CarbMate P-sync tool description

[Birdseyeview]

Does anyone have a good description of how the CarbMate tool works on a 912? I got nothing that describes the functioning of the tool when I purchased it from ACS and can't find any descriptions on line. I've watched a few P-sync videos on Youtube and Rotax's video, none of which describe the CarbMate's operation and why the various lights come on or what to specifically do about them when they aren't the center green LED. A few turns of the bowden cable clamps in one direction or the other don't seem to move the LED indications in the right direction (so far). If I knew what the LED numbers meant I could increase one carb 's throttle or decrease the other carb's throttle accordingly to get me to the center green LED. I had expected that the Mech sync should have gotten me a little closer to the optimal P-sync with only a few bowden cable turns needed. Frankly I'm confused and a little frustrated because the P-sync is taking forever to accomplish and I need some expert clarification.

Before I attempted the P-sync I had throughly cleaned the carbs again, checked the float levels, completed a mech sync per Rotax instructions, and performed a differential compression test before the P-sync attempt and all cylinders came in at 79/80 psi. I'm burning 100LL and the engine starts good and idles well at 1800-2000 rpm, in spite of some mild carb vibrations, which I hope can be reduced or eliminated with a good P-sync. No carb venting fuel at any speed nor with the CarbMate hooked up.

When I hook up the CarbMate to my 912 the "REF" side is plumbed to the right manifold (via the cross-over tube) and the "CARB2" side is plumbed to the left manifold fitting. At an engine speed of 2500 rpm the CarbMate readout lights up on the "REF" side as a red -4 LED. When I increase idle speed to 3500 rpm the LEDs starts to move towards the center green light indicating that the pressure/vacuum difference between the two carbs does actually decrease as the engine speed goes up.

When I disconnect the "REF" hose I can blow into it in order to get the same -4 red LED light seen during the test. When I suck into the "CARB2" hose it goes to the other side of the LED line where the plus numbered LEDs are at. Does red -4 LED mean that the right side (REF) manifold of my engine is positively pressurized or does it just mean that my left side (CARB2) has way less vacuum? I'm guessing that each manifold is actually seeing only a vacuum of some varying level. I'm also guessing that the positive pressure I'm seeing on the "REF" side means that the right hand carb's throttle needs to be increased to increase its vacuum level (and thus lowering the false positive indication. I assume vacuum level in a given manifold goes up as the throttle is increased. Are all my guesses and assumptions good ones or am I still confused? Any opinions on where should I go from here? I really appreciate any inputs.

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[jiott]

Larry, I don't have a Carbmate, but when I use the standard gauge setup for p sync I just push or pull a little bit on one throttle arm and watch which way the gauges move. This tells me what direction of adjustment needs to be made on THAT cable to bring the gauges into sync. I think you could do the same with a Carbmate and watch the LED lights.

[Birdseyeview]

Guys - SUCCESS finally, All your suggestions paid off because they helped me to better understand the entire sync process and the basic operation of the CarbMate tool, both of which were new to me. I didn't do the bowden cable adjustment while standing next to the spinning prop because it just made me too nervous, and I was doing it all by myself. As a result, the sync process took several adjustments with multiple engine starts and restarts, but I got lucky when my initial adjustment happened to be in the direction it needed to go. My 912 is finally purring like I suspected it should with no vibration at idle, or at any other rpm that I tried. It starts very quickly and I'm not getting any hint of the carb venting that plagued me a month ago before I flushed all the BSS out of my carbs and all the other fuel system components. During the hunt to find the real culprit responsible for rough running and carb vibration I have discovered that there are many potential causes. When I began to check for and test for each possible cause they were all eliminated one by one until todays victory. I've said it before and I'll say it again - I love this forum because its contributors have helped me identify all the potential causes for the various issues I've seen and what to do about each one. Thank you all.

[DHeal]

I don't want to stand directly behind a spinning prop to tweak my carb sync settings either! So I tie a long piece of monofilament fishing line to each of the two throttle arms and run each line back into the cockpit. Plumb the sync gauges (or CarbMate) into the cockpit as well. Then you can safely sit in the cockpit while the engine is running, set the RPM as desired, and pull on each monofilament line individually to determine which throttle arm needs adjustment. Then stop the engine, tweak the appropriate throttle sheath cable nuts, and repeat the process as required to bring the two carbs into sync.

