Honda-SOHC
SOHC.co.uk Forums => CB500/550 => Topic started by: badger_eng on September 01, 2026, 01:57:11 PM
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Gents, looking for a little feedback from the group.
I bought michtag’s Honda CB550K (US imported 1977) bike as my winter project for this year.
Bought unseen (I enjoy the lottery of it) and as was made clear in the advert, it had issues with erratic idle speeds and hanging revs. He’s enlisted your help in some previous posts around 2019/20 but as advertised, the issue is still there.
As it’s not yet winter enough to start the ‘project’, I’d thought I’d give it the first run up today after a week of it sitting in the garage.
Same sh!t, different day. Still hangs and idles at erratic speeds with no real pattern, other than it’s pretty random.
So, airbox off, and I thought I’d have a sneak peak in the entry to the carbs, looking for the obvious throttle bodies out of sync etc and tbh, they are not so far off that you can see by eye. I’ll go thru that procedure later in the week.
Anyway, my main question,,,,
I’m quite familiar with Mikuni’s and therefore more au fait with choke circuits rather than choke flaps.
However, I’ve noticed (and the carbs were still mounted at this time) is that when the choke knob is fully depressed to the OFF position, I can quite easily move the four choke flaps by hand in the region of 20/30 deg. The flaps are also not fully open as I’d probably expect and then I can push the flap down the 20/30 deg that then makes them fully open. There is also quite a bit of play in the linkages between the 1/2 and 3/4 carbs. Possibly a linkage.
So main cause for concern from the observations would be…
- Choke OFF and flaps not horizontal / fully open
- Play in linkages between the different carb choke flaps
Has anyone seen this before? Is it normal? Excuse my ignorance on the Keihins.
I will be dismantling and ultrasonic cleaning etc (also reading some of the guides on here) but just as a first pass it looks a bit suspect to me.
Thanks in advance.
Pics (Not the best) show the flaps with the choke OFF and the flap position.
[attachimg=3][attachimg=4][attachimg=4]
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Just an addendum….
My plan tomorrow will be to run the bike without the airbox and just see if the choke flaps are moving independently just to rule out or rule in.
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The Keihins on my 400f definitely sit horizontal when fully open and all 4 move together. There shouldn't be any flapping about or moving separately. Those screw heads holding the flaps on look much bulkier than the standard screws too. From memory they are small dome head screws so don't interfere with airflow too much.
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Thanks for the quick reply Gareth. Logic says it’s all wrong but other than logic, I don’t have the Keihin knowledge or experience yet to determine how much it matters.
My fear would be if there is enough free play in the linkage between carbs 1/2 and 3/4 where the butterfly’s could essentially be doing different things as the air is drawn in.
When I run it up tomorrow, I should be able to observe if it runs OK minus the airbox.
If only to settle my mind, it’s probably a worthwhile test.
Once I’ve done that and learned one way or the other, I can move forward with strippage, cleaning and rebuilding / synchro etc.
Cheers
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Questions I would ask:
How many miles on the odometer?
Any idea of the owner's history like how many?
How likely is it any previous owner has played with the choke flaps?
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22000 miles on the clock.
Bike imported sometime in 2016.
Changed hands to the guy I bought it off maybe a couple of years or so later and it’s hardly done any miles since 2016. I’m going to say less than 1000 as it came with a couple of old MOT’s.
Based on receipts I received and posts made on here by previous owner, the problem was attempted to be fixed by both the previous owner and an enlisted mechanic / workshop whilst in his possession.
Few parts changed but I have no exact details, just a few seals / bits n bobs in a box that suggest tinkering has occurred.
The bike was then stored from about 2020 till a decision to sell a month or so ago. Now it’s mine and mine to solve with help from the forum and my desire to never fail 😂
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Hi Pete. I had my PD46A carbs apart a while back to sort a fuel leak.
The butterflies are on two separate shafts, one for carbs 1 and 2 and one for carbs 3 and 4.
