Author Topic: 1978 CB750k Carb rebuild over the winter and tune up questions! (Read 13116 times)

fendersrule · « **Reply #75 on:** December 09, 2013, 02:22:05 PM »

I gave my dad this thread, so this is going to be a big help for him.

I want to direct everyone back to this post:

http://forums.sohc4.net/index.php?topic=131396.msg1474625#msg1474625

Does this spring need to be replaced?

flybox1 · « **Reply #76 on:** December 09, 2013, 02:28:07 PM »

No, but i would clean it.
I believe its the one associated with your fast idle cam.

fendersrule · « **Reply #77 on:** December 11, 2013, 10:57:23 PM »

He's finding good results so far. I'll ask him how dirty stuff was.

flybox1 · « **Reply #78 on:** December 12, 2013, 08:02:53 AM »

nice. thats all stock jetting for stock airbox/filter and exhaust. if you have different, jetting needs to change
new orings for the fuel screws (already missing one) and replace that missing washer.
new float valves as rubber tips can harden over time. if you didnt have carb overflow issues, reuse them.
clean out the holes in all the parts.
as long as the pilot jet holes are not compromised, those can be reused. they just look a little beat up is all.

fendersrule · « **Reply #79 on:** December 12, 2013, 09:37:25 AM »

My dad thinks that the missing washer/rubber is up in the cavity of the carb. He will look.

The only thing that isn't stock is my K&N filter in my stock airbox. That's probably not enough of a reason to rejet, but correct me if my assumption is wrong.

My dad said the rubber diaphram is in good shape of the accelerator pump. Still worthy of replacing?

bowl gaskets are probably not a bad idea to replace.

So far, bowl gaskets are probably the only thing I'll order unless you can think of anything else.

Did not have any carb overflow issues. I did find that one of my overflow tubes was clogged and needs to be cleaned out/replaced, but I'm even thinking of reusing the float valves too.

flybox1 · « **Reply #80 on:** December 12, 2013, 11:06:11 AM »

Check the diaphragm or tears and cracks. they are cheap. replace if questionable.
FYI..they do not react well to carb cleaner.
clean out the holes in the ears as they can easily obstruct the accel pump circuit.
replace that tiny o-ring at the top of the #2 carb bowl. its probably rock hard.
I got one at my local auto store. fuel resistant viton or buna-n only.

while he is waiting for replacement parts, he can work on the fast idle cam/choke linkage between #2&3 carbs. needs to engage well and lift the slides 3-4mm.

The K7/8's are lean to begin with. The freer flowing K&N will make you more lean.
I would re-jet for a richer mixture, even with a stock airbox/exhaust.
38/120's and one shim or down one clip position (depending on your model carbs)

SKTP · « **Reply #81 on:** December 12, 2013, 01:34:39 PM »

I'm just posting in this thread so I don't lose it...I'm about to do all of this stuff as well...great stuff here

fendersrule · « **Reply #82 on:** December 13, 2013, 10:16:20 AM »

Hmm...thanks for the info flybox. Do I really have to rejet in order to fix my throttle response issue as seen in the video? It just seems like a lot of work, but If you are 100% it will get me a better performing and more responsive bike then I won't argue with you! I just keep thinking it's going to add to the tuning complexity. If you are sure of it, do you know where I could get original 120 mains (no keyster crap) and 40 slows and what to get for shimming?

I'm trying to get my dad to sign into the forum and post stuff, but here's his latest reply:

These brass parts have been soaking in carb cleaner for 2 days now. The clear fluid has turned a cloudy green color with sediments on the bottom of the container.

I don't know how to remove the carb slides without separating each carburetor which I was trying to avoid. Any instructions?

andrewk · « **Reply #83 on:** December 13, 2013, 02:15:39 PM »

You might have to rejet to make it run its best. But run it with clean carbs first and see what it does. Key to tuning these things is to change one (and only one) setting at a time. So clean carbs, step one. It's tempting to put the cart in front of the horse, but don't.

To remove the slides, the carbs need to come apart. -- EDIT -- that's not true! see flybox's reply below-- There are felt seals on the pivot shafts and they are NLA, so beware of those and keep track of 'em.

flybox1 · « **Reply #84 on:** December 13, 2013, 02:23:59 PM »

Quote from: andrewk on December 13, 2013, 02:15:39 PM

You might have to rejet to make it run its best. But run it with clean carbs first and see what it does. Key to tuning these things is to change one (and only one) setting at a time. So clean carbs, step one. It's tempting to put the cart in front of the horse, but don't.
CORRECT
To remove the slides, the carbs need to come apart.
NOT TRUE. Slides are easily removed. remove the top caps and then unscrew the JIS screw that connects the slide linkage. remove the spring, and then gently slide the linkage off the pin, pulling the slide out. i will attach a good thread with how-to pictures

your father needs to see, read, and follow this thread for slide removal (and needle shimming).

http://forums.sohc4.net/index.php?topic=71245.0

andrewk · « **Reply #85 on:** December 13, 2013, 02:39:18 PM »

No idea what I was thinking. They totally come out without dis-assembly. Thanks, flybox.

