I have just became interested in converting whacker engines for aircraft use. I have several but I need help identifying them so I can get the right parts to convert them. Can someone give me some info on identifying the difference of Poulan and Ryobi. Also Identifying the carbs (such as P/N) and which would be more desirable to use.

Your help will be greatly appreciated to feed the compulsion of this terminal tinkerer.

Post some pics and we can tell you.Or do a search on each engine you have and there will be a thread devoted to that engine to convert it.
With part numbers and references in the most part.
As for carbs most of us use carbs in the 10mm range.
Im trying to open the websites addy i have for numbers but its not working right now.Just look at the individual threads and the best carbs should be listed in that thread for that particular engine.

Well I haven't tore any of them down yet. Their still in their intended use form. I am in the process of running them in their current form first, to determine their viability. Most are older WeedEater brand (Poulan?) and some are IDC (Ryobi?)
You mentioned a 10mm-11mm size carb. Where is this dimension taken from.

weedeater brand engines are made by poulan, they say their size on a label somewhere... the '90's and up engines all look the same and have interchangable parts (at one point they changed from a 4 bolt backplate to a 2 bolt is the main difference between them), range in size from 18cc to 25cc (including 18cc, 21cc, 24cc and 25cc), that is if they are the side intake and side exhaust engines, the older rear exhaust with the reed plate in the backplate or on the bottom of the crankcase are similar to the Ryobi but are different I believe, I have a few of the 32cc versions from sears/craftsman stuff, and one old green curved shaft weedie.

IDC are Ryobi, if they don't say a size they are either 28 or 31cc

Yeah, after spending time on the web. I have found you are very write. One notable difference I have identified in the older Ryobi/Poulan difference is the cylinder jug on Ryobi is bolted strait across with ears sticking out and Poulan's jug is bolted diagonally. Both seem to have rear exh. and intake. This is the best I can tell untill I get further in strip down.