Hi Paul, and welcome to Outdoorking. Hopefully people here can help you with this and any future projects. Can you post a picture or two of your mower. As a starting point, if the engine has a dipstick it will be a four stroke: it doesn't have to be that way (there are two stroke engines that don't have crankcase induction) but as far as ATCO mowers are concerned, that rule will hold. The clutch on the left usually connects the engine to the chain case on the left of the mower, so it drives both reel (cutter) and drum (rear roller). The throttle is usually a chrome-plated lever on the right. You referred to another lever on the right, and we need a picture of that. The only ATCOs I've seen rigged that way are big ones and the right side lever (which looks just like the clutch lever on the left) is a second clutch, so one controls the rear roller (drum) and the other the blades (reel), allowing you to engage and disengage the reel and drum separately. I've only seen this on professional-type ATCOs in the 30" or larger class. If you have one of those, that is quite a piece of equipment and should keep you entertained for a long time. I haven't even seen a picture of one of those on this site, but I admire them greatly and would like to see pictures to remind me of my lost youth. Incidentally on the small, single clutch mowers the clutch lever moves laterally and on the big, two clutch mowers the lever moves fore-and-aft.
We need to see a picture of your carburetor to get the starting routine clear. There is no priming bulb, but most of them had a flood button on the lid of the carburetor float bowl for starting enrichment. If it has the button, you just hold it down until fuel squirts out a tiny hole in the side of the lid. If it doesn't have the button, it will have a choke lever.
I recommend that you don't start the engine until you understand the controls. In particular, you have to know how to stop the engine, and how to disengage the drive. There are two usual ways to stop those engines: the throttle can be adjusted so it shuts off completely when pushed fully clockwise, or in the case of some four strokes, there may be a spark plug shorting device that you push down onto the top electrode of the plug. If you have a plug-shorter you would adjust the throttle to give a sensible slow idle when the lever is all the way clockwise. You also need to be sure the engine oil is clean and filled to the correct level.
