Amassing Parts To Balance The Rotax Engine’s Bing Carburetors

Now that the Rotax 912ULS engine has been run, at some point in the near future it will be necessary to balance the two Bing carburetors. This procedure is quite familiar to anyone who has owned a multi-carburetor car or motorcycle and has tinkered with the carbs. In a nut shell, the carburetors are adjusted so the vacuum on the intake manifold of each carburetor is the same at mid throttle and idle.


There are a myriad of gages available on the market that will do the job, including some digital displays that are really cool. Depending on how out of sync the carbs are, readings could vary a lot ... but expect the readings to likely be under 18 inches of water. After syncing the carburetors GUESSING the readings will end up be around 11 inches of water at 2,500 RPM and around 15 inches of water at idle. In fact, two long tubes containing an equal amount of water in each tube can even be used (if done carefully using slow RPM changes, especially when going from a high RPM to low a RPM) because the vacuum measurements are made in inches of water so using tubes much longer than the expected vacuum should help guard against water ingestion should one want to go that route ... I prefer the more costly dry approach in this instance.


While waiting for the new replacement 90 degree brake fitting, decided to work a little on making my own carburetor balancing fixture. Two quality gauges were chosen that read from 0 to 30 inches of water. The gauges will be screwed directly into a valve. The purpose of the valve is to create a dampener ... because if the vacuum fluctuates enough, it will cause the needles on the gauges to “bounce”. To alleviate the “bouncing”, the valves can be slowly closed off until the needle on each just gauge steadies out allowing meaningful carburetor adjustments to be made.


The Rotax 912 maintenance manual describes four methods that can be used to attach the vacuum gauges onto the engine … three of which involve removing/plugging various combinations of  hoses on the balance tubes. The last time I tried to mess with a balance tube hose, it needed to be replaced because it was destroyed trying to remove it, so I’m not keen on any of those methods. The fourth and most appealing method to measure the vacuum is to remove a 6 mm plug in each of the intake manifolds and screw a tube into the port that a hose coming from a vacuum gauge can be attached onto. While simplistic enough, this poses a problem here in the Americas because where do you go to find a M6 threaded tube with a hose barb on the end of it? I thought about finding a long M6 bolt, cutting the head off and drilling through the center of it … but that seemed like a lot of fussing around.


Then it came to me … go to a foreign car parts place and ask for a M6 brake bleeder valve. They are already hollow and have a hose barb on them already. M6 brake bleeder valves are used on VW’s and early models of BMW’s for sure … although I think M7 is more common these days. Anyway, I found some. Wade at the foreign car parts shop took the time to look up what I was looking for since I did not have a specific car year or model to give him. In the process, discovered he too was a pilot so we got along just fine. Wade was able to hook me up with M6 brake bleeder valves and also gave me some small rubber O rings to make a good seal when the valves are threaded into the intake manifold.


Because the tip of a brake valve is solid and pointy, the plan was to cut off the pointy part of the valve with a Dremel outfitted with a cutoff wheel then run a #40 drill down through the center to enlarge the small center hole a little bit. The result is perfect and should work great!

About to use the Dremel outfitted with a cutoff wheel to remove the pointy end on the M6 brake bleeder valve to reveal the hollow center.


After the tip was removed from the M6 bleeder valve reveling the hollow center, I noticed the hole was a little smaller than I desired so a #40 drill bit was run down through the center of the valve to enlarge the hole a little.

The setup for making the “home brew” carburetor balancing fixture. The meters will screw into valves that can be adjusted to dampen the meters if necessary. The valves will have a hose barb fitting for connecting to the hose between the valve and the M6 bleeder valves screwed into the intake manifolds.  The uncut M6 bleeder valve can be seen to the right of the one my finger is pointing towards which is the one I cut the tip off. The small O rings were added and will make a good seal when the fitting is screwed onto the intake manifolds.


I’m thinking of mounting the meters and valves on a piece of aluminum so the parts are not lose and may even try figuring out a way to read the meters with the engine running while inside the cockpit. I have a few ideas on that, but have not worked out the details as of yet.


For fellow builders that like to do it yourself, the big takeaway from this post is it appears that M6 brake bleeder valves can be easily modified to be used as an interface between the vacuum meters and the intake manifolds. If there are any unforeseen issues discovered with this setup, I will return from the future and amend this post accordingly.