SOHC.co.uk Forums > CB500/550
idle jet differences , 069's
K2-K6:
Could an overview be useful?
The original intake system on these it would be easy to describe as exquisitely resolved, for a 1970s engine with such smooth idle, linear response all the way to 10,000rpm without glitches and roughly 100bhp per litre, they deserve that wording as such a finely designed and specified production ready engine.
They are veeeery sensitive to any changes in their components though, that's either the vacuum characteristics, fuel levels or ultimately jetting.
There's effectively no facility to competently alter the vacuum curve, with any shifts in carburettor feed likely to need out of range jetting compensation to get toward where Honda set them. Even the airbox lid in different specifications will shift the balance.
As you note from experience with Amal and SU (I alter vacuum gradient on SU with oil viscosity to trim above and below target for transient openings) that tool is missing here.
The compression and cylinder draw on this engine is a serious consideration as they will run quite well with very significant upper cylinder damage, but with some odd response effects. This thread http://www.sohc.co.uk/index.php/topic,14049.0.html may give some idea if you look through at the state of the pistons when dismantled.
As already mentioned by others, current pattern brass components are often no match at all for the accuracy of the original parts. Kiehin where particularly good in production parts.
deltarider:
--- Quote from: fogrider on June 05, 2021, 10:41:43 AM ---One thing I noticed is a reference to a slow jet. There is'nt one on the parts diagram so presume it's a drilling. Must check that for size.
--- End quote ---
It should be in the parts diagram, part 17 in: https://www.cmsnl.com/honda-cb550f2-super-sport-550-four-1977-england_model17121/partslist/E17-1.html
Anyway, it's supposed to be a #38
fogrider:
The reference I saw was to a slow jet, but not stated as idle jet. I assumed it was meant to be slow air jet, I've seen them quoted as 150 and 220. 220 could be for the 750's though.
No problem with the idle jets, original 38's, the long ones you illustrated . They do seem to be nearer 40 though but it idles OK at the right air screw turns out.
I checked the air entry hole which is in the air box side of the carb mouth. I assume that is what is referred to as the slow air jet. It works out as a 180, much more air than a 150.
The original needles are now back in. The difference in diameter at set points amounted to nearly 2 needle clip positions weaker.
The new main jets with their O rings are a firm push fit, but just in case, I shall get "proper" O rings on Monday before re-fitting the carbs. Will put the ignition back too so it's a fair comparison.
Here's hoping.
fogrider:
Just found the reference to a slow air jet - it's in an addendum to the CB550K manual, shows a fuel level of 14.5mm so must be a different version of these carbs.
If anyone's got some 069a's accessible and can measure that small air drilling to the side of the venturi, that would be a
very useful bit of info. My guess is 1.5mm, but confirmation very welcome.
Anyone ?
deltarider:
--- Quote from: fogrider on June 05, 2021, 09:20:55 PM ---Just found the reference to a slow air jet - it's in an addendum to the CB550K manual, shows a fuel level of 14.5mm so must be a different version of these carbs.
--- End quote ---
That concerns the later K3 and K4 models which had totally diferent PD carbs. See pic below. Just to make sure: what type do yours have: the PD or the oldstyle?
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