Fayetteville RC Club

New Twin Needle Carb
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Crashed and destroyed a Goldberg Chipmunk in July 2010. It was flying with a Fox .74. As a result of the crash the muffler mounting broke in two places and I had to order a upper crankcase. I ordered the new Fox carb also. I broke it down  and thought I’d post my impressions.

This is a two needle carb. It has conventional rotating body with cam action. High speed needle is a short, fat assembly, sealed with an O ring. The needle has a shroud that seals around the outside of an O ring. The grip on the needle is deeply scored. The O ring gives a satisfying ‘break’ when the needle is first rotated and nice friction as you turn it. One can adjust the high speed with your thumb and index finger or use a #4 size hex wrench. That is very nice. One can hide the entire needle under the cowl and still get positive engagement of the needle with a hex wrench. I like! The low speed needle is also sealed with an O ring and actuated with the same hex wrench. Great feature. Nearly all low speed needles are actuated with a small slotted screwdriver and impossible to adjust with engine running.

When I close off the air inlet and outlet of the carb and blow through the fuel inlet nipple I cannot hear or feel any air escaping. Compared to the leaky high speed needles on the older MK two needle carbs and the EZ carb, this is a great improvement. I run a MK two needle carb on a Fox 50, it constantly drips fuel at the high speed needle.

Disassembly shows a small part count, just eight pieces. I like simple and clever, this carb is simple and clever.
Throttle body
Throttle barrel
Low speed needle
High speed needle
Cam screw
Fuel jet
(2) Fuel jet retaining screws

The throttle body is based on the EZ carb casting. I like the two screw flat mounting system that Fox has used since the sixties. Fox does not bother with effort to remove or smooth the flash from the cast. A idle stop screw is not provided. I never found use for that item. No casting defects were noted.

The throttle barrel is made from three inseparable pieces; barrel, low speed needle and servo arm, plus the O ring. Nice fit and finish on this piece. It moves smoothly, being ground to the throttle body. A small concern is that one cannot adjust the servo arm. I tried to unscrew from the assembly, but it started to bend so I accepted its position. I do not recall adjusting the servo arm position on other carbs, I usually do that at the servo. The barrel orifice is 5/16” or a little better. I do not have a caliper to measure. A 5/16” drill bit is a loose fit but the 21/64” will not go. The MK and EZ carbs both have a 21/64” orifice. So a little smaller orifice on the new carb. I hope that helps, I think the EZ and MK orifices are too large for 40 to 50 size engines. Fox does not use a spring to take up play between the cam screw and groove.

I really like the needles. The O rings seals and hex wrench actuation are valuable features. The low speed needle is made from a #4 socket head cap screw. A short smooth section near the head provides the seal against the o ring and a taper is machined into the end. I like it for ease of actuation with a hex wrench. It must also be cost effective, compared to the low speed needle on a Fox MK carb, just one machine operation.

The high speed needle is made from two inseparable pieces; the #4 needle pressed or perhaps tightly screwed, into a finger grip and shroud. The machine work is nice.

The fuel jet fits into a recess in the throttle body. The jet stands proud above the body and is held to the throttle body by two #2 slotted screws. The screws only press against the jet to the body with the shoulder of the head. I hate slotted screws on model aircraft, tolerate Phillips and love socket head screws. I replaced these with socket head cap screws. The fuel jet spray bar has a small hole on the bottom centered in the barrel orifice. As the barrel cams in and out the low speed needle restricts this hole. A long threaded fuel nipple completes the jet assembly.

I removed my MK carb from my Fox 50 and mounted the new carb. The paper gasket used between the carb and engine ripped. Alas! Fox does not include a new gasket with the carb. They do include two Phillips head screws to mount it. Luckily I had a gasket in my parts box. Bolted it up, attached the clevis and presto it required no adjustments to servo travel at all. I hope this is a good omen for tomorrow’s test flights.

Overall I am very impressed: O rings, needle actuation, appearance and feel all indicate a quality product. My only gripes: lack of gasket and those damn slotted screws.

The new carb will get compared to the MK carb mounted on a Fox 50 powering a 3D Hanger 9 Tango. I am happy with the MK two needle carb, but I think its fuel draw is low. One has to run the thing rich, it leans out a lot during the flight or in vertical maneuvers. Typical slight rich on takeoff results in lean overheat during late flight vertical maneuvers. The big 12.25x3.75 APC prop does not generate a lot of rpm or the Fox muffler a lot of pressure. I hope the slightly smaller throttle barrel orifice will increase the fuel draw and reduce the lean out during the run. My other complaint with the MK carb is the leaking high speed needle. Outside of that the MK throttles well throughout the range and holds its needle settings. If I like the throttling of the new carb as much as the MK, the new carb will be a success in my opinion. On the 40-50 size Foxes, the EZ carbs also worked well. On the Fox 60s and 74s I found the upper mid range lean and modified the throttle barrel with a little filing to richen the engine at this range.

I’ll be off to the field tomorrow, with the new carb, engine and aircraft ready to go. I’ll post my observations and opinions of flight test.

I love the Fox documentation that came with the carb and other parts I received. Looks like it was written on an old reporter’s typewriter then copied a few times. The carb instruction is two lines describing where the initial needle setting should be and stating that this is likely rich. They also included a sheet with a list of new products and prices. Some of this is old news but listed in the Fox 50 BBRC. The 50 is my favorite Fox but long out of production. It would be great if it really is back in production. I ordered my parts from Fox’s website and it did not list the 50 BBRC. However, the website lists the gas powered 50. In a similar vein, the gas powered 50 is not on the paper list of new products. I swear if Fox just hired a newly minted marketing guy part time, I’d think there would be more at the flying fields.

Meanwhile here are some pictures:

newtncarb2.jpg
New Fox Carb for 45-74 engines.

newtncarb1.jpg

I made three flights today. Starting with the Fox instruction everything was sloppy rich. The high speed needle worked great and it was quickly tuned. The low speed was very rich and took several interations to get a reliable idle and transition.

Leaving it as rich as I thought desirable I took off. Vertical manevuers were no issue, the engine noticably leaned with the nose point vertical. It seemed rich all over so I landed and adjusted. As the tank emptied the engine throtted well. The mid range is rich from 1/3 to 2/3 throttle.

Second flight I leaned it out. The idle transition improved and the Vertical manevuers still were without incident. The leaner setting made for better tranistions through out the range. The mid range is still rich from above 50% to around 75 or so.

Thrid flight same as first, but I think I will lean it out even more next time I am at the field.

Compared to the MK carb, I'd say the idle transition is nearly the same. The MK does run a little leaner in the midrange, there might be a small increase in fuel draw but not sure. THe engine did not seem to lean and run hot as tank emptied. This may be to my rich setting. The tip of the needle is just barely visible in the spray orifice at full throttle.

Overall a postive, really like the low speed ease of adjustment. I never had to shut the engine down to adjust the low speed. I just used a screwdriver type hex key. Could not find a leak anywhere in the needles. An improvement over the previous Fox carb. That opionion is due mostly to the sealed needles and their actuation with a hex key. If needing a replacment carb, buy this one without hesitation. IF getting a new engine from Fox insist they put this one on it.
 
By Scott Happel, August, 2010

Fayetteville RC Club
AMA Silver Leader
#680