Topic: assistance with 78 cb750k carb tuning... (i looked!!!!)

[timdhawk]

howdy all... recently bought this 78 750k. a carb cleaning turned into a full re-build by the time i was done. I have pods and a 750F header with a tiny muffler(if you can call it that) on the end (recently refered to as a rat #$%* exhaust system in a previos post).

anyway, carbs where carefully done by me. all jets are stock size and cleaned well, no problems seeing light through all of them. no leaks, and polished to a nice shine (good for 5 extra HP    )

also put new dyna coils and electronic ignition, plugs gapped correctly and wires on at the same time.

the issue:

bike will only run/start with full choke and at a high RPM. when choke is pushed in even a little, it dies and takes alot of full throttle with WO choke to get it to start again.

pilot screws are out 1 1/2 turns.
floats all good as can get without a float level tool.
never touched/adjusted the fast idle screw from before rebuild.
Manometer is on the way, but pointless until i can get it to run.


i think the carbs are from an F model. the # on the side is 42B A Pk9. (shop manual says 42B is from an F) the jets are stock for an F to 110, 20, 35) but there is no throttle return spring from the front stay plate to the RPM adj. screw base. just the one on the shaft.

SO, what should be my first move?

[eldar]

Ok, so there is no spring on the idle knob? You will want one of those otherwise it will be impossible to set your idle.

You sound like you had an air leak between the carbs and engine. Spray some wd-40 around on the boots, see if the running changes. if it does, air leak. Reseat the manifold rubbers and make sure they are clamped down good.

As for float levels, that is easy. Tilt the carbs so the float valve closes BUT does not push in the pin. Then measure at the middle of the float, from the base the bowl gasket sits against, to what would be the bottom of the float if the carbs were right side up. Should be 14-15mm book says 14.5 +- .5mm

[timdhawk]

sorry, there IS a spring on the actual idle screw itself. Some pictures of the carb set up on the 78 show a second return spring that attaches to the base that the idle screw makes contact with. i dont have that one...don't know if it was only on the K or F. mine is not set up for it...

if no air leak, then what?
there is a smaller hose next to the fuel inlet line on the carb that was plugged with a screw before i began that was not plugged when i fired it up this time. I was told that hose went to the air box for some reason. could that be part of the issue?

[eldar]

That little hose is a vent, it is never plugged. How far open are your slides? If they are open a lot, then you will idle high. Make sure the cables are set right so the carbs can close all the way, then use the idle knob to open them up just a bit.

[UnCrash]

If jetting is stock for a K8, it is probably not dialed in correctly for your 4-1 with a short muffler and the pods.

Stock jetting is accounting for the 4-4 with stock mufflers and an airbox and filter...

Try searching for the 4-1 conversion for which jets you should be running...

[eldar]

Most people do not have to jet, or jet very little with the later carbs. In most cases, turning the fuel screw out some is all that is needed. Hell I can run rich with NO filter on the stock jets.

[timdhawk]

okay, ....

WD-40'd and no air leak at the rubbers.
i have the mixture screws out about 2 turns so far... (how far is common to have to turn them?)
the idle screw is screwed in all the way and needs to be to get it to stay running even with full choke.

oh, and slides are about 1/8th to 3/16th open without choke.
what next?

[eldar]

You need to set all the slides to the same gap. Then turn in the idle knob so they are all open about 1/8 inch, you may need more or less.

Clean your plugs and idle the bike for a while, blipping the gas once in a while. Kill the engine and see what the plugs look like. Write down results.

Clean plugs again and get out on the road even if using choke. Start in first and gun it and keep the rpms up high and keep going. When in 5th and high rpm, Pull clutch and kill engine quickly and stop without letting the clutch out. Pull plugs and note what they look like.

Let us know.

[SiliconDoc]

okay, ....

WD-40'd and no air leak at the rubbers.
i have the mixture screws out about 2 turns so far... (how far is common to have to turn them?)
the idle screw is screwed in all the way and needs to be to get it to stay running even with full choke.

oh, and slides are about 1/8th to 3/16th open without choke.
what next?

Well whatever you've got going on your CB750K8, I've got on mine as well. Same symptoms, same deal. I replaced the points with DS-2 electronic ignition, that didn't fix it, but I do know the timing is right now - easy to set and check.
 I have 4 aftermarket airfilters on it , the "air case" had a rats nest in it ( was in a barn after being crashed before I got it repaired ).
 I did pull the carbs and clean them a while back - but I can seem to get it tuned this time.
I also discovered Cyl. #3 has an iffy spark plug wire.
Oh shoot old topic message

[Spanner 1]

Classic case of no accelerator pump function...IMO. These carbs wont work anything above idle without the acc. pump working....if you 'blip' the throttle, instant death!!
Pop-off your pods and observe the carb bores. There is a brass post sticking up from the bottom of each carb which has a tiny opening facing into the carb....each one must squirt fuel into the carb everytime you twist the throttle ( even 1/4 way ). Try that......

[climbingaz]

great info guys