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  #1  
Old 10-24-2017, 07:14 PM
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Tony_T Tony_T is offline
 
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Default Pneumatic Carb Sync

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 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.







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!)
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Last edited by Tony_T : 10-24-2017 at 07:23 PM.
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  #2  
Old 10-24-2017, 07:35 PM
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Now that's a pretty schmart idea right there. The more I use my CarbMate the less I like it.
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  #3  
Old 10-24-2017, 07:38 PM
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[quote=Tony_

(Don, using a caliper to mechanically set the linkage won't work!)[/QUOTE]

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.
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Last edited by DonFromTX : 10-24-2017 at 07:40 PM.
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  #4  
Old 10-24-2017, 08:07 PM
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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
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  #5  
Old 10-24-2017, 08:35 PM
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DaleB DaleB is offline
 
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Quote:
Originally Posted by DonFromTX View Post
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.
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  #6  
Old 10-25-2017, 01:41 AM
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Tony -

Just curious... how close do your L/R EGT track each other at idle, mid-range, and cruise RPM?
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  #7  
Old 10-25-2017, 09:39 AM
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Quote:
Originally Posted by Piper J3 View Post
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.
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Last edited by Tony_T : 10-25-2017 at 09:43 AM.
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  #8  
Old 10-25-2017, 12:46 PM
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Piper J3 Piper J3 is offline
 
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Quote:
Originally Posted by Tony_T View Post
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.
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  #9  
Old 10-25-2017, 08:05 PM
waterboy2110 waterboy2110 is offline
 
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I bought the dual gages from spruce. When they arrived one was slightly off of zero. I figured the error would be linear and syncíd 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. Iíll be doing this tomorrow and I will start with the carb mate. The single gage likes looks like the ticket.
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  #10  
Old 10-25-2017, 09:44 PM
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Quote:
Originally Posted by waterboy2110 View Post
I bought the dual gages from spruce. When they arrived one was slightly off of zero. I figured the error would be linear and syncíd 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. Iíll 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.
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