Balancing problems

[fayeslane]

Using the plastic tube filled with ATF manometer I can't get a good idle balance, in fact one carb wants to suck all the ATF into itself seemingly regardless of adjustment

At around 3000 rpm the balance is close going by the manometer.

Things I have done since the last balance (which was straight forward) are:
- fit Omega pistons
-new throttle shaft and o-ring in the carb that sucks too much.The other carb had new throttle shaft o-ring very recently when both carbs were rebuilt -gaskets, jets, o-rings, needle & seats etc.

Timing and valve clearances were done prior to attempting balance.

Working by ear & feel I have got the bike idling smoothly at 950 rpm.

What am I missing (apart from mechanical aptitude )?

 

[Beemerboff]

Sounds as if one throttle butterfly is not closing properly- always a risk when you remove them and the shaft.
You might have replaced it upside down or slightly bent the shaft.

By 3000 RPM both disks are open a fair bit , so the seal when closed doesn't matter.

I can vaguely remember some disaster in the dim and distant past when removing butterflys , probably on a SU, cant remember what it was but it put me off attempting it again for life.

IMHO a small leak at the shaft when the throttle is closed can be adjusted out with the throttle stop and mixture screws, and as soon as the throttle is open the the air will take the path of least resistance and come in the intake anyway, so I dont think I am missing much.

 

[fayeslane]

Checked the butterfly for correct installation, I had marked it with a scribe before taking it out so know that it went back in correctly. Hadn't considered bent throttle shaft though (don't want to pull that out again after doing such nice job of peening the screws ).

I will loosen the butterfly screws slightly and push the butterfly closed so it centres itself in the body and retighten the screws. It looks to be a snug fit but I don't know how many poofteenths of a gap make a significant difference at idle

If it's idling nicely and balanced thereabouts at 3000 rpm perhaps I should do nothing?

 

[GEOFFREY DEACON]

Are you carb manifold airtight?.
Make sure you have free play in the throttle cables where they fit the carb and is equal to each other,(2-3mm) recheck free play IF? you adjust the tickover screw.
Check mixture screws are equal (turns out) then adjust throttle tickover via the tickover screw, then recheck your 3000 rpm balance?.
If you need to adjust the cable free play at any point ,?It will mess up your 3k readings ,so reset again.

If you adjust the cable free play adjuster, then hold the throttle cable with pin nose pliers or finger to stop it twisting which upset the result later on.

After road test recheck your settings.

 

[ghiribizzo]

+1 for G D's advice. Do a bit of Aussie thinking on it - you may get a balance at 3000rpm but it should idle on the stops ok with free play in the cables, not either carb hanging on a cable.
That is - with the throttle cables totally removed you should still have the perfect balance at idle.
If not, there's work to do on the idle stops and mixture screws. (Assuming that the valve clearances are A1 and there are no other air leaks in manifolds etc.)

Remember that when you're trying to get a balance, the one doing the most "sucking" - pulling up more ATF or mercury is the one which is producing the greater vacuum. That's the one with the slide+needle sitting lowest.

Once your idle's sorted, as per GD's...

When you've sorted the idle - watch how the oil reacts as you open the throttle gently rather than just what you get once it settles at 3000. Remember the one that drops the most is opening first. The 'high' one is the one hanging back and needing to be adjusted to match the other. But again watch your free-play.

2 minute job once you have your head round it. Or send us some $$$ and 4 or 5 will come round for New Year to help you out!

 

[fayeslane]

That is - with the throttle cables totally removed you should still have the perfect balance at idle.

This where the problem lies -on start up no throttle, everything loose and stops wound right back to no contact point it wants to pull the ATF into the carb with the new throttle shaft. As per BBs advice I reckon the butterfly needs revisiting. Prior to installing the new shaft balancing was always a 5 minute job.

 

[ghiribizzo]

 

[fayeslane]

Do a bit of Aussie thinking on it -

What - crack the shits then go and get pissed?

 

[(RIP) pastyman]

Aussie thinkinjg at the mo would include setting the buggers on fire, sticking the remains in a little urn then they could have their own ashes

 

[fayeslane]

Oh, is the cricket on......................

 

[SJRoome]

Just a thought

Hi

If it idles smoothly it cant be far wrong. Have you checked that the vacuum pick up hole is not blocked?

I hesitate to ask, but did you did remember to take the cap screw off?

Steve