RodSOHC750 · « **Reply #86 on:** December 16, 2013, 10:47:19 AM »

I'm the father. I'm tearing down the carburetors on a 1978 SOHC 750. I'm down to trying to remove the slider on Carburetor #3. It appears I need to remove the roll pin through the throttle shaft. Is this the next step? All brass parts appear to be ok other than needing some soaking and cleaning. The connecting hoses are cracking and fit loose so they will likely need replacing. Any other hints? Any links to good carburetor rebuild instructions?

I'll post pictures shortly.

RodSOHC750 · « **Reply #87 on:** December 16, 2013, 10:50:57 AM »

I GOT THE THROTTLE SHAFT DISASSEMBLED. I'M TO THE THROTTLE CABLE WHEEL. DOES THIS ASSEMBLY COMPRISE OF TWO SEPARATE PIECES THAT ARE SUPPOSE TO SLIP WIHTIN THE OTHER?

flybox1 · « **Reply #88 on:** December 16, 2013, 11:18:06 AM »

ROD....
Sorry you disassembled them all the way as it really didnt need to be done. Epic cleaning job you are doing on those carbs, though.

there are 3 connecting hoses you need to be aware of. these are the three smaller tubes that comprise the accel pump circuit.
each of these has 2 small brass pipe sections inside. please reuse these brass bits as they help to reduce the volume flowing in the circuit, aiding in a more forceful accel pump squirt.

RodSOHC750 · « **Reply #89 on:** December 16, 2013, 09:35:14 PM »

I finally separated the hub from the wheel. They were corroded together. After separating and cleaning, they move freely. I applied some silicone grease. I hope I have enough pictures to figure out how this assembly goes back together.

RodSOHC750 · « **Reply #90 on:** December 17, 2013, 05:31:30 PM »

I have a plastic part that I can't determine where it goes. I think it goes in #2 carb between the slide lifter arm and the carb housing.

RodSOHC750 · « **Reply #91 on:** December 17, 2013, 05:45:33 PM »

There is excessive clearance in the choke shaft-to-carb housing openings. Are there replacement seals or bearings for these?

flybox1 · « **Reply #92 on:** December 17, 2013, 05:51:58 PM »

A picture of this excess clearance might help us...

RodSOHC750 · « **Reply #93 on:** December 17, 2013, 07:26:53 PM »

here's a picture of the clearance issue. There's at least 1 mm clearance between the shaft and bore. Its at the choke valve and upstream of the carburetor so my major concern is contaminants entering the carb.

RodSOHC750 · « **Reply #94 on:** December 17, 2013, 07:30:16 PM »

The hoses between the carb for fuel feed and accelerator jets need replacing. They appear to be different diameters. Can anyone suggest the correct hose size? I found 4.5 and 5.5 mm ID hose for sale.

flybox1 · « **Reply #95 on:** December 17, 2013, 09:23:52 PM »

4.5mm. Same as the OD of the brass restrictor tubes hiding in those old fuel hoses...

fendersrule · « **Reply #96 on:** December 28, 2013, 06:44:09 PM »

Turns out the "gulp" sound was most likely the function of improper timing on 2-3.

We installed the rebuilt carbs with no issues, other than a cracked casing in #3's bowl which causes a continuous leak out the overflow tube. New bowl has been ordered.

We set the fuel screws with the exhaust analyzer.

We are now syncing all the carbs.

Even without the carbs synced there is no "gulp" sound anymore. Likely, the timing was the culprit. However, I'm sure the carbs needed a cleaning anyhow as it fixed one thing mentioned below. It's going to be fun to get out and see how well it performs.

The carb cleaning has also fixed decreasing the RPMs by moving the throttle sleeve forward. It turns out I was wrong about a loose pull or push cable being the reason. The reason for mine doing this is because throttle cable arm/wheel had gunk and were seized to the throttle shaft. The brass bushing was corroded and needed cleaning. Steve, be aware of that.

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Author Topic: 1978 CB750k Carb rebuild over the winter and tune up questions! (Read 13116 times)

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