CB550 PD46A carbs

[badger_eng]

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.
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[badger_eng]

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.

[taysidedragon]

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.

[badger_eng]

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

[deltarider]

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?

[badger_eng]

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 😂

[Harold 400/4]

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

[badger_eng]

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

[Harold 400/4]

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

[badger_eng]

Will do.

Thanks for everyone’s input 👍

Much appreciated

[Bryanj]

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

[badger_eng]

Can’t beat a bit of good luck 😂👍

[Oddjob]

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.

[badger_eng]

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 👍

[deltarider]

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