Topic: Frustrating CB500K3 carb issues

[bryanj]

Not your lucky day, the ony carbs with same needle is the USA 78 550K3 the other 550K3 have a different last number

[jonda500]

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

Also note the spacing between the carbs is slightly different on the older alloy inlet manifolds with the vacuum ports.
John

[scottly]

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

Also note the spacing between the carbs is slightly different on the older alloy inlet manifolds with the vacuum ports.
John

It might be easier to just fix the  original carbs, as the bike ran well enough with them when it was new?

[Deltarider]

P.4  and p.33 in the parts list inform you what exact carbs your model was supposed to have. Compare your serial number. https://www.honda4fun.com/dwnload/Part-List/CB500/CB500-K3-CB550-K3-K4-Parts-List.pdf
I have added a few pics.
There's no need to separate carbs from each other. There hardly is ever!!!
Issues reported on these carbs usually had to do with:
Inactivity for too long. This can possibly be adressed by adding a socalled maintenance dose of whatever fuel system cleaner at every tankfill (15-20cc). The first dose should be a much bigger quantity however.
Venting. Make sure all 4 carbs vent their floatchambers to the open. If your model happens to have tube(s), cut the bottem end (at the rear of the transmission) by an inch.
Allthough it concerns a CB550K4, you may find this post worth reading: http://forums.sohc4.net/index.php/topic,184833.msg2140378.html#msg2140378
Fuel tube(s) must be installed correctly. NO inline fuel filters!
These K3 models have a typical behaviour at cold starts. They usually shoot up to 3000 rpm and it's hard to bring it to an acceptable idle of 1200 rpm. When warm there should be no problem.
In contrast to the models CB500 with oldstyle carbs, the slow jet does not so much play a role in higher rpms. It's a different concept.
I have no experience with aftermarket carb parts.
Are you sure advancer operates as it should? When synced do the dials of the vacuum gauges come down quickly - ideally simultaneously - or do they hesitate?

[Teddyhoeg1982]

P.4  and p.33 in the parts list inform you what exact carbs your model was supposed to have. Compare your serial number. https://www.honda4fun.com/dwnload/Part-List/CB500/CB500-K3-CB550-K3-K4-Parts-List.pdf
I have added a few pics.
There's no need to separate carbs from each other. There hardly is ever!!!
Issues reported on these carbs usually had to do with:
Inactivity for too long. This can possibly be adressed by adding a socalled maintenance dose of whatever fuel system cleaner at every tankfill (15-20cc). The first dose should be a much bigger quantity however.
Venting. Make sure all 4 carbs vent their floatchambers to the open. If your model happens to have tube(s), cut the bottem end (at the rear of the transmission) by an inch.
Allthough it concerns a CB550K4, you may find this post worth reading: http://forums.sohc4.net/index.php/topic,184833.msg2140378.html#msg2140378
Fuel tube(s) must be installed correctly. NO inline fuel filters!
These K3 models have a typical behaviour at cold starts. They usually shoot up to 3000 rpm and it's hard to bring it to an acceptable idle of 1200 rpm. When warm there should be no problem.
In contrast to the models CB500 with oldstyle carbs, the slow jet does not so much play a role in higher rpms. It's a different concept.
I have no experience with aftermarket carb parts.
Are you sure advancer operates as it should? When synced do the dials of the vacuum gauges come down quickly - ideally simultaneously - or do they hesitate?

Thanks very informative! My frame VIN is CB500K-1002762 and engine VIN CB500E-2202813, so that checks out to be a CB500K3.
My carbs are, as can be seen below PD46A (C), while they should have been PD46B (B)-(D) for a CB500K3. So the carbs are from a CB550K3. Though since the engine completely dies already from 50% throttle, I believe it is not just because of the 50cc less engine size. Allthough the carbs are a slightly wrong model, I think I will try to stick to them.

My belief is that my needles are wrong, since I from the past already have bad experience with Keyster aftermarket brass. From my memory the taper of the Keyster needles are different and further need the Keyster aftermaket needle sleeve in the carb body. I can see that the needle is a new Keyster, but the sleeve in the body looks like original Keihin.

I will take the carbs off and check the vent holes for the bowls - I did not check these yet. Also I found out that my fuel tank cap vent was actually blocked, though it made no difference to clear it so far.

In regards to the advancer I had it off and confirmed that it is lubricated and seems to workly freely, but I could not confirm at which rpm it expands. It looks original. The bike has done 56.000 miles but has great 150 psi compression across all cylinders. I can not confirm whether perhaps the springs in the advancer could be loose. Also only aftermarket replacements seem available, so I am hesitant to go for that.

Here is the carb ID:





I will further try add this additive to the fuel after I have checked the bowl vents, since I have it available and used it for another bike already.

