What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

balancing carbs with new springs

seagull

Well Known Member
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?
 
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?
 
Last edited:
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.
 
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...
 
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.
 
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.
 
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
 
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.
 
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 didn’t want to remove the torsion springs for only a test.

I have new torsion springs and the safety plates coming, I’ll see what difference that makes.
 
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:
I had a similar experience. When the new springs were installed I had a difficult time adjusting the carbs to balance (using a Carbmate) at both 1800 RPM and 2500-3000 RPM. Lubricating the cables didn't help. I reinstalled the original springs, leaving the new springs in place, and balance across RPM's was spot on. I also noted the engine ran smoother with both springs. I have the venier throttle control fully tightened and have no throttle creep. Perhaps the routing of my throttle cables resulted in enough friction to require a stronger "pull" to keep the cables taught.
 
I have added the original springs and my carbs are spot on through out all throttle movements, it makes sync. the carbs MUCH easier, thanks for this suggestion!
 
Where to buy the ‘old’ springs?

For those of us with only the new back plate and torsion springs... any pointers one where/how to order the ‘old’ springs?

I’m going to balance my carbs this week with a carb mate and hopefully balance the prop too. Having these springs handy if I run into issues would be nice.
 
For those of us with only the new back plate and torsion springs... any pointers one where/how to order the ‘old’ springs?

There have been a few iterations of "old springs". The original Rotax springs are a pull of around 8-12 lbf/in ... way too much and constantly pulled the throttle forward.

When Van's switched over to the vernier control throttle from McFarlane ... lighter springs were adopted for the McFarlane vernier throttle.

I know you can find McFarlane springs at the following McFarlane link:
https://www.mcfarlaneaviation.com/products/product/7140/

Happy flying,
 
Last edited:
I ordered from Lockwood Aviation, it made a big difference keeping the carbs in sync but you'll need to keep a little more friction to stop throttle creep, no big deal.
 
I installed the extension springs (Home Depot Aviation Dept.) in conjunction with the new Torsion springs just to see if mid-range RPM would improve, and I believe it has. I didn’t do a re-balance yet because I plan on installing vernier throttle during Annual Condition Inspection in the Spring.
 
Thank you! But jeez - $33 for a spring?! ....So, about those Home Depot springs? Any idea on part/aisle?

There have been a few iterations of "old springs". The original Rotax springs are a pull of around 8-12 lbf/in ... way too much and constantly pulled the throttle forward.

When Van's switched over to the vernier control throttle from McFarlane ... lighter springs were adopted for the McFarlane vernier throttle.

I know you can find McFarlane springs at the following McFarlane link:
https://www.mcfarlaneaviation.com/products/product/7140/

Happy flying,
 
Thank you! But jeez - $33 for a spring?! ....So, about those Home Depot springs? Any idea on part/aisle?

I wish I could tell you, but it's been a few years since purchase. For $33, I think you buy every spring in the whole store... :D
 
Per a few requests I sourced a spring that will give the same results as the ones I made from my 'junk" drawer.

These are new springs I just made for this instructional. The spring I used came from McFadden-Dale, a little research shows it as a Hillman #61, cost $3.10. This spring will make (2) carb springs.

I counted down 20 coils and separated the spring bending a hook. The finished spring should be the same length (un-stretched) as the OEM spring. When extended to 2.5” the spring will have approximately 3 pounds pull.

NOTE; I am using this spring in conjunction with the coil spring that fits around the throttle shaft, the one that Van's supplies.

3 pound spring 0.jpg
The drawer label at McFadden-Dale.

3 pound spring 1.jpg
Spring as I bought it.

3 pound spring 2.jpg
Separating coil to bend second spring hook.

3 pound spring 3.jpg
2 springs made from one.

3 pound spring 4.jpg
Testing with the scale at full extension.

3 pound spring 5.jpg
On carburetor at idle stop.

3 pound spring 6.jpg
On carburetor at full throttle.
 
Walt....absolutely genius. Thank you for taking the time to spell it out, along with providing photos. What an elegant fix.
 
Very thorough, Walt. I used the TLAR METHOD (that looks about right) with some Home Depot springs about 8 years ago, and they have served me well.
 
Very thorough, Walt. I used the TLAR METHOD (that looks about right) with some Home Depot springs about 8 years ago, and they have served me well.

TLAR, PBA (probably be alright) was used on the first ones in this thread. Explaining how to do that was too involved to share, and sourcing the basic spring was a problem without giving access to my junk drawer. 😂

So.....I found a spring that everyone could have access to and showed my method.
 
Back
Top