Topic: What (else) is wrong with these carbs?

[edwardmorris]

Before I tear these open, I wanted to make sure of a few things, most important one being Are they correct for a 72 CB750 K2?. I've highlighted a few things that I thought were off, is there anything else I should look out for?

[flybox1]

Before I tear these open, I wanted to make sure of a few things, most important one being Are they correct for a 72 CB750 K2?. I've highlighted a few things that I thought were off, is there anything else I should look out for?

Yeah.... Look at the drain screw orientation...none should point towards the center of the bike.
2&3 bowls are reversed....

[flybox1]

...and that rogue screw in the center is plugging up one of the vacuum sync ports.  Let's hoped they used a m5 with the correct thread pitch and didn't just Fk up the threads ...

[Stev-o]

Before I tear these open, I wanted to make sure of a few things, most important one being Are they correct for a 72 CB750 K2?. I've highlighted a few things that I thought were off, is there anything else I should look out for?

Yeah.... Look at the drain screw orientation...none should point towards the center of the bike.
2&3 bowls are reversed....

But thats no biggie, someone just put the wrong bowls on 2&3.

Take the bowls off and let us see inside...

[edwardmorris]

Thanks all, I'll try and drop the bowls tonight after work and add more pix. Anyone have any ideas about the left most one in the first pic (big red circle)? That's the only one that's different from the rest.

[Stev-o]

The choke is closed on that one, choke must not be sync'd.

[Henning]

Quite a project you have going there. Looks to me like your no. 2 carb is really a no. 4 carb. The idle air screws/vacuum screws on 1 and 2 point out to the left, and on 3 and 4 out to the right. It will work, but it's ham-fisted and will be a pain to tune. And something is wrong with the choke on no. 4.

[LesterPiglet]

Yep, #2 has the mixture screw pointing inwards instead of out.

[fmctm1sw]

Yep, #2 has the mixture screw pointing inwards instead of out.

I had a set just like that and I still have a bowl drain that points to the inside.  Since the 750 bowls are so easy to get off, I never bothered with it.  I have a 750 carb body laying in my garage.  If any of yours turn out to be beyond repair, let me know and I'll see which it is...

[TwoTired]

Does they even HAVE choke plates for 3 of the carbs?

What a mish-mash.  Someone either knew exactly what they were doing to make them work properly or didn't have a clue and just assembled 'til they ran out of parts.  Either way, restoration will be a challenge, particularly for a noob.  I would expect to examine each and every internal part for dimensions, correctness, and settings.

Look on the carb bodies for a number like 7A or B750A. Those would be correct for a 72 according to this:
http://www.motorcycleproject.com/motorcycle/text/specs.html

[harisuluv]

Yeah, with that one choke plate down and the others not even visible, it's possible they aren't even installed.

It is correct that the #2 body has been replaced.  I agree that it is a #4 body, which means between 1&2 you basically have no duct to the outside atmosphere, which would not help proper filling and depletion of the bowls.

I will add that the #2 body itself is not the same casting.  It is probably from an "F" model or a 76 cb750.  Which would make it an 087A or 069A.  I would also add that the screw caps appear to be from a 72 or earlier model.  Post 72 sometime they moved to a cap with a larger bevel on the bottom.  Your 1,3, and 4 bodies if from a 72 should be 627A. 

The body of a 76 carb or supersport carb is most recognizable by the little point "horns" that are right above the air screw, and is symmetric to both sides.

The bowls are different too, but there are perhaps a half dozen different castings that vary, your bowls all appear to be the same.  So it looks like the body was primarily replaced.  However, whoever did it may have transitioned some internals over, so I would check the jets, and the needle clip (the number on the needle itself and the clip position relative to the others. 

There is a variation on the throttle slide rod from different years/models where some had a beveled edge on the threaded portion as opposed to a more squared top.  Looking at the tops of your threads it looks like some are different, this COULD mean that slides were swapped, which means you should go through all of them, and check out the needles and clip positions.

The backplate is correct for a K model, the F model had a different configuration for the spring and where the spring hooks to the back plate.

I would also check your emulsion tubes because the configuration of the holes is the same between the two different types I know of, but the diameters are different, which will affect your mixture.