[Teddyhoeg1982]

UPDATE

I could not find a set of PD46B carbs at all, so i ended up buying a whole bike for 200$ which has the right carbs on it. So I will now cross my fingers they contain original brass and can be used to make my bike fully original. At least I have the right carb bodies then, and also this bike contains a couple of other original parts I could use. I should be able to sell it again for 200$ when I have taken what I need.

[bryanj]

For 200 that was a bargain!

[Teddyhoeg1982]

I took the new spare carbs apart from the spare CB500K3 I bought, and they look good inside and has all original Keihin brass.
The needles say E2349F. I cannot find any source of information to confirm whether that is original for a CB500K3, but I assume it is, since all other internals look original.

[bryanj]

Those pilot jets will come out, grip gently with vice grips or parallel pliers the pull and fwist at same rime. Clean out thoughroughly along with passages and refit with gentle tap from nylon hammer

[Teddyhoeg1982]

Those pilot jets will come out, grip gently with vice grips or parallel pliers the pull and fwist at same rime. Clean out thoughroughly along with passages and refit with gentle tap from nylon hammer

Yes i Got the others out by heating the carb body slightly, I will also take these out to properly clean

[scottly]

While you have the carbs apart, I would consider opening up the tiny vent holes to 1mm or more, if they are like the ones I posted the picture of with the drill bit in the hole.

[Deltarider]

[...]
The needles say E2349F. I cannot find any source of information to confirm whether that is original for a CB500K3, but I assume it is, since all other internals look original.

Then you must have missed p.178 in the Shop Manual Honda CB500-550 which you can download here in a good resolution. http://info.sohc.co.uk/download/cb500-550-shop-manual/

[Teddyhoeg1982]

[...]
The needles say E2349F. I cannot find any source of information to confirm whether that is original for a CB500K3, but I assume it is, since all other internals look original.

Then you must have missed p.178 in the Shop Manual Honda CB500-550 which you can download here in a good resolution. http://info.sohc.co.uk/download/cb500-550-shop-manual/

Yes I missed that, thanks! Looks like I need a E2350F needles, so I Will be looking for that.

[Deltarider]

Umh... don't know. If they were not in yours and not in the carbs on your recently acquired 500K3, I hold it for possible Honda has fitted the same needles as in the 550K3 after all. I mean, it's not very likely both previous owners have replaced their needles, is it? Realise that Honda assembled some 4200 of these CB500K3s at the most. Also, I doubt it will make so much difference.*
What's the air intake like? Any leaks? Is the airfilter element OK and not wettened? Is that breather cleaner element underneath it there, well sealed and does it have its tube attached? Any possible air leaks there?
I agree with you these carbs can function well, when bike is regularly used. If possible upload a vid that shows how it runs now. I know these K3s can run wonderful.
* Surprisingly the same needle with part number 16012-404-630 was on the CB550K3 USA and K4 USA (aka CB550K 1978).
https://www.cmsnl.com/products/needle-setjet_16012404630/ says:
CB500K3 FOUR EUROPEAN DIRECT SALES
CB550 FOUR CB550K3 1977 USA
CB550K 1978 USA

[Teddyhoeg1982]

Umh... don't know. If they were not in yours and not in the carbs on your recently acquired 500K3, I hold it for possible Honda has fitted the same needles as in the 550K3 after all. I mean, it's not very likely both previous owners have replaced their needles, is it? Realise that Honda assembled some 4200 of these CB500K3s at the most. Also, I doubt it will make so much difference.*
What's the air intake like? Any leaks? Is the airfilter element OK and not wettened? Is that breather cleaner element underneath it there, well sealed and does it have its tube attached? Any possible air leaks there?
I agree with you these carbs can function well, when bike is regularly used. If possible upload a vid that shows how it runs now. I know these K3s can run wonderful.
* Surprisingly the same needle with part number 16012-404-630 was on the CB550K3 USA and K4 USA (aka CB550K 1978).
https://www.cmsnl.com/products/needle-setjet_16012404630/ says:
CB500K3 FOUR EUROPEAN DIRECT SALES
CB550 FOUR CB550K3 1977 USA
CB550K 1978 USA

My Old set carbs had Keyster needles in them, not Keihin. I have had problems with Keyster brass before. I Will try the 2349 needles and see how it goes.

Air intake has new OEM manifold rubbers and the right narrow clamps. Cleaner element is OEM and cleaner chamber has the right lid. New rubbers between carbs and airbox

Cleaner element is in place and has a rubber O ring on top. The breather tube is attached and plugged in the end.

Breather tube for the valve cover is not mounted, but should not make a difference.

One thing I saw mentioned was that the intake rubbers should actually not be clamped too tight since it can pinch and create air leaks. I replaced the JIS screws with Allen bolts and tightened them quite hard, maybe that is also a problem.

