Help.. starts and idles well but skips and pops and is sluggish

Posted by demenacer on
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/Help-starts-and-idles-well-but-skips-and-pops-and-is-sluggish-tp461025.html

Background:  Hello!  I am excited.  I bought this bike for 600 bucks.  I am not a mechanic at all.  I am learning, but this problem is way over my skill set.  The bike is a 1980 CB750F in good condition.  It starts and runs very well.  First 3-4000 RPM's is fine if you are slow on the throttle.  If you hammer on the throttle or you get past 4k RPM it spits and pops.  

The guy I bought it from said it need the carbs synced and it would be fine.  Um... not true.  My buddy and I synced the carbs and it still ran horrible.  The gas tank was filled with rust flakes from the sea foam.  I cleaned out the tanks, shook rocks around in them then cleaned them out.  Still bad.  

So now I pull out the big guns and request the help of another buddy that just got out of two years of motorcycle school.  He is a bit young, but he has tinkered all of his life and 2 years of school, so he is way above me.  He took the carbs a part and dipped them, checked compression, put silicon in the boots to make sure they were sealed and basically looked through everything he could think of.  We replaced one air cut off valve and the accelerator pump just because they were a little crusty.  Mikey went through everything from the air box to carbs and he is stumped.  Anybody have any idea?