What's new
Van's Air Force

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

Pneumatic Carb Sync

Tony_T

Well Known Member
Patron
Syncing the carbs is one of the most satisfying aspects of the annual condition inspection — or the most frustrating.
When I took my Repairman course the instructor showed us how to use the CarbMate tool to pneumatically synchronize the carbs. That looked pretty cool, so I bought a CarbMate. Bad decision. The fittings that it came with were US automotive dimensions and the Rotax carb tubing is metric. I had to make adapters. After working with it I found the moving LED lights hard to interpret and confusing and had to school myself each time I used the tool to relearn how it worked. Syncing took a long time. The CarbMate went on the auction site as a motorcycle sync tool. Hopefully another Rotax mechanic didn’t buy it.

Then I bought a pair of gauges advertised as for the Rotax. These worked better and were a bit easier to interpret but the needles were very jumpy due to intake pulses and the little valves did not take out all the pulsations. Whenever the rpm was changed even slightly the anti-pulsation valves had to be adjusted.

I rewatched the Rotax-Owner video and the manifold pressure gauge that they were using looked like a much better solution. I was able to find a twin engine aircraft gauge for about $100, about the same as a gauge set cost.

This works really well. I have the tubing set up so the left and right carbs are on the left and right needles (naturally!)

i-gw3Qh2Q-L.jpg


I intentionally sought out a gauge that was made for normally aspirated engines so the maximum reading is 35 inches. I figured this would give the most resolution on the scale for the purpose of carb syncing.
The needles are internally damped so they don’t bounce around and there is no fiddling with valves to damp them out.

i-zNZJPm3-L.jpg


i-JdjDWJm-L.jpg

i-Qh7s5kC-L.jpg



The procedure I use is to hook up the gauge on the starboard side and place it on the passenger seat. Then I get in and lock the canopy in the hold open position, start the engine and make a reading. Then shut off the engine and make a carb cable adjustment. Real easy to interpret what is happening. Only takes a few tries to get the needles right on top of each other. This makes syncing the carbs actually fun!

(Don, using a caliper to mechanically set the linkage won't work!)
 
Last edited:
Now that's a pretty schmart idea right there. The more I use my CarbMate the less I like it.
 
Tony_ (Don said:
Ah ha, another challenger! I have to do the caliper thing now just to prove I am right! Anyhow, I do have a set of gauges to confirm my theory. Shucks, I don't even understand WHY someone would say it won't work! I have been looking for a setup like yours, but have not been successful so far, just using two vac gauges and have to read both.
 
Last edited:
I know there are a lot of folks who agree with Tony and far prefer the dual analog gauge approach. I understand that, in the hands of someone who knows how, there's a lot of diagnostic potential using those as well.

Just on the flip side, though, I love my Carbmate. I bought the Rotax adapters (for something like $20 IIRC) so the hookup was easy. The connecting hoses were long enough so I can Velcro my Carbmate to the RAM mount I use for my tablet inside the cockpit.

I "calibrated" mine during the first session (aka experimenting with the carb adjustment nuts to see how many "flats" on the adjustment nut resulted in light movement at each of the sensitivity settings) and "documented" that on a strip of masking tape on the front of the Carbmate.

Now it's hook it up, fire it up, see how many out-of-synch lights we have on which side, shut down, adjust the appropriate adjustment nut the appropriate number of flats, fire it up again and check my work. It often takes a couple of iterations but rarely more than that.

Not taking anything away from the value of the analog approach, just an alternative. YMMV
 
Ah ha, another challenger! I have to do the caliper thing now just to prove I am right! Anyhow, I do have a set of gauges to confirm my theory. Shucks, I don't even understand WHY someone would say it won't work! I have been looking for a setup like yours, but have not been successful so far, just using two vac gauges and have to read both.
I'm all for experimentation.

With cables like that, there are really two adjustments. One is the position of the throttle arm on the inner cable. That's really just set for no slack when the throttle arm is on the idle stop. The other is the length of the cable sheath, which will affect how far the throttle arm moves as you advance the throttle. Who knows? Your way might even work. It's a pretty touchy adjustment, though. I would expect a measurement at 3/4 or near full throttle to get me in the ballpark, but not dead on. Let us know how it works for you.
 
Tony -

Just curious... how close do your L/R EGT track each other at idle, mid-range, and cruise RPM?
 
