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  #1  
Old 10-07-2020, 10:37 PM
seagull seagull is offline
 
Join Date: Dec 2019
Location: San Bernardino
Posts: 344
Default balancing carbs with new springs

I have been chasing carb balance, trying to get a perfect balance at 3500 and also 1750. I could get one or the other but not both. I settled for 3500 balanced and then at 1750 they would be off unless I pulled hard on the throttle cable. I found the left carb was not hitting the idle stop unless I pulled hard. Once on the stop they were in balance. Chasing down the issue i found the left cable has the slightest bit more drag than the right so the right made it to the stop and the left was stopping .040 short of the stop, enough to keep them out of balance. My carbs have the coil return springs. For a test I added the expansion springs in addition to the coil and the carbs now run in perfect balance regardless of RPM. I can tighten the friction to eliminate most all of the creep. I know this setup isn't ideal but is loads the cables and keeps them traveling equally.

Has anyone else seen this or has a better idea?
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  #2  
Old 10-07-2020, 11:47 PM
NinerBikes NinerBikes is offline
 
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Location: L.A.
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Can you lubricate the throttle cables inside the housing with some LP-2 ? Short of that, new throttle cables are, I think $225 to $250 to replace. Have you also checked the brass throttle stops, that they too are in specification, both left and right?
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Last edited by NinerBikes : 10-07-2020 at 11:49 PM.
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  #3  
Old 10-08-2020, 03:39 AM
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Piper J3 Piper J3 is offline
 
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The Piper Cub I used to own had a throttle cable much like a lawnmower. The outer sheath was the typical spiral-wound jacket and the inner moving cable was a solid wire. With hundreds of hours of use, vibration had worn the inner solid wire with spiral grooves that matched the outer sheath. When the spiral grooves entwined the cable would lock up.

The RV-12 uses the same spiral-wound outer sheath but the inner cable is a multi-strand design. The solid wire on the Cub was fairly soft and I believe that is what caused the wear problem. The multi-strand cable is tempered wire and more flexible that solid wire. Think multi-strand electrical wire Vs. solid copper.

If there would be a friction problem, it would be where the cable is in a radius bend. On the RV-12 that occurs forward of the firewall where the two cables double-back in order to get a straight path to the carburetors.

You could try lubricating the cable by applying light oil / silicone / WD-40 to the outside spiral-wound sheath. The spiral-wound sheath is porous and lubricant will seep through the spiral to reach the center multi-strand wire. There is no need to disassemble the cable.
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Jim Stricker - EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 694

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father & CFI) - USAAF 1st Lt. Captain B-17H
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  #4  
Old 10-08-2020, 08:25 AM
bobg56 bobg56 is offline
 
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Location: Peachtree City, GA
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According to Lockwood Aviation the balance should be done in the lower rpm range, 2000-2500 with the balance tube disconnected/plugged...I've done the carb. sync. many times, IMHO when you push the throttle past 2500 rpm the reason the sync starts to change is the vacumn slides/needle are starting to actuate and there maybe be a little more drag on one than the other, when reaching about 4000 rpm the carb balance seems to return to where it was were I made it at 2000-2500...from 2500-4000 is a transition stage from the idle jet to the main jet.(IMHO) When you get it completed, re installing the balance tube helps to equal out the vacumn. The springs help to pull the stranded cable forward since this type of cable is made to be pulled not pushed. I've noticed what your talking about with the slight difference in cable movement and like your idea of adding the other spring...
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Old 10-08-2020, 08:48 AM
NinerBikes NinerBikes is offline
 
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Don't forget, also, a new service bulletin came out on Oct 7th, 2020, requiring a new sheath underneath the new coil springs that sit around the shaft of the butterfly of the carburetors, that needs to be installed to prevent wear, next annual.

May as well get it on order now, get it done now, since you are still fiddling with springs, throttle cables and synching the carburetors.
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  #6  
Old 10-08-2020, 10:09 AM
seagull seagull is offline
 
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Location: San Bernardino
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Just to clear a few things up,
I have lubricated the cables and the radius's are generous with no kinks. I have been in conversation with McFarlane and have determined the cables are ok.

If you wanted to see what I am experiencing try this. Engine off move the throttle from idle to 1/2 - 3/4, not full. Go to the carburetors and pull on the cable, one will probably extend a bit more, .040 or so. the other one will most likely not. If your springs are strong enough there will be no additional moment.

I use an electronic tool to balance it measures in millibars, about 33 times better resolution than inches of mercury. It has a real time visual display.

When the springs are in place both sides track each other perfectly through the whole RPM range and there is a noticeable smoothness to the engine.
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  #7  
Old 10-08-2020, 11:59 AM
pilotyoung pilotyoung is offline
 
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Default balancing carbs with new springs

SEAGULL,

Would it be possible to post a picture of your setup with two springs on each side. I don't know what you are refering to as expansion springs.

Thanks.

John
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Bought it as a flying airplane in Feb. 2018
Just passed 300 hours flight time in RV-12, and 10,000 hours mostly in corporate jets. I am a CFI; CFII; MEI; and a Advanced Ground Instructor, CFIG; and hoping to be able to help new RV-12 owners by doing some transition training for new builders and owners in RV-12's.
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  #8  
Old 10-08-2020, 12:35 PM
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Piper J3 Piper J3 is offline
 
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Quote:
Originally Posted by pilotyoung View Post
SEAGULL,

Would it be possible to post a picture of your setup with two springs on each side. I don't know what you are refering to as expansion springs.

Thanks.

John
He is advocating the use of the new "torsion" spring in conjunction with the old "extension" spring that was prone to break.
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Jim Stricker - EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 694

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father & CFI) - USAAF 1st Lt. Captain B-17H
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  #9  
Old 10-08-2020, 02:38 PM
seagull seagull is offline
 
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Location: San Bernardino
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Quote:
Originally Posted by Piper J3 View Post
He is advocating the use of the new "torsion" spring in conjunction with the old "extension" spring that was prone to break.
Jim is correct on how I currently have the springs arranged. I suspect using the old extension springs by themselves would be enough. I didnít want to remove the torsion springs for only a test.

I have new torsion springs and the safety plates coming, Iíll see what difference that makes.
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  #10  
Old 10-24-2020, 10:11 AM
seagull seagull is offline
 
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Location: San Bernardino
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I did get a new set of torsion springs from Vans and the new backing plates that eliminate the shaft wear caused by the springs. I already had torsion springs and the replacements were the same and felt like the same tension.
The original expansion springs I added (discussed above) tested at 5 pounds. I discovered that it only takes 3 pounds to keep the “slack” pulled even on both cables. I have now made a set of 3 pound springs they work fine, during carb balance tests both carbs follow each other throughout the RPM range.
The additional 3 pounds does not cause any throttle creep with only light friction on the vernier control.

Last edited by seagull : 10-24-2020 at 12:33 PM.
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