Topic: 1972 CB500 Carb “Missing a plug?”

[kudos2CB]

Hey folks,
I’ve been admiring these forums from a far for a while. Been saved numerous times and really admire the knowledge and the community here.
I finally made an account because I’ve ran into a very specific, peculiar problem.
I’m rebuilding the stock carb on my 1972 CB500 and after I ultrasonically cleaned it and blowing through all holes with compressed air, I noticed that there was a unfamiliar hole on the upper carb body, right where the throttle valve slide goes up & down. Photo attached for a more clear explanation. It’s the hole to the right of the brass vacuum line. Second picture is with a flashlight from the inside.
After some googling, it seems like there’s suppose to be a plug/notch there, which guides the throttle valve slide to be aligned to the proper position when it goes up & down. Strange thing is, it’s the same for all 4 carbs - none of them have it! The bike never ran properly (no power, barely idles, possible vacuum leak which I’m working on) so maybe this is part of the problem?
So here are my questions:
1) There isn’t suppose to be a hole there, correct?
2) My guess is that having a hole that’s not suppose to be there on the side of your carb is a big problem. Is that the case?
3) Do they sell the piece/tab/plug that’s suppose to be there individually? I’d really love it if I didn’t have to source another whole set of carbs.
4) If they don’t sell it individually, does anybody have any alternative suggestions to plug it up?

I appreciate all and any help! You guys rock!

[Bodi]

Other than the seals and brass, no carb parts are available as far as I know. Check the parts fiche (ie on CMSNL.COM) and probably the only parts listed are the complete carbs, a gasket set, and the removable brass parts.
There should be a small brass key that engages a slot in the slides to keep them from turning, but I'm not super familiar with 500 carbs. I doubt that there should be an open hole, the air leak down the slot would mess with idle mixture with the high vacuum then.
Can you see any keys in the slide bores? Do the slides have slots?
You could make them from brass rod with a file I suppose, and use epoxy to keep them in place.

[kudos2CB]

Thanks for your thoughts Bodi.

I scoured the web for the exploded diagrams and OEM parts but it doesn’t look like it’s sold separately - understandable because it’s probably never suppose to come off in the first place.

All four carbs have that notch/key so cleanly missing that it was hard to believe I’m the first one to encounter this problem. But I haven’t seen anything else on the internet about it.

And yes, all the slides have slots and I believe that missing brass key is what’s suppose to be in that hole.

I’m almost thinking that the slides doesn’t even really need that key to keep them from turning, because of how the slide is linked. Maybe I just need to cover that hole with epoxy... although that will be a really tricky one.

[StockRider]

I'm currently working on my set of 1970 CB750K0 carbs. I was curious about what material the throttle valve "guides" are made of. It almost looks like plastic/epoxy on mine. I'll go grab some pics of what I have.

Below are a few I found of your model carb that seem to show there should be something in there and the throttle valve is "keyed" on it.

[I thought we were talking about very similar things. Corrected by newday777. I'll keep the pics I took, which may help explain kudos2CB's question, out of the topic.]

[newday777]

I'm currently working on my set of 1970 CB750K0 carbs. I was curious about what material the throttle valve "guides" are made of. It almost looks like plastic/epoxy on mine. I'll go grab some pics of what I have.

Below are a few I found of your model carb that seem to show there should be something in there and the throttle valve is "keyed" on it.

Start a new topic rather than tagging on a totally different carb question so you don't put confusion into the op's question....(delete your post here)

[kudos2CB]

Thanks StockRider!

Thanks for the pics. I found those pics on the web as well and have been staring at them for days.

It is indeed that throttle valve guide that is missing from mine. And yes, I’ve been trying to figure out what that material is as well. Definitely not the same material as the carb body itself. It’s epoxy-ish plastic, seems like.
I’m guessing it was either not there to begin with or I knocked them loose while ultrasonically cleaning them.

Bummer that I have to look for another set of carbs just for that tiny piece...

[StockRider]

kudos2CB,

I'll go ahead and attach the pics in case they help. Again, not the same carbs as yours but might enlighten someone else down the road.

1st pic shows an unused port that is not machined all the way through on this version (1970 CB750K0). 2nd pic is the opposite side where the valve guide is on mine..., circular on the outside, rectangular on the inside (3rd pic). 4th pic is the same as the 2nd, but illuminated inside.

[Bodi]

On most Keihin carbs the slides are made in pairs, two with slots on one side and two with them on the other. The bodies as well: the key can be installed at the factory on either side and there is a key on one side and an undrilled spot for it on the other. The outside pair faces one way and the inside pair opposite. You can then install a pair or all four slides backwards by mixing this up, not an uncommon mistake that makes the engine run extremely poorly below almost full throttle as the cutout makes the carb work at low throttle. The cutouts must face away from the engine ports.

[StockRider]

+1 - I definitely can say that I learned something today! (And, just in the nick of time.)

[ADW]

Thanks StockRider!

Thanks for the pics. I found those pics on the web as well and have been staring at them for days.

It is indeed that throttle valve guide that is missing from mine. And yes, I%u2019ve been trying to figure out what that material is as well. Definitely not the same material as the carb body itself. It%u2019s epoxy-ish plastic, seems like.
I%u2019m guessing it was either not there to begin with or I knocked them loose while ultrasonically cleaning them.

Bummer that I have to look for another set of carbs just for that tiny piece...

Before you go looking for another set of carbs, I'd try to buy some brass rod stock just slightly over-diameter to that hole so it would have to be installed as a press-fit. With some careful measurements and test runs with say, some stiff wire, or tiny dowel (shishkebob sticks?) etc. you could determine:
1- how long they'd have to be
2- how deep the slot for them is in the slide; clearly enough to go into the slide trough but not enough to scrape on it. Actually you could build a fake throttle slide that you could put in the carb to tap in the final brass rod until it touches the fake slide set to the right length. Then cut off the excess on the outside. That way you wouldn't potentially damage your real carb slide when installing the new brass pin
3- what diameter the final brass stock needs to be to be a good press fit but not big enough to get stuck or damaged
4 - etc....you get the idea. If you're careful you might be able to make some that you could tap-tap-tap in there juuuuuust right once you get all your testing done. Yeah, it'll be a PITA to do it, but it could be done.
5- The end of the brass rod that touches the carb barrel slide should be rounded off, every one I've seen is.

EDIT: just thought of something else that would be easier...you might be able to find some tiny screws that are the right diameter to thread into the hole, and you would be able to use a dremel tool to cut them off (before installation obviously) and round the end over. Would be easier than press fitting and more permanent. Could put a touch of epoxy over the screw heads once installed to keep them in place.