Topic: Frustrating CB500K3 carb issues

[Teddyhoeg1982]

Sorry if this is a long post, but I am really losing hope to get this bike running right.
I have a 1977 CB500K3 that I spent quite a bit of time on converting back to original. When I got it, it was a poor and stranded cafe racer project. Unfortunately I can not make it run right. Too bad since I did a bolt-up restauration, including new paint job and a long list of OEM spareparts.

In Scandinavia (Denmark in this case) they sold these CB500's that were similar to the US and UK CB550 K3's, only difference being that cylinders were 500cc instead of 550 cc.

This means also that it was equipped with a set of PD carbs from the factory. Apparently 1977 CB550K3s was running PD46A carbs, while these CB500K3s were maybe equipped with PD46B carbs. I think it is only jetting and possibly needle being different to account for the 50 cc less engine volume. This bike is equipped with a set of PD46A carbs.

It has 42 pilot jet, 90 main jet - both OEM Keihin brass - and float level are confirmed to be 14.5 mm. as original. It has new connection rubbers and new OEM air filter, also new rubber O rings on the intake manifolds. I noticed the slide needle has a "D376" stamp, which indicates they are Keyster aftermarket. This may be a problem.

Now to the running issue: With anything more than 50% throttle, the engine dies. I believe it comes from running too lean when you open up the throttle.
To make it idle acceptable, I need the fuel mixture screws 4 turns out. I believe with 4 turns out, they are supplying more fuel than the standard 1,5 turns out. This is surprising to me. If you leave it idling for 1-2 min., it gradually slows down and dies, which I see as running too rich idling.

Even though it can idle ok like this, it still does not like to accelerate. With a light touch on the throttle it accelerates ok, but will only reach 4-5000 rpm until it then starts to bog and die, since you are applying too much throttle.

Can someone please help me get my motivation back and suggest what I should check.

I considered buying new OEM slide needles, but they are 200$ for a set of four... I am almost considering just buying a new set of Murray Mikuni VM34 carbs to forget the carb problems, but it would be a shame since I sourced so many other original spareparts on this restauration. There must be a way to make it running right.

[denward17]

Nice looking bike....

Did you check your fuel cap vent?

Sounds like it is running out of fuel, how clean is the tank and have you checked your petcock for correct operation?

[Teddyhoeg1982]

Nice looking bike....

Did you check your fuel cap vent?

Sounds like it is running out of fuel, how clean is the tank and have you checked your petcock for correct operation?

Yes tank is cleaned with Evaporust and fitted with a new petcock. I will double check the ventilation, but doubt it to be the problem. Fuel flow seems good at all times.

[denward17]

I'm not sure of the float height adjustment, have you performed a clear tube test to actually see if the level is correct?

[MauiK3]

Very nice bike

[Oddjob]

Why not consider ditching the PD carbs which TBH are not the best carbs out there. It's essentially a normal Cb500 so think about fitting a set of standard carbs for that model, 627B are one example. Lots of spares around for them. You will need to change the air box rubbers to fit these carbs but again they are available. You could also ditch the engine recycling system and fit the airbox off the 500, filters are generally cheaper for the 500s as well.

[Teddyhoeg1982]

I'm not sure of the float height adjustment, have you performed a clear tube test to actually see if the level is correct?

Yes looks fine I think

[Teddyhoeg1982]

Why not consider ditching the PD carbs which TBH are not the best carbs out there. It's essentially a normal Cb500 so think about fitting a set of standard carbs for that model, 627B are one example. Lots of spares around for them. You will need to change the air box rubbers to fit these carbs but again they are available. You could also ditch the engine recycling system and fit the airbox off the 500, filters are generally cheaper for the 500s as well.

There are plenty of people with these bikes running great, there must be a solution I would believe. Also I have gone to quite some lengths to make everything original, so it would be sad to throw it all away I believe.

