SOHC.co.uk Forums > CB500/550

500 carbs what does what question.

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McCabe-Thiele (Ted):
When I rebuilt the carbs on my 400 it worked out fine, runs well, no hanging throttle issues - I used the full Keyster Kit that came with the bike when I bought it from the PO. I know some folks have had issues with Keyster Kit - mine were new boxed, purchased by the PO many years ago.

Fast forward almost three years  - my 500 seems to be giving me some unexpected problems - not helped by my superficial knowledge of understanding the carb design arrangement.

A bit of background is worth a mention as they were sent by the PO to Gerben so had quite nice shiney tops & bowls.
On dismantling the carbs I found one of the two bearings in the manifold mounting bracket  was half broken presumably when the locking pin had been removed. I replaced the bracket with one sourced from Ken plus a much better shaft from Florence as mine was quite pitted. The four needles were set one groove lower than the centre position so the needle was effectively raised a notch - set slightly richer. One of my needles was bent near the tip - I do not belive it was bent by me but cannot be 100% certain only 99% certain it was not me. I purchased a set of 500 needles & emulsion tubes from DS as the kit was not too expensive. I have fitted the needle clips into the centre position of the five grooves on all four sliders. Carbs only bench sync at the moment.

On first start up the bike ran reasonably well but by the next day I noticed that my petrol pipe that came with my temporary tank kit was sticky on the outside. When I started the 500 the next day  it was clearly not running properly on number one carb as the manifold was almost cold, plus the float valve was now leaking.  I've binned the crap petrol pipe, removed the carb bowl on number one carb only  & cleaned the screw in jet that had a small amount of muck in it. I opened the petrol tap with the bowl off and a bit of muck was washed out as well.

At the moment I am mentally resisting taking the carbs off - as Roo has said I need to take them off and do a proper check on the other three carbs. I'm now resigned to taking them off when I'm in the right head space. I hate having to do things twice.

I fired her up again yesterday and there was some heat on number one exhaust manifold but nothing  like the other three. At the moment I have all four carbs set with the side screw 1.5 turns out. After about 5 minutes of engine running with my Stand Fan blowing air over the front of the engine I removed the spark plugs to find 2,3 & 4 all sooty.

I understand the basic principale of the main jet/slider needle - it's very similar to the old SU car designs.
I am not sure when you adjust the Brass coloured side screw what that actually does - is it just essentially for the engine idle mixture?
The other jet that screws into the carb body inside the bowl - the Pilot Jet - is the Brass coloured side screw linked to this internally?

I know we understand what a sooty plug looks like but added the photo to show how mine looked.

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500 plugs by Macabe Thiele, on Flickr
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K2-K6:

 "I am not sure when you adjust the Brass coloured side screw what that actually does - is it just essentially for the engine idle mixture?
The other jet that screws into the carb body inside the bowl - the Pilot Jet - is the Brass coloured side screw linked to this internally?"

The idle circuit, in isolation for this description;- the small jet 40 ? Is connected to the venturi on engine side of slide to feed fuel in concise metered amount for slow running, this is given a gradient response by having an air passage from airbox side of slide into the vertical passage from float bowl to venturi through that 40 jet.
The size of that jet sets the maximum this assembly can flow, then with the external adjustment brass screw, the air passage can be controlled to alter the fuel air ratio and how quickly it gains fuel flow by changing the amount of air is allowed to pass the adjustment screw.

In effect it could be considered a small carburettor (essentially running in parallel to main slide, needle and main jet) but obviously set and fixed after adjustment to give the required fuel air mix at low air volumes in the venturi. 

Effect;- if you turn the brass screw fully in, closed, there's nil air going through, the venturi drop in pressure just pulls full fuel as soon as you start the motor.
Too far turned out, there's excess air that's easier for vacuum to pull in rather than lift fuel from the float bowl.

Bryanj:
When you changed the needles you say you changed the emulsion tubes/needle jet in the carb body Ted were these Honda parts or aftermarket parts? Everybody on the US site says that the needle size and taper on "kit" parts are not the same as original and give problems.
The mixture screws on the side of the carbs are interconnected with the pilot jets, i thought there was a diagram in the Honda manual showing what went where but trouble is i dont often look in the book anymore, its all mostly in the dustbin tween me ears mate

McCabe-Thiele (Ted):
The needles & emulsion tubes were from DS. I still have the old tubes & 3 needles iir.

Bryanj:
Yes but were they pattern or genuine Honda

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