SOHC.co.uk Forums > CB500/550

CB550 PD46A carbs

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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

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