SOHC.co.uk Forums > CB500/550
idle jet differences , 069's
fogrider:
The compressions have been mentioned on here, finally got 'round to a check :
cold engine, 150 psi on all 4, give or take 1 psi. Looks OK to me for a mere 9 to 1 engine.
Hopefully the 1.2x4.2 O rings will turn up today and I can put the carbs back on and try the latest changes and adjustments. Trial and error. I think I now know what was going on, soon find out !
K2-K6:
Compression looks pretty decent, interesting to hear what you get when you feel you've all the obvious bugs out of it.
fogrider:
The new aftermarket needles back in, 1 clip richer as previous runs showed the new needles made it way too weak .( much fatter profile than the originals). It's clear to me that the needle jet is worn ( it's done nearly 67,000 miles ).
Incredibly, the Lambda reader showed over-rich the entire range, right up to 70mph. I took the air filter out at the roadside, no difference, just lots of extra noise .
After suggestions on here, I had set the air screws slightly on the rich side to partly improve the pick-up off idle. Still ticked over OK.
As an experiment, I turned the screws out to 1.1/2 and had another run. All the richness gone, right up to 65 70mph. The idle system was feeding the whole carb range, BIG style. That's a new one on me, I've had this Lambda set-up in my Austin Gipsy (Zenith), my Citroen DS (twin choke weber) my Triumph T100 unit twin (Amal concentric ) and my Cortina estate (Ford/Weber), none of them had the idle system working the whole range. Only the bottom end.
The idle jets are stamped 38 but measure 41, next step , change the idle jets, but , for now, it's running the best so far.
Glory be.....
philward:
Just seen this that shows jet range - may help?
https://www.caferacer.net/threads/cb500-four-4-to-1-exhaust-dilema.23465/page-3#lg=thread-23465&slide=0
fogrider:
Thanks Phil, I had spotted that chart a week or so ago. Quite different to some I have seen, much bigger overlaps. The idle system on mine is'nt just an overlap, it's a whole range mixture feed ! Without putting the Lambda reader in I would not have seen that. I suspect that the bigger size of the idle jet in it is possibly allowing too much volume into the choke area. I'm waiting for some 35 pilots to try as a test.
I had spotted something else somewhere regarding Keihin type 1's, an article describing how someone had worked out that going to a smaller idle jet helped the transition off idle. I wish I could find it again. That's what I will be trying next,
although to be fair , she's running really well now with the changes I've made, clean acceleration all the way except the slightest stumble just off idle, good tickover. Good plug colour.
All this effort has turned a very badly neglected bike into a pleasant ride.
I've learned a lot about CB550f's too !
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