Topic: Refitting an airbox to a CB500K3 with 649a carbs + carb questions.

[Malovski]

Hello everyone!

I have a Cb500K3 that I want to refit an airbox to. A previous owner have installed 649A carbs from an earlier model CB500K2 on it, together with pod filters. The carburettors I am just glad about, as I have been told these "old style" ones are easier to work on, and more trubble-free than the original PD carbs. However, when I now want to refit an airbox to the bike, I got some questions that I hope some of you here can help me with!

I did'nt get any airbox with the bike, but I managed to find and buy an airbox from an CB500K3 some time ago. This airbox however is'nt complete, so my first question is what crusial parts I'm missing? The things I can see are missing and needed are a paper filter insert, one of the top mounting brackets and some general brakets/nuts/bolts, the spring that secures the filter insert (is there any DIY replacement I can use?) and lastly the airbox to carburettor rubbers. Im not going to use the part of the airbox that connects to the crankcase-breather, so I exclude those parts.

My second question is that since the bike and airbox is K3, but the carburettors are from a k2, what inlet manifold rubbers/airbox to carb rubbers should I buy for everything to fit? Should I buy the rubbers meant for the K2? Will these work with the k3 airbox/intakes? At least the intake rubbers seems to vary in appearance between K2/K3 if I look at Davidsilverspares.

Here are some pictures of the airbox, I have dismantled the lid/snorkel so that I can remove the rust and paint the metal parts:




My last question regards the carburettors themselves. The gaskets in them are all dried up and needs to be replaced, so I was wondering what gaskets/rebuild kit I should buy? At the moment it stands between buying genuine honda gaskets or this kit from cruzinimage_co. If I buy the kit i will only use the gaskets and not the brass.

https://www.ebay.com/itm/71-73-HONDA-CB500K-CB500F-CARB-REPAIR-KIT-SET-OF-FOUR-KITS-INCLUDED-20-1050CR/162449453546?hash=item25d2bdf1ea:g:itYAAOSwGzlTt3P0&frcectupt=true

Either option will cost me around 65 dollars shipped to Sweden (for some orings... ). The genuine gaskets will without a doubt be of good quality, but what makes me lean towards the kit from cruzinimage_co is that I get three sizes of jets for every carb (78,98,100) aswell and that it seems to have gotten good reviews here on the forum ( http://forums.sohc4.net/index.php?topic=149097.0 ). The carbs have size 95 jets in them at the moment so if I'm not extremely lucky I will need to change the jetting after installing the airbox (the bike has a hooker 4-1 exhaust and was running lean when I bought it with the pods). Therefore  those jets in the kit will come in handy!

[TwoTired]

The air filter element sits on the recirculator assembly at the bottom of the filter case.  You'll need that or something to locate the bottom of the filter and hold it in proper position.
I think the 649a carbs also need an inlet restrictor plate.  It goes along with the small main jets.

Cheers,

[Deltarider]

Difficult to say. Good you have the oldstyle carbs. I had no idea they'd fit well into the intake manifold engineside. Or has that manifold also been changed? Two things to realise: your CB500K3 has the same frame and air intake setup as a CB550K3. The 649A carbs as a rack, dimensionwise is the same as a rack of 627B carbs that was on the CB500, K1 and K2 elsewhere. Carbs were only jetted differently. The racks are dimensioned the same. You do not need that cover (btw I would not call it a restrictor) only when you continue the #78 main jets. The old model CB500s (so not your K3) have the same frame as the CB550, CB550K1 and CB550K2. These models (not imported in EU) happen to have the same dimensioned oldstyle carb racks as were on the CB500, CB500K1 and CB500K2. So an owner of a CB550K3 that was tired of the PDcarbs and had them replaced by the oldstyle carb rack, could give you first hand advice. I hope he will chime in. 
As far as running I don't foresee so much difference in having the K3 filtercase set-up or the K2's. The engine itself is 100% identical to the CB500 models before yours, so it will run. The K3s with their #90 main jets were known lean. You have a 4-1, so I'd start with the #100 main jets and then have some trial and error with the position of the airscrews. I would leave the needles as they are.
My choice for the gaskets would be aftermarket (cheaper). So far I haven't read bad reports. The original CB500 filter element is available through Dave Silver https://www.davidsilverspares.co.uk/CB500K2-FOUR-1973-USA/part_61806/. Silver also has a K3 aftermarket airfilter https://www.davidsilverspares.co.uk/CB550K3/part_28991/ If the steel spring that fixates the filter element is not available, you can make one yourself. I have seen that steel used in packaging, to keep cargo together like on on pallets.
If you are going to use the K3 airbox and all, you will have to close the breather's entrance (which is a nuisance anyway, that breather I mean). Whatever setup you are going to use, as far as tuning, I'd concentrate on the main jets. I can understand your dilemma as far as the rubber boots airfilter side. Sorry, I just don't know if K3 boots fit well into an oldstyle plenum and vice versa. Others may chime in.

