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Hi everyone. Finally did it. After reading the haynes manual and all the great tips on these threads I balanced the carbs on my wife's 1999 Thunderbird with 6000 miles on the clock.
Be interesting to see what the readings are when I do my 2002 TBS.

Thought I would post the results, because I don't really know what they mean. The gauges were Davida. Expensive but all metal and built to last. The scale on the gauges was cm. hg (centimetres / mercury)

Vacuum gauge readings before making any adjustments.

Revs Cylinder1/2/3
1000 27 34 27
3000 28 30 28
4000 26 30 30

After completing adjustments
1000 30 30 30
3000 32 32 33
4000 30 30 32
5000 30 32 30

To my thinking, the scale is arbitrary. It's the relationship the between the readings for each cylinder that is important??

A bit naive, I thought the vacuum would increase with revs??

Haynes says the reading should be 12.7 - 15.2 cm / hg

So my vacuum is either twice what Haynes says it should be, or half of the Haynes figure.

Should I be concerned with these readings??


Tomsdad
 

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...The scale on the gauges was cm. hg (centimetres / mercury)

Vacuum gauge readings before making any adjustments.

Revs Cylinder1/2/3
1000 27 34 27
3000 28 30 28
4000 26 30 30

After completing adjustments
1000 30 30 30
3000 32 32 33
4000 30 30 32
5000 30 32 30

To my thinking, the scale is arbitrary. It's the relationship the between the readings for each cylinder that is important??

Exactly. The object is to balance the throttle butterfly openings with no throttle applied so your idle will be smooth. As the the butterflies open the balance becomes less important until it has virtually zero impact at full throttle.

A bit naive, I thought the vacuum would increase with revs??

Just the opposite if there's load on the engine since intake manifold vacuum decreases as the throttle opening increases.

With a loaded engine, more throttle means that the butterflies are open more and the vacuum is lower. With no load on the engine it only takes a minimal throttle opening to produce high rpm.

Haynes says the reading should be 12.7 - 15.2 cm / hg. So my vacuum is either twice what Haynes says it should be, or half of the Haynes figure. Should I be concerned with these readings??

The Triumph shop manual gives the same specs but I got similar readings to yours on my Morgan Carbtune last time I did a balance. There are lots of mistakes in both the Triumph and Haynes manuals. ;)

The main thing is that they're all as equal as possible and it sounds like you did well on a first attempt! :)
 

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Davida Gauge Readings Mikuni

Toms dad,
I get approx 30cm hg at 1000rpm...................
I definately think my haynes manual is incorrect.............
30cm hg = 11.81 inhg
= 300 mmhg
The davida gauges are the best,
i think your theory, in doubling the haynes figures is on the money-

2 x 152 mmhg = 304 mmhg or 30.4 cmhg...........people should be shot for these hairpulling, spanner throwing blunders! Ride on & listen to that rumble of the triple!

So we're point four under the max hg range.............jeff


The gauges were Davida. Expensive but all metal and built to last. The scale on the gauges was cm. hg (centimetres / mercury)
After completing adjustments
1000 30 30 30
Haynes says the reading should be 12.7 - 15.2 cm / hg
So my vacuum is either twice what Haynes says it should be, or half of the Haynes figure.
Tomsdad[/QUOTE]
 

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To my thinking, the scale is arbitrary. It's the relationship the between the readings for each cylinder that is important??
Tomsdad
This is exaclty why, you can balance your carbs with a simple manometer. I don't need no stinking expensive gauges, but if you don't need yours anymore .... :D

Secondly, being a novice, it dawned on me that carb balancing is essentially a mechanical adjustment. The throttle cable is on the center carb (master) and the other two are linked with those 2 adjustment screws. Balancing assures that when you open the throttle, the other two butterfiles on carbs 1 and 3 open with the exact same distance as the master carb. :idea: I think it was Jimmy, who wrote a post on "bench balancing" ..... which is measured mechanically. They measure the butterfly opening with drill bits! Now, no one in their right mind would remove carbs to bench balance, rather that is for when you have them out for some other purpose and don't have a manometer or synch guage.

You can learn a lot from these guys here. When they start talking electronics, they often lose me, but I'm learning as I go.
 

