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I am having a real problem balancing my carbs. I believe I am following the training video put out by Rotax for a 912ULS but seem to have a problem getting results. Engine shakes to the point that the carbs bounce out of their couplings. Has anybody had this problem? Can you give me any advice. Thanks. Bob Toth
Bob,
It is important to follow the procedures in the Rotax manual; and, a person needs to have a ballpark starting point on any carb adjustment - I'll get into that a few paragraphs down.
I cannot emphasize how important it is to fully consult the manual.
Before changing anything - be sure that there are no fuel flow problems with your system or either of your carburetors. Fuel should flow equally out of the hose at the carb connection on each side if you disconnect it at the carb fitting. If there is a plugged line or a stuck float valve - that needs to be corrected first. It is amazing how small a piece of crud can muck-up fuel flow. It is not a bad idea to take off the float bowl and be sure the float and needle are working correctly and are completely clean. It is not beyond possibilities to find a constricted fuel line either.
It is a good idea to visually check to be sure something unique hasn't happened like a vacuum leak anywhere between the carbs and the engine heads. Is everything tight on the balancing tube - no leaks? Plug wires on the right plugs? etc.
A couple points to check on the carbs.
1) Be sure that both throttle valves are open the same amount (this sets the basic idle speed) - if one is way more or less than the other - it is not a good starting point and needs to be corrected.
To check this - a) disconnect the throttle cables from the carbs to be sure they are not affecting your primary setting of the main throttle valves. b) (Note: Since the Rotax system fails to Wide Open Throttle you have to hold the throttle arm to the idle position against the idle speed screw to check this). Get a note pad and a screwdriver and determine (and write it down so you know where you started) - back off the idle speed screw and see how many turns each side is open - check it carefully - use a feeler gauge if necessary to see when the screw breaks contact with the arm - they should both be the same - if not - they need to be made the same - see if the engine manual gives you a figure for how many turns the idle speed screws should be turned in. If they are both the same - adjust them back to the exact point they were. FYI - Before I started my engine the first time - I checked this and found that the factory setting had exactly the same number of turns on each side. I put it back and left it there - ended up being real close for the correct idle speed.
2) Be sure that both Idle mixture screws are set the same - again - count the turns it takes to screw the idle mixture screw in on each side till it stops - and write it down (be gentle - a person can wreck the idle mixture screw seat if more than a light contact occurs). You should find that each side is backed off exactly the same number of turns - if not they need to be made the same. Once they are the same - any need to change the mixture to make the engine idle correctly must be changed exactly the same on both sides. An engine with a mal-adjusted idle mixture will run like crap - but you have to have an equal starting point on each side then change both the same amount to get a smooth idle. Again - check your manual to see if it provides a basic setting for the mixture screws and use the same setting on both sides. I checked this on my engine before hitting the starter and found that the factory setting was exactly the same on both sides.
Sincerely,
Dave S
Kitfox 7
912ULS
Bob,
In addition to the suggestions the others have offered, could you be trying to idle it too low? Rotax's like to idle around 1,800 rpm, I suspect if you tried to idle it at 1,200 or so it would be extremely rough.
Just a thought,
Thanks for the suggestions on Carb. Balancing. I appreciate and will go back through everything.
Bob Toth
Hooray I got them Balanced today. Was a real battle but finally got it. Thanks to everyone for their advise. Bob Toth
I have heard , but not seen, that someone removed the carburetors and fabricated a manifold (tubing) that fed both intakes and used a single throttle body injector. Has anyone seen anything like this?