CB900F rej-etting question?

MB-5 Joined: Sep 02, 2019 Posts: 11 Location: Florida

Hello All, with all what is going on right now, we are confine to stay home, thanks god i have my garage and toys to work on..
Im rebuilding my 1982 CB900F but got some issue with my carb that i need help with..
Important to know, i got an 4into 1, air pod and purchase an Dyno jet kit ,so install kit three as per instruction 130 Main jet and drill (w/5/32) the 3 slide lift holes and change the needle installing on groove 3 and adding the 2 spacer.
Turn the air fuel mixture to 3.5
Now the bike start properly and idle like an charme, as soon as i give trottle the engine seems to gasp for air and died, if i put the chock it work, very sporadicly but much better..
I check for leak spraying carb cleaner, very minimum change in idle (like very minimal) i want to check the spark plug but bike didn't run enough to read them,,,
Today i take the carb out and place 126 as main jet and close the air fuel mixture to 2.5,, seems a bit better but note much more, still died as soon as i give gas !!!
Also very important i notice my chrome exhaust turn blue especially to cylinder 1, much less on the other one, is this sign of lean mixture? 130 and 3.5 turn????
Any suggestion/comment?
will appreciated some recommendation or comment....

Posted: Sat Mar 21, 2020 12:58 am

Dynojet kits are terrible, it will never run good. Go back to stock, try stock slides with an 80/120 jet combo. Make sure they are clean. Buy a Mike Nixon book.

Just my experience.

Toddk

MB-5 Joined: Sep 02, 2019 Posts: 11 Location: Florida

Thanks for your reply... now that i drill the slide and take out the air box it will be difficult , that been said i may try, nothing to loose
thank you

Posted: Sat Mar 21, 2020 12:35 pm

+1 on Toddk's post ... dynojet kits suck. The reason you are bluing your pipe is due to an overly rich condition (continuing the combustion process out of the cylinder and into the pipe.) "Back in the day" bikes ridden in cold weather with the choke on too long would blue the pipes...... Cold dense air + rich fueling.

As far as the jetting goes on an 900 stock is a 68 primary jet (slow speed) / 105 main jets. The altitude where you live & ride matters as well. At sea level +/- 500 to 1000 feet go up to 70's on the primary and 110 on the mains. I am not sure if you can get drilled out slides to work correctly but return the parts to stock and try.... worst case if you get a "bog" then sudden rush of acceleration when the slide come up source a set of unmolested slides from someone here. Lean primary jets will cause a surge on part throttle and closing the throttle slightly during the surge will cause the engine to speed up a little as you reduce the airflow into the engine. Lean mains are harder to feel and you would be surprised at how much of normal riding is done on the primaries.

Sudco ..... http://www.sudco.com/files/assets/basic-html/page-1.html .... has the jets you need (NLA from Honda). I spoke with Mike Nixon regarding his recommendation to disable the "idle cut valves" on the sides of the carbs and he does NOT recommend it with an aftermarket pipe ..... I have had little or no success with carbs I have done for piped bikes and with a couple 1100 it causes a weird mid-throttle surge. The 750 / 900 carbs have a completely different cut circuit and tolerate the mod much better.

If you want to judge the jetting on your "properly running" bike(s) put the bike on the center stand and fill it until the fuel just touches the lower ring inside the tank filler neck then ride moderately to reserve and return to the same fuel station the same day (fuel density changes day to day / load to load in the storage tanks) and precisely fill the tank to the same level. 40 mpg to 45 is average to very good. This is assuming good maintenance correct compression and a smooth running engine (plugs / secondary ignition operation ect.) Most of the "F-er's" I ride with go 150+ miles before they hit reserve and 180 is not uncommon. Excessively rich jetting makes these engines run hot, and pollutes the engine oil with unburned fuel getting past the piston rings.