[Ozzybud]

Did this problem just aart up after rebuilding the carbs?
I had a hard time believing on my bike after restoration. My battery was not fully charged  and the short rides thataround the neighborhood were not sufficient to give  it a full charge. After giving it a bench charge the issue you are having went away. Our bikes don't charge much under 4k rpm. Simple fix if it works for you. I was happy not to take my carbs  off again!

[Teddyhoeg1982]

Did this problem just aart up after rebuilding the carbs?
I had a hard time believing on my bike after restoration. My battery was not fully charged  and the short rides thataround the neighborhood were not sufficient to give  it a full charge. After giving it a bench charge the issue you are having went away. Our bikes don't charge much under 4k rpm. Simple fix if it works for you. I was happy not to take my carbs  off again!

That is a seperate problem, that the bike is actually not charging the battery. But it has a brand new AGM battery that I have tried fully charged - no difference.

[Ozzybud]

Did this problem just aart up after rebuilding the carbs?
I had a hard time believing on my bike after restoration. My battery was not fully charged  and the short rides thataround the neighborhood were not sufficient to give  it a full charge. After giving it a bench charge the issue you are having went away. Our bikes don't charge much under 4k rpm. Simple fix if it works for you. I was happy not to take my carbs  off again!

That is a seperate problem, that the bike is actually not charging the battery. But it has a brand new AGM battery that I have tried fully charged - no difference.

Check the static voltage at the coils when static, idling and while reving. You might be surprised at the voltage drop especially if the bike is not charging.

[bryanj]

Your symptoms are of blocked pilot jet or pathways, i know the pilot jets are a press fit but they can be removed with careful use of vice grips twisting and pulling at same time.

These jetways block up for a passtime like leave the bike for 3 to 4 weeks and it plays up.
Usually a long ultrasound clean with jets removed will clear it but not always.
The only place i have ever been able to get pilot jets is Sirius consolodated in Canada but their website really sucks.

As to converting to 627 carbs you would need the 500 air box, plenum chamber with rubbers, inlet manifold rubbers and alloy inlet manifolds, the last to get vacuum take off points, quite a big job.

The PD carbs are so notorious that i point blank refuse to work on them as nobody belies the time you have to spend and so dont want to pay it

[Teddyhoeg1982]

Your symptoms are of blocked pilot jet or pathways, i know the pilot jets are a press fit but they can be removed with careful use of vice grips twisting and pulling at same time.

These jetways block up for a passtime like leave the bike for 3 to 4 weeks and it plays up.
Usually a long ultrasound clean with jets removed will clear it but not always.
The only place i have ever been able to get pilot jets is Sirius consolodated in Canada but their website really sucks.

As to converting to 627 carbs you would need the 500 air box, plenum chamber with rubbers, inlet manifold rubbers and alloy inlet manifolds, the last to get vacuum take off points, quite a big job.

The PD carbs are so notorious that i point blank refuse to work on them as nobody belies the time you have to spend and so dont want to pay it

I find the when applying choke it generally runs better when warm.
I did remove the pilot jets and cleaned everything, but who knows.

Since the problem is that the motor bogs when applying 50-100% throttle, I was thinking it was rather needle and/or related to main jet.

[Ozzybud]

Your symptoms are of blocked pilot jet or pathways, i know the pilot jets are a press fit but they can be removed with careful use of vice grips twisting and pulling at same time.

These jetways block up for a passtime like leave the bike for 3 to 4 weeks and it plays up.
Usually a long ultrasound clean with jets removed will clear it but not always.
The only place i have ever been able to get pilot jets is Sirius consolodated in Canada but their website really sucks.

As to converting to 627 carbs you would need the 500 air box, plenum chamber with rubbers, inlet manifold rubbers and alloy inlet manifolds, the last to get vacuum take off points, quite a big job.