These shafts link together between carbs 2 and 3, and there are two springs here which hold the two shafts so the flaps are in the horizontal position. You can see one of the springs to the right of the first photo in your post.It was a right fiddle firstly getting the shafts in the correct position, and secondly attaching the two springs.
I would check the springs to make sure they have not snapped, or become dislodged. Also make sure the two shafts are positioned/connected correctly.
I agree the screws on the butterflies look to have been replaced. The originals were peened to ensure they wouldn’t come loose, but it is tricky to do this with replacement screws. As a minimum, the replacement screws need a good dob of threadlock.
As an aside, if you do have to separate the carb bodies, I recommend you renew the O rings on the 3 tubes which allow the fuel to flow to each carb. It’s a 2 minute job once the carbs are apart but a right pain to do otherwise.
Hope this helps.
Cheers
Harold
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Thanks for that Harold, I will heed your advice on the O rings for sure and verify that the butterfly valve screws have loctite (by adding loctite)
In the box of spares that came with the bike is a bag of very small headed screws. I would bet a few bob that they are the ones that were originally fitted.
Whilst they may be larger and a possible slight restriction, if they are identical for each of the butterflies, at least it’s a common restriction for each carb.
The ability to wobble the flaps albeit by finger pressure concerns me more, if they are blowing in the wind….. due to the springs or wear in the assembly…. that airflow change ‘could’ be a factor in the erratic and random engine speed and hanging revs.
Many thanks for the info 👍
Pete
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Pete. Just had another thought. It would be worth checking the choke cable as well. If the inner cable has corroded, or the cable is kinked, it may be fighting against the return springs on the choke shafts and not allowing the chokes to return to their fully open position??
Keep us posted!
Cheers
Harold
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Will do.
Thanks for everyone’s input 👍
Much appreciated
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PD carbs hate being stood doing nothing, if you get it sorted you must run it for a decent amount of time at least once a week..
The pilot jets are a press fit in the body but can be removed with a twist and pull using decent grips, they do block for a passtime as do the passageways.
Pilot adjust screws are VERY delicate DO NOT BOTTOM TOO TIGHT, each one has a tiny spring, O ring and washer on them.
Parts are not easy to get, only place i have seen pattern parts is Sirriusonic in canada but their website was abysmal to navigate, genuine parts, if available, are better.
There are 3 different types of pd carb, each has different settings especially float height.
Literature on carbs is difficult to find, even the K3 supplement is only about 4 pages i think its in Ashs dropbox, you start with 500 then get all the different supplements of 550 to K3.
Mark Paris aks hondaman on the US site knows the differences by model number
All i can say is good luck
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Can’t beat a bit of good luck 😂👍
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Someone's definitely been tinkering with those carbs, as you suspect wrong screws in the choke flaps. I use a small dome headed stainless steel screw with a very small spring washer these days. A bit of locktite won't hurt either. The choke arrangement wasn't great on the PD carbs, not detent bearing to stop them from moving around like the 500/550F. Best advice would be fit the older type carbs off the 550F 069a or even off the 500 627b. You'd need to change the inlet manifolds though as they ain't the same on the K3 and the airbox rubbers to carbs are different but they are easier to keep running well and far easier to get parts for.
Disconnect the choke cable to see if the flaps sit more horizontal.
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PO has admitted in previous thread on here (2019/20 ish) to either tinkering himself and having someone else tinker on his behalf. Nothing I didn’t expect.
In fact, id be more shocked if a bike with carbs, that’s half a century old, hadn’t been tinkered with. Imagine that!!
Looking forward to the challenge and learning about these carbs. Doesn’t mean they won’t get filed under B1N but as said, looking forward to it anyway.
Sooner I know the view is t worth the climb, the sooner I make a different choice and the bike strip / rebuild will commence.
Thanks again for everyone’s input 👍
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I just remembered having seen a report in the French site on rebuilding CB550K3 PD46A carbs. Lots of pictures. https://www.cb500four.com/viewtopic.php?f=16&t=623
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Merci monsieur 🍾
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So just a quick update on today....
