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  #21  
Old 06-08-2018, 07:20 AM
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DanH DanH is online now
 
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Quote:
Originally Posted by rvbuilder2002 View Post
Personally I don't consider EGT as a reliable indication of carb. synch/balance.
Quote:
Originally Posted by RFSchaller View Post
Above about 4000 RPM the intake manifolds will have the same pressure unless your throttle cables are completely out of whack, so I would not expect cruise RPM to be a good indicator of synchronization.
Ditto and ditto. Sync is mostly about low throttle openings. Sync 'em on the idle stops, then sync cable lift.

Back in the day a BMW airhead with CV Bings was synced by ear, and with an index finger on the cables. Not saying you can do the idle stops on a Rotax by ear, but I don't doubt that a fella can get the cable sync real close with a manual method.
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  #22  
Old 10-12-2019, 07:49 AM
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Piper J3 Piper J3 is offline
 
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Location: Hinckley, Ohio
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Default Carb Synch Issue...

I posted this in the Rotax 912/914 Technical Forum but I thought Id also ask here

I have done carb synchronization on two RV-12s with 912ULS using CarbMate electronic synchronizer with good success. Synchronization in each case was completed within about ten minutes and engines run smooth and idle 1400-1500 RPM

Fast forward to today when Im helping a friend synch his 912ULS RV-12. When connected, the CarbMate shows huge imbalance with indicator all to one side. If I manually move each throttle arm by hand a little bit to determine which carb is producing power and which one isnt nothing changes on the CarbMate scale its pegged to one end. This is weird. I reverse hoses to the CarbMate and the scale switches ends so CarbMate is functioning correctly.

Looking at the idle adjusting screws it appears that the left carb is completely closed by the throttle plate itself and the idle speed adjusting screw is lifted off the idle stop. I have adjusted cables to make sure throttle pulls the arms against the idle stops and yet it appears the throttle plate is what is actually setting the idle position and not the idle stop screw.

So, in addition to huge vacuum imbalance I cant get a low idle. Engine is running 1800 2000 RPM. Right carb can be adjusted to lower idle speed but left carb cannot. I will check cable lengths and mechanical setup per Vans instructions.

This is a S-LSA RV-12 with less than 100TT so I think this carb imbalance problem has been around since the git-go. Engine doesnt run horrible at idle, runs smoothly at higher speeds, and CarbMate shows good balance at 3000 4000 RPM.

I have looked for vacuum leaks and everything looks good. Another thing to note is the engine runs much smoother with manifold crossover tube connected. With CarbMate connected and intake manifolds separated the engine runs rough.



Any idea what could be causing such huge vacuum imbalance at idle?
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Jim Stricker
Hinckley, Ohio (1OA2)
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
RV-12 E-LSA #120058 AWC July 2012 N633CM
RV-12 Bought Flying Oct 2015 with 48TT - Hobbs now 522

LSRM-A Certificate 2016
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father & CFI) - USAAF 1st Lt. Captain B-17H
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  #23  
Old 10-12-2019, 09:08 AM
John-G John-G is offline
 
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Location: Northeast Ohio
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Jim

Because the CarbMate uses one carb as a reference, the user does not actually know what the actual vacuum reading of each carb really is (which is usually not an issue). However, if there is a problem, it is not a good tool for troubleshooting. Consider locating a for real vacuum gauge and take readings on both carbs .... you will quickly discover which side of the engine needs all the attention.

A couple of easy things that you may want to check out first:

Clean air filters, carbs are properly seated in the rubber boot (and the boot is in good shape)and properly tightened, verify the carb piston slides move up and down freely and are in the same position on both carbs, verify the choke lever on both carbs is resting on the stop when the choke is off, check that the floats are within weight specifications, check that the intake manifold flange mounting bolts are tight and the rubber O ring is in good shape, also make sure the manifolds are tight at the cylinder head.

Find figure 73-6 in section 73-00-00 of the heavy maintenance manual. Item 6 is the idle air jet make sure that the air jet and passage have no blockages. Item 5 is the idle jet remove the idle mixture screw and verify the idle jet is clean of debris. Reinstall the idle mixture screw and gently screw it in until it is fully seated then back the idle mixture screw out 1 turns.