[TwoTired]

You are going to need the manifolds with vacuum ports when using the old style carbs.  PD carbs had them in the carb bodies.  Otherwise you won't be able to vacuum sync the carbs.

Cheers,

[Malovski]

Difficult to say. Good you have the oldstyle carbs. I had no idea they'd fit well into the intake manifold engineside. Or has that manifold also been changed? Two things to realise: your CB500K3 has the same frame and air intake setup as a CB550K3. The 649A carbs as a rack, dimensionwise is the same as a rack of 627B carbs that was on the CB500, K1 and K2 elsewhere. Carbs were only jetted differently. The racks are dimensioned the same. You do not need that cover (btw I would not call it a restrictor) only when you continue the #78 main jets. The old model CB500s (so not your K3) have the same frame as the CB550, CB550K1 and CB550K2. These models (not imported in EU) happen to have the same dimensioned oldstyle carb racks as were on the CB500, CB500K1 and CB500K2. So an owner of a CB550K3 that was tired of the PDcarbs and had them replaced by the oldstyle carb rack, could give you first hand advice. I hope he will chime in. 
As far as running I don't foresee so much difference in having the K3 filtercase set-up or the K2's. The engine itself is 100% identical to the CB500 models before yours, so it will run. The K3s with their #90 main jets were known lean. You have a 4-1, so I'd start with the #100 main jets and then have some trial and error with the position of the airscrews. I would leave the needles as they are.
My choice for the gaskets would be aftermarket (cheaper). So far I haven't read bad reports. The original CB500 filter element is available through Dave Silver https://www.davidsilverspares.co.uk/CB500K2-FOUR-1973-USA/part_61806/. Silver also has a K3 aftermarket airfilter https://www.davidsilverspares.co.uk/CB550K3/part_28991/ If the steel spring that fixates the filter element is not available, you can make one yourself. I have seen that steel used in packaging, to keep cargo together like on on pallets.
If you are going to use the K3 airbox and all, you will have to close the breather's entrance (which is a nuisance anyway, that breather I mean). Whatever setup you are going to use, as far as tuning, I'd concentrate on the main jets. I can understand your dilemma as far as the rubber boots airfilter side. Sorry, I just don't know if K3 boots fit well into an oldstyle plenum and vice versa. Others may chime in.

Thank you for taking the time to write that answer Deltarider, it's truly of great help! The manifolds are stock K3 and while they do fit the carbs size-wise, they don't have any vacuum ports as you said Twotired (thanks to you aswell!!). I therefore plan to drill and thread my own 5mm holes into them, as new K2 manifolds don't come cheap on David Silver. Any objections or advice regarding this?

As for the airbox, I will keep my eyes open for something to replace that steel spring, close the breather's entrance and fabricate a metal replacement for the breather that the filter can rest on! I actually found a thread here on the forum which shows the difference in length between the K2/K3 rubber airbox boots, with the K2 boots being shorter. I think the best thing to do will be to buy K2 boots for both Intake and airbox side (+ a K3 air filter, as the airbox is K3) and simply hope for the best .
The thread I referred to: http://forums.sohc4.net/index.php?topic=91801.0

Thanks for the tips regading carb tuning! Do you possibly know what the stock needle position would be? As the carbs have been tampered with before the needles may not be in their original groove. Or should I initially leave them be no matter what position they may be in?