Ran the bike with no airbox just to see if I could see anything happening with the choke butterflies. As mentioned yesterday, they are not sitting horizontally and there is a lot of slop between the choke butterfly / shaft assemblies for carb bank 1-2 and carb bank 3-4.
Fired up OK on full choke and quite quickly depressed the choke button to the OFF position. Semi warmed the bike giving it some throttle up to 3-4 k and as yesterday, engine RPM was slow to fall and hang around 2.5k before slowly dropping. No change to issue with airbox fitted or no airbox at all. When idling or revving the throttle I could NOT see the choke butterflies moving by themselves and potentially varying the air volume into the carb. Good news as this counts out one possible cause.
Removed the carbs and got them on the bench, just for visuals only at this stage, and its quite obvious that the springs that join the two choke shafts are not in the best shape - I'm sure there will be an abundance of these somewhere - NOT ;D
Tried a drill bit in the carb inlets to check mechanical sync of the throttle bodies and whilst they are slightly different, they are certainly not MM's out - I've ordered a Morgans Carbtune balancing kit so I can get them synced when all the cleaning and checking is done. - Wish I'd kept the set I bought for pennies back in the 80's!
I'll do some reading of the resources on here and elsewhere before I start any serious strippage.
Probably radio silence for a while until I get them stripped, parts ordered / delivered, re-assembly and subsequent re-start.
Thanks again
Pete
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Try removing the throttle cables and testing again, the hanging could be caused by the cables binding. I am sure someone on here has broken a set of PD carbs and may have the springs available. A lot get wrecked by people trying to assemble them after and not understanding how they go back together. Only ever did one set of PDs and vowed never to do another.
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Sorry, I should have added... But good for me to add now for clarification.
It was suggested to remove the choke cable by someone to rule out the butterflies not being fully open and I tried this as part of the experiment.
I also removed the throttle cables as part of the troubleshooting AFTER i'd done it with the cables still attached and functioning as normal.
Bike was left ''as normal'' with no airbox, run up and checked, all control cables then removed and tried again.
Just trying to eliminate the obvious before digging deeper.
I've found some springs on a overseas site, cheap'ish but crazy shipping prices. Still got more searching to do. It looks like the exploded diagram below shows identical springs (pictorally anyway)
[attachimg=1]
However, my carbs have different springs. One that looks correct and one that looks like it belongs in a watch :-X
I'll do my searching then maybe make a call in the Forum 'wanted' section.
Thanks for your input
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If the carbs are off remove the inlet manifolds and check the orings in them. Most have gone rock hard over the years and fail to seal, allowing air to get in which will cause the symptoms you describe. The rings are hard to source in the correct size and most sellers say theirs are the correct size when it fact they are not, I’ve seen some that I refused to fit because they were so bad. They are the same size as the tappet covers, you get 12 in a genuine top end gasket set so use something close on the tappet covers and save the correct ones for the manifolds is my advice. Simple to change a tappet covers and save oring, not so simple to change the manifold ones.
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Excellent tip. Something else to fettle tomorrow after my morning trip to the dentist. Wisdom tooth possibly 🦷 coming out tomorrow so I’ll probably be even more cerebrally challenged in my remaining years 😬
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My K3 was always a pig to start even with carbs stripped and cleaned thoroughly. I increased the pilot jets from 42 to 45’s. End of starting issues and runs great.
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Thanks Johnny, starting is very good actually, it’s just the erratic / random response after giving it some throttle I’d like to solve.
I’ll be opening up the carbs over the next few days and I’ll see what’s fitted. It’s got a 4-1 Delkevic pipe on it so may not be on std jets.
👍
Pete
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Yes of course, mine is standard 4 into 4 exhaust plus factory air box etc. Good luck, interesting project!👍
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Thanks, I’m sure I’ll be leaning on you guys plenty more.
I’ve belonged / belong quite a few forums (as I’m sure many are) but this one is very lively…. Nice to see one thriving.
