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Low Vacuum / erratic on side 1 and 3

in 912 / 914 Technical Questions 12 years ago
I am unable to sync carbs pneumatically since side 1&3 have lower vacuum by about 5"hg. The needle is jumping by about 2-3 inches as well on my vacuum gauge, but smooths up some around 2100-2200rpm.
I did find a leaking gasket at the carb to manifold and corrected and improved somewhat.
Any thoughts of items to look at next?
What is the possibility of this being a carb issue vs. leaking intake seat etc.?

Thanks

  • Re: Low Vacuum / erratic on side 1 and 3

    by » 12 years ago

    Wellslake,

    If you suspect a valve leak, Best test is a differential pressure test. If you have a leaking intake valve, you will hear it when you put your ear to the carb. If you don't have a differential tester, a standard compression gauge will probably show you if #1 or #3 have low compression.

    You can also visually observe the butterfly valves as you have someone open and close the throttle and see that they are opening and closing simultaneously. They MUST begin to open at the same time exactly.

    Also hold the butterfly shut and see if you can move the throttle lever at all. There should be no free play between the lever and the butterfly.

    You can use a can of starting fluid and spray a SMALL amount with the engine running anywhere on the carb or manifold where you suspect a leak. If there is one, the engine will suck it in and momentarily speed up.

    Also check the EGTs. They will be higher where you have a manifold or carb vacuum leak.

    Eliminate these possibilities and that gets you back to throttle cable adjustment. That is usually where the problem lies.

    good luck,

    Bill.
    Thank you said by: Wellslake Morse
      
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  • Re: Low Vacuum / erratic on side 1 and 3

    by » 12 years ago

    I have a compression tester I use on Lycoming and Continental engines, but haven't used in on the 912 yet.
    Un-scientific as it is, when pulling the prop through, all cylinders seem to has similar compression.

    Mechanical sync should be correct and throttles advance at the same time and distance.

    I used carb cleaner spray around the manifold and this is how I found the leak between the carb rubber gasket / drip pan and manifold. Now corrected. So far unable to uncover additional manifold leaks.

    The carbs previously had new floats added a couple years ago. I pulled the air cleaner on the carb and looked in and found the slide rotated maybe 20 degrees out from normal. I opened the top and found the diaphragm not properly located with the tab in the slot and corrected that. I don't know if this was responsible for the leak I was seeing. I didn't get a chance to retest with vacuum gauge yet.

    Thanks for the great feedback. I am new to Rotax, but diving in learning everything I can.

    Wells
      
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  • Re: Low Vacuum / erratic on side 1 and 3

    by » 12 years ago

    wells,

    The diaphragm could definitely allow outside air into the manifold if it is leaking. I'm not sure, but I would think it would take a pretty large leak to affect the vacuum very much. Someone else on the forum who knows carbs better than me could probably elaborate on this further. If the tab on the diaphragm was not seated properly in the detent, It could cause a leak and affect manifold vacuum. One quick check for diaphragm leaks is to pull off the air cleaner and put your finger over the small hole in the carb entrance that allows ambient air pressure to reach the underside of the diaphragm. If you do have a diaphragm leak, this would change the manifold pressure almost immediately. Other way of course, is to pull off the diaphragm cover and inspect the diaphragm. If that diaphragm was out of position and you fixed it, you have probably already corrected the problem.

    As to being un-scientific, I still use the old thumb in the spark plug hole trick occasionally. It works.

    Bill.
      
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