[Birdseyeview]

Jim,

I wish it was that easy but on my installation the two throttle cables are each .090" diameter and tied together tightly where the two separate cables join the push pull rod (a McFarland friction lock throttle assembly). As a result, a little push or pull on one cable is transmitted completely to the other cable and there is no independent movement, not even a little. I'm completely forced to use the individual bowden cable clamps to adjust only one or the other carb at a time. That's why I'd like to understand better what the CarbMate is telling me so I can move the bowden cable in the correct direction. I hope there is another option that I may have overlooked.

[rv9ralph]

I used a CarbMate to sync my carbs 3-5 years ago, I will try to remember the process. As you stated, first do the mechanical sync. Then connect the CarbMate using the crossover tube as the connection point with one CarbMate to the crossover and the other to the connection that would normally be connected to the crossover.

The led lights on the CarbMate show that the vacuum is more or less to one side or the other. The "Ref" is the carb that you decide not to adjust. The other carb is adjusted by turning the adjustment at the end of the cable housing on the carb. By turning the adjustment at the end of the cable you will slightly change the vacuum until the carbs are in such. Start at around 2500 rpm with the sensitivity set to where the faceplate recommends. Then check at higher rpms until you get it the best you can.

I did the sync alone and did not feel comfortable doing the adjustments with the engine running and shut down between adjustments.

That is about it, good luck.

Ralph

[Birdseyeview]

Quote 1: "The "Ref" is the carb that you decide not to adjust.

Quote 2: The other carb is adjusted by turning the adjustment at the end of the cable housing on the carb. By turning the adjustment at the end of the cable you will slightly change the vacuum until the carbs are in sync. "

Ralph - Thanks - Your 1st quote above is very helpful. I couldn't find a definition of what the "REF" side was until reading your definition - it now makes perfect sense for that piece of my puzzle.

Relative to your 2nd quote - do you have a suggestion which way I should adjust "CARB2"? Should I increase or decrease the throttle setting at that bowden cable? Or should I just experiment with both directions until I stumble into the right direction? I like knowing how deep the water is before I dive in. Example: If it should require me to increase the throttle setting of "CARB2" I can do that but I'm nervous that the P-sync may be far enough out that when I do get it balanced that both throttle levers may not hit their full throttle stops at the same time. Is that a problem at the full throttle setting or is it a logical consequence of a normal P-sync and therefore considered as OK as long as the carbs are sync'd? On the other hand, if I need to decrease the throttle settings on "CARB2" there may not be enough butterfly valve angle left to go closed before it stops rotating internally. I guess in that case I may need to start over with the mech sync or move to adjust the "REF" side carb to compensate? Or could I switch the "REF" and "CARB2" by reconnecting the hoses to the opposite sides of the crossover tube. If I did that I'd probably have to redo the mech sync first and start again at the beginning.

I may be making a mountain out of a molehill but from my questions above you can see that this all gets complicated fast and I hope to document the definitions and rules for all this when I finally get it all figured out. Any further thoughts, recollections, or discussion would be helpful to clarify all this.

[jiott]

Assuming the mechanical sync is fairly good, I have always found that you can choose either carb, whichever is the most convenient, to do the adjusting. It is never a certain carb that is out of adjustment, it is just that they are not both the same. So you can adjust either carb to bring it into sync with the other. This is nothing new to most of you, but maybe it might help some newbies.

[jrevens]

Assuming the mechanical sync is fairly good, I have always found that you can choose either carb, whichever is the most convenient, to do the adjusting. It is never a certain carb that is out of adjustment, it is just that they are not both the same. So you can adjust either carb to bring it into sync with the other. This is nothing new to most of you, but maybe it might help some newbies.

Good point Jim!

[avidflyer]

I know this post is about using a carb mate tool, but I'll throw this into the mix incase some one reading these messages is wondering what to buy to sync their carbs. I've been using a single vacuum gauge with a T in the line coming into the vacuum gauge. Each line going away from the T has a valve in it. Those lines then go to the ports on the intake manifolds. With it all hooked up, open one valve and you see the vacuum there. Close that valve and open the other and see if both readings are the same. Adjust the linkage so they are reading the same. Even if the vacuum gauge isn't accurate, you will still be able to compare one side to the other, and that's what you are really trying to do. JImChuk