Motorcycle Repair: 1980 Honda cb 750 custom, needle nose pliers, honda cb 750

Question
hey, thanks for your time and anything you can help me out with. My problem on my 1980 honda cb 750 custom is with the carbs. it will idle just fine and will rev up just fine but when reved up the rpm's stick at about 3 grand then slowly go back down to 1 grand where it idles just fine. when i asked about this at my local honda shop the mechanic told me that cb carbs are known to do this and my problem is most likely with the o ring under the vacuum cylinder in one or more of my carbs. when i took my carbs off i found that the vacuum cylinder in the 3rd carb was stuck partially farther up than it should be, or atleast compared to the other 3 carbs. i took all the top thirds of all 4 carbs off separately and cleaned them all with carb cleaner and reassembled them the way my clymers manual shows they go back together, which is the way they came apart too. my problem is still there, do you have any suggestions? should i be looking for worn out parts and if so how do i know they are worn out? im almost to the point where im out of ideas and have to bring it in to the shop, which i can't truely afford but i need my bike to get to and from work so any advise or thoughts you have are greatly appreciated and very welcome. Thanks
-david

Answer
If you think the carbs are set correctly, I'd look at the timing advance unit.  Sometime the springs relax a little allowing the timing to stay in advance a little longer than it should.  The way to tell is to take the unit off and advance it with your hand and see if it snaps back to the retarded position if it doesn't return completely or quickly, that's probably the problem.  I've sometimes been successful by taking needle nose pliers and pinching one or both loops at the end of the spring into a little tighter loop. Not too much or it won't have the right advance curve.

Good luck
Rich