Topic: 1978 cb750 carb adj.

[TwoTired]

PD41A and B had needle clip adjustments.
PD42A and B did not.

[eldar]

You can however jimmy up a method. Just get thin washers and mic them to make sure they are the same thickness. slip them on the needle so they are between the clip and clip seat. Make sure they are tight or somehow fixed so they do not slide though. Or you can just get the kit for the 77 and be good.

[medic09]

Thanks.  Now I am corrected about the 77s, and essentially correct about the 78s.  I knew the fundamental design of the carbs was the same for both years.  I did not know about subtle differences like the needle clips.  I knew about Eldar's adaptation.

Lloyd, are the 41s from 77 then, and the 42s from 78?  Was there a difference between K and F models?

Thanks TT and Eldar!

[TwoTired]

Lloyd, are the 41s from 77 then, and the 42s from 78?  Was there a difference between K and F models?

Yes and yes.

http://www.motorcycleproject.com/motorcycle/text/specs.html

Cheers,

[medic09]

Thanks!

I forgot all about that great table.

[eldar]

The f bikes have 105 mains and the mix screw is turned out 1 3/4 turn. Other than that, the f and k carbs are the same.

[grwmotocycles]

Hey!   

Back to the original topic.  The two outer idle jets were plugged, which is weird cause you would think the outer jets plugged they would be lean, but the innner carbs were lean!  Just put them back together and will be on the bike tomorrow for a restart.   

[78CB750CAFE]

I have stripped and rebuilt 78 carbs twice.  Those needle do not have clips.  They're fixed on my bike.  Of that, I am certain.

 

+1 on that. No clips on 78 carbs. 100% sure.
Unless someone rebuilt them before and used the wrong kit.

crap....

no wonder that set of carbs has given me such a hassle.
here is to P.O.'s who F stuff up.

[eldar]

Well it is not really an Fup. It was done to meet epa regulations. Most likely the PO bought a keyster kit and many have reported problems using the needles in those kits.

[BobbyR]

I have a K8 and have 120 mains with no fouling. The PO put them in when he first got the bike and rode it for 2K miles. I must admit my carbon footprint is a bit large.

[78CB750CAFE]

What are the manufacture dates on everyone's K8's?
Mine is Dec 1977 and the further I dig into the thing the more bits on it I find that just don't fit the K8 bill.... standard stuff... weird stuff... I am now questioning if my carbs are actually PD41a models from a K7, since the spring and linkage is not at all what it looks like in the K8 manual.

Was there a mish-mosh rollover period where they used up the last of the K7 parts on 1978 titled bikes or is this PO tampering???

[medic09]

The carbs are marked on the side, so identifying them ought not to be too difficult.

[eldar]

My K8 is listed as december 77 also, remember, that is when it was made in japan. It then had to be shipped over here and sent to what ever dealer location. So it IS a 78.

[78CB750CAFE]

My K8 is listed as december 77 also, remember, that is when it was made in japan. It then had to be shipped over here and sent to what ever dealer location. So it IS a 78.

I know it IS a 1978 K8, but I wonder if there is any cross over of parts, or if it was a PO swap

[eldar]

probably a swap but why dont you take a pic of it and we can tell you.

[grwmotocycles]

Seem to have gotten the Carbs. in order.  Outer idle jets were plugged solid.  I am running velocity stacks and wonder what main jet is stock and how much of an increase I should use to richen it up.

[eldar]

Depends. The K is 110, the F is 105.
You may not need to change the mains at all. I know some will say you have too but in the later carbs, that is not always the case. Do a wide open plug chop and see what the plugs look like.

[grwmotocycles]

Sounds good eldar,  just thought it may need a little fuel cause right off of idle you have to feather the gas a bit to accell. after that anything above idle the throttle response is amazing!

[TwoTired]

Sounds good eldar,  just thought it may need a little fuel cause right off of idle you have to feather the gas a bit to accell. after that anything above idle the throttle response is amazing!

Accelerator pump working to full capacity?

[grwmotocycles]

Acell. pump was my first thought, but I thought that it would affect the throttle response at any rpm?

[TwoTired]

Acell. pump was my first thought, but I thought that it would affect the throttle response at any rpm?

The highest vacuum in the carbs throats (which is what pulls fuel from the jets) occurs when the slides are near closed.  Twisting the throttle opens the slides and vacuum drops significantly, pulling far less fuel from the jets.  The accell pump squirts raw fuel into the carb throat so that the engine will pick up and when the air velocity in the carb increases, venturi effect once again drops the carb throat pressure so fuel can be drawn from the fuel jets again in the correct proportion.  At higher engine RPMs, the slides are much farther open and the air velocity and venturi effect are already making fuel draw from the jets. Opening the slides further has much less effect on pressure drop than from low RPM, and requires much less additional fuel from the accell pump.

If your Accell pump is weak, stumble off idle is an expected symptom.

Cheers,

[eldar]

Check the brass nozzles on the airbox side of the carbs. Twist the grip and you should see a squirt of gas from each nozzle. There is one and they stick up.

[grwmotocycles]

I will be on it this weekend.  To bad you just can not go to the corner for parts anymore!  Thanks

[754]

You can, if you get a new bike, but you may not like the prices.

Back to yours, The carbs only had to be cleaned once.. properly... once that is done you should be fine..

[grwmotocycles]

Old Timer, (and everyone)

Must of done it properly, just could not wait till the weekend.  Got off early today, carbs together, adjusted and repaired a fuel leak at inlet line and it is a great experience.  Just went for a ride 18 degree wind chill!!  Let it warm up 10/15 min.  what a ride.  Did not realize it was cold out.  Could not go to far with no plates and registration but the 5 mile ride was worth all the effort.  Thanks to everyone and all the advise.  I always wanted a 750 since 73 when I was riding dirt, finally got one and after taking it down to bare min. and back up to finished  bike it is a great experience.  Thanks for all the help.   Geno