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Engine basics => Engine set up tips => Topic started by: frank mccune on June 23, 2018, 06:13:29 PM
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Hi Everybody:
I was getting my OS .40S fired up in the Nobler and I was experiencing trouble getting a good setting. Upon removing the spray bar, I noticed that it was a bit small in diameter for the .40. Perhaps it would fit a .15-.19 engine. When the new NVA was installed, I noticed that the hole in the spray bar was not in the centre of the venturi. I tried about 4-5 other mfg. nva and they too would not centre. Would this cause any problems with getting a good needle valve setting? If so, what is a remedy?
Tia,
Frank McCune
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Hi Everybody:
I was getting my OS .40S fired up in the Nobler and I was experiencing trouble getting a good setting. Upon removing the spray bar, I noticed that it was a bit small in diameter for the .40. Perhaps it would fit a .15-.19 engine. When the new NVA was installed, I noticed that the hole in the spray bar was not in the centre of the venturi. I tried about 4-5 other mfg. nva and they too would not centre. Would this cause any problems with getting a good needle valve setting? If so, what is a remedy?
Tia,
Frank McCune
Hi Frank
You just asked about 2 problems you lsted, the small diameter ?? are all of them too small in diameter?
The NVA not being perfectly centered will not cause that problem
The diameter being too small can cause that problem
what is the diameter of the NVAs you are trying to use
Randy
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Randy, if the hole was significantly closer to one side, is it possible some of the spray could re-condense on the side of the venturi throat and dribble into the crankcase, fouling the f/a mixture... maybe too wet and flooding the crankcase?
Rusty
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Ever seen YouTube clips on two stroke engines with transparent heads and back plates?
Condensation seems to be prevalent no matter how its atomised at the front end.
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Hmmm, no. I'll have to look it up.
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Hello All:
Thanks for the prompt replies!
Here are the diameters of spray bars that I have to use:
Enya .15-.19? .150"
Enya .29-.35 .155"
ST .29-.35 .155"
The recommended size is 3.9 mm or .154"
The diameter of the venturi is about No Go .2818", Go .2969" These measurements areoapproximate as I only had a 9/32" and a 19/64" drills to measure the diamaeter of the venturi. Perhaps this is too large for good performance. I was told that the case was quite wide where the carb was installed on this series of engines. I guess that I could get the hole in the middle of the venturi by turning a bit off of the shoulder of the spray bar. I do not have a lathe anymore. Lol
I should point out that the engine is in good nick and strarts neary everytime with one flick. The engine is started in an inverted position!
In any case, how can .005" make much difference in spray bar diameter make? I am thinking that I need a venturi with a throat diameter of about .270" or less. It was suggested that a 7mm, .284", would be the correct size. ????
Tia,
Frank McCune
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Frank,
Just seal around the spraybar boss and nut with high temp silicon sealer. Use a toothpick to get a thin bead around the surface. Don't overdo it as it can wooz (sp?) into the venturi. Mark where the hole is pointing on the spraybar boss (I file a tick mark with a hand file) so you know were it is pointing and don't have to spin the spraybar once you get the sealer smoothly on. Let it cure out for several days (a week is real good) before getting fuel on it.
Best, DennisT
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Randy, if the hole was significantly closer to one side, is it possible some of the spray could re-condense on the side of the venturi throat and dribble into the crankcase, fouling the f/a mixture... maybe too wet and flooding the crankcase?
Rusty
Most likey NO, that would never happen, unless maybe... the hole was almost, or touching the side.
I would not think anything he listed to be a problem unless the hole in the case of the engine is much larger than the spray bar diameter.
in that case sealing the hole very carefully
Randy
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I lost the original answer so I will condense what I attempted to post.
The engine now runs perfectly. New nva. was installed. In level flight the engine turns a great 4 cycle of 8500 rpm produces .49 hp and has an air speed of 68 mph. Turn the nose to vertical, and a smooth 2 cycle develops 10500 rpm., produces .87 hp and has an air speed of 84 mph. A 10-7 wood prop was used for the tests. In a couple of days, flight tests will happen and I will see how this thing works in real time. Why an 10-7 prop? I can't use an 11-5 due to lack of prop-ground clearence and the fact that I have a scad of these props. Lol Hand launching the Nobler can be very exciting! Lol Perhaps there may be a 10-5 or a 10-6 in my future.
Oh yes, 84 mph with a Nobler and 60' lines is a real test of endurance. Lol
Thanks again,
Frank McCune
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Frank,
To absorb the excess HP while getting useful pulling power at a reasonable lap time you could try a three blade Master Airscrew 10x5 (you may have to depitch from a 6). They are a good price for a test prop, can be heat pitched and look cool on a Nobler with a needle nose spinner.
What lap time did you get? For 68 mph on 60' lines you would be about 3.8 ish sec. and be over 11K rpm. At 8500 and a 7 pitch your lap time would be around 4.95 -5.1 depending on how much slip the prop has.
Best, DennisT
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Hello Dennis et al:
My lap times are 38 seconds for 7 laps or 5.4 seconds per lap. This is a nice speed for the Nobler. However, I would like to get an earlier break into 2 cycle. How may I achieve this?
I used 11-5 and 11-6 props and they appeared to give good flying results. I do think that I may have to lean the needle valve a bit to get a smoother running engine. As is, the engine is not smooth firing in either 2 or 4 cycle. Any suggestions?
Be well,
Frank McCune