SOHC.co.uk Forums > CB500/550

Hanging Idle and poor throttle response.

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Oddjob:
Well.

Checked 2 sets of jets and there is something in there. At first it's not really apparent but when you look through the jet whilst holding it up to an intense white light like an LED you can see what looks like a * in fact that exactly how it looks, 3 lines intersecting in the middle. I checked a set of 069a carbs and a set of 627B carbs and both show this odd little feature. I have no idea how they manufactured them like that. Probing the jet with a piece of wire off a wire brush they all felt the obstruction in exactly the same place, you can actually get the wire down past it by wiggling it a little but the obstruction is right where the body and the narrow tube meet. I measured the wire and it's right there on all of them.

I doubt a wire would break these lines but a drill certainly would. Not wonder my probing with a interdental brush didn't work when I was cleaning these jets. I thought maybe the brushes were too big but it's just that they are too soft to break the mesh.   

Sesman:
Now perhaps Trigger is not taking the piss.

I managed to probe through with a 0.4mm wire with some effort, obviously clearing the ‘web’ as I’ve come to describe it.

I’m about to remove a new OEM jet I bought last year for the 550……sound of drums slowly building.

Sesman:
And the winner is……

Brand new #38 jet for the 550. A 0.2mm and 0.3mm wire passes straight through.

I’ve had a good look through a bright light wearing sunglasses and I’m convinced it’s an optical illusion…light refraction.

deltarider:

--- Quote from: Skoti on August 22, 2022, 05:32:06 PM ---
--- Quote from: JezzaPeach on August 22, 2022, 04:56:57 PM ---Very many thanks. Good to understand. I get some plug sooting but probably a combination of general wear, valve guide seals, and jets worn slightly oversize. It seems to be getting better with more decent longer runs, but fresh jets would be a nice treat in future.

--- End quote ---

The carb needle jets wear out far quicker than other jets due to needle vibration from the engine when the throttle slides open and close.
Most people ignore them and waste cash on the easy to change jets instead, just like I did with my CB750.

New needle jets instantly cured my lumpy slow running and sooty exhaust problems.

You probably require 1601-232-3004 needle jet kits  X 4.
Not cheap for a CB500K1 tho...

--- End quote ---
Although needles worn by bibrations is known for one and two cylinder bikes, I have yet to see the first needles worn by vibrations on a 4 cylinder bike. Vibrations on a 4 cylinder are very very little indeed.

McCabe-Thiele (Ted):
I've seen visibally oval main jet needle seats on SU carbs regularly on cars - more so when BL spring mounted the main needles on later designs.

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