Lastly, I just want to double check that this is the correct carb kit for my 649A carbs? It's sort of a dump question as the carbs definitely are from a cb500 k1/k2, but I rather be safe than sorry . ( The cb500 k1/k2 were the only "cb500/550" bikes sold in Sweden prior to the cb500K3 what I know, I don't think any cb500f supersport were ever imported or existed?)
https://www.ebay.com/itm/71-73-HONDA-CB500K-CB500F-CARB-REPAIR-KIT-SET-OF-FOUR-KITS-INCLUDED-20-1050CR/162449453546?hash=item25d2bdf1ea:g:itYAAOSwGzlTt3P0&frcectupt=true

I'm asking since the seller also stocks this kit that to me (not a trustworthy source ) seems to have the right amount of orings for my carbs? Or what is the reason for the 71-73 kit having so many orings? 
https://www.ebay.com/itm/74-76-HONDA-CB550K-CB550F-CARB-REPAIR-KIT-SET-OF-FOUR-KITS-INCLUDE-20-1220CR/142068817153?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D55187%26meid%3D15b4c82b501640409bba6403ac46fa68%26pid%3D100005%26rk%3D2%26rkt%3D2%26mehot%3Dlo%26sd%3D162449453546%26itm%3D142068817153&_trksid=p2047675.c100005.m1851

[Deltarider]

I therefore plan to drill and thread my own 5mm holes into them, as new K2 manifolds don't come cheap on David Silver. Any objections or advice regarding this?

You could do that.

As for the airbox, I will keep my eyes open for something to replace that steel spring,

You will be able to improvise something. Even some wood used as a wedge would do.

Thanks for the tips regading carb tuning! Do you possibly know what the stock needle position would be? As the carbs have been tampered with before the needles may not be in their original groove. Or should I initially leave them be no matter what position they may be in?

Apart from this here forum, not many go at the needles at all. At least not in the other fora I frequent.
Standard the 649A stamped carbs had the needle in 3^rd (middle) position, the airscrew at one turn out ⁺/_- ¹/₈ and had the #78 main jets. These carbs were on the models for continental Europe that also had the AIR DUCT. Models for France excepted. France and UK had the 627B stamped carbs, which had everything the same as the 649A, only the main jets were #100 and they had no AIR DUCT. But needles and airscrews are identical. In this forum I learned that the models for US, that also had the 627B stamped carbs, had theirs slightly differently set: the airscrews 2 turns out and - probably to compensate - the needleclip in 4^th slot, also no AIR DUCT. Looks like the US wanted less emission at idle.

Lastly, I just want to double check that this is the correct carb kit for my 649A carbs? It's sort of a dump question as the carbs definitely are from a cb500 k1/k2, but I rather be safe than sorry . ( The cb500 k1/k2 were the only "cb500/550" bikes sold in Sweden prior to the cb500K3 what I know, I don't think any cb500f supersport were ever imported or existed?)

You're right, the CB500 only came with the 4-4 exhausts, there was no supersport. But the CB500 was often called CB500Four or CB500F for short.

... Or what is the reason for the 71-73 kit having so many orings?

It's clearly an aftermarket set, as no CB500 anywhere had #98 main jets. You may have the impression there are a lot of O-rings, but realise each set for one carb includes two identical O-rings ment for one side of the T-joint fuel supply. BTW, I would not separate the carbs from the rack just for that as these rings there seem to live eternally. At least on mine they do.
Each set has an O-ring
for the floatbowl
around the main jet,
for around the drain screw,
for around the floatvalve
and 2 for the T-joint.

[bryanj]

If you go to the UK forum a member there was reproducing the air filter retaining spring clip.

[Deltarider]

That 20-1220CR set mentions a #86 main jet. That main jet is not known on any CB550, so I take it the seller read 98 upside down. 

[Malovski]

Thanks again Deltarider! I'll order the 71-73 CB500 carb kit then.

As for the air filter retaining spring clip, thanks for the tip bryan! However, I actually found the steel needed to make my own retainer clip at work holding cargo onto pallets. I will source some of it next week and make my own spring. Score!