[Deltarider]

For a reason I dont know, the rubber boots engine side should be oriented like in the pic.
Courtesy: the French Forum http://www.cb500four.com/viewtopic.php?f=16&t=623
BTW, I do not recommend to separate carbs from the rack unless there really is a need, which is rare, very rare.

[newday777]

For a reason I dont know, the rubber boots engine side should be oriented like in the pic.
Courtesy: the French Forum http://www.cb500four.com/viewtopic.php?f=16&t=623
BTW, I do not recommend to separate carbs from the rack unless there really is a need, which is rare, very rare.

Rare in your mind but the gas in the states did major damage leaving deposits in the ports of the carbs that have to be deracked to fully clean out. Please stop saying that over and over.

[Deltarider]

For a reason I dont know, the rubber boots engine side should be oriented like in the pic.
Courtesy: the French Forum http://www.cb500four.com/viewtopic.php?f=16&t=623
BTW, I do not recommend to separate carbs from the rack unless there really is a need, which is rare, very rare.

Rare in your mind but the gas in the states did major damage leaving deposits in the ports of the carbs that have to be deracked to fully clean out. Please stop saying that over and over.

First: I put it like this: "... unless there really is a need, which is rare, very rare." Must be my limited knowledge of English, but what do you mean by 'ports'?

[Oddjob]

Plus how do you change the orings on the fuel T pieces without deracking? There is no downside to deracking IMO as long as you are competent, it allows you to really clean the bodies, do maintenance and it can demystify how the carbs work for some people. I've done so many I've lost count. On some of the later bikes it's also the only way to change some of the diaphragms.

[bryanj]

The PD carbs are a pain and very fiddly to reasemble to the rack but its just time and patience.
If not used for a while, which can be only 2 weeks, the ports or body passages can and do get full of oxide and varnish, as few of us can afford an ultrasonis bath big enough to fit the complete rack in this means  you have to bite the bullet and derack

[Deltarider]

Plus how do you change the orings on the fuel T pieces without deracking? [...]

Thank you, that is exactly my point. When they have the fresh bought carb gasket sets on the table, few can leave the O-rings for the T-joints unused in the package. Well, I never had to use mine. Mind you: thanks to Honda The Netherlands' generosity I have 16 (!) of them. They only (semi-)leaked once, when I had left the carbs dry in hibernation. After ten minutes of being in contact with fuel again, they had arrived at the proper sealing size again. I have more parts still in packages. Another example: so many here repeat over and over again that you should replace the rubber boots between carbs and engine after a few years. Again: not mine. If there's a leak, it will be noticed quickly by me. I may be lazy, but I see no point in replacing parts that are still good. If the O-rings at my T-joints survived 48 years, why not 10 more? Same for the O-rings around the fuel valves. The carb O-rings I did need to change were the tiny ones on the main jets, the big ones on the bowls and one or two on the drain screws. All my carb ports can be reached easily. An ultrasound bath is no guarantee for success, experts in the field - back then I have phoned three of them specialised in carbs - have told me and as far the oldstyle carbs it can lead to parts missing (see pic).

[Deltarider]

[...] One thing I saw mentioned was that the intake rubbers should actually not be clamped too tight since it can pinch and create air leaks. I replaced the JIS screws with Allen bolts and tightened them quite hard, maybe that is also a problem.

If it's about the rubber boots between airfilter plenum and carbs, it is not that critical. As far as the boots between carbs and the head however, an air leak will definitely have its effect. I have read more than once in this forum that you should tighten them real well. Well, not me. I was instructed differently: tighten them just enough, not to let air in. Probably that's why I still have the originals the bike got at the assembly line in 1976. I prematurely bought four new ones, but they are still in the packages. An air leak is not that difficult to detect. Some use carb- or brakecleaner. You can also rev the engine and monitor if the rpm drop immediately to idle when the throttle is released. A vacuum meter can also help you determine which one of the 4 has the leak. If the dial comes down in a hesitating manner, it could indicate to an airleak.

[Oddjob]

You seem to forget that fuel has significantly more ethanol in it than it has in the past. This affects old orings far more than modern ones, it’s why it’s recommended to change the rings to viton ones. Saying they’ve lasted so long they’ll last much longer is false.

As for parts missing, not really a problem if you know what you’re doing, taking them completely apart helps to check on parts hidden when they are intact, parts do break and start to crack etc.

What you should say is that you personally don’t want to take yours apart, maybe it’s beyond your skill set, others though have no problem taking them apart, it’s just a matter of confidence.

[Deltarider]

Up to 5% ethanol has proved to be no problem. I avoid E10. And it was me that uploaded an article on the subject E10 which I had translated from German into English many years ago. Yes, I will replace O-seals by Viton when needed. Not now. Yes, I don't like to do work that's not needed and often is IMO rooted in a neurosis of some kind.

[Oddjob]

From what I understand the states have higher levels of ethanol in fuel than we do.