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Heck Pete,
What's with having a wisdom tooth extracted? I've been trying to get some put in!
Anyway, if you were to decide that the PDs were too much hassle, I have a set of 627Bs here (500) but jetted for 550. They were on my F1 that I got from JamesH on here and went right through them, zinc plating fittings and the ultrasonic bath individually. They are on original brassware and the only small issue is corrosion around the outside of the main jets - inside where the jets/o rings locate is fine on all 4. I ran the bike for a short time with them before locating a set of the correct 069s from the Netherlands and went through them too and that's what I have on the bike. Obviously the manifolds would need changing and airbox rubbers.
[attach=1]
Ian
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I’ll message shortly Ian. At the point of extraction now 🥳🍾🎂
Many thanks
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and i have a set of the earlier manifolds if required for the cost of postage.
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Wow, this must be 1st!
Hull and Scunny working together as a team ;D
Appreciate the offers gents. I will message you if thats OK?
Please be gentle as i'm now 25% down on wisdom
Pete
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LOL I like people from Ull as my mum was yorkshire born, dad was a Lincolnshire yellowbelly so I am mixed race! also the Hull accent makes my accent sound posh!!
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Wierd eh, but I always thought the Scunny accent was waaaay posher than Ull's.
My accent is soft now, I spent a few years living in Hessle & Barrow upon Humber before moving up towards Newcastle way. Bit of a mongrel if I may say so :-)
I'm going to PM you now Mick. Hopefully its not too difficult.
Pete
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Well, after much tough negotiations (Not), I’ve a set of CB500 carbs coming from Ian later in the month and some manifolds from Mickwinf arriving shortly.
Many thanks guys.
I decided to have a look at the PD46’s today and see why the choke flaps were not fully open as I found earlier in the week.
Also check float heights, fuel cut off valves and also check if they were jetted as standard.
- Float heights good at 14mm
- Standard hardware I believe is fitted at 90 mains and 42 slow. No blockages in the jets.
- Rigged up the remote fuel tank and checked the cut off valves were not passing with the fuel bowls off and over a bucket - All good.
- Checked the fuel / air mixture screws and they were randomly different, ranging from 1-3/4 to 2-1/4 turns open from gentle closure (Not checked if all the right hardware (seals/washer etc are fitted or missing yet but I’ll get to it)
Anyway, apart from one of the choke closure springs being from a child’s watch rather than the correct item where the two shafts join, I think I’ve found the issue. May be of help for anyone else.
The choke crank arm is touching the cast stop on carb 2 before it’s bottomed out. It’s not travelling its full rotation as a result.
You can see in the pic that the end of my scribe shows a gap between the casting and the crank arm
[attachimg=1]
The pic here shows the crank arm bottomed and the choke flaps are now horizontal. Only required mild finger force to overcome the bind point but obviously more than the closure spring gives.
[attachimg=2]
Well well well…..
Had a look at the fuel / air screws and possibly found the cause of the hanging revs and erratic idle.
Carb 2 had an extra washer / O ring and carb 3 only had a washer.
[attachimg=3]
Carb 3 and Carb 4 have small pieces of broken O ring in the chamber between the fuel / air mixture screw and the slow jet. Two pieces I’ve been able to remove. Many more I have not.
[attachimg=4]
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After yesterday’s orifice purge of all things rubber and re-assembly of the carbs, today I bench synced the throttle slides.
Slide 3 was good, slides 1 & 4 were off a little from the master.
Set them all even then adjusted the little screw (not sure of its proper name) that sits between carbs 2 & 3 that sets the throttle slide rest position. On Mikuni’s I’d call it the throttle stop screw.
Anyway, as delivered, with the bike before I started my tinkering, they were set about 3.5mm open, thats measuring on the engine side of the slide!
They are now set at zero + an unmeasurable tad. Just enough to see torchlight shining thru.
There is still the big thumb screw on the r/h side of the carb bank to increase the idle if required.
Refitted the carbs, piped in the remote fuel reservoir and hit the go button.
