I have a 1999 yamaha vstar 650. Ithe seems to be popping and back firing while at low speeds and it will pop once in a while while idling. I've had my carbs cleaned, new sparkplugs,and new air filter. I'm at a loss. If I'm running too rich or too lean I'm screwed cuz the last owner stripped the adjustment screws on the carbs. I'm supposed to be going on a longoing ride next month and want it running right before I go. Thanks and please help!!!!