SOHC.co.uk Forums > CB350/400

400 four knocking noise.

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Lynx:
Thanks Bitsa. I think you're right, especially as the knock goes at higher revs when all cylinders are running. So where in the carb do I find this fault/blockage?

mick:
Probably in your idle & slow running circuits you may have to give your carbs more than 1 good clean to shift the stubborn dirt etc , ultrasonic is probably your best bet but if you haven't got that then carb cleaner and a a compressor for blowing through usually helps  ;) cheers Mick.

Clem2112:
Interesting..... been reading everything I can find about mysteries of 400/4 carbs ...here and elsewhere !!

Listened to that engine knock... heard that before !

My completely rebuilt engine (and carbs etc) does a similar-ish thing from cold.

After learning that the ethanol in petrol is a problem for our carbs and seals, I try to remember to run the bowls empty after a run. But if I don't....!!
...then after a day or two without running it will start only after lots of cranking, but No2 will not fire. Always No2 !!

I usually have to drain a bit of fuel from that bowl and then after a couple more attempts it starts on choke after a bit of coaxing. Bowl isn't full??
After a brief warm up it will  idle at 1200 rpm but only with the air screws set just one and a quarter turns out. Any more and it knocks and the idle speed drops could this be related to the float levels which I adjusted to stop the plugs sooting up.

On the road the engine now pulls well , especially above 5K if asked,  so the fuel supply is good. Only done 900 miles since the complete rebuild and it's been a tale of shortish rides and carbs-off for work.
Exhaust is standard OE (apart from the two pattern headers DSS supplied) as is the airbox etc.
I think the exhaust note is louder than I remember with my 1978 F"2" (yellow .. reg WGM508T) -  but that may be a symptom of this particular engine's state of tune - or just my memory of how quiet and civilised they were?.

Work on the carburettors....
Stripped for re-plating and inspection.
Were u-sonically cleaned individually (I bought a small cleaner from Maplin) and lots of compressed air to clear.  Fitted NOS floats, new OE needles, needle jets (tube), all new O rings. The floats were set using the clear tube+modified drain plug method (about 23/34mm after many attempts) so fuel all four levels equal and 7mm below joint faces.
Engine wise..
Tappets set to .002", ignition (Dyna-S) also spot on at idle and full advance.
Cam chain tension also OK. Camshaft measured OK when rebuilt.
Re-bored to 1st oversize, new rings, pistons. Re-shelled the mains and big-ends too.

Balanced the carbs to within half a cm Hg according to my old Davida vac gauges.
Each time the carbs have been off (half a dozen?, lost count) the bowls are spotless with no signs of deposits, gunk etc.
No longer overflow either-  I junked the Keyster kit items and re-fitted the original float valves after a through clean up and polish.
My thoughts are that maybe some excess air is getting in - could those felt seals be critical or some other item worn or corroded.
The bodies are apparently very sound and all the passages seem clear. I did read that new pilot jets sorted out idling problems on a  500 four, but mine look fine to me.

Nostalgia..
I got 400/4 fettling "experience" from working on my 1978 bike and clocked up 30,000 miles in all from new.
But the most I ever did or needed to do to the carbs was fit new main jets to suit a Yoshimura 4 into 1 (what a noise) and the bike never missed a beat... unless it rained or the points needed setting. Finally had to fit new tensioner blades and a head gasket to stop the infamous oil weep and chain rattle. 

 

Lobo:
.... I'm at a loss too, though in my case doubt leaks in the felt washers areas: mine were in poor shape; and having failed to replicate successfully I fitted good quality / tight fitting rubber 'o' rings (which appear to seal nicely) in place of... ie can't believe any leaks possible here now.

Which begs the question why are they used in preference to standard 'o' rings - was it to provide a source of shaft lubrication; or to minimise throttle 'stiction'.... or what?

Lynx:
Wow. Thanks for that. I did make my own felt rings. I haven't checked them since installing (such a pain disassembling that complicated lifting mechanism). If one of these is leaking air, will this effect the idle? I might have to go for the O-rings too. Do you have the right size handy Mick?

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