I am NOT surprised at seeing these posts.
As more 1977-78 bikes start to be rebuilt/cafe'ed and run you will see more of these posts.
The reason is that the 1977-78 carbs with pods. or stacks and a 4 into whatever exhaust
cannot just have a larger main jet and run properly.
Look at TWO TIRED's post above at the graph and you can see the main jet influence and its range. And in the graph above and others like it there is no accelerator pump and squirt nozzle influence shown.
The idle jet will have to be increased. Zero-1/4 throttle.
The main jet will need to be increased too, but you cannot just increase the mainjet and have the entire midrange FIXED. The midrange 1/4-3/4 throttle does have some overlap from the main jet but not all the way through the full range.
Also on those 77-78 bikes you have to make sure the accelerator pump and spray nozzles are working.
Look into the intake with filters off and engine off. Carbs full of gas. quickly turn the throttles a couple of time and you should see gas squirt out of the brass spry nozzle sticking up in the venturi.
Since the needle on the slide of this 1977-78 bike does not have the adjustable clip on the needle you will need to add some shims to raise the needle and richen up the midrange.
The shims should equal one position space on the needle .Try 1mm or .039 thousandths. make sure the little washers or shims sit down all the way into the slide piston. SEE PHOTO below showing the recessed pocket.
Make sure the washer will fit all the way down in the recess.
The brass washers vary in thickness so you have to measure them. Also there is a burr on the back side that needs to sanded or filed smooth.
If you use a stainless washer they are more precision in thickness but they would be harder to file or sand if you wanted to alter them.
NOTE: I actually made a mistake and put .044 thousands worth of shims on the needles of my 1978 carbs and was not paying close enough attention and did not notice that there was this recessed pocket at the bottom of the slide and so my washers were too large on the OD and did not sit down all the way.
The recess pocket at the bottom was .012 deep. so I actually I had raised the needle .056 thousandths!! It ran very strong but boy was it rich. LOL
So you can see that it is very forgiving. I had a #45 idle jet too. But that was a little large and it was not easy to start. I am going to reduce that to a #42mm.
The #40mm idle jet would not work it needed the #42mm
The main jet should be about 120 or larger. I would not go over 130.
Remember you are not going to be at wide open throttle that much.
After test riding and the engine cools, pull out a plug and check its color.
Maybe #1 and #4.
BTW while you are taking the slides out to put the shims back in let the slide on #2 carb set down to the bottom(make sure the idle screw is backed off), and with a caliper measure from the top edge of the slide to the top edge of the carb body and as you get the shims on all of the slides set the other carb slides to the same measurement as #2 slide.
The measurement should be about 1.205(something close to that) + or - .007
Then all of your slides will be with in .001 of each other.
Click ONCE on the photo to get a larger image.
After I run my 1978 CB750 with #42 idle jets,120 mains and .037 shims under the needles I will report back and let you know how close I got to perfection.
I would like to hear your results as well.
Between all of the owners of the 77-78 owners we will get the jetting down to a science. Make it easy for the ones that come after us.
Video link to my 78 bike running.
1978 CB 750