Tony -

Just curious... how close do your L/R EGT track each other at idle, mid-range, and cruise RPM?

I haven't watched them track but my right EGT is running 30-35° more than the left at cruising rpm. Don't know about idle and mid-range as I don't run the engine at those speeds for any extended length of time.
 
Last edited:
I haven't watched them track but my right EGT is running 30-35° more than the left at cruising rpm. Don't know about idle and mid-range as I don't run the engine at those speeds for any extended length of time.

Tony - next time you fly take note of EGT's as engine is warming up. They should be pretty similar if carbs are balanced for idle and off-idle.

Most guys report that right EGT is higher than left at cruise power settings. Mine is ~ 75F higher on right side. Perhaps from air flow through cowling or close proximity of R EGT thermocouple to hot oil tank.
 
I bought the dual gages from spruce. When they arrived one was slightly off of zero. I figured the error would be linear and sync?d with the error.
Later a buddy brought his carb sync box over and he turned the nuts while the engine was running. Seemed to be a better process as far as I observed. Then I checked my gages against his cal and found that at speed my two gages read the same. I?ll be doing this tomorrow and I will start with the carb mate. The single gage likes looks like the ticket.
 
I bought the dual gages from spruce. When they arrived one was slightly off of zero. I figured the error would be linear and sync?d with the error.
Later a buddy brought his carb sync box over and he turned the nuts while the engine was running. Seemed to be a better process as far as I observed. Then I checked my gages against his cal and found that at speed my two gages read the same. I?ll be doing this tomorrow and I will start with the carb mate. The single gage likes looks like the ticket.

When you're finished you can hook your gages up in reverse L->R / R->L to verify any gage errors.
 
When I took the ROTAX course the instructor had a home made setup using the manifold pressure gage from a twin engine aircraft. It worked great, but the Carbmate seems to have a better resolution than what you can see on the analog gage. On the other hand maybe ?close enough? is good enough.
 
When I took the ROTAX course the instructor had a home made setup using the manifold pressure gage from a twin engine aircraft. It worked great, but the Carbmate seems to have a better resolution than what you can see on the analog gage. On the other hand maybe ?close enough? is good enough.

Richard...what airfield are you based @ in Phoenix?
 
I am seeing a 0 to 56 degree delta (right side higher).
I was told (at my 200 hour / annual) this was good and no balancing was necessary by the mechanic doing the work. Is this consistent with what you are finding?
 
Personally I don't consider EGT as a reliable indication of carb. synch/balance. If it was reliable I think Rotax would have put it in the maint. documentation.
 
I balanced my carbs with two vacuum gages. When I was satisfied that they were closely synched I then transferred the vacuum gages L>R to make sure they was no gage bias. I then looked at EGT's for idle and off-idle and they are very closely matched. I frequently monitor the EGT's and will re-balance if differential ever changes...
 
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. The ROTAX instructor told us that is why you do the ignition check at 4000 RPM: to eliminate carb synch as a factor in rough running during the check. It makes sense considering the throttling profile of a butterfly valve. After about the first 50 degrees of rotation they have little restriction to airflow.
 
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. The ROTAX instructor told us that is why you do the ignition check at 4000 RPM: to eliminate carb synch as a factor in rough running during the check. It makes sense considering the throttling profile of a butterfly valve. After about the first 50 degrees of rotation they have little restriction to airflow.

Hi, can changing spark plugs or prop dynamic balancing or adjusting/correcting prop blade pitch affect carb sync?
 
I would hardly call myself the ROTAX expert, but I don?t see how prop pitch or spark plugs would affect carb balance.
 
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. The ROTAX instructor told us that is why you do the ignition check at 4000 RPM: to eliminate carb synch as a factor in rough running during the check. It makes sense considering the throttling profile of a butterfly valve. After about the first 50 degrees of rotation they have little restriction to airflow.

I?ve tried several dual gauge set-ups; but I like best the dual-needle gauge like Tony uses. Set your off-idle adjustment first @ 3800 rpm. Then adjust idle sync & idle rpm with your idle screws....she?ll purr like a kitten.
 
Personally I don't consider EGT as a reliable indication of carb. synch/balance.

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.
 
Carb Synch Issue...

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

I have done carb synchronization on two RV-12?s 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 I?m 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 isn?t 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 can?t 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 Van?s 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 doesn?t 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?
 
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.
 
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.
 
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.
 
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.
 
Back
Top