Topic: weird carburetion problem

[mazingerzeca]

Hello all.
The bike is a CB750 K7, 4 into 4 exhaust, standard air filter and Dyna S ignition. The bike runs well at low rpms, but when you go on high speeds and high rpms, it hesitates, and sometimes I notice that is only going on 3 cylinders. My first thought was that the needle was too high, but checked it and it is on its lowest position (the top notch). Checked the plugs and they look a bit black, so I turned the pilot idle screw 1/2 turn out, so now it is 2 turns out from totally tighten. I'm not so sure, but I think that in this model when you unscrew the pilot screw it increases the air on mixture, but I think that it only affects the idle, and on my bike the problem appears at high rmps.
What else should I check?

[Rg72chop]

this is my post it may help...  http://forums.sohc4.net/index.php?topic=92905.0

[KingCustomCycles.com]

sticky float needle in that carb not allowing fuel to flow freely, partially plugged main jet, float level too low on cold cylinder carb, sparkplug or lead firing intermittely at high rpm, your needle should be in the middle notch for your basically stock set-up. Idle adjust screws have nothing to do with high rpm operation.

[mazingerzeca]

I cleaned and rebuilt the carburetors, but I left the adjustments as the previous owner had set. I wonder if the previous owner put the needle on the lower position because the needle does not go tight on its jet, and tried to solve the problem that way. I think that I should dismantle the carbs again and check it.

[bollingball]

I think you have the wrong carbs 77-78 750 PD42a & PD42b do not have adj needles Well you can put washers in but they don't have slots. Look to see what number is stamped on the boss of the body. Are you sure you don't have a K6? or earlier bike or engine?

                    Ken

[the technological J]

have you done your timing? i would do the 3000 mile tune up after you go over thefloat heights.and make sure the carbs are synched up

[mazingerzeca]

The carbs are stamped PD41AA (the second A inside a square). Maybe the previous owner changed the needles, if you say that the original ones don't have slots. Tried to find new needles and jets, but they are out of stock for PD41AA carbs, they are only available for PD41AB, but I don't know if they will fit on my carbs.
Yes, I did the timing and the carbs are synched.
Thanks for your help.