Hanging Idle and poor throttle response.

[JezzaPeach]

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.

[Sesman]

FM. Judging from that historic post it’s still unclear.

Personally I very much doubt the jet is more than a simple drilling, unless somebody would care to produce evidence to the contrary.

[Skoti]

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.

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...

Good luck

Skoti


   

[Oddjob]

I have about 3 sets of carbs apart right now. I'll check all the pilot jets to see if any show any sign of ANYTHING down inside them.

I do think some jets have this feature just not these, the small holes are meant to be the atomisers for the fuel.

[Sesman]

Cheers Oddjob.

Looking forward to the outcome.

[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]

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.

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...

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.

[3scs]

There’s definitely a spider in there I can see them in both sets of f2 carbs I have but only when I take my glasses off

[Sesman]

Bet you can’t. Try passing a 0.2mm then .3mm wire through a clean jet. If it goes through, there is no spiders web.

[McCabe-Thiele (Ted)]

It might not be relevant but back in the 1960 blocked jets on Vauxhall Vivas caused misfires due to them blocking up - the factory fix was to produce similar jets fitted with a screen filter as a remedy.

Eventually they just fitted better in line fuel filters that fixed the issue completely.

[3scs]

Bet you can’t. Try passing a 0.2mm then .3mm wire through a clean jet. If it goes through, there is no spiders web.

exactly push any wire through and there will be no spiders web left I know what I can see in my jets it’s definitely there 😊

[Skoti]

"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."

Ted makes a interesting observation regarding oval wear on needle jets because it's apparently not only engine vibration that wears them but also the inlet vacuum effect.
When accelerating hard on full throttle the vacuum draws the needle forward hard against the needle jet and then the back and forth motion causes this oval wear pattern which was also evident on my CB750 when I changed my first needle jet  set at 24k.


It's obvious that I'm not the only one on this forum that worked in the motorcycle trade in the seventies or others who perhaps professionally repair that stuff nowadays so to them please forgive me for offering this advice.

The needle jet and needle control the correct amount of fuel passing through the main jet up to about 3/4 throttle opening, after that the main jet size defines the correct fuel supply.

My point is that no amount of fiddling with slow jets will considerably alter rich running problems on the road as the slow jets only controls engine idle and the transition to the needle jet and main jet fuel supply and is effective up to around 1/8 throttle opening.

Worn needle jets alter the air/fuel ratio and cause rich running and possibly throttle hanging etc.

Damaged slow running jets enlarged by cleaning with oversized wire probably won't improve things either...
 


Please correct me if you think I'm wrong, I'm never to old to learn!

Regards

Skoti