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Carb Sync

todehnal

Well Known Member
Hy Folks.
Having trouble syncronizing carbs that were serviced at Lockwood. Following the instial static adustments, I installed the carb mate for the final adjustments. The carb mate shows solid red left led on the referance carb at 2500 rpm, however at 4000 rpm and up it's centered solid in the green. No amount of cable adjuting will get the reference carb indication off of the far left red led below 2500RPM. What am I missing??
After hours of frustration and I am looking for advice. Thanks....Tom
 
Isn’t the carb adjustment done at idle? I don’t think it’s possible to have them synchronized throughout the whole rpm range..
 
try 2 manifold pressure gauges

I never had much luck with the carbmate. For me, using the gauges shows the numbers, in-hg, for each carb while advancing through the rpm range. It might show something not seen with carbmate.
YMMV
Tom O.
 
Though others offering advice here in the forum have focused on balancing the entire throttle range, the focus should be at the low end. Particularly near idle.

The reason is that slight imbalance it higher throttle settings will have very little influence because the throttle plate is open so much and once the balance cross tube is reconnected it will help some as well.

Focus on the low end below 2500 RPM with particular attention at idle and just above idle. Your gear box will thank you.

BTW, I have never like the carbmate much either.
We use a manometer type gauge set but dial gauges are a good choice as well.
One tip is to tie both gauges you are using together with a tee fitting and connect to a vacuum source to make sure they both read the exact same value. If they don't, you can apply a correction factor when you use them, to get more precise with the balance.
 
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Do you have torsion springs on throttle plate shaft? I have both torsion spring and old-style extension spring. The added spring force keeps the throttle cables more uniformly "pulled".

I notice smoother engine throughout entire RPM range...
 
Do you have torsion springs on throttle plate shaft? I have both torsion spring and old-style extension spring. The added spring force keeps the throttle cables more uniformly "pulled".

I notice smoother engine throughout entire RPM range...

Hey Jim, You are absolutely correct about the springs. I had been fighting vibration for almost 2 years. I was trying to do another carb balance a few days ago and everything just kept changing and I couldny get a satifactory sync; passable, but not great. There is just too much slop in the throttle cable and that little spring just can't cut it. So I decided to try adding the 3# pull springs and was trying to dial it in again. That's when I couldn't get the guage off of the red peg and I put out a help post here. This morning I went bsck out, determind to fix it. I found that I had a poor hose connection to the carb mate and it must have been leaking some. Anyway, after fixing that and doing another full static and dynamic carb balance everything dialed in perfectly. It hasn't run this smooth in a looooong time. Thanks to all of you for your thoughts.....Tom
 
The stranded cables used for throttle and choke work best when being PULLED. If the SOLID wire cable was used the direction would not matter, stranded cable needs the springs to pull it along, I added the original Rotax spring to my carbs and get a much better sync.
 
The stranded cables used for throttle and choke work best when being PULLED. If the SOLID wire cable was used the direction would not matter, stranded cable needs the springs to pull it along, I added the original Rotax spring to my carbs and get a much better sync.

Hi Bob, I agree that the stranded cables need springs. They pull great, but they don't puch worth a darn. Plus the fact that we are trying to balance these 2 carbs to within a few thousands of an inch of travel, and any slop makes that a challenge. The springs do a good job of compensating for that. I've never seen solid throttle cables on an RV-12. I think that all of the bends that we have in the routing would prevent their use. That being said, these added spring have totally solved the proble. With the added springs, I also now have my RV-12 running smooth again, but it took me a long time to figure that out.
 
Anyone having trouble with the performance of the torsion throttle springs should be sure they have complied with SB 18-03-06 Rev. 1, which references SB-00026 which details installing VA-00276, Throttle Spring Retainers.
The retainers not being installed, has an influence on the rotational friction of the spring which can effect carb synchronization.
 
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Anyone having trouble with the performance of the torsion throttle springs should be sure they have complied with SB 13-03-06 Rev. 1, which references SB-00026 which details installing VA-00276, Throttle Spring Retainers.
The retainers not being installed, has an influence on the rotational friction of the spring which can effect carb synchronization.

Scott,

I have complied with SB-00026 but your comment about SB 13-03-06 Rev.1 made me think maybe I missed something. I went to the Vans site and could not find SB 13-03-06 Rev.1 with a search or in the SB's.

Can you provide a link?
 
Scott,
18-03-06 was what I had followed.

I had my springs installed with the tail against the bolt head as shown in 18-03-06. When I tightened the throttle arm nut the last turn created friction on the arm rotation. This happened on both carburetors. the friction was caused by the spring being squeezed.

My solution was to let the tail drop into the notch of the retaining plate.

This kinda defeats the purpose of the backing plate since the last coil is now riding on the shaft.
 
SB-00026 refers to KAI page 50-05 (attached below) for installation details of the VA-00276.
In figure 2 it shows that the inboard leg of the spring is supposed to be against the tang of the VA-00176 Spring Retainer.

For possibly some additional clarity... What the Spring Retainer does is constrain the inboard end of the spring (the end that will bare against the throttle shaft because of the wound up torsion load in the spring) so that it is held in proper position with the throttle shaft projecting through its center. Additionally, with the leg of the spring rotated to bare against the tang on the Spring Retainer (highlighted with red arrow in attached drawing), it adds a small amount of additional spring tension.

If the torsion spring and Spring Retainer are properly installed, and there is no abnormal friction in the throttle cables, we have found that the system performs very well and the carb synch is very accurate through the entire throttle range (as checked on multiple ULS RV-12's).
 

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Scott,

That is the way I have them now, your pic with the red circle, no binding.

What if you add that pic to SB-00026 along with the pics that are there, just a thought.
 
If the torsion spring and Spring Retainer are properly installed, and there is no abnormal friction in the throttle cables, we have found that the system performs very well and the carb synch is very accurate through the entire throttle range (as checked on multiple ULS RV-12's).

I don't want to start a battle with the expert, just sharing my experience.

My carbs would not stay in balance, the problem was tracked to the cables were not moving evenly, measured with calipers. I discussed this with McFarlane and they offered to check my cable for problems. I thought maybe there was problem where the cables came to the "Y" into one cable.

I pioneered the idea of adding a 3# additional expansion spring and it solved my balance issue as far as the cables traveling equally.

I was felt I was using a bandaid to hide a cable problem so I bought a new throttle cable, installed it with the upmost care and large radiuses. Then I removed the additional springs. the travel on the cables would occasionally be different, I could lightly pull on the short cable and it would move another .040 out. Maybe engine vibration of unbalanced carbs will nurse the cables into position with just the tension of the torsion spring, but I felt they should move equal without vibration help.

I put the #3 spring back on with the torsion and have not looked back. This was well over 100 hours ago.
 
I agree with Seagull, I had replaced the throttle cable and still had some slop in the cable movement, adding the additional springs did the trick making the sync easier and more accurate.
 
The outer sheath on these cables is spiral wound wire which has a few drawbacks. Dirt and grime can penetrate the cable housing along its length. Vibration can impart the spiral grooves onto the surface of the sliding multi-strand inner cable.

The J-3 Cub I owned developed a stiff throttle cable. Upon disassembly, the solid inner wire had grooves that matched the outer sheath and caused binding.

Eye-opener for sure…
 
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