If all the above is good try to follow the procedure in section 12-20-00 of the line maintenance manual that covers idle speed adjustment.

This is an interesting problem ... let us know the outcome.
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  #24  
Old 10-13-2019, 03:10 PM
bobg56 bobg56 is offline
 
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Location: Peachtree City, GA
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I have a Carb Mate and the first time I used it confused me. What I did was tie a piece of string to each throttle arm. When I had the engine running I would gently pull on one of the strings, the effect the pulling had was to bring the light on that side closer to center which told me what direction to adjust that cable and about how much. If you over adjust that one side it would now have the light on the other side lite up and moving outward off the center. I do start the rig process first using a caliper to get the cables synced but there is some cable slop even after applying a preload by pulling on cable end then tightening retaining nut, it is minimum slop but it is there which will have an effect on the balance, which shouldn't be much. I have it synced good at idle and off idle then the light starts moving off center as I increase power then at about 3500+ the light returns to center. After re installing balance tube the engine idles nice and has smooth acceleration. EGTs are within a few degrees at idle but in cruise can be 40 difference.
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  #25  
Old 10-13-2019, 11:19 PM
pilotyoung pilotyoung is offline
 
Join Date: Jan 2018
Location: Birmingham, AL
Posts: 131
Default Pneumatic Carb Sync

I purchased a CarbTune from an English motorshop. If my memory is correct it was less than $100 shipped to my house. My airplane has a nipple on each side of the crossover tube with a rubber cover on them. I simply take the rubber cover off, put the tubes from the CarbTune on, and then use a special pair of vicegrips to block the crossover tube on one side. It only takes 5 minutes to set it up. I put the CarbTune in the cockpit and run the tubes out through the air vent on the passenger side. I have started syncing the carbs after every oil change. It is basically a check but this time I lowered the idle rpm to 1650 and synced them at that speed. I also checked the high speed idle but it was still fine.

I highly recommend the CarbTune for its accuracy and price.

One caveat, be sure to loosen or remove the screw in the idle stops on the cable before you try to adjust the idle speed. If you don't loosen the screw in the idle stops on the cable, you can change the screw on the actual idle arm and nothing will happen. Unfortunatly I have made this error twice.

Without making any error like above, it only takes a few minutes to check and/or set the synch at every oil change.
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Serial Number 120022, N6812Y
Bought it as a flying airplane in Feb. 2018
Just passed 200 hours flight time in RV-12, and 10,000 hours mostly in corporate jets. I am a CFI; CFII; MEI; and a advancd 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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  #26  
Old 10-13-2019, 11:34 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by pilotyoung View Post
I purchased a CarbTune from an English motorshop. If my memory is correct it was less than $100 shipped to my house. My airplane has a nipple on each side of the crossover tube with a rubber cover on them. I simply take the rubber cover off, put the tubes from the CarbTune on, and then use a special pair of vicegrips to block the crossover tube on one side. It only takes 5 minutes to set it up. I put the CarbTune in the cockpit and run the tubes out through the air vent on the passenger side. I have started syncing the carbs after every oil change. It is basically a check but this time I lowered the idle rpm to 1650 and synced them at that speed. I also checked the high speed idle but it was still fine.

I highly recommend the CarbTune for its accuracy and price.

One caveat, be sure to loosen or remove the screw in the idle stops on the cable before you try to adjust the idle speed. If you don't loosen the screw in the idle stops on the cable, you can change the screw on the actual idle arm and nothing will happen. Unfortunatly I have made this error twice.

Without making any error like above, it only takes a few minutes to check and/or set the synch at every oil change.
The carb. tune tool is what we use in our shop. We tried the carb mate but never liked it because as already described, if the synch is off very far (pretty common) it is much more difficult to determine what direction you need to adjust to begin to get closer to a proper synch.

We did find that our carb tune tool has a slight mismatch between the two indications (checked using a Tee and applying the same vacuum to both and check the readings). We have a label on it reminding us of the slight compensation factor to apply to the side the reads higher.
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