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Post  KMatch on Mon Oct 19, 2020 8:37 pm

I have a babe bee with a stripped needle so I figured a larger tank with backplate would be a good thing. Then I read otherwise anout plastic tanks... First the screws were about .020” too long. After shortening them enough to work this engine has issues. To say the needle is all over the place is a joke. It runs fine a few moments at a time, but it may be 2 turns out or it may be 6. I believe it is seeping where the backplate venturi connects to the tube in the center of the tank. Air feed?. Old timers with these feel free to set my terms straight! It’s been a year or 40 since I tinkered with these! Anyhow, there is no gasket or o ring between the backplate and tank where the screwdriver is pointing as in other engines I’ve seen. The tube outer diameter portion that should seal is a few thousandths below flush. The tube’s smaller center portion isn’t a tight fit. The backplate and venturi are flush. So this gap has to be leaking. I have two new tanks and neither have a seal here. Is this the potential problem or am I looking in the wrong place?

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Post  Admin on Mon Oct 19, 2020 10:49 pm

There absolutely should be a venturi gasket on that tank. Having no gasket will cause the problems you describe.

In a pinch, you can cut some new ones out of fuel tubing: https://www.coxengineforum.com/t308-quick-and-easy-way-to-make-venturi-gaskets

But eventually, you'll want to get some real gaskets.

https://coxengines.ca/cox-.049-venturi-gasket-3.html
or
http://www.exmodelengines.com/product.php?productid=17645&cat=268&page=2

Many use these too with success:
https://www.amazon.com/25-Orings-2BA-inch-Washers/dp/B000JEQ4CK

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Post  NEW222 on Mon Oct 19, 2020 10:53 pm

Yep, definitely missing an o-ring there. They are commercially available, but if you have some rc fuel tubing, cut yourself off a sliver and put it on the tank side. About credit card thickness or less is what you need. Using a fresh and new razor blade makes this easy. It is a venturi gasket. With it installed, it should run good. If not, post again and we will help you diagnose it again.
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Post  NEW222 on Mon Oct 19, 2020 11:01 pm

And just another thing you can do to help the run. Take out the needle and remove the spring. Slide on a small washer (#4 ) I believe, then slide on a little piece of fuel tubing onto the needle. The washer holds and keeps the small section of fuel tubing from sliding up all the wsy. I apologise as i do not know the length needed, but if you screw the needle in first all the way, you can just measure that and add a hair or two. It does two things. It seals the needle from leaks, as well as it helps keep it stiffer so it does not vibrate and change setting. If you don't understand this and need a picture, just let us know and I can post one tomorrow as I'm laying down typing from my tablet now.
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Post  KMatch on Mon Oct 19, 2020 11:15 pm

Thanks for the heads up! I believe I can make a seal, I just wanted to make sure this wasn’t supposed to be some “precision fit” intended to not need one.

Sealing the needle is an old trick in other engines I had forgotten. Thanks to all for the hints! I’m confident the lack of gasket is the issue. The engine told me it was seeing fuel on and off somewhere it shouldn’t have and I just need to make sure this wasn’t some magic surface meant to go without. Now the smallest hose I have at the moment is medium silicone so I may need to get creative for a quick fix to verify.
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Post  fredvon4 on Tue Oct 20, 2020 9:59 am

get a Dubro or any Ace hardware 2-56 tap...make it a " bottoming tap" if it is not already

basically grind off the point but not the lead in tapered first threads

the cox crank case holes are not drilled all the way through...and the threads are not cut all the way to the bottom of the holes

chase the threads another 1.5 to 2.5 turns and blow out the chips

Now you can snug down the 4 back plate screws...gently ....too much and the plastic distorts and leaks.

this site has a lot more hints for Cox engines....if you don't find stuff just ask
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Post  KMatch on Tue Oct 20, 2020 10:47 pm

Well a seal from fuel line fixed it 95% but that’s due to a bit of inconsistent running still. It needs two or three minor needle tweaks per run still just to stay in early 2 stroke stages. It’s minor but 15% fuel may be part of that.. I have some 25% to feed it when it deserves it so it looks like it’s time.
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