Rotax 912 Carburetor Synchronization

Doing a vacuum sync on my Rotax 912
Note: there are replacement floats available for the Bing. www.aircraftsp... I have had these in for a year or so, seem to be fine. FWIW...

[@bobirving7460]

Hi, I suggest that when setting the throttle idle stop, you also check the setting of the idle jet. Also, I have never used gauges to synchronise my carbs: what I did instead was a static setting, adjusting the cables so that both carbs start to move off their full throttle stops at exactly the same moment as the throttle is gradually closed. I then checked the same thing from the throttles fully closed position: that both carbs began to move off the idle throttle stops at the same moment as the throttle is gradually opened. They do (provided the cables etc are set up and routed well) but since most running is done at more open throttle settings, my emphasis has been on adjusting for perfect full throttle synchronisation. That, combined with very accurate pitch setting of a reasonably light prop has always given me very smooth running. I check the above regularly and during the first 50 or 75 hrs made minor adjustments as the cables etc bedded in. Since then there has been no need for adjustment, though I still perform these same simple and quick checks any time the cowl is off.

[@kitfoxn990tt5]

There is a far easier way to determine which way to move the adjuster. While the engine is running and you are looking at the two gauges, just grab and slightly move the throttle arm on the side you want to adjust and see if the gauge needles move toward or away from synchronization. This will immediately tell you which way to adjust. Absolutely fool proof and easy.

[@LightAndSportyGuy]

Thank you for the suggestion. And the comment.

[@johnlucas2037]

What idle rpm are you using?

[@LightAndSportyGuy]

It idles at about 2100 rpm

[@cardocarnut4665]

It reads like you neglected the imbalance at low RPMs to make it balance at higher RPMs - and only at 3000 RPM. The task per the Rotax maintenance manual is to get them to balance for the full range of RPM starting at low idle RPMs and then at progressively higher RPM - all the way to as high a possible/reasonable. I do like your Ashcroft gauges and hope to obtain them in the future. Currently using the Aircraft Spruce gauges for the Rotax carbs which are actually Harbor Freight gauges. I have added gauge snubbers which work well above 2000 RPMs. And glad to see you compared gauges as I found mine 1" Hg different through out the range. BTW we only need each gauge to display the same value for the same vacuum and calibration is not required. The challenge is getting the ranges to match as one carb will show a tighter/smaller range between 2000 RPMs and 4000 RPMS. I believe the throttle cable attachment point affects this. So the purpose of balancing carbs I have read is to smooth the engine and should increase engine life. But a good motorcycle mechanic I know says the balance tube does a good job of balancing the manifolds at higher RPMs on multicarb motorcycles.

[@DesiChhore]

Well explained 🎉🎉

[@benfrost5317]

thanks for leaving in your mistakes, it helps us all learn

[@LightAndSportyGuy]

If I left them out, there wouldn't be much video left :-) Thank you.

[@michaelvadney5803]

Question: how do you actually calibrate the two gauges? I see your cross tube and valve setup, but how to you actually get the two gauges to read the same? Thanks!

[@LightAndSportyGuy]

They have always been pretty close - I just double check each time.

[@michaelvadney5803]

Thanks

[@brgranger]

Jim, i just bought a Merlin GT and am looking for other owners to chat with, are you aware of any user groups?

[@LightAndSportyGuy]

The only group I know of is on facebook. facebook.com/groups/2356797377881273

[@brgranger]

@@LightAndSportyGuy thanks!

[@Germancho1970]

Hi sir , how do you make this system with double gauges . Does it handmaid?? How do you hand it , I mean what valve to close and what valve to open to test Manifold ?Thanks

[@LightAndSportyGuy]

Yes - I made it. Gauges were probably from Amazon - 3 valves , 2 Tees from local hardware store. Sized to fit the valves. Each gauge is screwed into the long side of a T fitting - the short sides of each T is connected with a valve. The remaining ends of the Tees also have valves and finally fittings to adapt from the threaded valve to the hose (I don't remember the size - but one fits over the metal part of the connection between the manifolds and the other has an adapter to go into the rubber hose connections. To start, I open the valve between the two gauges, close the valve to one of the hoses and open the last valve enough to get a vacuum reading but not enough that the needle bounces. With the two gauges connected to the same manifold, I can compare to make sure that the gauges have the same reading. Then I can close the valve between the gauges and open the valve to the other manifold enough to get a reading but not so much the needle bounces.

[@tatolin92]

Brilliant

[@johnpro2847]

you edited your errors which is normal in any first take....even the pro's do numerous takes before getting it right ..thanks.

[@ingvarrichardolsen7453]

many thanks for a very instutive video

[@LightAndSportyGuy]

Thank you for the comment.

[@MartinSage]

Go to Terrell’s Classic workshop. He’s the master tuner for webber Ferrari’s. He uses a rubber tube and a screw driver. I learned to sync my Dino Ferrari and Performance VW. It’s an art not a science.

[@Carlosio1001]

Pretty useful. I made it in my Rotax 912 ULS and I eliminated some vibrations at lower rpm. Now the engine runs smooth in the whole range. Thanks a Lot.

[@MrHanowski]

Wondering why you don't use the CarbMate or equivalent electronic balancer which I'd think would be easier and more accurate.

[@LightAndSportyGuy]

Gauges are what I have.

[@LeonardLayne]

Who does the condition inspection and how much if I may ask? I may need one soon

[@LightAndSportyGuy]

My AI does the condition inspection - I get it all prepped (change oil, check compression, lube bearings, etc. etc. etc.) and he inspects $260 - $300 depending, and I fix any discrepancies.

[@danbeck6741]

Thank you for the tutorial! I need to reduce my idle slightly. Do you recommend a balance at the same time? Or can an idle decrease of 100rpm be made without a balance check?

[@LightAndSportyGuy]

I would be inclined to do it as part of a balance - you may be able to improve the balance and get 100 rpm at the same time by just closing the throttle that has the lower vacuum/ higher manifold pressure.

[@patway4683]

Great video

[@LeonardLayne]

Thank you