Let me know if you need any parts, I can supply just about anything you would need, including another body, here are most of my spare "round top" bodies, soda blasted and ready to go. 

[edwardmorris]

Thanks Harisuluv for the detailed explanation, these things are just a new addition to the pile of horse$hit this bike has been since purchase. I should have looked at these sooner, Ron/MCRider just sold his nice K2 stock carbs 

Haven't had much time, and wasn't really planning on getting into the carbs just yet, but I guess now's a good time as any. I took some pix with the bowls off, there's cobwebs in #1 and this one was always leaky. The float looks busted on it too? I just Keihin on all the bodies, didn't see any numbers, but I may not have looked in the right place. The choke plates are all there, but the last one doesn't move. I suppose I'll start tearing these down for practice anyway and start looking for a good K2 set. In the meantime, can these be salvage/corrected?

[edwardmorris]

Closer

[harisuluv]

Who knows what's going on internally.  However it looks like someone has installed new rebuild kits in it.  Which is actually not the greatest, hopefully he didn't do the needles, aftermarket needle jets suck.

They can be fixed for sure, but not exactly a standard rebuild, lot of curve balls.  Stamping can be seen if you're looking down from the top, on the cylinder head side.

[edwardmorris]

I made some time to separate them today, and like harisuluv said, 1, 3 and 4 are 657As and 2 is 086A. Hopefully I'll tear them down fully this weekend and post back here. If the jets are correct and OEM, then these might be worth holding on to.

[edwardmorris]

Further teardown, pic 1 and 2 are #1 657A. Same story for #3 and #4 but I can't tell what jet sizes etc. are. How do you tell. Obviously these need cleaned out, I might borrow my buddy's HF ultrasonic cleaner and give it a shot. Pic 3 and 4 are #2 086A, looks like the rest except for the body. Good news on that nasty long screw sticking out, its the proper size metric one and unscrewed effortlessly. I haven't dug deeper on the needles, not sure how well these were synced but for now I left it alone. I can tear those down too if you guys need to take a look inside. Thoughts?

[LesterPiglet]

I think that first picture Harisluv posted would be a great theme in my kitchen.
Bugger all those shelves of teapots and teddys.

[lucky]

Check this out.

[harisuluv]

I think that first picture Harisluv posted would be a great theme in my kitchen.
Bugger all those shelves of teapots and teddys.

Lol, that would actually be nice.  Some sort of carb cabinet system.  That drawer is actually about a third of my spare bodies, I could use it.

[edwardmorris]

I ultrasonic cleaned the #2 body (details here), things are a little cleaner now but I'm still not sure how to determine the jetting. The main jet has the number 20 on it and I didn't see anything on the rest. What's the best way to find out?

[ekpent]

Do any of the jets have a sylized "K" logo for Kehin. The 20 may be #120 on the main. The slow should probably be marked 40.

[harisuluv]

The float valve is aftermarket, question is did they install a whole rebuild kit?  or just the float valves and some other select brass.  There is a number on the brass needle jet, check them all and tell us what they are.

[Henning]

Like the guys are saying, you need to go through the carbs to see what you have. Grab a piece of paper and write down what you find. Something like this (here for my K1):

         1         2         3         4
Carb         7A         7A         7A         7A
Needle      K27201      K27201      K27201      -
Clip pos.      3         3         -         -
Slide         102K2.5      -         -         -
Idle jet      K40         -         -         -
Main jet      K115      -         -         -
Needle/seat   K2.0         -         -         -
Air screw      K         -         -         -

K is for Keihin. If you ever post a carb question, guys will want to know what you have in there. Now is the time to find out.

[lucky]

Do any of the jets have a sylized "K" logo for Kehin. The 20 may be #120 on the main. The slow should probably be marked 40.

Some main jets are stamped on the top and some are stamped on one of the sides of the hex nut portion. If a very responsible mechanic worked on it and it has no number then it means it was drilled out. But of course a really responsible mechanic would etch the number on the brass.

[lucky]

I would replace that needle jet and seat. That is if you really want to be sure of the results.  For me it is a matter of my reputation.