SOHC.co.uk Forums > CB750

750 K4, poor running

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ianmck:
My 750K4 was running fine apart from a slight "stumble" when accelerating gently from say 30 up to 40 mph, also showing signs of being rich i.e. sooty deposits on plugs - hardly any choke required to start.  After some investigation I found that there were 120 main jets in carbs.  Searching through both SOHC sites, speaking to David Silver and looking at CSMNL website it was decided that 105 mains should be in carbs so duly ordered 4 of these and fitted at the same time I checked tappets, points, timing, carb synch, adjusting timing chain etc. New plugs (D8EA).  Bike now doesn't have a flat spot at low revs, but "holds back" "surges" on harder acceleration, doesn't want to go past 3500 to 4000 RPM.  Re-checked all settings, re-fitted 120 jets, still the same.  Checked fuel supply - cleaned petrol tap, still the same, any help/advice would be gratefully received. Thanks

Ianmck

Bryanj:
What exhaust and air filter is fitted?

Is mechanical advance free?

Is battery good and charging?

Plug caps new?

BigAl (Alan):
Did you also replace the needle jets? sounds as if the machine is running too lean. Had a similar problem with my K4, re-built the carbs with Keyster kits, but had to order 105 jets and needdle jets from Dave Silver to solve the problem.
I have assummed every thing on your bike is standard spec.

Alan

ianmck:
hello bryanj, standard exhaust, (new a couple of months ago), standard air filter (new), battery is new, havn't checked charging, plug caps have been swapped from my 400/4, which runs like a sewing machine makes no difference. have advanced the timing slightly, I can then clear 3500/4000 rpm, but starts surging again around 6000 revs, if I try to cruise at a steady 30, or 40 mph engine is rough,light acceleration is fine, hard acceleration is fine up to around 6000 rpm, if I set timing back to spec, then problem happens back down at 3500/4000 rpm, have checked advance, is free, also checked with timing light and the ign advances to spec.
hello Bigal, hav'nt changed needle jets, they are as they were, but have cleaned and checked the carb seems OK. regards ianmck

Bryanj:
It would suggest to me that a PO has "rebuilt" the carbs with the wrong bits or cheap aftermarket kits. It seems that Keyster and aothers dont get the needle profile right.

I have also heard of, but not actualy seen, that its is possible to mix the slides up by putting 1&2 in 3&4 which puts the slide cuttout on the wrong sideas the location pegs are "handed".

Sorry to say i think you are in for several carb removals before you cure it

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