I recently acquired a 1977 Batavus for the low low cost of "pick me up and take me away." The previous owner had obviously tried to start it and failed, because all sorts of bits were missing from it, not the least of which was the air filter.
After discovering that the problem was no spark (compression, spark, fuel), I traced the problem back to the taillight (far away from where he had been meddling), where some wires had fatigued off. After soldering those back together, the engine started on a few shots of starter fluid, and then she started smokin' the way she was meant to.
However, it ran like crap up hills. We had jury-rigged a u-tube to keep airborne junk from blowing into the cylinder, but we didn't have enough intake restriction, and it would bog out at open throttle. You almost had to nurture the spark, and keep the engine at the right revs to keep firing, no matter what it did to your speed. Not quite right.
I took a brass cap from some plumbing assembly that was a close fit to the carb air intake, and drilled a hole through it and stuffed it with a sandwich of brass screen (considered periphenalia in some states), filter material, and screen again. The engine ran a little better on this, but it began to stall out. I drilled the cap out almost completely, leaving just enough lip to hold the sandwich in, and I've been running that way ever since. It now beats my roomate's Yamaha QT50 up hills, where it had been losing sorely before.
Bottom line- get an air filter! That engine's tuned to have that intake restriction.