I had the same symptoms lucid mentions for the same reason. The air hose connection on the outlet of the AMM had been pulled off but still was pressed against the AMM. The car would start but had poor acceleration. This was because under acceleration the engine torques to the right. When it did, it pulled that air hose off the AMM a little giving false air and a huge vacuum leak. It may be that in reverse, the engine movement is less or the air hose doesn't move off the AMM. In my case, the mechanic who recharged the AC reached down in behind the AMM to connect the AC hoses and dislodged the air hose wothout noticing what he had done.