1978 cb750 carb swap

[Lucky 1]

The needle controls the midrange from 1/4 throttle to about 3/4 throttle.

About your mixture screw.
Gently turn it all the way in and when it stops do NOt tighten it .
Then turn it out to the desired amount.
With that #45 idle jet that you drilled with a #77 (numbered size drill)
Your mixture screws should be out about 1/2 to 3/4 of a turn. Are they?

To tell which idle jets you had originally, look on the idle jet
near the top with a magnifying glass and you will see the number.
 If you drilled that jet out, file the number off!!!!

[shultz_1978]

Well, my main needles has multiple grooves in them. They are adjustable. I left them where they where. It took 3 full turns to get my mix screws in. So I turned them back out 1 full turn. I think I will see what that does. Rode it around a little and it still runs nice with a good pick up. If I leave it here I will take the mains out and get a picture of them to show where they are set at. Sofar so good. Maybe the main jets have been changed? I still need to change the plugs.  

[Lucky 1]

You say it is a 1978 but it has adjustable needle with grooves and a clip?

What position is the clip.?

I am thinking maybe it is 1977.
Very interesting!! Maybe THIS is the cure for the 1978. 1977 needles.

I just looked up the 1977 CB750K on Bike Bandit OEM parts breakdown and it only shows a needle with one clip position (non adjustable needle).
Maybe someone had this almost figured out except for the idle jet.

Wow This sounds like a breakthrough.

I am really excited to hear back from you about this needle with adjustable clip.
I would like to see a photo if at all possible.

The 1976 DOES have a adjustable needle.
Maybe what you have is a 1976 needle in a 1978 carb slide.

[shultz_1978]

It looks like rain here for the next few days. I will have to see if I can get them off and take some pictures. We'll see what happens. Thanks for the help.

[Lucky 1]

Thanks Shultz!!
You may have discovered a very good remedy for this 78 carb for
all the owners that want pods or stacks instead of the stock airbox!

What did you have for a muffler?

[Lucky 1]

BTW Shultz.. If you just put a pan under the bike and open all of the float bowl drain
screws it is a fast way to get them ready for removal.
It is much easier to do it that way instead of draining them after you take them off.

Make sure the drain pan is not close to a gas fired hot water though!!

[shultz_1978]

Lucky this is my bikes set up no mufflers  I still don't know if it is runnuing right and it looks like rain for the next few days. I will see about getting the pictures up tomorrow.

I don't even know what year the carbs are. Someone may have changed them. They changed everything else. But I will try to find out.  

[Lucky 1]

Hard to tell from the photo but those look like 1976 carbs.
They have the float bowls with spring clips instead of screws like the 77-78's.

That does explain the needles with clips on the slides.
1976 carbs do not have the accelerator pump on the #2 carb either.

Much simpler and easier to deal with.

[shultz_1978]

See now you are making me want to get the carbs off and snap some pictures. I think there is a pump on number two carb. Is my memory getting that bad. lol

[Re-run]

The picture above ARE the 77/78 carbs. The 77 needle had slots in it. Only the 78 did not have adjustable needles.
He could have 77 carbs, they look the same.

[Lucky 1]

Thanks for clearing that up ReRun. I wonder why the Bike Bandit OEM drawing shows only one clip position for the 1977 version carb?

[Lucky 1]

Thanks for clearing that up ReRun. I wonder why the Bike Bandit OEM drawing shows only one clip position for the 1977 version carb?

Do you think he has the 1976 version needle and clip in that 1977 carb?

[shultz_1978]

Thanks guys I was about to go to the garage and start working. Thought I was losing it.lol I will still pull them tomorrow and post pics of what I find.

[Re-run]

I think honda discontinued the 77 needles but otherwise, bike bandit is wrong. The 77 had slots on the needle. I suppose it saves minimal space to use the same fiche for both.
I think the 76 needle can be used though.

[shultz_1978]

Sorry
  I don't know what I was working on that had the adjustable needles but it was not this bike. Here is a picture of what I found.The slide has a 83b on the bottom of it and needle has 15a on it. It looks like some has made shims that look to be about twice as thick as the c-clip. (Sorry don't have mics to check it). :( I don't know if this is all stock? Don't think the shims are thow.

I have not had a chance to take it out since playing with the mix screw but I will post when I do. Thanks for any info.

[Lucky 1]

I wondered how your bike could be running good without a shim on a non adjustable needle without a shim.

The thickness of that shim is VERY VERY important. Could you please take it to a machinist to measure or
get some calipers.

You do not need a micrometer,just vernier calipers that measure 0-.100 in one revolution. A pair of 6 inch calipers.

Do not waste your money an $25 calipers.

Maybe a NAPA auto store could measure it with some calipers. Don't lose that shim!! LOL..lol

Thanks Shultz!!!

BTW...Is that shim plastic or metal?
That one shim in that FIRST photo looks like the plastic washer on the mechanical linkage of the throttle
mechanism. Was that shim on the slide needle??

Oh, sorry ..One more thing.
Does that shim sit down in that pocket in the bottom of the slide?

That pocket is about .012 deep and that amount would need to be figured into the equation.

[Lucky 1]

Were on the edge of a breakthrough?  Lets hope so.

[shultz_1978]

OK the shim is 1/32 = .03125 it sets in the pocket -.012 =

[shultz_1978]

sorry = .01925 that was the shim in the picture you saw. It is brass

I still do not know if it is right but it is better then what it was running.

Lucky I am in Ohio. Our climates be different. But who knows our weather has been mid 60s to high 60s and a little wet.

[Lucky 1]

Thanks for your efforts Shultz!!!

Brass shim .031

And you said it was accelerating smoothly? Right?

So if the shim is .031 thousands  AND it fits under the clip on the needle, AND it sits down in the pocket like the needle clip does, it is raising up the needle .031 thousandths. No need to add the pocket depth.

The needle is being raised up .031 thousandths. Makes sense. I know .044 thousandths was too rich.

What is interesting to me is that those exhaust pipes are form the 70's too and I knew someone had fiqured all of this out but the information got lost.

So when you get your carbs back on and you know the accelerator pump system and spray nozzles work
please tell us that it runs perfect.


To check those nozzles on the bench, make sure the float bowls are full of gas (carbs level) and flip the throttle a couple times, and make sure all 4 brass nozzles sticking up are squirting fuel.