I recently purchased a 71' CB350 and after a little work got the engine running. I used seafoam in the fuel tank and in the carbs to try and work out some of the gunk that has built up. I rode the bike around for a few days and noticed it wasnt firing on one of the two cylinders all the time. I ran the bike up for a while. The oil got extremely dirty so I changed it. It starts up great now, however as soon as I use the foot lever to drop the bike into first gear it leaps forward and stalls. Any ideas on what is going on?