The PD carbs are so notorious that i point blank refuse to work on them as nobody belies the time you have to spend and so dont want to pay it

I find the when applying choke it generally runs better when warm.
I did remove the pilot jets and cleaned everything, but who knows.

Since the problem is that the motor bogs when applying 50-100% throttle, I was thinking it was rather needle and/or related to main jet.

Bogging at 50-100% throttle has nothing to do with the pilot circuit!

[Little_Phil]

I have a 550K3 with PD42b carbs and took them apart three times before getting them clean. Used a whole can of carb cleaner last time and had some success. But my issue was starting and tickover mostly. Have you had the individual carbs separated from the mount plate? I found getting the choke connecting springs a nightmare between 2 and 3 and then making sure the choke opened/closed correctly as it would stick. When you close the choke to start the idle should shoot up to about 3000rpm as it also open the throttle slide slightly.
4 turns out for the pilot petrol feed adjustment is too much. Are you sure you have the tiny washer and o ring on these correct?

[Teddyhoeg1982]

I have a 550K3 with PD42b carbs and took them apart three times before getting them clean. Used a whole can of carb cleaner last time and had some success. But my issue was starting and tickover mostly. Have you had the individual carbs separated from the mount plate? I found getting the choke connecting springs a nightmare between 2 and 3 and then making sure the choke opened/closed correctly as it would stick. When you close the choke to start the idle should shoot up to about 3000rpm as it also open the throttle slide slightly.
4 turns out for the pilot petrol feed adjustment is too much. Are you sure you have the tiny washer and o ring on these correct?

I did not seperate the carbs. Yes i checked that the o ring, washers and spring were present in the idle screws. Maybe I need to look for air leaks.

[bryanj]

Ozzybub, ok i have only done the 550 carbs but in every case, no matter what the system it was pilot circuit.
Those pilot screws are super delicate and easy to damage if tightened tpp much. The PD is super finicky in just about every way possible, if the needles are not original you need originals, somewhere i have a parts book and could compare, if i can find it

[Teddyhoeg1982]

Ozzybub, ok i have only done the 550 carbs but in every case, no matter what the system it was pilot circuit.
Those pilot screws are super delicate and easy to damage if tightened tpp much. The PD is super finicky in just about every way possible, if the needles are not original you need originals, somewhere i have a parts book and could compare, if i can find it

I am also suspecting the needles are my main problem, being from Keyster. I am not sure where to get original needles, except paying 200$ for a set, which seems too much. Maybe I should rather just cross my fingers and buy another set of used carbs.

[madmtnmotors]

At 50% throttle you are transitioning to the main fuel jet and needle. If the needle position is set too high then the main jet fuel circuit is opened too soon and runs overly rich. Are the main jet needles adjustable via a clip?

[scottly]

Is it rich or lean? What do the spark plugs look like? Do these PD carbs have the teeny tiny bowl vents like the ones used on the 750s?

[Teddyhoeg1982]

At 50% throttle you are transitioning to the main fuel jet and needle. If the needle position is set too high then the main jet fuel circuit is opened too soon and runs overly rich. Are the main jet needles adjustable via a clip?

Yes I checked the needles and they are currently set to the 3rd out of 5 grooves, so in the middle. Originally they are supposed to be second from the top, so a little leaner than this. I believe that when it dies above 50% throttle it is because it is too lean.

[Teddyhoeg1982]

Is it rich or lean? What do the spark plugs look like? Do these PD carbs have the teeny tiny bowl vents like the ones used on the 750s?

Spark plugs are grey/white and look quite good. No soot. So I believe it is generally running lean.
I am unfortunately not sure how the bowl vents look like.

[scottly]

This is the vent on a K7 PD carb. The drill bit is only .014". There is another version that has a small spigot, but with the same tiny hole.
 

[scottly]

Here is the other style vent, from a '77 F 750:

[Teddyhoeg1982]

Here is the other style vent, from a '77 F 750:

Ok I will try to locate them and make sure they are not blocked