My old 83 was bad about plugs. New spark plugs are relatively cheap. The Venture does not like to run smooth with the air cleaner top removed. It will miss and backfire. As your looking down into the carburetors while its running look to see if all the sliders are fluttering about the same. If so,good. Check to see that the connector from the stator is OK. You will find this about half way up the frame rail on the left side of the bike. About top of your ankle. This connector will burn and cause bad mojo. Some clean it some cut and solder the wires. Whatever you decide. If the sliders are not fluttering relatively the same in the carbs, inspect why. The diaphragms could be bad. They are easily removed. Dans advice on the Seafoam is spot-on ,do it. Just peruse the service Manuel on this site for all its worth. Right now $12. Best $12 ever spent. Good luck and ask away until you get it running right.