help needed with 75 Johnson

[ALLAN DENNIS]

Need help with a 75 Johnson .It is on a F3 Tunnel and seems to go only 1 lap then dies.Have checked Fuelpump and line s , tried another fuel pump, Runs on trailer OK for ages. Does just one lap and if re-started after about 5 seconds ,does the rest of the race most times.
Any advice would be appreciated.
Allan