I just purchased a '77 15 HP which starts and runs pretty good except for one problem. If I set the idle mixture slightly richer than peak RPM at lowest throttle setting the engine runs a bit rough at part throttle (worst is when the throttle is barely off the idle stop). If I open the idle mixture another quarter to half turn the mid-throttle operation smooths out but the idle is too rich and it eventually loads up. Worse yet (and likely related) if I go from full or near full throttle to mid throttle very abruptly the engine dies completely and is difficult to start without choke. With the low speed mixture set properly it idles nice and slow albeit with a little shaking and above half throttle it runs strong. If I decrease power gradually it doesn't die, only if I close the throttle quickly to about the "start" position on the tiller.
The throttle to ignition advance looks OK based on the marks on the throttle cam although I didn't use a clip-on throttle movement indicator, just eyeballed the throttle shaft. I'm thinking that perhaps some of the low speed fuel openings behind the throttle plate might be plugged up but don't know if that would cause this particular symptom. Has anyone seen this issue before? Does it sound like a carb problem of some sort?
The throttle to ignition advance looks OK based on the marks on the throttle cam although I didn't use a clip-on throttle movement indicator, just eyeballed the throttle shaft. I'm thinking that perhaps some of the low speed fuel openings behind the throttle plate might be plugged up but don't know if that would cause this particular symptom. Has anyone seen this issue before? Does it sound like a carb problem of some sort?
