Topic: Tuning questions..... need some help

[J.Webster Designs]

Hey guys, I am trying to tune the carbs on my bike (78 CB750 F motor with 76 CB750 F carbs) I have done all the other tune up processes (i.e. set valves, set camchain). The bike is running the stock airbox with a K&N filter and what i believe is a MAC exhaust without a baffle.

the problems i am having are as follows:

first problem i am having is an idle that doesn't want to settle down after revving up and letting off the throttle. I have to physically twist the throttle forward so it will settle back down. I had this problem at the beginning of the bike build. I tore the carbs down and cleaned them thoroughly. The carb to head rubber manifolds are brand new. I have researched and found that most of these problems are caused by air leaks in the rubber manifolds. but mine are new and still having this problem. The carb slides are closing fully after i rev. I can hear them click closed. I am also syncing the carbs as i haave read this can cause the problem as well. I am able to get them practically spot on level with eachother and still doesnt want to settle down.

I am also running very rich. let the bike idle and it practically fills up the garage with unburnt gas fumes that practically make your eyes water. the idle mixture screws are set at 1 3/4 turns out. 107.5 main jet, and 40 slow jet. plugs come out nice and soot covered.

also, when syncing, i am having a problem with keeping my settings when i tighten the lock nut. as soon as i tighten it down, that carbs vacuum skyrockets up the tube.

Need your help here guys.

[J.Webster Designs]

TTT

[jjwaller]

I guess I should look first, but,
Those jets seem small.
Normal 78F mains 112 or 115, from experience.
My original 78F with a K/N drilled airbox and opened up
exhaust, used 120's,  My current one with stock filter, but drilled airbox
and stock exhaust runs 115's.
Don't know if the 76 carbs are that different.

[Mdub]

I know that the intake manifold clamps wear out to the point that they will not seal the tubes even when completely "tight". That is to say, that the ends of the clamp come together before enough sealing force is applied.
I replaced all of mine with heater hose clamps.

I've also seen the rubber grommet in the top of the carb, that the slide rod goes through, wear to the point of sucking air through it. I think you can still get grommets from the stealer.

Is it more willing to idle down without the sync device attached?
If so, you may have leaks where the sync tubes screws into the carb.
There should be a little tiny o-ring on the tubes that, if not careful, rides up over the sealing "shoulder" when tightened to much and fails to seal. Just snug should do it.

You'll probably have to go down to 105's (102.5's?) and maybe drop the needles a notch or two.
I bet if you put a baffle in your exhaust, it'll be about spot on with the 107's you have.
Good luck and let us know how it turns out.
Mike

P.S. With an open exhaust you get more air flow, dragging more fuel out of the carb = rich

[J.Webster Designs]

im willing to put a baffle in. infact i need to with how loud this thing is. do you guys think that if i put a baffle in it will really solve my rich problem? i thought that an open exhaust and higher flowing filter would cause a leaner condition.

[J.Webster Designs]

TTT

[J.Webster Designs]

need some help here guys

[J.Webster Designs]

TTT

[HondaMan]

Start with the air screws: go in to 1.0 turns, which is correct for those carbs. Also check your spark advance, make sure it is not past the "F" marks at idle. Those 2 together will cause the overrun symptom you're having.

The 750F carbs have different settings from the PDs of the K8, as they are more efficient and have less EPA rules applied. The mainjet should be about 105 to 110, depending on tuning of pipes (and non-stock cams). The needles should be in the 2nd notch from the top, to start with.

Synching: Honda stated that the difference between carbs should be less than 2 cm Hg. On a tube-type synch bar, that's 4 increments. 

[cycleman]

A rule of thumb that I always try and follow is only change one thing at a time, when trying to track down a problem.  Start with the simplest & work to the harder. Carbs have to be really really really clean and it's not uncommon to have to clean then more than once to get them right.

Always go with adjust valves, adjust/check timing & then carbs.

[Mdub]

need some help here guys

Did you check for leaks in the places mentioned?
How about the clamps or the carb caps???
Yoe have a leak somewhere.
Mike