CB550K3 carb balancing - the never-ending story

[Ashdowner]

Rigged up the vacuum gauges yesterday. The needles on 3 of the gauges are rock steady through the rev range but no matter what I do the gauge on pot 3 fluctuates wildly. Any ideas what disastrous problem I might have? Other things to note is that tickover is still very lumpy (due to pot 3?) and if I try and up the revs with the throttle stop screw and then twist the throttle it races and won't come back to idle unless I turn the screw back. Note I've checked all the cables and they're all as they should be.   

[totty]

Have you tried swapping the gauges between 3 and 4 to see if the fluctuation is the gauge or the pot?

[Derckey]

Have you checked for air leaks between carb and head?

[K2-K6]

After checking the other mentioned items above, have a check of the number 3 inlet valve clearance as if the valve is leaking even a small amount it'll probably pulse the inlet fo that cylinder.

A more extreme check would be to run it with the number three plug removed to see when you have no compression in that cylinder it is still pulsing the inlet vacuum.

If you check the compression it may also give you a read on what's going on internally.

[ka-ja]

Hi,
    Double check HT lead/plug cap for loose fitting

[Ashdowner]

Thanks everyone. Compression tester defunct or would have checked it. Yes swapped the gauges. Plug caps all tight on HT leads. Will check inlet valve and carb manifold next week and report back

[Ashdowner]

Replaced 0 rings on carb manifold. No change.

[Bryanj]

Tiny O ring on mixture screw missing along with washer, only available with a screw kit

[Ashdowner]

Was that a question Bryan?

[Bryanj]

Nope, what I think may be wrong and you have to buy a screw kit to get the correct one

[Ashdowner]

Ok I'll take the screw out and check it. A mate has got a box of very small o rings so one may fit.

[Ashdowner]

Just to bring this story to a reasonably happy end, and to admit that the problems I was having were entirely of my own making by not giving the bike a proper post-winter service. I got a compression tester and got awful results which suggested valve problems. I checked the tappets to find that the valve clearances had tightened and in the case of two valves completely disappeared  - so pot 3 certainly had a valve that wasn't closing. I redid the tappets and redid the comp test with much better results Pots 1, 2 & 4 around 110 lbs and pot 3 at 90lbs.  The lower compression on pot 3 I put down to the valve seat probably being burnt Anyhow, the bottom line is that the bike now ticks over very well at 1200rpm and over 3000rpm it pulls superbly without hesitating on the throttle and has that lovely singing noise. Even the clacking from the clutch has almost disappeared. Maybe I set the tappet clearances wrongly last time but it really brings home just how important it is to get them right. Next winter I'll take the head off and reseat the valves but for now it does everything a CB550 should do.

[Trigger]

If the bike is run for sometime with the valves not fully closed, the valve face will burn and pit. This will also effect the valve seats, the valves will need re-dressing and valve seats cut to seal.

[Ashdowner]

No doubt about it Trigger, but maybe I'll be lucky because it's hardly run since I last set (mis-set?) them. Anyhow I'll give an update when I pull it apart in a few months time.

[Trigger]

Try setting them at 8 & 10, instead of 5 & 8.