Press in jet size? Only runs with choke open

[Maria24060]

Hello, I'm new here. I have an '80 cb650c. 13k original miles.. I bought it for $500. The guy had gotten the carbs rebuilt and hydroblasted. When he got it going it would only run on full choke. It sat for a year before I purchased it. First thing I noticed was that the tank interior was in bad shape. Lots of flaking rust. I cleaned it and coated it in red kote. I cleaned the carbs and installed a rebuild kit because I noticed the jets were bent and marred. Still have the same problem. Tried bench synching them. Maybe a slight improvement, but not much. Going back and looking through everything the press in jets are 35's.. is that correct?
Did some more investigation and the air cutoff diaphragm on the end was totally torn. And the accelerator diaphragm was nasty and the piston looking portion was rusty.

Now, I need to unrack the carbs and check the other diaphragms. Any instructional on that?

Anything else I should investigate for my choke issue?

Thanks in advance

(Yes, I have the air box and rear signal lights)

[Volker_P]

Welcome here!

bike looks like good basis with lots of work.

The guy had gotten the carbs rebuilt and hydroblasted. When he got it going it would only run on full choke. It sat for a year before I purchased it. First thing I noticed was that the tank interior was in bad shape. Lots of flaking rust.

I'd say these diaphragms look like they did sit there since 1980.

I cleaned the carbs and installed a rebuild kit because I noticed the jets were bent and marred.

Bent jets mean mistreatment. Be prepared that other things in your carbs are bad, too. E.g. it is basically possible to mount the floaters upside down.

Still have the same problem. Tried bench synching them.

Vacuum synchronization is fine tuning that only makes sense when everything else is all right. Just adjust the slides that all four open simultaneously to the same tiny gap when you turn in the black idle adjust knob. This is sufficient for nice idle and riding if nothing else is really wrong.

Going back and looking through everything the press in jets are 35's.. is that correct?

Yes.

Did some more investigation and the air cutoff diaphragm on the end was totally torn.

Do not expect a bike running reasonably that way.

Now, I need to unrack the carbs and check the other diaphragms. Any instructional on that?

The air cutoffs in the CB650 PD carbs are somewhat special. All carbs have the bolted covers but only few have the diaphragms below (like yours, probably PD50B carbs). Therefore most carb rebuild kits do not contain them.
The carbs without diaphragms do not have the corresponding holes bored through. This means you cannot just leave them out.
However it is possible to plug the corresponding passages to avoid splitting the carbs and buying diaphragms.
Have a look on page three of viewtopic.php?f=4&t=722 and to viewtopic.php?f=4&t=8781&start=6

Anything else I should investigate for my choke issue?

Guess you need the choke to compensate for the "air leak" through your torn diaphragms.

Good luck!

[Maria24060]

So I gather that you can bypass the air cutoff valves by plugging off the hole beside (not below) the butterflies. Someone soldered, and another plugged with toothpicks.

Do you think JB weld would be sufficient?

[Maria24060]

I'm running into a new problem. I had started to take the carbs apart until I found out they could be plugged. Now the throttle sticks open. Any ideas where I should look for the issue first?

[themaddrebel]

Putting the cart before the horse but, when you get done, this tool is only 25 bucks on ebay and will allow you to sync carbs

https://m.youtube.com/watch?v=nh4OqKzuRvk&t=397s

Sent from my SM-G955U using Tapatalk

[Volker_P]

The throttle spring is very strong. So either the spring is not in proper position or something serious blocks the throttle mechanically.