CB550 PD46A carbs

[badger_eng]

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

[badger_eng]

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
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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.
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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.
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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.
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[badger_eng]

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.

[Oddjob]

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.

[badger_eng]

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.

[K2-K6]

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.