Topic: cb750f running lean?

[.jordan]

I'm towards the end of the process of reviving my 1977 cb750f, i've rebuilt the carbs(jet needles, gaskets, accel pump), adjusted the valves, cam chain, points, and new plugs. I was forced into using pods and bought a main jet kit up to 120. I'm running the 120's right now and everything seems pretty good, but when I get on it the bike hesitates a bit, then takes off like a raped ape. Could it still be running lean and I need to go up to 125's or 130's even? I don't have my license yet so I can really go out and do a plug chop. What are other guys with similar setups running for main jets?
Oh and btw I'm running a 4 into 1 (pretty much straight pipes).

[Johnny5]

I have a 77 750K, so it may be a bit different than yours.

What setting do you have your needle at? Most pods like the 2nd richest setting.

Also try 2 turns out on the Fuel screw.

[Flying J]

Ok on my 78f im running 125s. I have not decided if that is my final jetting yet but the hesitation could be your accelerator pump not working. When you twist the throttle do you see fuel spray out of the tube that sticks up in the throat? My carbs were gummed up and had to clean all the little passages in the accelerator pump set up. Thats my first guess. Second i would say you might need to raise the needle up 1 notch.

[MCRider]

I have a 77 750K, so it may be a bit different than yours.

What setting do you have your needle at? Most pods like the 2nd richest setting.

Also try 2 turns out on the Fuel screw.

Agreed. Remember the mains only work from about 5000 rpms up. You are indicating a lean stumble earlier than that?

The needles are likely where to look.

[Flying J]

Turning the screws out more will cause the idle to be more lean, correct?

[MCRider]

this is a total noob question, but raise what needles up? also how do i know when they're seated?

Generally speaking there are 3 controls in the (all?) carbs. THe air screw/slow speed jets for 0 to 1500rpm (in CB750 case) the needles for 1500 to 5000, and the main jets for 5000+ to WOT.

THe mid range, where we usually are when we go to open hard, is controlled by the taper on the needles and how deep they hang into the emulsion tube which has the main jet screwed on the bottom of it.

Straight to your question, the needles to which I refer, are brass, about 2 inches long and hang down from the slides, suspended by a clip inserted in one of many grooves. By changing the position of the clip you change the mixture. Raise the needle by lowering the clip will richen the mid-range, and vice versa.

This is VERY basic, doesn't apply to all carbs and I will rollover and defer to anyone claiming to know more, as they probably do.

[punch455]

Jordan, your carbs do not have an adjustable main needle.  That needle clip adjustment is only available on carbs up to '76 I believe.  Your idle mixture screw is located on the engine side of each carb body, and consists of an o-ring, washer, spring, and needle. The head of the needle is slotted for a flat blade screwdriver.  Stock setting for the 77F is 1 3/4 turns out from seated.  You can seat the needle by GENTLY tightening the needle into the carb body. Don't over tighten it; stop as soon as you feel resistance.  Then back it off 1 3/4 turns.  With the engine running, adjust each needle (I would start at stock setting, and work in 1/4 turns) for highest rpm.  Once all four carbs have been adjusted, you are good to go.  Keep in mind this is an adjustment for idle circuit only.  Your hesitation, as stated earlier, may be caused by a clogged accel circuit.  But that's easy to check too. Just take of the pods, start it up, make sure the choke is off, and blip the throttle.  You should see fuel squirt into the throat of each carb from the brass posts sticking up in each carb inlet.  If even one of those circuits is clogged, additional cleaning is required.  On the image below, #4 is the idle mixture screw.


Good luck!

[Flying J]

the 77 is adjustable needles. The part that sprays is on the pod side of the carb.

[MCRider]

Okay thats what i was thinkin, when someone mentioned a clip it got me all confused... I think right now they're about 2.5 to 3 turns out and it idles great. I'll check the accel curcuit, I'm thinking this is my problem.

I was thinking I might be off-base, my experience is with the older carbs, which have the clip-type adjustable brass needles.

Still, the idea that you need to tweak the mid-range, however that is done, rather than the main jet, is likely correct.

[Flying J]

MCrider you are correct. The mid range is controlled by the brass needle and adjusted using a small clip at the top. It is accessed from the top side of the carb. In the carb faq is the stock settings. 77 was 2nd notch down.
http://www.motorcycleproject.com/motorcycle/text/specs.html

[MCRider]

MCrider you are correct. The mid range is controlled by the brass needle and adjusted using a small clip at the top. It is accessed from the top side of the carb. In the carb faq is the stock settings. 77 was 2nd notch down.
http://www.motorcycleproject.com/motorcycle/text/specs.html

Cool, well in this case i was right by accident, nevertheless...

Assuming there are 5 notches as usual, if it starts in #2 from the top, that would be a lean condition relative to earlier bikes.

And so Jordan, appears you still have the needles to look at.

PS: That motorcycleproject.com looks like a great resource. Thanks!