Topic: simple question at 75 to 80mph

[KRONUS0100]

bike is 76 cb750f with 78 cb750f3 motor and the 76 f carbs.  stock airbox  with k and n filter and points, finally timed correctly with new advance springs  wrapped kerker 4 into 1 with 2 1/2 inch homemade baffle
120 mains 42 pilots clip in middle notch
 idles great at 1000 now, runs very strong to about 70mph
as i appraoch 75 it starts to feel like a fuel flow issues...................no pinging or anything
plugs are good, maybe just a bit sooty, but thats the way i keep em
thinking i need to move the needle clip down 1 notch, does this sound right?
thanks in advance.

[KRONUS0100]

reckon i should add that the floats are correct at 26mm  and the petcock is new with new in tank filter, with a 3/16 y splitter in the fuel line.  fuel line is level, no ups and downs.

[Dave Voss]

Sounds like the engine might be running lean under load, possibly due to the exhaust system flowing more freely than stock.  I had a similar issue with my CB550K, and I was able to make a significant improvement with a combination of slightly larger main jet size (from 90 to 95) and raising the carb slide needles two clip positions, so might be something to consider.

[HondaMan]

bike is 76 cb750f with 78 cb750f3 motor and the 76 f carbs.  stock airbox  with k and n filter and points, finally timed correctly with new advance springs  wrapped kerker 4 into 1 with 2 1/2 inch homemade baffle
120 mains 42 pilots clip in middle notch
 idles great at 1000 now, runs very strong to about 70mph
as i appraoch 75 it starts to feel like a fuel flow issues...................no pinging or anything
plugs are good, maybe just a bit sooty, but thats the way i keep em
thinking i need to move the needle clip down 1 notch, does this sound right?
thanks in advance.

I would estimate that you are correct: lower the needles one notch. You may also find that the mainjets need to drop to 115 or even 110 as well. With the late-opening "F" cam, having a rich midrange will make it fall flat on its face at about 5000-5500 RPM. The F cam also tends to make the plugs whiter than the earlier cams, so if your plugs are a bit dark then it is definitely rich.

If you ever take the engine out again, may consider getting a slotted cam sprocket and advance the cam timing a little, like 3-5 degrees. This will reduce that flat spot considerably and will make the tuning simpler overall.

[KRONUS0100]

hmmmmmmmm  it does have the slotted cam sprocket  i remember that when i degreed the cam to make sure it was indeed an F3 cam.  I did not however think to advance it a wee bit.  I do have a frame kit installed which will help so i dont have to pull the motor.
i want to turn the sprocket counter clockwise to advance right?

and i will lower the needles one notch.

ummmmmm.................one more question.............if i advance the cam 4 degrees, i will have to reset the points,correct?

[70CB750]

Yes, counter clockwise to advance. 

[Jerry Rxman Griffin aka MuthaF'er]

Why do you have 120's in there with a stock airbox?!

[KRONUS0100]

according to a jetting chart, with the 2 1/2 inch thrush baffle in the kerker and the k and n filter i should have 118's.  i am going to drop the needle a notch and see what happens.  if i need the 115's i will order them this fall.

incidentally, i still cant quite get over how much smoother and quieter she runs on points over a failed electronic unit.  starting to wonder how long exactly the electroni unit was actually in fail mode.
   plug chop today 9-4-13 at 10am
my guess is im lean even with 120 mains

[KRONUS0100]

crap missed a photo   but it looked like other 3

[ekpent]

What brand is that spark plug with the different end.

[bjbuchanan]

Looks to me like he did the old sidefire trick where you cut the end of the electrode off. Homebrew stuff, cyclex talks about, others mention it here and there

The idea is the electrode is just in the way so trim it back and gap properly and it is unobstructed. Haven't done it myself

[KRONUS0100]

yeah  the side gap trick.  idles a wee bit better.  not really noticeable going down the road.

[seanbarney41]

according to a jetting chart, with the 2 1/2 inch thrush baffle in the kerker and the k and n filter i should have 118's.  i am going to drop the needle a notch and see what happens.  if i need the 115's i will order them this fall.

incidentally, i still cant quite get over how much smoother and quieter she runs on points over a failed electronic unit.  starting to wonder how long exactly the electroni unit was actually in fail mode.
   plug chop today 9-4-13 at 10am
my guess is im lean even with 120 mains

those look maybe a tiny bit rich but really need to look down inside the insulator...that's where it really counts.  Aim for dark grey/light tan

[KRONUS0100]

real light grey down inside, wearing off the red color from a fuel additive. the black around the edges is leftovers from very rich idle.  have new pilots  just havent installed em yet

[TwoTired]

Side fire is great for racing.  Plugs are cheap.
For the street, expect to change/replace plugs at about half of recommended life, as with those mods, reconditioning is near impossible.

[lucky]

I agree that you need to move that stingy stock 76F slide needle clip down to the fourth notch.

 2nd notch was stock for the 1976F model carbs.
Also I would change that fuel line configuration.

I would use a larger fuel line (3/8) splitting into 2 lines .
One line feeding 2 carbs each. Much better.


Remember ...only one change at a time.

If you advance the cam AND change the needle clip position you will not know which one caused the engine to run better or worse.