throttle is opened.

Closing this needle all the way in will only make it fall into the barrel.
The O-ring only serves to hold it in place against rotation; not to seal a fuel-passage.
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Old Feb 01, 2015, 12:31 AM
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Dar is correct about how this carb operates. It would seem that the area in which the low speed needle meters the fuel flow has been damaged or the needle its self has been damaged. I have a Surpass 91 that has had the high speed needle screwed in tight so many times ( previous owner ) that it runs best with the needle open only 1/8 turn.
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Old Feb 01, 2015, 04:49 AM
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Quote:
Originally Posted by DarZeelon View Post
Closing this needle all the way in will only make it fall into the barrel.
The O-ring only serves to hold it in place against rotation; not to seal a fuel-passage.
Well.... whatever the case, the LSN should at least, if not closing off completely, be able to close sufficient for the engine to die of fuel starvation, otherwise it is not able to cover the full adjustment range.

So there is definitely soething wrong here, since ALL engines I own, regardless of make, will die of fuel starvation if, with properly set HSN, the throttle is set at idle and the LSN is consequently turned all the way towards lean.

It might be that the LSN is not supposed to completely seal, but at least it should NOT allow sufficient fuel to pass, to allow the engine to continue idling...

Indeed, the O-ring does not seal off fuel, it seals of air, and keeps the needle from vibrating out of adjustment.

Brgds, Bert
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Old Feb 01, 2015, 07:56 AM
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I suspect at one or more times that LS needle has been forcefully tightened till it damaged either the taper or seat at the end of the fuel distribution tube. That tube is fixed in the carb body while the needle rides in the rotating barrel.

The same can occur if the LS needle were threaded in with the throttle opened any amount. In that case, when the barrel closes the needle can impact the seat.

As the throttle is opened a spiral groove has the barrel move away from the HS needle and tube, opening the gap between the LS needle and tube. When opened enough the only mixture control is then adjusted with the HS needle which varies the max amount of fuel allowed into the carb. This is why remote needles react exactly the same since they control total fuel flow. Anything below that amount is set with the LS needle adjustment and shape.

By design the HS needle can affect all needle settings, but really only comes into play when the throttle is opened approx 3/4 and above. That's why you need to adjust the HS needle at full throttle, then work with the LS needle for idle and mid throttle. The shape of that LS needle taper controls mixture at mid throttle and in the case of some Fox carbs is altered to get better mid throttle behavior. Not usually required for OS carbs as they seem to be pretty stable when running the proper fuel, (5-15% nitro). In fact, I have grafted OS carbs on other engines to tune or improve their behavior.

I have seen where damage to the carb seat O-ring or bad front bearing can affect mixtures, (a worn bearing can cause the crank to run off center and wear the inside of the case bore allowing air to bypass the normal internal crank passage). In some cases an incorrect bearing type, such as metal shielded rather than rubber lip sealed, can cause this by allowing air leakage in through the front end, but if the crank to bore clearance is correct the bypass should have minimal effect. This was by far the biggest problem with GMS and some other clones and my own quick fix was to replace the metal shielded front bearing with a rubber sealed to help prevent air getting in that way.

Regardless, that will not prevent closing the LS needle from stopping the engine.

Having said that, if either the needle or tube have been bent from true you may not get a full seat with the needle screwed all the way in. Same effect if the tube or needle have been damaged or the tube split open. Faulty manufacturing may be the cause, but from my experience would be low on my list.

Did you buy this engine new? What carb number?
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Old Feb 01, 2015, 10:55 AM
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Thanks for all the replies. I checked the low speed needle, the spray tube, etc. and they are all in perfect condition. I have owned this engine since new and it runs great, just a little rich at the bottom. I'm thinking that maybe it just loads up with fuel in the crankcase, so when I turn the low speed needle all the way in it keeps running. Like I said, when the needle seats into the spray tube it should completely shut off fuel, but I guess if there's enough fuel still in the crankcase it would run awhile. So, does that sound possible? And when setting the low speed needle, do you run the motor first at full throttle to burn off that extra fuel or what?
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Old Feb 01, 2015, 11:17 AM
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You first tune the HS needle at wide open throttle, setting for peak then a few clicks rich, (this accounts for leaning encountered in flight and reduces the chance for engine overheating or cutting out altogether). Once set slowly reduce RPM. As you get below half you may need to rough set the LS needle to get at least a relatively smooth idle. From there you see how it reacts to rapid throttle opening.

