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Carb Balance Video - Reasonably Balanced?

Balance the carbs at just-off-idle where throttle arm comes off the idle stop. Mid-range is a **** shoot and can’t be adjusted. Likewise, full-throttle can’t be adjusted, but should be very closely matched (even if carbs are out of synch) because of large mass air flow with throat of carb nearly open.

You need to match carbs at just off-idle because you want power pulses (L-R) to be very similar so engine runs smooth at low speed. This is critical for the gearbox. The gears don’t like to experience chatter or strong power pulses at slow speeds. At high speeds the gears can tolerate a fair amount of abuse. This is also why you want to warm engine up at something above 2500 RPM so engine is smooth and plays well with the gear reduction. I run my engine ~ 2800 RPM for warm-up.
 
Also remember that once the balance tube is reconnected any differences will be less...one thing I've noticed doing it is that mid range was always off a bit, however someone posted that they added the original springs back to the throttle arms which help pull the cables there by removing some cable slop...I added the additional springs and found the carbs stayed better in sync through out the range, however you must keep a little more friction lock since the tendency will now be dragging the throttle forward, not a problem if you keep the friction adjusted. The coil springs that Vans put on the throttle arms is a bit weak IMHO. Stranded cable works best when pulled so you need good springs to be pulling the throttle forward eliminating slop.
 
Also remember that once the balance tube is reconnected any differences will be less...one thing I've noticed doing it is that mid range was always off a bit, however someone posted that they added the original springs back to the throttle arms which help pull the cables there by removing some cable slop...I added the additional springs and found the carbs stayed better in sync through out the range, however you must keep a little more friction lock since the tendency will now be dragging the throttle forward, not a problem if you keep the friction adjusted. The coil springs that Vans put on the throttle arms is a bit weak IMHO. Stranded cable works best when pulled so you need good springs to be pulling the throttle forward eliminating slop.

What I did, and suggested, was adding additional springs in the original tension spring positions. The Van's tension springs are roughly 10# pull. McFarlane tension springs are 5# pull. I made springs that are 3# pull. This keeps both carbs traveling in sync without causing much issue with the throttle friction.
 
I use the gauges with damping needles so I don't have enough experience with your system - but that looked weird. One side might have been lagging the other.

I usually start mine at 3000. Once I get all that lined up, it usually works for all RPM ranges. I will check it idle, 2300 or so, then 3500 and 4000 as well.

But you want to pause at each RPM and let things settle for 30 seconds or so before you note the difference in vacuum. YMMV. Concentrate on one off-idle RPM and then get that wired in.

Vans' RV12 MM - Section 12:

OFF IDLE SETTING
15.After warming the oil temp to the minimum required for takeoff power application, run the engine to full throttle and verify the engine is developing proper power with an indication of at least 4700 RPM.

16.Verify that both vacuum gauge readings are the same when the throttle is slowly moved between near idle and full power. Use of damping valves (see Figure 30) may be required to smooth out the vacuum gauge readings. Install valves if required and slowly close them until the vacuum gauge readings stop fluctuating rapidly.

If the vacuum gauge readings are not the same, shut down the engine and verify that the choke controls on both carburetors are in the full off position. Also verify that both throttle cable leads (see Figure 27) are resulting in full travel of the throttle arms to the full throttle position on both carburetors. If both are satisfactory move to step 17 for making adjustments.

17.Determine which carburetor has the higher vacuum reading. Loosen the aft nut on the off idle adjustment hardware of the carburetor with the higher vacuum (see Figure 27) and then tighten the fwd nut. This will adjust the throttle (for this carburetor) open an additional amount.

18.Restart the engine and run the engine through the middle of the throttle range. Repeat the adjustment done in step 17 until the vacuum gauge readings for both carburetors are closely matched. If you go too far and the carburetor that you have been adjusting ends up having a vacuum gauge reading lower than the other reverse the adjustment and bring it back to where the vacuum gauge readings match. Once the off idle vacuum gauge readings are balanced confirm that the nuts on the throttle cable locking hardware are tight on both carburetors.
 
What I did, and suggested, was adding additional springs in the original tension spring positions. The Van's tension springs are roughly 10# pull. McFarlane tension springs are 5# pull. I made springs that are 3# pull. This keeps both carbs traveling in sync without causing much issue with the throttle friction.

Any chance you could tell me where you bought or what springs you are using so I can add springs that are 3# to mine as well, since I like that idea in addition to the new style weaker springs I just put on.
Thanks!
 
Tony,

I made the spring using a much longer spring out of my spring assortment, then cut it to length, (same length as OEM). I experimented with several to get the pull I was looking for, something less than the McFarlane springs 5#.

If you are not in a hurry I will get over the Home Depot and see what they have whether "as is" or maybe modified, and post back here.
 
Tony,

I made the spring using a much longer spring out of my spring assortment, then cut it to length, (same length as OEM). I experimented with several to get the pull I was looking for, something less than the McFarlane springs 5#.

If you are not in a hurry I will get over the Home Depot and see what they have whether "as is" or maybe modified, and post back here.

Thanks .yeah that would be great if you find something that I could use.
Thank you!
 
I found the springs at Lockwood Aviation, they are the Rotax original springs. adding them will help getting a more accurate carb. sync.
 
Thanks .yeah that would be great if you find something that I could use.
Thank you!

I checked at Home Depot today, nothing usable. Tomorrow I will go to a store that has hundreds of these type of springs, McFadden-Dale. I will be able to test them and get a mfg & part number, stand by.
 
I checked at Home Depot today, nothing usable. Tomorrow I will go to a store that has hundreds of these type of springs, McFadden-Dale. I will be able to test them and get a mfg & part number, stand by.

Walt,
Thanks for taking the time..yes Mc Fadden dale...used to go there a lot!
 
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