912 ULS no more than ~4800 RPM problem is back!!!!

On many 912 installations that last part of the throttle advancement doesn't actually open the throttle any further, Certainly on mine, the throttle flap is fully open when the throttle control in the cockpit is about 80% advanced.

You could easily check if the throttle flap is fully opening by removing the air filter and looking into the venturi. If you suspect that the problem only occurs at certain points, then stop the engine when the problem is evident and take a look at the throttle flap positions.

What is the arrangement of your throttle cable? Is it a single cable that splits into two in the engine bay or two separate cables right from the control in the cockpit? Could you have a broken strand that is getting stuck somewhere in the sheath?

Assuming that the rpm limit is not accompanied by any form of roughness, then the problem is likely to be something that is common to both sides: fuel pressure/blockage or throttle cable. Could you rig up a separate fuel supply from a container that you elevate to give you a head of pressure? Feed this to the 2 carbs via new temporary hoses. If this fixes your static rpm problem then you know there is a fuel supply problem.

Hi all

I think we all need to understand that the carburetors on the Rotax are constant depression.  What that means is the throttle butterfly only opens what the cable is set to for WOT.  The slide is open only by pressure depression of the MAP and outside air pressure.  if the load is too much, as in excessive pitch, it will never open fully.  If the engine is low on power you can't pull the pitch set for a normal engine is another way of thinking about this.  Without a MAP gauge you can't really tell what the issue is in relation to the load vs manifold pressure.  

There is no mechanical link from the butterfly to the slide.  

Cheers

Thanks Kevin, but, there is not a direct cable connection between the throttle handle, throttle lever, throttle shaft and throttle valve? having said that and having in mid other publications that describes that the last 1/3 of the throttle handle is 100% functional, I would say that there is not reason that supports that the last 1/3 of the throttle movement doesn't do anything. In my case, I have verified that and I do have a full moment of the throttle handle that match with the movement of the throttle valve lever and other associated parts in a 1:1 relationship

Again, no A&P here, but, this is solid metal and if there are not levers, deferent component radius, etc. in my humble opinion that could not be happening.

Thanks Rotax Wizard, There is some doc that explains/shows the relationship between RPM and MAP?

To be honest, I don't think this that this is related to external conditions.

Unless I have misunderstood, let's look at the evidence here:

1. The static RPM was OK during the inspection test.

2. The max RPM is correct most of the time but occasionally is limited to 4800.

3. There is no roughness or misfire during these RPM limitation events.

4. The standard 912 installation requires a direct link between the throttle control in the cockpit to the throttle flap in the carb venturi. There is no 'fly by wire' that I know of ;-)

5. Unless it is fitted with a variable pitch prop then the static load will not change.

I would therefore assume that whatever is happening is happening to both sides otherwise it would run rough. I fully accept that the throttle piston height will vary with load due to the pressure in the venturi and this is not a function of the cockpit throttle control position.

However if both pistons were sticking then this would restrict the fuel flow from the main needle. I would imagine that it is unlikely that both are sticking in the same way. This is why I thought that it might be more likely that the actual throttle cable might be getting stuck and simply preventing the butterfly valve from fully opening.