If it quits it is too lean. If it stumbles, smokes then picks up it is too rich. Work one click at a time either way until it gives a reliable transition. If tweaked right you should be able to idle forever and still throttle up without any problems.

This may not be possible until the engine is broken in, (or if there is a mechanical issue). Also, due to the small metal particles that wear off in the initial process, the glow plug can be compromised, affecting performance or cutting out completely. In some cases it may only keep running with heat applied, to die when the power is removed. Swapping for a new plug often gets it back, but I usually keep a set of older plugs for break in since no great loss if they quit.

A couple of other factors may be affecting how it runs. Ensure your tank is at the correct height, specifically the middle of the tank should be even with the carb fuel inlet. This minimizes the chance for radical mixture changes as fuel is consumed. If the engine is mounted inverted it may be difficult to get that tank down low enough.

Also, inverted has a tendency for fuel/oil to wash down onto the plug, dampening out the element and affecting how it stays lit. In those cases I usually run a 4-stroke F type as it stays a lot hotter.

Other than inverted, I find the stock OS #8 plug performs fine most of the year, but once temps drop close to freezing or below the #3 plug seems to work better.

ps. Check the back cover. I've found as with a carb base leak or bad front bearing, if not sealed you will need to richen the mixture so much you can get the case awash with fuel.
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Old Feb 01, 2015, 03:00 PM
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Hey Cougar, I understand what you're saying and I have done it just like you said. The deal is that I set the high end like you said. Then bring it to idle and let it idle about 15-20 seconds. Then I go to wide open. If it stumbles and blubbers, I lean the low needle clockwise and try again. This is the way I have always done it, including on this motor, but what I have found on this one is that I have gradually turned the low needle in and have seen improvements off idle, but now the low needle is all the way in at idle and it still idles and transitions pretty good, but I would like to lean it a little more on the low end - can't because the low needle is all the way in now. That's why I was thinking that I need to really run it wide open to maybe burn out the fuel sitting in there before testing the off idle bit.
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Old Feb 01, 2015, 03:13 PM
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The only way fuel would build up is if the LS is set too rich or you have an air leak that requires so much fuel you may build up.

From what you describe you have the opposite, where the engine continues to run with the LS needle screwed in all the way. Only way fuel will continue to feed is if there is a physical problem between the needle and tube or the tube was not installed into the carb correctly.

On some carbs, removing the HS needle base and thread, (most common is 8mm hex or nut) will also allow the tube to extract. May be necessary to do that and have a look.

If you can provide the carb number I may be able to see a breakdown.
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Old Feb 04, 2015, 10:57 PM
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Took a closer look at the tank position like you mentioned, and since the engine is mounted sideways, the tank center line is about 3/4 inch above the carb center line. It's a Great Planes Patriot ARF so no way to relocate the tank. I'm thinking maybe that is the root of the problem. Next time I'm out I will hold the front of the plane up while checking the off idle transition. I was thinking maybe I could try it without muffler pressure as well since that probably aggravates the condition. Flew it today and it ran real good, but the low speed needle is bottomed out - maybe one click out to keep it from binding when the throttle is closed.
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Old Feb 04, 2015, 11:59 PM
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I've seen the low speed bottomed out before on a 46 fx.... Ran just about right
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Old Feb 05, 2015, 09:55 PM
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I thought of one more thing to try next time I make it to the field. Since I think the tank is too high relative to the carb, and the low speed needle is all the way in, I thought that taking the baffle out of the stock muffler would tend to lean it out some and maybe give a little more power to boot.
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Old Feb 06, 2015, 10:11 PM
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Would only lean out the top end. Will still get rough transition.
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Old Feb 06, 2015, 11:43 PM
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Well you're probably right about that. I guess I'm stuck at running with the low needle all the way in and just a hair rich still. Go figure.
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Old Feb 07, 2015, 05:18 AM
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Or spend the $60 and buy a new carburetor.
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