So I think after I change the oil in my '75 MKIII I'm ready to try and start it, but was wondering if there is anything I should do to the motor to prep it since it's been sitting for about 15 years since it last ran. Any advice would be great.

I've never had anything sit for that long, but after a rebuild I make sure the top end is primed with oil and after it is started I watch to make sure oil is returning to the oil tank.

Charging the battery might be worth considering

I'd pull the rocker covers and coat the rockers/valve springs with a little oil. Maybe a shot down the sparkplugs holes too.

Hobot's dead on. Well, maybe I should rephrase that. Hobot is right. I can guarantee you there is water in the tank, Trans. Forks, Brakes, and you electrical connections are corroded. All from condensation.

Ride close to home, for a while.

Thanks for the input so far. Yeah, I've already rebuilt the front brake caliper and master cylinder and replaced the rubber brake lines. I've replaced a bad ignition switch and replaced the battery, as well as both fuel petcocks. The carbs were rebuild when my friend parked the bike and they haven't had fuel through them yet. The gas tank was removed at that time and cleaned out and coated and hasn't had fuel either. Right now, my biggest concern is possible rust in the pistons and rings and the valve train. So any other info on this would be great or if there is something I'm missing post it up. Thanks again.

This isn't relevant to commando's, but when I bought my 65 Duc 250 it had been stored in poor conditions outside and in a barn for 30+ years. I changed the oil and cleaned the points, carb and fired it up and it's been fine that way for years. Amazing how an engine can withstand a beating and the things we fear aren't always reality. If you suspect rust in the cylinder pull the head wouldn't hurt and it will only be a few hours of extra work plus a lot of cussing, but it may make you feel more at ease. I assume you have taken the primary cover off just to have a peak, while in there you can see the clutch and if it's functional. Make sure the tranny has gear oil, shifts through the gears. I'd do the basics and fire it up see what it sounds like and how it runs at idle then ride it around the neighborhood see how it functions

If the engine was in good condition, 15 years ago, there really isn't any reason why there should be a problem. Mine was off the road and stored in my garage for 25 years, only turned it over every couple of years (oil in the cylinders to be careful). Drained the oil and cleaned the oil tank. Fuel tank was cleaned, coated with Caswell and repainted. Put in new oil and rebuilt the carbs. Started 2nd kick - before putting the Boyer on.
Currently has 1100 miles on it since mid-April with only a failure of the head gasket (copper). Running fine now. Essentially if you're sure it was in good condition when parked start it up and see what happens. These engines are much tougher than many give them credit for. Of course as you described there is a lot more to do than just starting the engine.

I would drain the sump and the oil tank, flush them with some mineral spirits or oil, and make sure you don't have a bunch of crap in the oil lines, sump or oil tank. Fill it with your appropriate oil and fire it up, check for oil coming into the tank. I'll probably catch some flack on this one as usual.

Dave
69S