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Secondly, being a novice, it dawned on me that carb balancing is essentially a mechanical adjustment. The throttle cable is on the center carb (master) and the other two are linked with those 2 adjustment screws. Balancing assures that when you open the throttle, the other two butterfiles on carbs 1 and 3 open with the exact same distance as the master carb. :idea: I think it was Jimmy, who wrote a post on "bench balancing" ..... which is measured mechanically. They measure the butterfly opening with drill bits! ....
Setting the butterflies to equal mechanical opening in the way you suggest is maybe not quite the whole story, tho' it's useful especially if the linkages between the carbs have been reassembled after a complete strip down. It should get things thereabouts.
Other small differences in the air/mixture/exhaust flow path thru' individual carbs, cylinders & exhaust system will also affect the vacuum on each one. Such things might be relative cylinder head intake port smoothness or the single rear airbox connection to the air filter housing being over on one side maybe. Hence the reason for directly reading (really comparing) air pressure (vacuum) at the same point in each carb as an accurate 'proxy' of it's actual air/fuel mixture flow rate.

In my own experience the balance screws are very sensitive in getting a good tickover & nice smooth pick-up from throttle off. IMO it's one of the most satisfying bit's of maintenance that an owner can do :)
 

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Other small differences in the air/mixture/exhaust flow path thru' individual carbs, cylinders & exhaust system will also affect the vacuum on each one. Such things might be relative cylinder head intake port smoothness or the single rear airbox connection to the air filter housing being over on one side maybe. Hence the reason for directly reading (really comparing) air pressure (vacuum) at the same point in each carb as an accurate 'proxy' of it's actual air/fuel mixture flow rate.

In my own experience the balance screws are very sensitive in getting a good tickover & nice smooth pick-up from throttle off. IMO it's one of the most satisfying bit's of maintenance that an owner can do :)

Excellent points ... I wasn't saying the bench balance was a better option. To me it explained what actually occurs when you tweak those screws. Thanks for explaining the other internal happenings the affect the vacuum pressure for each cylinder. I definitely would recommend the running bike synch option. It's easier, more accurate and like you said "satisfying"!

Also agree that synching is easy and most satisfying, especially using a home made manometer with red fluid (stabil) in jars. The tiniest tweak can move the fluid from one side to the other. When you get it right, it bubbles evenly in both jars like a mad scientist experiment. MUHAHAHAHAHA ...:elated:
 

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cool!

We should have a sticky..'mad scientist photo maintenance guide' :D :beerchug:
 

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I think it was Jimmy, who wrote a post on "bench balancing" ..... which is measured mechanically. They measure the butterfly opening with drill bits! Now, no one in their right mind would remove carbs to bench balance, rather that is for when you have them out for some other purpose and don't have a manometer or synch guage.
Nah. Wasn't me, but I've done dry settings like that after completely disassembling a carb rack for other reasons. It's very helpful in getting a bike to start after some serious disassembly so you can fine tune it with gauges.

Jim
 

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Hi interesting post. I would like to ask where do I start with the synchronising. I was told to set the idle screw at a high idle then adjust 1 and 3 to tge same tgen use tge idle adjustment to bring it down to 1000RPM is this correct. My carbs are running at 0.4 0.3 0.4. Both exhausts are smoking a bit running rich I expect. What is tge correct procedure
 

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adjust 1 and 3 to tge same tgen use tge idle adjustment to bring it down to 1000RPM
OK, is this a letter substitution code? if "g" = "h" then, yes, that's the right procedure, except slower is better, as long as you get steady readings. Sync makes the biggest difference at the smallest throttle openings.
 

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One notice: in the past at first I set the pilot screw position ( 2.5half out as a starting point and then final position with hightest rpm....
Then I synchr the carbs.
AS a happy ower of AFR gauge when I did procedure in the same order now (setting the pilot screw position at idle according to the best AFR ), I found out that after balancing carbs afterwards, AFR got changed (in stroke the carb I had to balance)quite lean at idle (AFR 16). Its clear cos we all know how sensitive synchr screws are at idle and what big vacuum changes ( and fuel venturi effect as a result ) it makes. (No surprise the gap around butterfly almost invisible with close trottle)
So for me after carb overhaul :
-set pilot screw position the best you can
( hightes rpm at idle)
-Synchr carbs first
-set final position pilot screw as a second step
 
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