CB550K3 PD46 carb jet sizes - the eternal querstion

[Ashdowner]

My 1978 CB550K3 PD46 carbs have got main jets of 90 and slow-running jets of 38. Is that correct? I struggle to get it to run smoothly at low revs and it always feels like it's starved of petrol when I open it up. The Haynes manual only refers to the earlier type carbs.

[Bryanj]

You can just about guarantee that the pilots are blocked and possibly pilot circuit. The K3 takes a long time on choke before it will run properly. Last one i did took 3 carb cleanings at monthly intervals and new pilots before it ran anything like right

[Ashdowner]

Cheers Bryan. I've discovered from the SOHC specs file that I have the right jets. The jets weren't blocked but I've put them in the ultrasound tank. So to clean the circuits I assume requires ultrasound? Only got a small tank which means the pain of dismantling the carbs from the rack and bath them one by one. Groan.

[Bryanj]

DO NOT split the PD from the rack as the choke mechanism is an absolute nightmare. I managed with carb clean and air line. I did find new pilot jets at Sirius in Canada.

[matthewmosse]

On my wifes old nighthawk I had issues with carbs gunked up on stale fuel. Being as time was short and I had limited patience for a bike I regarded purely as a utility, I just jetted enough carb cleaner into it to get it running like a bag of spanners then ran it with redex in the fuel for 3 months d a daily rider - well actually 2 or 3 years but 3 months was how long it took to get the cabs running decently. Improvement was gradual, the first few hundred miles was not very fun as it kept stalling and dieing. My shortcut probably cost £10 in redex but removing the carb would have taken longer than an hour and a halfs wages. Vibration and solvent both present when riding. Obviously only effective if the blockage is one that will dissolve or flush, so stale fuel it works but alloy  oxide or dirst still needs removing the old fashioned way, but I have had some luck with my lazy way over the years and would say its worth a shot.

[Seabeowner]

DO NOT split the PD from the rack as the choke mechanism is an absolute nightmare. I managed with carb clean and air line. I did find new pilot jets at Sirius in Canada.

If you have split them getting the double spring connection be tween 2-3 is certainly challenging.
I think that the pilot was opened up to a 42 for the US 78 bikes.
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[Bryanj]

Sirius in Canada do new pilots for these and a couple of sizes if i remember correctly

[K2-K6]

I'd echo Bryan and Mathewmosse,  in that if they've been blocked on those really small low speed jets,  it can take quite a while before you can get them back up to scratch.
Initially, as soon as you get them visibly clean and running,  then they accumilate more problems.

You can get some idea of which carb is problematic by warming the engine and one at a time screw the adjustment in and out.  If it's blocked you get no response from that cylinder.  so at least you can pinpoint where you need to work.

Also,  with them dismantled,  you can use one of those domestic steam cleaners with a nozzle for ovens etc to clean the tubes within the carb bodies. It's used in chemical analytical labs to clean all the glassware,  removes tarring, crystalline deposits etc and may help you get them fully operational.