Topic: CB500k3 choke flaps not aligned

[Teddyhoeg1982]

This is a Danish 1977 CB500, which is similar to the American CB550 k3, only difference being 50cc less. I have been fighting so much with this bike, often giving up and leaving it for weeks until I felt like coming back. Carbs are Keihin PD46C with all original brass.
First problem is that it can not take full throttle. Applying full throttle will make it die - seems to run too lean when applying full throttle. I have not found the reason for this yet. No air leaks or any other obvious problems.

Second problem for a while was that it has been very hard to start, and would only run on cylinders 3&4, and then after several minutes would start to sputter on cylinder 1, which would then come to life. Cylinder 2 would remain quite cold.

Finally I just observed that when pulling the choke, choke flaps on 1&2 are not closing as much as 3&4. That explains why it would not run cold on those cylinders. Though I can not seem to find the reason. I can push choke 1&2 closed with my fingers though. Maybe someone had the carbs apart and did not assemble them correctly.
Any guides available to troubleshoot this? Seems hard to find assembly instructions for these PD carbs.

[HondaMan]

What most often causes this on the 46C carbs is mis-alignment (or bent part) in the coupler that joins the 1-2 carbs' choke rod to the 3-4 side's choke rod. I have rebuilt/cleaned a number of them and ended up with the same situation yours has now, and found that diassembling and reassembling that "junction" between them made the correction.

Short of having some pictures to try to point out the "why" of it, if you'll look closely at that coupler gizmo, you'll probably find that it just wasn't aligned right on reassembly at some point. I use a thin wire, wrapped 270 degrees around the coupler, to pull the 1-2 rod to the right position before meshing the left- and right-hand sides together before putting them in the rack again. Sometimes I have found where someone before me actually bent the thin sheet metal plate of that coupler, which then wouldn't let them mesh up correctly. It has always been a mis-assembly problem on all of the 46C carbs (and the similar ones on the 750 bikes) when I have seen this issue.

The 500/550C is very cold-blooded, due to the PD46C calibration. There are 2 pilot jet (idle circuit) holes toward the front of the carbs, one is directly above the [pressed-in] pilot [idle] jet and the other is downstream of the slide, directly above the idle-mix screw. Most of the time, the tiny O-rings in this idle-mix screw are now dried out and won't seal, which lets too much air in below the slide, making idle almost impossible. Renewiing those tiny O-ring and clearing the hole into the throat with a thin wire fixes it up: you'll have to [heat up the body and] pull out the idle jet to get to its hole into the throat, so be gentle with those pressed-in idle jets.

There is a tiny washer above that idle-mix screw, too, which is used to squish the O-ring. Make sure it is still there?

Update:
And, lest I forget - the PD46C carbs are jetted differently from the PD46A/B: the "C" idle jet is #42, the mainjet is #95 (unless you're in Europe, where it might be #78 instead) and the float level is 12.5mm deep. I have also seen #90 mainjets in the PD46C, but don't know if they were factory original.
(The float depth in PC46A is 14.5mm, in B & C it is 12.5mm).
The PD46C needles should be in clip slot #2 (2nd from blunt end) and the idle airscrew is set at 1.5 turns, +/-1/4 turn there.

[Teddyhoeg1982]

What most often causes this on the 46C carbs is mis-alignment (or bent part) in the coupler that joins the 1-2 carbs' choke rod to the 3-4 side's choke rod. I have rebuilt/cleaned a number of them and ended up with the same situation yours has now, and found that diassembling and reassembling that "junction" between them made the correction.

Short of having some pictures to try to point out the "why" of it, if you'll look closely at that coupler gizmo, you'll probably find that it just wasn't aligned right on reassembly at some point. I use a thin wire, wrapped 270 degrees around the coupler, to pull the 1-2 rod to the right position before meshing the left- and right-hand sides together before putting them in the rack again. Sometimes I have found where someone before me actually bent the thin sheet metal plate of that coupler, which then wouldn't let them mesh up correctly. It has always been a mis-assembly problem on all of the 46C carbs (and the similar ones on the 750 bikes) when I have seen this issue.

The 500/550C is very cold-blooded, due to the PD46C calibration. There are 2 pilot jet (idle circuit) holes toward the front of the carbs, one is directly above the [pressed-in] pilot [idle] jet and the other is downstream of the slide, directly above the idle-mix screw. Most of the time, the tiny O-rings in this idle-mix screw are now dried out and won't seal, which lets too much air in below the slide, making idle almost impossible. Renewiing those tiny O-ring and clearing the hole into the throat with a thin wire fixes it up: you'll have to [heat up the body and] pull out the idle jet to get to its hole into the throat, so be gentle with those pressed-in idle jets.

There is a tiny washer above that idle-mix screw, too, which is used to squish the O-ring. Make sure it is still there?

