Topic: CB650 Custom carb clean up

[chrisnoel]

A friend of mine, Billy, got an 81 CB650 custom from another friend, Lucas, and now I'm rebuilding the carbs for Billy.  I've been itchin to help this bike out since Lucas first showed it to me but Lucas thought he could do it with no help.  I rode the bike a long time ago and it had a bad stuttering problem around 3K RPM and it also would to out around 55mph.  It has an aftermarket set of two into one pipes, probably MAC, and a K&N air filter in the original air box.  Billy was given the bike a few days ago and promply called me to help him rebuild the carbs.  I've rebuilt the carbs on my CB750 a couple of GSX-r's and more than a few GS motorcycles so I'm fairly well versed in doing so.  I'm thinking the problems lie in poor previous maintance of the carburetors rather than jetting so thats where I plan to focus on untill I get the carbs in clean and repaired condition. 

So far I have discovered 120 main jets and I'm curious what the original jets were?
Also found the vent t's beween 1,2 and 3,4 are rotted so I'll replace them.
Also read the tech buliten on the vent tube that should be routed to the air box intake cover. 
Should I rout this tube to the cover to help the top speed problem or should I focus on the carburetors first and the vent tube later?   
Thanks in advance, Chris. 

[cb650]

120's are stock for the 81.  After playing with it try running with the air filter cover off.  With the K&N it shouldnt help.   What condition is the tank in?  Clean it no matter what.  Some peoples idea of tank maintenance is less than desired.

[chrisnoel]

I'm planing to look into the tank before we fire up the bike and if need be we'll clean it out with acid then rinse it out with gas. 
Not a fan of the Kreem as a good clean bare metal tank cant be beat.
I found the carb breather line is linked to the air box so Honda must have fixed this problem by 1981. 
I like the air box cover removal suggestion as this is what I had to do to my GS1100 when I went to Vance & Hines / K&N. 
If 120's are stock I have some 125's we can try but I think I'll run it once and do a new plug read first. 

[cb650]

No matter what the tank looks like clean it good!!!!!  CV carbs are very forgiving to jetting issues.

[chrisnoel]

Found some old gas varnish issues to clean out of the tank but no rust.  The varnish is now a thin film that wipes off with my finger but wont seem to rinse out easily.  I tried about 1/2 cup of Chemtool diluted with 1 cup of gas but this didn't do the trick.  Any suggestions to go more aggressive?  I'm thinking of going with a whole 15oz can of straight Chemtool next.  Yes the petcock and filler cap gasket have been removed from the equation.

I'm at the point now with the carbs where 1 and 2 are done and joined again.  I have #3 ready to join and I'm not sure the choke links are right.  with all of the valve plates lined up they don't seem to join beteen 2 and 3 very well to me.  With the cable mechanism slack the stay open well and good but when I pull the cable mechanism the valve plates dont move till I push on them.  If I push 1 and 2 first 3 and 4 don't move but if I push 3 and 4 first then 1 and 2 move with them.  Letting the cable mechanism go returns all four to the open position.  Does this sound right? 

[cb650]

Check out the 900 carbs thread.  same basic carb and a couple good links in there.