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

Thanks Kevin,

1. The static RPM was OK during the inspection test./ CQ: was ok before the condition.

2. The max RPM is correct most of the time but occasionally is limited to 4800. / CQ: 4800 just 2 times in the last 5 or 6 months.

3. There is no roughness or misfire during these RPM limitation events. / CQ: Correct. sounds and feels great.

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 ;-) / CQ: Correct, if the diagrams are right and what I saw below the cowling, yes.. is a wire.. that is what i said that is weir that you reach 100% butterfly with 80% or throttle. I dont thing you have some expo or dual rate thing installed in you plane. I haven't installed the fly by wire option in my small guy :):):)

5. Unless it is fitted with a variable pitch prop then the static load will not change. / CQ: Agree. That is what I would like to see.

I would therefore assume that whatever is happening is happening to both sides otherwise it would run rough (CQ: agree.. air box problem/obstruction? not this time, the previous one, when I was at full throttle i opened the carb heat expecting to see some rpm change to confirm some obstruction but no change was noticed)

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. (CQ: agree, at the end of the day the Throttle Valve is the only piece moved by the throttle)

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. (CQ: there is full movement on both throttle lever. maybe one needle is stuck and the unbalance is absorbed by the compensation tube?)

Thanks!

Was the exhaust system removed during maintenance?  A partially blocked muffler will limit the RPM. 

Assuming the engine is in reasonable operating condition (ie not warn out) there are only a very few things that can limit rpm. I no particular order;

Air in - double check condition of air filter(s) Foreign object in the air intake, etc

Gas out - partial blockage of the exhaust (usually muffler baffles collapsing) can occur at almost any time.

Fuel in - Several factors can reduce fuel low - pump performance,  partially blocked fuel line (foreign object constricted/damage hose) or filter (including any in tank sieves). Does your fuel have to come through an in line boost pump (internal damage)? Does the boost pump have a by-pass system? is there a non return valve on the main fuel supply (malfunction).

Prop pitch - unlikely I am sure but worth checking that no adjustment has been made that may "govern" the prop to 4800 rpm.

I am discounting ignition issues because you have two, making this unusual factor even less likely however should both your ignitions be "breaking down" at  4800 rpm, even pigs might fly?

hi Carlos and all

I went back are reviewed the first thread from your problem.  it would seem we are just going over the same fix variations.  I am inclined to think that Sean is correct in that you may be just a bit overpitched.  

Consider this:  The engine is not new anymore, there will be some degradation of some parts.  I would seriously consider some blockage of the exhaust as a possibility.  In your installation manual there is a value for this back pressure, you would however have to be able to measure this.  An alternative is see if you can find another canister and borrow one for a test.  (assuming it is a Rotax one and not custom made) 

What I am missing, perhaps I just did not see it in all the thread, is what is your WOT RPM when everything is working correctly in flight?  This might be an issue if you are not getting full RPM I go back to what Sean is thinking, overpitched.  Remember that this is sensitive in that 4800 is still on the climb to peak torque.  (5252) Cylinder pressure at 4800 has not hit its maximum value within the head.  That last bit is where the engine is fighting you right now and makes me think it is overloaded.   Perhaps you have someone else at your airfield with a similar aircraft and prop combination.  See if they can share what pitch they are using.  If not contact your OEM and see if they can make a suggestion of what they used, if it is a common aircraft type. 

Just trying to give you a straw to grasp at...

Cheers

Interestingly, 4800 is the maximum static RPM for the Eurostar fitted with a 912UL under the LAA rules in the UK. This is my maximum static RPM and I know from experience that relatively small pitch adjustments of the prop will change this value. I also know that the analogue Rotax tacho is not linear over its entire range. Mine is fairly accurate at about 5000 RPM.

This being the case, you could adjust your prop pitch to make it slightly finer and then see if the problem reoccurs. I would mark/label the blades and hub so that you can easily set it back to what it is now.