LOBO PUP TWIN

TESTING

By Gail Graham

So, off to the test stand. Full fuel tank and a almost charged battery for the starter and away we go. Open the needle valve about 1-1/2 turns and back off the compression on both cylinders. Hit it with the starter and the fuel jumps right up to the needle valve.  Bring up the compression on the #1 Jug and it starts to show signs of life. Play with the compression and needle valve setting and it starts to unload the starter and make better noise. Still won't run with out the starter, of course, as there is nothing but drag coming from the #2 cylinder. Bring up #2 cylinder compression and it starts to run for 5 seconds or so without the starter. Everything is still stiff, but it is loosening up a little bit. I am still running it rich until things wear in a little bit.

The whole 10 seconds of run routine is repeated until the fuel tank is empty. It's a small fuel tank, less than a fluid ounce. Refill the tank and start over again. This time I lean it out a little bit and it runs on it's own. And the first problem shows up. The compression adjustment works loose. To keep it running smooth I have to hold the compression adjust lever in position by hand. This is probably means the contra piston is a little bit too loose and a new one will have to be made. I won’t know until I pull the #2 jug off and inspect it but I am pretty sure. It takes less pressure to move the contra piston down with the adjusting screw than it should, and less than #1 cylinder takes.

Second problem is the needle valve spring makes the needle jump during adjustment. Close it a 1/4 turn and it jumps back an 1/8 turn when I let go. I probably need a stronger spring and a smoother grind to flatten the ends.

Third tank of fuel. I start it and let it run the whole tank out. My left hand is frozen holding the #2 compression adjustment in place in the slipstream. I smell of ether my fingers taste of castor oil and the starter battery is low so it's time to call it a day. Remove engine from test stand, clean it up and retire to the shop. Wash hands. They no longer smell of ether, but left hand still has little feeling other than cold. Wash hands again, this time with hotter water. Much better!

Time to remove the #2 jug and check the contra piston, and probably make a new one. And make a new spring for the needle valve and maybe put a washer on each end of it to smooth it out.

A new contra piston was made. The old one was borderline on my rule of thumb test. If I can push it in with my thumb then it is too loose. I just put washers on each end of the spring to stabilize the needle valve. It could use a heavier spring with properly ground ends, but it works OK with the washers.

Video below. The Lobo is running about 6900 RPM at the beginning of the video and climbs to about 7100 RPM with a Master Airscrew 8-4 propeller. It is still running rich while I break it in, but these are acceptable numbers for a long stroke engine of this displacement.