Engine stops after about 40 to 50 sec

Hi, does the engine run with the needle valve closed? How much fuel is left in the tank after it quits? Try using a syringe to pull remaining fuel out to measure. You can also use a syringe to pressurize the tank to find leaks. Close the needle valve and see if it holds pressure. If fuel leaks out the intake it could be the tank o-ring or a bad needle seat. If all else fails try swapping parts from a good running engine to see if it makes a difference.
Aloha

Tank o ring is new, With needle valve fully closed it is not running although needle does not seal 100% when closes but i ques this is problem only if you need 100% closed needle. As for the remaining fuel i think i checked it and it has some fuel left but i will double check to be sure. I pressurized tank as you said and it leaks from intake but i am pretty sure it has to do with bad needle seat.

On a second check i noticed that it leaks fuel from the four bulkhead screws.

One thing I do on almost all of my engines is to cut a short piece of fuel line tubing and slip it over the needle valve and spring.
It does 2 things, it reduces the chances of leakage and the other is it holds the needle valve settings better (from vibration).

Now for the back plate screws leaking.
I notice that you have a metal back plate. If the screws have been over tightened any times on the metal back plate.
The holes for the screws become oversized.
If you want to still use that metal backplate, you can cut (from surgical tubing) small "O" rings tat fit very snugly around the screws.
Or you can look on line for very small "O" rings that will work as well.
In a pinch, I have used the Cox Tee Dee needle valve gaskets to do the job as well.

My best suggestion is, if the screws are leaking, just get a new back plate. That also would take care of the needle valve problem as well?

Another thing I use for my backplate screws is to use a small bit of cotton from a Q-Tip wrapped around just below teh head of the screw before tightning it up. That also helps seal them.

Dental floss is another Method to seal the screws.

If you have a syringe you can pressurize the tank with air and listen and feel for leaks. Tank to backplate, tank to crankcase, venturi and backplate screws are all common air leak places.

Cribbs74 wrote:Dental floss is another Method to seal the screws.

If you have a syringe you can pressurize the tank with air and listen and feel for leaks. Tank to backplate, tank to crankcase, venturi and backplate screws are all common air leak places.

Sometime, not very often, but sometimes the screws seem too long and bottom out in the crankcase. Sometimes the crankcase is not properly tapped to the correct depth. IF you have the correct type and size tap, you can retap the hole in the crankcase an additional 0.1inch or so. This is a rare case, but I have seen it happen, and heard of it happening to others here on the FORUM.

OR you have screws that ARE TOO LONG and bottom out in the crankcase. This can happen when you purchase screws on your own from outside sources.

I have done this and wondered why the screws bottom out. Then I checked on the Cox websites for the proper length and WHOOPs , I had ordered the screws too long.

happydad              

I made 4 seals out of silicon tubing. Leak stopped, only a tiny leak between tank and backplate but only under pressure. I will test it and let you know. Maybe engine stops because she was out of fuel. Sullivan bulb tank is ideal to pressurise the tank

To keep this post updated I checked engine fuel and still has fuel in it after engine stop so it is not from fuel, I switched backplates with another engine and seems that problem is gone. The other engine with the "defective" backplate also run with no problems!! I will have to run it again to be sure but neighbours can only take 3-4 minutes of engine run at a time.

Double check that the backplate isn't bulged rendering the O-ring with a poor seal. This happened with one motor I had. I put it in a vice and very softly tapped it straight again and have made 30-40 runs without a hitch after that. Before I straightened the backplate it leaked terribly and sputtered and ended early all the time. I don't know who had almost ruined it but I made it come to life again!
Take a look.
Thanks
Ice

It is a little because of over tightening but that cause the seal inside to touch first and lives a gap between bulkhead and tank edge. But i don't want to hit it and brake it, i managed to overcome any leaking problems. I might buy a good spare metal bulkhead and then try to repair mine back to true.

Yes always need to be careful. If you tap the buldged backplate softly and bend it true slowly it will not crack.

Thanks
Ice