c-90
fireball111: I assume the main problem is hesitation when advancing the throttle? This is cause from a lean condition from either an intake leak, or insufficiant fuel mixture. Late ignition timing will cause this symptom also.
The propane is a good check normally, but too much of a prop hazard and too windy to be successful with an aircraft engine. Do a check on the intake rubbers for no splits and the clamps tight. Check the intake elbows at each head. Make sure the carb is tight on the spider. Everything must be perfect for the Stromberg to work with-out hesitation.
If all this checks out several things with the carb may cause this hesitation. The mixture control should be wired full rich, cause the damn things never worked right anyway. The fix was to leave them wired rich.
The big screw at the back of the carb is the idle mixture screw. Open it 1/2 turn at a time while checking throttle response. The engine will still idle reasonably smoothly with just a slight touch of richness. It will run lumpy with black smoke puffing out the pipes if you go too far. Idle speed should not be below 600 rpm when hot. Lower than that , you may be looking for trouble, especially when cold weather comes. (ice) Mine is set for 700 rpm hot, and cold start it runs around 500 rpm.
If that doesn't do it , the float level may be too low, which has the effect of having to draw fuel from a lower , further point, which will make all rpm ranges leaner.
The overall condition of the carb will have major effects on things. If the throttle shaft bushings are too loose, vacuum needed to draw fuel will be lost, again causing it to lean out, especially at lower rpms. The carb may have dirt in the idle jet not allowing sufficiant fuel to be delivered. If turning the idle mixture screw out has not effect, this may be the culprit.
It's hard to fix the problem with a key-board, but this would be where I would be looking for the problem. Good luck.
Ron