Topic: CB750 K3 Airbox and Carb Clean and Reassemble (pics)

[Sigop]

I bought my bike new in 1973 so any issues with it I must own. My bike sat for quite a while so ethanol gas evaporation issues related to carbs was obvious.  Got to where it would barely start then only with full choke. It was time to learn how to fix it--thus this forum. (Must confess that Hackaweek was motivating also.)
Airbox patches, stiff and misshaped tubes and sloppy gasket shown below. Ordered replacement gasket and new tubes.  Also learned that wide gap of the tubes outside the box faces up, with fabrication line of tube centered.

[Sigop]

Carbs next. Took pre pics before disassembly so I could find my way back.

[Sigop]

Cleaning time with Simply Green soak, small carb wires, and carb cleaner spray. Jet sizes are shown as 40 slow jet and 120 main.  This bike was manufactures October of 1972 but marketed as K3. I think the 120 jet was standard on K2's and was carried forward a little into K3 before change.

[Johnie]

Don't forget to change out those fuel T o-rings.

[Sigop]

Done. Check.
Replaced t o-rings, drain screw o's, bowl gaskets, and carb top rubbers.
Thanks.

[Sigop]

Final assembly.

[ekpent]

Cleaning time with Simply Green soak, small carb wires, and carb cleaner spray. Jet sizes are shown as 40 slow jet and 120 main.  This bike was manufactures October of 1972 but marketed as K3. I think the 120 jet was standard on K2's and was carried forward a little into K3 before change.

I have had 1971's with 120's but I think 110's started sometime in 1972 with the switch to HM341 exhaust pipes.  Also I have never seen a stock 1973 with a 120, just the 110's.  One way to tell if you still have stock jets both slow and main is to look for a stylized "K" stamped on them, do not see that on your slow jet pic. Stock parts get tossed to quickly when people do an unnecessary carb "rebuild" in the past instead of just a good clean. Some aftermarket jets can be iffy.
  Also in some pics the bowls are on the wrong carb bodies, you want the brass drain screw to the outside so you can get to it with a screwdriver easily.
   Here is a carb chart also--  http://www.hondachopper.com/garage/carb_specs/carb_specs.html
  This is a genuine Kehin slow jet with logo.

[Sigop]

Fair enough.  Thank you.  All reassembled now so do not recall seeing another mark on the jets.
I was looking at the spec tables for these bikes on this web site, so that's why I assumed a transition on the main jet from '02 to '03.

http://www.sohc4.net/cb750k2/
http://www.sohc4.net/cb750k3/

[ekpent]

 Hopefully some others with the same years will chime in, I am not an expert. I have a bone stock 1972 in my garage right now, when I get a chance I'll drop a bowl and see what the main is for kicks and report,been quite awhile since I did the carbs and can't remember the size.
  The chart you posted for the 1973 says a 105 main jet but all I have ever seen in them are 110's. Think I read on a parts fiche once that a 1974 had a 110 main until a certain engine serial number then dropped to a 105 which was then used through 1976 before the 750K totally changed the carb design.

[Sigop]

Great catch Ekpent. Duh. Turned carb bowls around. Thank you. Lost sight of the trees for the forest. Nothing like a stupid mistake to improve one's humility.
Thanks again for noticing.

[ekpent]

 I and am sure many others have done the same thing-No biggie- glad to see you digging in there and doing them yourself