I recently changed the belts on my YardMan mower and now I have to press hard on the gas to make it go at all - now it runs very slow. Is there a sequence that has to be followed when replacing belts?

You mean it moves slow or the engine runs at a lower/slower RPM? Likely doesn't move as fast as it did, could be a slipping belt. Lets start by making sure you installed the correct belt. Mower model# and belt number you used to replace. Yes there is a proper sequence but lets get the belt # first

It moves slow, engine is running fine. I'll have to check the model and belt numbers.

Hi,
If you did not use the OEM belts on a Transmatic drive and instead substituted 5/8" belts; there you are. Most all the belts are custom widths/lengths and substitutes rarely work.
See this to see what I say:
MTD Belts Listing (http://outdoorpowerinfo.com/belts/mtd_belts.asp)
Peace,
Clarke
PS be glad to help, but this is a starting place.