Engine Pops

Just got a new Fox .25. ( New to me ) I tried to test run it this weekend with not much success. It would start and run about 3 seconds then quit. Needle setting - I started with 4.5 turns out and adjusted from there. My question is it is now making a real loud popping sound when I flip the prop. It is just a like a popper going off. Does not start, just makes a loud pop!
Anyone heard of this before?

LAST EDITED ON Jan-12-04 AT 08:27 AM (CDT)
 
Hi Paul,
OK, to further this discussion, what fuel, what plug and what type tank and how is it mounted? All these can cause similar problems.
When the engine does the short run, can you see the fuel being drawn through the fuel line?
Sounds like it might run out the prime, then it's not drawing enough fuel to stay running. The "pop" might then be a flooded condition from re-priming. All conjecture until the questions above are answered.
Might be as simple as turning the needle in a little bit, but with further info, I'm sure a lot of people will jump in!
Bill <><

>Just got a new Fox .25. ( New to me ) I tried to test run it
>this weekend with not much success. It would start and run
>about 3 seconds then quit. Needle setting - I started with
>4.5 turns out and adjusted from there. My question is it is
>now making a real loud popping sound when I flip the prop.
>It is just a like a popper going off. Does not start, just
>makes a loud pop!
>Anyone heard of this before?
>
>
>Thanks
>Paul

Fuel or fire? Fuel or fire! That's always the question you need to ask yourself: Fuel or fire?

Check to make sure the glow plug is lit (fire). The plug could be bad. The battery, or hook-up could be bad. The plug could be flooded. If you have fire, then the problem is fuel.

Assuming that the fuel itself is good, then the problem is getting it to the engine and mixture. We know nothing about your set-up. Did you put the tank on the floor and the engine on the bench and you are trying to draw all the way up? Did you mount the tank higher than the engine and the fuel runs in on its own? Location of the tank makes a difference. We are assuming that the needle valve body is located correctly so the fuel will draw properly. We are assuming that your back plate bolts are tight so it can draw and that the head bolts are tight so that it can compress the mixture. We are also assuming that there is no "junk" in the line (or needle) keeping the fuel from flowing into the venturi. (I said "We" are assuming. These are all things that you will need to check. Oh, and make sure the glow plug itself is tight).

From here determine whether the engine is starved or flooded. If it is starved, prime it. If it is flooded, burn out the flood. To burn out the flood, close the needle or remove the fuel line so it can't draw any more. Then burn it out. If it is starved, you can give a small prime in the venturi if the engine is upright (which it usually is on the test stand. If each time you prime it, it runs, then quits, it is a sure sign that the engine is starved for fuel. Again, if the tank is reasonably level with the engine, and you have the needle turned out a sufficient number of turns, I would check the needle for junk and proper orientation.

Oh, we are also assuming that the engine is "as sent" from the factory. I have seen engines that have been torn down and the liner was skewed off in some outlandish position. But if the engine is as it should be, when it doesn't start it is either fuel or fire.

I am learning so much stuff here!!!
OK I may have my tank mounted wrong. It is just about 1/4 inch above the center line. And my fuel line may be too long. It is about 7in long. Loops down and back up. I mounted the engine as it will set on a plane. Needle valve up. When I walked away from the engine and came back a few minutes later, all the fuel had flowed out onto the floor. I would post a pic, but I am at work. I will lower the tank just a tad, and shorten the fuel line.

Every thing that Leonard is very pertenant to the problem. But to add to it, the Fox engines are VERY tight when new and often give off the large POP sound when you are trying to start them.
Pinch off the fuel line (or disconnect and cap it). Remove the plug and look at it to see if it is coated with liquid fuel (wet). Connect the battery to it and see if it glows a good red. With the plug removed, flip the engine over and see if fuel droplets are emitted through the plug hole (if so, the engine is flooded). If it does, keep flipping the prop through until you get no more fuel comming out the plug hole. Install the plug, and connect the battery. Flip the prop through a few times. If you get pops, continue until you get no more pops. With an open exhaust, (no muffler installed) put three or four drops of fuel in the exhaust with the piston at the bottom of the stroke. Now, give the fuel in the exhaust port a hefty PUFF of air (lung power) to blow it back into the cylinder. Now flip the prop through. The engine should fire right up or "kick back" against you instead of going through compression. When it starts it will only run long enough to burn the prime out, so you will have to re-prime it. do this "prime and start routine several times to get the rhythm down and then connect the fuel line and see if it continues to run. If it kicks back, rotate the prop in the normel direction of rotation until compression starts to build, then sharply WHACK the prop in the BACKWARD direction so that the tendency to "kick back" will occurr and the force of the kick back will start the engine in the proper direction. That is known as "back flipping" and is used extensively by users of the high performance BB engines.

By-the-way, The prop you are using for these initial starts,, What size and pitch?? I recommend for that engine, a Master Airscrew 10 X 5, cut down to an 8 1/2 inch diameter and rebalanced and deflashed.. If you still have trouble, let us know and we will try something else. At the worst, send me the engine, I will check it over for problems or trouble and start and set it and break it in some so that the tightness is overcome.
If you have more questions, you can call me on the Ma Bell at (316)838-8494 almost any time.
Hope this helps

Bigiron aka Marvin Denny

apparently it ran out a prime and is now just popping. Symptom of not enough fuel. So you have to figure out why it isn't drawing fuel. Try priming it(a few drops in the exhaust or intake) and get it to run out the prime consistently. Then try holding a finger partly over the intake to see if the motor will draw fuel while the prime if running out. Then proceed backwards from the needle valve to try and determine what is causing the lack of fuel flow. Loose needle, leaks, wrong position of the fuel hole, leaky fuel line, plugged fuel line, tank too low, etc. Also check carefully for a leaky head or backplate.

If this is a used engine like the old baffle Fox 25 with the bolt-on venturi a previous owner may have bored the intake out. The newer BB 25's are pretty marginal on venturi size as they come. Lots of power, but the venturi tends to be too big for good suction.

Thanks guys,
I will try some of these things this week when I get some time.
BigIron - I might take you up on your offer.

>If this is a used engine like the old baffle Fox 25 with the
>bolt-on venturi a previous owner may have bored the intake
>out. The newer BB 25's are pretty marginal on venturi size
>as they come. Lots of power, but the venturi tends to be
>too big for good suction.

If this is an old bolt on venturi fox 25, make sure the venturi does not leak where it bolts on. This is a tough problem to find, there should be a gasket and the little screws should be tight.

When I ran one of the bolt on venturi Fox 25s, I liked to run a bead of permatex silicon on the bottom of the venturi flange, then the gasket, then bolt it in place, snug but not REAL tight until the silicon set. I would then tighten it on down. This seemed to work real good in preventing leaks. I use a similar procedure on the pipe headers now, without a gasket.
Bill <><