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Throttle balance puzzle.

D

dikman

Tuned and Synch'ed
Joined
Jun 27, 2012
Messages
47
Location
Sth. Oz
Today I re-balanced the throttle bodies on my 1200 Breva, using the excellent how-to's posted here.

Closed air bleed screw, attached manometer, adjusted the fast speed screw to balance at 3000 rpm, then adjusted the idle by opening the air bleed screw. So far so good, but if I then take it to 3000 rpm again the balance is now out because of the extra air coming in on one cylinder via the open bleed screw!

It seems to me that it's back-to-front. Balance the idle first, then do the high speed which will balance allowing for the extra air via the open bleed screw.

Am I missing something here? Where am I wrong in my reasoning?
 
The amount air induced by the air bleed screw will have a negligible effect in cylinder temperature. It can take a few of the balance adjustments to get it spot on. A large fan blowing on the motor will help in keeping the temperature from getting too high during the adjusting procedure.
 
Sorry draidt, you've confused me. I don't see how the temperature is affecting the balance - the problem is airflow. I get the high speed balance right, but as soon as I open the idle bleed screw then that high speed balance is no longer correct because additional air is going into that cylinder via the bleed screw.

The idle bleed has to affect it, otherwise why say to close it before adjusting the balance? This doesn't make any sense to me.
 
dikman said:
Sorry draidt, you've confused me. I don't see how the temperature is affecting the balance - the problem is airflow. I get the high speed balance right, but as soon as I open the idle bleed screw then that high speed balance is no longer correct because additional air is going into that cylinder via the bleed screw.

The idle bleed has to affect it, otherwise why say to close it before adjusting the balance? This doesn't make any sense to me.
Click to expand...

When you do the high speed, it won't settle in on the first try. Get it spot on, let it idle, bring it back up and it is off is normal. It take a few tries to get the high speed spot on in a repeatable fashion. The amount of air passed by the bleed screw for idle sync is minimal and has very little, if any impact on high speed sync.
 
john zibell said:
dikman said:
Sorry draidt, you've confused me. I don't see how the temperature is affecting the balance - the problem is airflow. I get the high speed balance right, but as soon as I open the idle bleed screw then that high speed balance is no longer correct because additional air is going into that cylinder via the bleed screw.

The idle bleed has to affect it, otherwise why say to close it before adjusting the balance? This doesn't make any sense to me.
Click to expand...

When you do the high speed, it won't settle in on the first try. Get it spot on, let it idle, bring it back up and it is off is normal. It take a few tries to get the high speed spot on in a repeatable fashion. The amount of air passed by the bleed screw for idle sync is minimal and has very little, if any impact on high speed sync.
Click to expand...


That's what I trying to say :dry:
 
Belive what John is trying to say is to do the high speed sync a few times to make sure it is spot on, then let it idle. Don't adjust the bleed screw yet. Once you have the high speed sync repeatable then reset the TPS and open the air bleed to balance idle.

I run mine a little different. On my carb mate I can set the resolution. I will set the high sync at 0.5 cm hg. Make sure it is repeatable and then reset the TPS. I will then do the idle balance at 0.75 to 1 cm hg (which is actually the spec). When I was trying to balance both at 0.5 cm hg I too was chasing my tail. I had to have my bleed screw open a couple of turns which does make a difference on the high speed sync. When I decreased the resolution on the idle sync, my bleed screw is only open a 1/2 turn and everything stayed balanced on the high speed. Not sure if it is the right method, but works for my 2012 Stelvio.
 
Thanks guys, it looks like I was jumping the gun a bit and need to do several passes at it. I think my bleed screw is open more than 1/2 turn, which may account for part of the problem. (The oil-filled manometer is also, I suspect, very sensitive to adjustments).

My apologies, draight, the way I read your post I thought you were talking about the temp causing my problem.
 
Finally got around to re-doing the balance. And yes, it took several passes, every time I adjusted it I dropped it back to idle, then up to 3k again, bit more tweaking, down to idle, back up etc etc. I finally got it reasonably stable, and the idle screw is out 1/2 turn.

Even though it was a cold day, with the bike sitting in the open shed doorway and a fan blowing on it that motor gets damned hot!! I think I'd better save this sort of adjustment for the Winter months (not our 35C Summer days!).

Just a thought - for anyone else using an oil-filled (tube) manometer, what weight oil are you using?
 
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