could it be an air leak????

[stuart]

hello all .i have an 1980 sd 900 it has been standing for a while 12 months i know i need to balance the carbs they are 40mmdelorto,s now the question is if i close off it seems to be trying to give me a few revs on the rear cylinder intermittently no is it an air leak this has only just started as i have only done 300 miles it started about 50 miles ago . now i know the answer is out there with you guys many thanks stu