I have a '77K CB750 that I can NOT get to come off idle and rev up! It starts (badly) and will idle after a cupla minutes of warming and choking. Then when I twist the throttle, the engine sounds like it's being strangled and dies. BUT IF I pull the choke way out, the engine will race to hi-rpm in a split second. I have the mesh cone air cleaners, a 2-into-1 dual pipes setup wide open exhaust, and 115 man jets with the pump accelerator on the #2 carb. The carbs have been cleaned and checked a cupla times- they are clean enuf to eat offa. I 'think' the only thing I have not done is fiddle with the 'notches' on the needle rod(s). HELP!!
I had an *identical* problem last month. The bike would only start on the choke and it wouldn't rev without the choke. If I turned off the choke the idle became very rough and would often just kill the motor.
I figured "okay in order to run I have to cut down the air supply" so I figured it was running lean, but the spark plugs were dark and wet with gas.
The pilot jets were clogged, they were blocked shut. I'm thinking only the float bowl fuel level, which was correctly set, allowed this engine to start at all.
I used carb cleaner and 100psi airgun until I could see that the pilots were open when held to light. The bike would then start normally and would build revs smoothly as I opened the throttle. I put all my carb jets in a gallon-can of Berrymans carb cleaner for TWO SOLID DAYS and I still had to use a thin electric guitar string and 100psi air to get the junk out of the pilots, so hold your pilot jets up to the light and *make sure* you can see that the pilot jet is 100% open. If you just spray carb cleaning into a pilot jet that may well *not* open them up.
Your potentially 'slightly off' needle jet does *not* explain why the engine dies when you have the choke off and you open the throttle unless you analyze it carefully.
The needle jet only kicks in after 1/4 throttle open (literally, 1/4 of the full turn of your throttle grip).
You said
"when I twist the throttle, the engine sounds like it's being strangled and dies" -- can you try opening the throttle just a bit off idle, from 'not open at all' to 1/8 or 1/4 turn (you can use a piece of tape as a marker on the throttle to establish what a 1/2 turn, and what 1/4 turn of your throttle is).
If you open the throttle 1/4 turn or less and it dies, a minor needle adjustment will not fix that 'engine dies' problem.
A needle that is 'off' by a notch or so has this symptom, more often than not: when your carb transitions from 1/4 open to more fully open and the needle jet begins to influence the fuel/air mixture into the motor -- the incorrectly-set needle will often sound/feel like a 'stumble', so that you *clearly* feel that the needle is not set right.
When the carb transitions from 1/4 throttle to more open, and the needle jet starts influencing air/fuel mixture delivery to the motor, with a correctly-set needle there is no 'stumble' or 'hesitation' in the engine revs. With a correctly set needle, the engine revs just keep building smoothly once the throttle is beyond 1/4 open.
If your bike dies with just a tiny throttle opening, a minor needle adjustment will not fix that.
A minor needle adjustment can cure a 'stumbling' in the buildup of motor revs, but not fix an engine that dies as soon as you open the throttle a tiny bit.
