I have a 20hp Twin Briggs Model 461707 Type 0143 E1 Code 9911295B that's in a 13A6699H192 1997 LTD Yard Machines lawn mower. It's slow to throttle up & loses power when it's under a load. I've cleaned the carb, new spark plugs, checked the coil, checked good fuel supply & no black smoke when...