Fired up great once fuel starting flowing and chokes off almost immediately (ambient is about 16 according to the barometer)
I’ve not got any cables connected at the moment so choking & throttling is being done by hand but it started good, ticked over at around 1200-1300 and revs returned to tickover speeds quickly after giving it a rev. Very different to all the other starts I’ve done since i acquired the bike.
I’ll repeat the experiment again from cold in morning and see what transpires but looks promising.
Gives me much needed confidence and learnings before going into the big strip in late October.
Thanks for all your inputs and the wealth of info in past posts and documentation.
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I’d have thought with virtually no gap between the slides at rest the tickover would be really low. So low you’d need to use the throttle thumb screw to raise it a little. Normal tickover for a K3 differs from the rest, normally 1000 rpm but the K3 with its PD carbs is set at 1100. However, at that speed most older engines can be noisy, primary chain banging about and the clutch basket knocking, so most up the idle speed to mask it a little.
Unlike the normal carb 550 the PD carbs air screws work in reverse. In is weaker, out is richer. If it starts with minimal choke I suspect it’s a bit rich already.
Good bit of detective work on the carbs, at least it’s running much better, which is always a result on these old engines.
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Thanks for the feedback.
I'm just a happy bunny as I wanted to make sure the bike was running, at tickover and low revs in the garage, at least somewhere near or something like you'd expect. i.e. It starts, idles, revs when you turn the throttle and returns to idle when you close the throttle.
It now does that now and that gives me a lot of confidence going into the 'big strip' towards the end of next month. The last thing I wanted was to put a lot of effort into making the bike sparkle then having the carbs on and off ten times ruining paint, screw heads, clips etc etc
Pilot air screws were all set to 1.5 turns open so could possibly need a tweak as you mention.
Like I said previously, Keihin carbs are new to me. I've never needed to strip the few previous bikes Ive had with them so its been a good learning curve.
I'm still going to replace them with Ian's 627's using the manifolds from Mick as visually, they look very clean compared to the 46's I have. As long as they behave the same, it'll offset the cost of me vapour blasting, ultrasonic cleaning and zinc'ing the ones I already have.
They'll get the same fit and check test once Ian is back off his hols and finds the time to send them to me.
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Thanks for the feedback.
I'm just a happy bunny as I wanted to make sure the bike was running, at tickover and low revs in the garage, at least somewhere near or something like you'd expect. i.e. It starts, idles, revs when you turn the throttle and returns to idle when you close the throttle.
It now does that now and that gives me a lot of confidence going into the 'big strip' towards the end of next month. The last thing I wanted was to put a lot of effort into making the bike sparkle then having the carbs on and off ten times ruining paint, screw heads, clips etc etc
Pilot air screws were all set to 1.5 turns open so could possibly need a tweak as you mention.
Like I said previously, Keihin carbs are new to me. I've never needed to strip the few previous bikes Ive had with them so its been a good learning curve.
I'm still going to replace them with Ian's 627's using the manifolds from Mick as visually, they look very clean compared to the 46's I have. As long as they behave the same, it'll offset the cost of me vapour blasting, ultrasonic cleaning and zinc'ing the ones I already have.
They'll get the same fit and check test once Ian is back off his hols and finds the time to send them to me.
I think it's worthwhile pointing out the value in not carrying out a "gauge carb sync here.
That can get the bike into a doom loop because the vacuum gauge method needs a verified vacuum source from the engine and hardware to avoid sending the carbs off into unneeded offset that just builds in a fault, inappropriately.
Completing a accurate and diligent bench sync avoids falling down that false rabbit hole.
For anyone trying to get out stuck jets, or frazzled jis screws when working on carbs, Knipex make both a "Cobra" type waterpump plier in ES (extra slim) model and "Twin-grip" in small size. Both of these have a "fluted" arrangement in their jaws to accept cylindrical objects, making them particularly useful for compromised components that are difficult to extract.
They are a very good on the spring fuel pipe clips that are used on more current bikes too.