Update:
And, lest I forget - the PD46C carbs are jetted differently from the PD46A/B: the "C" idle jet is #42, the mainjet is #95 (unless you're in Europe, where it might be #78 instead) and the float level is 12.5mm deep. I have also seen #90 mainjets in the PD46C, but don't know if they were factory original.
(The float depth in PC46A is 14.5mm, in B & C it is 12.5mm).
The PD46C needles should be in clip slot #2 (2nd from blunt end) and the idle airscrew is set at 1.5 turns, +/-1/4 turn there.

Thanks I actually bought your book to troubleshoot so far.
I just separated the rack and took out the choke arm for 1&2. The end metal plate has come loose, and caused the plate to not close. See YouTube video below. I will replace the lever. So far so good!

https://youtube.com/shorts/2A8LubZ1w6c?si=BjM_eOTotdNOi2Tc

And by the Way, I just Saw it is PD46B carbs, not C. I will check all jet values according to spec.

[bryanj]

Losseness there suggests that in the past it has been assembled wrongly and forced when the choke pulled, the pd carbs are a nightmare to assemble and clean properly and were always hated back in the day.

When we had used bikes on the showroom floor it was found those with pd carbs needed to be started and run to a hot engine at a maximum interval of weekly, and that was before ethanol!

[HondaMan]

What most often causes this on the 46C carbs is mis-alignment (or bent part) in the coupler that joins the 1-2 carbs' choke rod to the 3-4 side's choke rod. I have rebuilt/cleaned a number of them and ended up with the same situation yours has now, and found that diassembling and reassembling that "junction" between them made the correction.

Short of having some pictures to try to point out the "why" of it, if you'll look closely at that coupler gizmo, you'll probably find that it just wasn't aligned right on reassembly at some point. I use a thin wire, wrapped 270 degrees around the coupler, to pull the 1-2 rod to the right position before meshing the left- and right-hand sides together before putting them in the rack again. Sometimes I have found where someone before me actually bent the thin sheet metal plate of that coupler, which then wouldn't let them mesh up correctly. It has always been a mis-assembly problem on all of the 46C carbs (and the similar ones on the 750 bikes) when I have seen this issue.

The 500/550C is very cold-blooded, due to the PD46C calibration. There are 2 pilot jet (idle circuit) holes toward the front of the carbs, one is directly above the [pressed-in] pilot [idle] jet and the other is downstream of the slide, directly above the idle-mix screw. Most of the time, the tiny O-rings in this idle-mix screw are now dried out and won't seal, which lets too much air in below the slide, making idle almost impossible. Renewiing those tiny O-ring and clearing the hole into the throat with a thin wire fixes it up: you'll have to [heat up the body and] pull out the idle jet to get to its hole into the throat, so be gentle with those pressed-in idle jets.

There is a tiny washer above that idle-mix screw, too, which is used to squish the O-ring. Make sure it is still there?

Update:
And, lest I forget - the PD46C carbs are jetted differently from the PD46A/B: the "C" idle jet is #42, the mainjet is #95 (unless you're in Europe, where it might be #78 instead) and the float level is 12.5mm deep. I have also seen #90 mainjets in the PD46C, but don't know if they were factory original.
(The float depth in PC46A is 14.5mm, in B & C it is 12.5mm).
The PD46C needles should be in clip slot #2 (2nd from blunt end) and the idle airscrew is set at 1.5 turns, +/-1/4 turn there.

Thanks I actually bought your book to troubleshoot so far.
I just separated the rack and took out the choke arm for 1&2. The end metal plate has come loose, and caused the plate to not close. See YouTube video below. I will replace the lever. So far so good!

https://youtube.com/shorts/2A8LubZ1w6c?si=BjM_eOTotdNOi2Tc

And by the Way, I just Saw it is PD46B carbs, not C. I will check all jet values according to spec.

Wow, a broken weld! I guess someone bigger and stronger than you was in there before? 
I'm glad you don't have the "C" carbs. They have to be set up like the factory just built them in order to work well, and with little info to go on. The "A" and "B" (and the rare, early 'no letter' version) are not as picky. I often have set those according to the straight edge of the floats themselves: making it parallel to the seal surface of the carb body sets it at the 12.5mm or 14.5mm that Keihin originally used, like a "hint, hint" sort of thing. I started doing this after I came across some of the "no letter" carbs long ago and couldn't find any reliable info about them: one of the floats was broken, so I think I still have a set of 3 around here somewhere, and I set them to the "parallel" setting for the owner - that became the 14.5mm depth. He said the bike fired right up when he got them back, so I've leaned that way since. The CB750 PD carbs are similar, with similarly-shaped floats having a flat top on them.

I've also had some good luck with sometimes setting the 14.5mm versions to 12.5mm instead, usually on bikes with damaged airboxes or non-OEM intake filters, like foam elements. Those that restrict airflow above 5000 RPM are the troubled kids (foams, mostly) and the vacuum lift in the mainjet emulsifier is less when the inlet air is being choked. These bikes were used in large cities like Chicago and New York,where they seldom exceed 45 MPH, so they work OK. They don't do well when trying to use the freeways, though, with those foam air filters.