When I sold my Honda CB-750 motorcycle I thought I'd never have to sync carbs again (that ill-handling monster had four carbs). At least the Rotax only has two. The idea here is to ensure that the two cylinders on each side of the engine see the same throttle opening across the rev range. This is done by starting with the throttle arm on each carb 0.004 inches from the mechanical stop using a feeler gage, then opening each carb 1.5 turns of the adjuster (clockwise). The lock nut on each arm is then screwed down, attaching the arms to the two cables. The two cables merge into one, which is the throttle in the cockpit. Idle is now set for each carb and can't be adjusted individually with the throttle-stop screws once the cables are locked to the arms. Adjusting the throttle plate opening of one carb relative to the other must be done with the ferrule adjusters on each cable. The following didn't dawn on me initially and I learned it the hard way: the friction lock on the throttle plunger in the cockpit must be tightened down before adjustments are made to the ferrules. This prevents relative motion between each cable and its sheath. The ferrule only attaches to the sheath, but the cable and sheath must move as one at each carb during ferrule adjustment. Otherwise it'll screw up the 2.75 inch plunger travel needed in the cockpit to ensure the achievement of full throttle on each carb with the plunger full forward. Francis Miller, who administered my private pilot check ride back in 1969, told me that the origin of the expression "balls to the wall" came from this: propeller, mixture and throttle "balls" all the way to the instrument panel. I thought it meant something else. Also, the RV-12 only has one ball. Returning now to the topic at hand, the Rotax setup is different from anything I've seen in that the carbs are spring-loaded wide open. The throttle cables allow the springs to open the throttles or pull them closed, a safety feature, I guess, in the event of cable breakage.
In the pic, the ferrule adjuster can seen on the cable at left, the lock nut which secures the cable to the arm on the right, and the throttle idle stop (with orange torque seal) just to the right and below.
The carbs are now mechanically synced and must be tested over the rev range, with adjustments made to the ferrules as needed. This is accomplished by measuring the manifold vacuum on each side and making ferrule adjustments as needed to balance them. The vacuum readings can be obtained with two individual gages or a single differential pressure gage (the bast way).
A couple of years ago when I was insanely optimistic about when I'd have the engine running I bought a kit from Aircraft Spruce which included two vacuum gages (which I later found for $20 each at Harbor Freight) and some tubing which was supposed to facilitate hooking everything up to a Rotax. As I recall I paid a few hundred bucks for this. Both gages were faulty, but I found them on line under a different name, still $20 bucks. This rig would have worked, with poor accuracy, but probably good enough.
Fortunately, I was able to borrow a CarbMate, which uses an electronic differential pressure transducer (shown below). Adjustments are made until the light is centered.
After some puzzling attempts at a balance I started to doubt the device (couldn't be me, right?) so I rigged up a large syringe (below) hooked up to each leg of first the analog gages and then the CarbMate, exposing both legs to the same vacuum. Both measurement methods passed the test, each analog gage showing the same reading and the CarbMate exactly in the center of its scale. I went with the CarbMate.
I already had the syringe on hand since I had used it to test the pitot-static system earlier (discussed in a previous post).
With renewed confidence in the measurement system, I hooked it up to the Rotax. The intake plenums on each side of the engine are connected by a balance tube which can be disconnected, allowing the two legs of the measurement device to be attached. Although it's a bit crowded, the pic below shows the hookup.
The two black tubes in the center with brass fittings are the hookup. The two brass fittings with the teflon thread tape go to the two legs of the CarbMate. The initial confusing readings were due to a vacuum leak on one of the rubber hoses, easily fixed with a hose clamp.
Once all this was sorted out the process was easy. Balance is checked at idle and at approximately 3000 rpm, this higher rpm allowing the carbs to get off the idle enrichment circuit. Mine was perfect at idle (1800 rpm) and at high rpm but showed a slight variation (1/2 of a light on the CarbMate) part way between the two. I called it good, figuring the engine spends most of its life either near idle or around 5000 rpm.
More Colorado wildlife: I had a few elk in my back yard.
