Topic: CB750K3 Idling issue

[earth tones]

Have a stock 1973 CB750K3.  It is difficult to start, even though it's in the 70's and 80's down here in Texas.  Sometimes have to choke the carbs and then ride the throttle for a minute or two before it will idle on it's own without dying.  Once it's idling steady on it's own, it's at about 950-1000rpm.  This is not altogether that big of problem, however what is weird is that once I am riding the bike, the idle speed with slowly increase.  Eventually, it idles at 2Krpm and slowly rises up to about 2.5/2.8Krpm on it's own.  I manually reverse the throttle to keep the idle at about 2Krpm when I am sitting at a stop, but often when I am riding in the low range of a gear and I pull the clutch, it actually idles higher with the clutch engaged than when in gear.  Does this bring any immediate thoughts to mind for anybody out there?

[Old Scrambler]

You are 'idling' on the main jets.  Clean your primaries....the small jets.  You can do this by dropping the bowls and using a mirror to see the small jets for access.  Its best to remove them with a small screwdriver to be sure they are clean.  Use a very small wire or the cleaning tips for a gas-welder.  With clean jets you will be able to turn the idle adjustment screw back to a normal setting so that the main jets are completely closed.  If you want complete satisfaction........remove the carbs as one unit, remove the jets and the emulsifier tube for a manual cleaning, then take them to a shop with a sonic cleaner.  To prevent this from happening, either ride the bike regularly throughout the year, or drain the carbs when parked for more than a month.

[EverettShadowbrook]

I have a 76 CB750. Initial morning start is sometimes tough. It seems a lot of these bikes are like this. I run throttle for a few seconds until she idles on her own. Then drop the choke and go. After a few miles of warm up I reach down and turn the idle screw until I get the  RPM's in the sweet spot (1000-1100 RPM's). I don't think you have a problem - sounds more like you have a 30+ year old bike. I would try adjusting the idle screw after you warm up the bike before you start messing with other possible issues.

Lets see what others think...


 

[Duke McDukiedook]

You are 'idling' on the main jets.  Clean your primaries....the small jets.  You can do this by dropping the bowls and using a mirror to see the small jets for access.  Its best to remove them with a small screwdriver to be sure they are clean.  Use a very small wire or the cleaning tips for a gas-welder.  With clean jets you will be able to turn the idle adjustment screw back to a normal setting so that the main jets are completely closed.  If you want complete satisfaction........remove the carbs as one unit, remove the jets and the emulsifier tube for a manual cleaning, then take them to a shop with a sonic cleaner.  To prevent this from happening, either ride the bike regularly throughout the year, or drain the carbs when parked for more than a month.

+1.
Better yet, pull off the carbs and manually clean all orifices with compressed air and wire, then bench synch and synchronize with a carb synch tool.

[andy750]

I have a 76 CB750. Initial morning start is sometimes tough. It seems a lot of these bikes are like this........I don't think you have a problem - sounds more like you have a 30+ year old bike.  

I appreciate you thinking that its the age of the bike but its not normal to have such a tough start in the morning. Myself and others have clean carbs/clean idle jets and good spark...both my 72 and 74 750 start up first thing in the morning...flick up the choke lever...start the bike...push down the choke lever...idle at 1400rpm for 60 secs and then take off...no worries.....today for example was 33F - no problem. If you are having an issue starting its not the age of the bike but the general maintenance or lack thereof.

Good luck the other posters seem to have hit on it.

cheers
Andy

[earth tones]

That's what is strange, the carbs were off completely for replacement of the main jet and the needle jet so they were completely cleaned.  Of course, I will not discount the possibility that debris has made it's way into a tight spot already.  The carbs are synced...condenser and points are new as well.  Prior to the most recent tuning changes the bike started instantly and idled correctly, but it always ran rich.  We replaced some items that were not at stock settings and the fuel mix seems to be right, however this weak start/high idle issue suddenly began to occur.

[Duke McDukiedook]

Air leak in the intake insulators/airbox rubbers? You running stock airbox and pipes?
Did you clean the other orifices besides the jets?

[earth tones]

Yes, stock airbox and pipes.  The airbox filters look good.  Haven't noticed, or checked for, a leak in the intake/airbox rubbers.

[earth tones]

Can anybody confirm the main jet size for a stock '73 CB750K3?  I have read 120, 110 and 105. 

[andy750]

Should be 110s. Check the stamp on the side of your carbs though as they indicate stock jets...657B or 657A?

going by this webpage...

http://www.hondachopper.com/engine/engine_mods/modifications.html

cheers
Andy