SOHC.co.uk Forums > CB750
miss at 3.8K
Bitsa (Ralph Wright - RIP):
Billy
All depends on how much rust and crap is floating around, my tank is rust free and yes the tap filter is also new but being over cautious wont hurt.I even have little filters on the float valves,
Cheers
Bitsa
billy192:
went out today between showers and crap still missing. so I thought at 60-65 which is about 3.5 k it starts hesitating so i pulled up the choke halfway and boom no missing it flew along at 70-80 no problem.
so off with the carbs again an check the main jet and the needle clip position. clip is in the middle one (correct for std) the main is 105 and according to the book it should be 120. would this make it miss? i assume by pulling the choke its running lean.
the bike before i stripped had a four into one would they have had to re jet lean for that?
so next step 4 new main jets 120.
Billy
K2-K6:
Looks like you are getting there billy, as when you choke it you obviously richen it at all revs and the fact if took off ok then shows it needs more fuel. I'd go with the spec and return the mains to std.
I was slightly confused as with your second post you said the choke made no difference but it's always worth a play with at different revs to see what affect you get and can give a cheap and quick diagosis.
Just to cover a bit more of what is happening here, an engine will run at a steady state (continous revs and load conditions) with quite a lean mixture but it then needs quite a bit of a kick with extra fuel to get a nice clean acceleration (they do this on later F2 engines with an additional pump as you open the throttles) but older carbs like the K6 will be set as a compromise of steady running AND enough fuel to accelerate without going too lean and causing a missfire, yours appears to be jetted right at the bottom or even less than required to achieve that balance. If you jet as per spec then any small adjust should be possible with the needle clips if it needs any trimming at all, but it should be much better if you change the mains.
hairygit:
Silly question Billy, but are they the correct carbs for your bike? maybe some previous owner swapped them over, as 105 is the main jet size for a 750F1 and possibly they have been replaced at some time with F1 carbs (probably thought F1 has 4 into 1 as standard, easier than changing jets, and cheaper which it was years ago!)
billy192:
original carbs
billy
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