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Old Dec 19, 2005, 04:20 PM
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Pressurized fuel tank question..


Everything that I have uses a pressure line from the muffler to the fuel tank... I just installed a new engine in my 1/4 scale Sig Cub and got a J'Tec in-cowl muffler for it... This muffler does not have a fitting for that line going into the tank... Can I simply plug that line and forget about it, or do I need to raise that like up above the fuel level like way back in my U/C days...??? Thanks in advance. Signed; Confused Dave
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Old Dec 19, 2005, 06:05 PM
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You shouldn't really need to pressure the fuel tank, however if you seem to have a problem then perhaps you might. what I would do is run the engine and see how it runs without pressure - you may find that you're good to go. If on the other hane you find that she doesn't perform the you can take measures to add pressure. I have a few 1/4 scale Sig cubs and I run pressure because the nipple is on the muffler, otherwise I would try things without and see what happens.. two of mine have 120 Saitos, one has a 150 Saito - but one has the O/S 160 Gemini twin (without pressure) and it runs super fine, maybe yours will also. Jim
Old Dec 19, 2005, 06:06 PM
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What goes up must come down
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I think you will need to drill and tap the muffler for the pressure feed, depending what type of engine it is. If it's a pumped and regulated engine try this page. I know it's for YS, but it should help.

http://www.probuild-uk.co.uk/factsheets/ys_engines.php

JIM
Old Dec 19, 2005, 06:18 PM
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1/4 scale cubs


A couple more pictures of my cubs
Old Dec 19, 2005, 06:20 PM
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The new engine is an OS .91 ...
Old Dec 19, 2005, 06:24 PM
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A bit light on engine for the 1/4 scale cub, huh? Is it a two stroke?? I sent you a PM , check at top of page for private message from me - Jim
Old Dec 19, 2005, 06:29 PM
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Quote:
Originally Posted by Maineiac63
A bit light on engine for the 1/4 scale cub, huh? Is it a two stroke?? I sent you a PM , check at top of page for private message from me - Jim
I flew it for quite some time with an old OS Max .61 and it flew just fine... It didn't "struggle" for altitude, but it didn't clime fast either... Once at altitude and throttled back, it looked and sounded very real... From that I know the .91 will be fine...
Old Dec 19, 2005, 06:32 PM
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mmmm I'd pressure the tank as JIM says... I'll ensure a nice fuel flow in any condition. Besides, I doesn't requires much work/money.
Cheers!
Old Dec 19, 2005, 06:38 PM
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Quote:
Originally Posted by Guille
mmmm I'd pressure the tank as JIM says... I'll ensure a nice fuel flow in any condition. Besides, I doesn't requires much work/money.
Cheers!
I'll probably give it a go using that line as a vent, but if it's necessary drilling and tapping for a fitting is no big deal...
Old Dec 19, 2005, 06:54 PM
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I don't know if you specified a 2stroke or 4stroke.... it really does make a diff though... a 2stroke will run WAY more consistently with a pressure line from the tank, whereas a 4stroke doesn't necessarily need pressure for consistency and usually runs just fine without pressure. Dubro makes the fittings you need, in different sizes.. just get the one that matches your engine/tank so your assured uniform fits on everything.
Old Dec 19, 2005, 07:25 PM
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When you do decide to add a pressure nipple, just look where it would fit nicely and easily installed. Make sure that is somewhere high above the mean center of the exhaust, and close to the inlet. The 8-32 sized one is the easy one, not too small.
HTH.
Old Dec 19, 2005, 08:46 PM
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Quote:
Originally Posted by dgrant
I don't know if you specified a 2stroke or 4stroke....
2 stroke
Old Dec 19, 2005, 10:31 PM
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Almost every 2 stroke nowadays needs muffler pressure because the manufacturers use too large a carb so they can claim high HP at astronomical revs to sell their engines. Too large a carb doesn't want to draw fuel so they use the muffler pressure to force fuel from the tank. They'll still run perfectly without muffler pressure on the bench but the needle needs to be opened much more than normal. In flight though you can go very lean with the nose up or even just running the tank near empty (after it's been tuned with a full tank).

If you try it without muffler pressure then don't block that line unless you've got a third vent line (the air has to get into the tank from somewhere ).
Old Dec 19, 2005, 10:54 PM
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Problemo el solvedo....... I took a fitting out of an old muffler, found the right size bit, drilled, tapped with a same size thread screw, and now I have pressure other than that which my wife bestows upon me...
Old Dec 20, 2005, 07:04 PM
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Well since it's a 2 stroke 91 O/S I see no problem with the power. Those engines were some strong - I still have one from a Cessna 182 which was plenty powerful! on the other hand the 91 Surpass 4 stroke would be a bit weak such as the one on my Tweety Bird - ya gotta make sure the valves are right on with 4 strokes which isn't an issue with 2 strokes...I am more into 4 strokes myself, however I still have quite a few 2 strokes that do just fine. I've been around the model plane business for over fifty years - such a fun hobby!! I'm just setting up the bench for another Winter building session - took delivery of a couple of C G Anniversary Cubs and a Bird Of Time Glider last evening from Tower Hobbies. So I'd better get busy !!


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