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912 ULS Carb Sync Using Option 3

Tacco

Well Known Member
Option 3 in the 912 manual uses the two screws on top of the intake manifold. Seems like it would be easier than removing the crossover tube every time. Anyone ever try this? Seems like completely clamping the crossover tube would be difficult.
 
I never have. The reason being that for it to be accurate you need to be able to still isolate the two sides from each other by applying a clamp to one of the hoses. I have never been confident that I could clamp the hose 100%.
 
I tried the top manifold ports early-on using individual vacuum gauges normally used for motorcycles. I made plugs for the rubber crossover tube to seal it off. I had to install needle valves inline with the vacuum gauges to dampen the very sharp intake pulses. It worked OK but a friend bought a digital CarbMate that connects directly to the crossover tube and that is now our standard setup...
 
I use that option. I put two clamps on the crossover tube. I tried such a clamp on an other piece of hose and that blockage was airtight.
 
The crossover tube probably doesn?t have to be completely isolated to do a good carb balance. You want to get the carbs closely synchronized and then the crossover tube will mitigate small differences in balance. The crossover tube is relatively small diameter and comparative long length ? not an ideal design to make two separate intake manifolds act as a single vacuum chamber.
 
I have always used the third option as well to sync the carbs. For making the connection to the intake manifolds, I went to a foreign car parts shop and purchased two metric wheel cylinder bleed valves and cut off the pointed end ... now the hole at the ball end goes all the way through the valve.

A small rubber O ring is placed on the valve so it makes a good seal on the intake manifold. Below is a link to a photo of the clamping arrangement I WAS using consisting of a small C clamp and two pieces of scrap aluminum. If you look very closely at the first photo in the link below, to the right of the spring, you can see half of the brake bleed valve screwed into the intake manifold (partially hidden by the fuel line) ... I just slip a piece of hose over the ball end and run that to the vacuum gauge or CarbMate. Also, there are a couple of links below to photos that show how the end was cut off the brake bleed valves to make them usable for our needs.

I no longer use a C clamp anymore to pinch off the rubber hose on the balance tube. Now I use a pair of locking duck bill pliers from Harbor Freight with narrow jaws that seem to come together fairly flat. A couple layers of tape are placed over the teeth so they don't cut into the rubber hose ... the locking duck bill pliers do a great job of pinching off the rubber hose connected to the balance tube.

https://3.bp.blogspot.com/-f8vV98iKdg0/WAhVbDPuihI/AAAAAAAAMdk/0bS6D5IXeikyEeM59zH9UEiN0rTF1XMXwCLcB/s1600/DSC02390.JPG

https://3.bp.blogspot.com/-UosFVRmsuQc/V4z_5uqIxoI/AAAAAAAAMRk/z36WKw13tIQorN3HpEmPPx1E1WWkFQcbwCLcB/s1600/DSC02237.JPG

https://4.bp.blogspot.com/-4KrCVeS1gOo/V40AHE2IE6I/AAAAAAAAMRo/5IruMelNMaQeN_4GT_85TTXiGnuUSK0UACLcB/s1600/DSC02238.JPG

Not sure if the locking pliers I now use are the same as the one shown in the following link ... but if they are not, they are very close to what I'm currently using to clamp off the hose on the balance tube.
https://shop.harborfreight.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/6/3/63870_I.jpg

Happy flying,
 
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Metric hose pillar

Slang-pilaar.jpg


Plastic hose pliers

Slangklem.jpg
 
Maintaining many Rotax's (Tecnam singles and twins and the RV-12s), we always used the inlet manifold mounted adapters and clamped one of the balance pipe hoses with a hose clamp.

It works perfectly and there is absolutely no doubt that the carbs aren't as balanced with the hose clamp in place - you can feel the difference.

Don't try and balance the carbs with the engine running (some do...) and easy to monitor if you're doing it on your own by running the Carbmate hoses in through the fresh air inlets.

Carbmate solid smiley green light every time! ;)
 
My take on the iS versus ULS is that the ULS can be worked on while the iS seems to follow the society trend of component replacement with little opportunity to repair. Maybe I?m just an analog dinosaur, but having two lights that tell you, ?I?m Train A (or B) and I?m not happy (or I am happy).? while needing a $1000 interface cable to elaborate seems to take a lot of options out of the maintainer?s hands.
 
Does anybody know the thread size for the port plug? I would guess M8? Metric thread to imperial tubing fittings are available here from a company specialising in fuel injection.

Jack
 
The site I was on doesn’t list anything that small. Will have to look further. Jack

Jack- Truly no need to search for special fittings. Have a look at post 6 in this thread and check out the photos of the M6 brake bleed valves I procured at an auto parts store and modified. Just cut off the tip, slip on an O ring and presto you have a threaded fitting to screw into the intake manifold with a ball end to slip your vacuum hose over. Works great!
 
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Thanks, John.
I’ve found M6 brake bleed screws are available locally. They are used on BMW, Audi, VW and likely many others.

Jack
 
Ok, used this method and here are my observations:

You have to wrestle the fuel lines out of the way to get to the ports which necessitates cutting off any zip ties you've secured them with. I also had to disconnect the fuel line clamp on the right side to clear the right port so I could get to it.

Second, although I was initially pretty sure I squashed the balancing crossover hose enough to isolate the two sides, I was never quite sure. Here is a clue I might not have: I was having trouble balancing with the CarbMate so I switched to my basic motorcycle mercury filled cages. As I increased or decreased throttle, one side would lag but eventually even out. It was the "long side", i.e. the side furthest from the clamp.

I did manage to get them balanced. One advantage of this approach is that you can remove the clamp from the crossover tube, when you're done balancing individually, and see immediately how the carbs normally perform.

All in all, I didn't find "Option 3" any more convenient than the other approaches. Given the stuff you have to disturb to get it done, I'm going to stick with Option 2. That way I'm absolutely sure I have isolated the sides and besides it's the same of work.

Here are the fittings I used:
 

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