RE: exasperated (@#$%) Jett .50 owner
Look at this first before you adjust anything else..... (this is something we have seen in the past a few times)
Did you get the engine hot along the way? A hot/lean run will cause the head bolts to come loose. Usually will blow a plug too.
Remove the muffler - look into the exhaust port. Make sure the sleeve port lines up proprely in the crankcase. They should center left to right (dont worry about up and down).
If you get a hot run the head bolts can/will come loose. If you change the glow plug with the bolts loose, the liner will rotate when you tighten the new glow plug in place. A mis-aligned sleeve will make the engine either not run, or will make it run VERY poorly.
If the sleeve is not lined up, loosen the head clamp bolts (head bolts) put the piston at the bottom of the stroke. Then use a glow plug wrench on the glow plug to turn the sleeve and head button so it lines up properly in the crankcase.
(if this happened, go over the fuel system again - chance are it went way lean in flight - maybe fuel foam - be sure the tank is padded and not touching the airframe)
Sleeve position is ok? Other things to look at ....
Did you adjust the low speed mixture at all? This is not a standard type idle needle valve. It is a mid-range mixture more than anything else. Dub sets this at his shop before the engine ships. If you turned that inward more than about 1/4 turn, it will do exactly what you are describing (It will cut off fuel flow entirely below about 1/2 throttle and can mess up with fuel delivery on the top end too). There are instructions on the website about setting the carb and how to start-over with the low speed mixture. Starting point would be with the low speed mixture screw appx flush with the surface of the nut.
Another possibility - Not running at all or drawing fuel when in low throttle - there is an air leak in the engine. Make sure the backplate screws are all in place and are tight, make sure the head bolts are tight, make sure the carb is fully seated down. Since you can not choke it to draw fuel, please check for these.
More to look at if the above does not work....
You may have an obstruction in the needle valve assembly (dirt, debris) blocking the venturi. Fuel should flow easily through the needle valve. Re-installing the needle to make the installation work well is not a huge problem - others have done this (When that goofs up, you can not lean out the engine usually - needle wont thread in - opposite problem than what you have). Disconnect the feed line to the carb, turn the needle full in, back it out about 3 turns.... blow into the pressure/vent line. Fuel should easily flow. If not, there is an obstruction in the needle/spray bar. Take the remote needle apart and clean it out (Easy to do).
Also...
Once you have checked everything - be sure to remove the engine from the aircraft and mount it on a solid test stand. That is the only place to effectively diagnose an engine issue. Run it there, make sure it runs properly on the bench, make any adjustements needed on the bench. If it runs properly on the bench, this will get you very useful baseline rpm information. Then reinstall in the airframe and run it there - should run identical.
If it does not run properly on the bench under controlled conditions, then it might be worth sending it down for a look-see.
Let me know how you make out.
Bob