rough running 912ULS

Does your fuel system have a "return line" between the "T-fitting" where the supply line spits to go to the two carburetors, so excess fuel can return to the fuel tank (or header tank, as the case may be)?  If not, you could be encountering some fuel vaporization in the lines due to higher temps. Having that return line is recommended by Rotax, and I've seen several posts where folks experiencing "vapor bubbles" in the fuel resolved the issue by adding the return line.  Perhaps one of the "gurus" can explain it better than I have...

Bret, sounds like you have a good grasp on things.  I'm nowhere near the expert you are but I was thinking venting or fuel return possibly.  Since it's a temp/heat issue, you might call the rotax places in hot regions like the one in Mississippi or Florida.  I'll be watching.

One last stupid question - of course the first thing we think of with rough running is a carb sync issue.  But you said you did that.  Is it possible it's a cable issue - stretched or something?  So while on the ground the sync issue performs fine, but then maybe airborne, maybe both carbs don't open the exact same?

I'll shut up now.

Bert 

Hi Guys thanks for the input.   I have the recommended set-up where after the mechanical pump the line splits off to both carbs and also back to the gascolator through a 0.02" restrictor.   This has given me a fairly reliable 3-4 PSI fuel pressure and I checked to make sure the return line was patent recently.   

The carbs are synched and to avoid issues such as the one you mentioned I used music wire for my throttles so they are a rigid push/pull setup and shouldn't have any ability to flex or stretch.   With how rough the engine was it would have needed to have one carb almost completely closed with the other open so it is definitely not flexing that much.   

So far there seems to be a consensus the issue is still likely fuel/carb related.  I would have to agree the heat causing vapour or mixture related issues makes more sense than that the oil or CHTs being hot would cause such a massive issue in itself.   It is technically possible to have adequate fuel pressure but not adequate flow, so what I might do as a next measure is take off the fire covers on all the hoses to make sure none of them are starting to kink.   They all look fine with the fire hoses on but if one has a bit of a kink when cold then when it gets hot it could be severely restricting.   I will also check the filters off the tanks to make sure they are not getting packed in (I replace them on my 100 hourly either way so they should be fine).   If it is a vapour issue that for some reason has newly come up then I could also fill one tank with 100LL and see if the problem goes away or gets better running Avgas and if it does than at least I know what system to be focusing on.   

"The carbs are synched and to avoid issues such as the one you mentioned I used music wire for my throttles so they are a rigid push/pull setup and shouldn't have any ability to flex or stretch.   With how rough the engine was it would have needed to have one carb almost completely closed with the other open so it is definitely not flexing that much." 

One more thing to rule in or out.

The issue may be with your throttle cable. I would put a set of carb sync gauges on the engine and run it from idle to around 3300 - 3500 rpm and see if the gauges read the same on each and every run up. I have seen may throttle systems that do not go back to the same place each time the throttle is advanced and it makes trying the sync the carbs a pain and sometimes you just have to  live with those results. I have found thinner cabling to be smoother and work better than thick heavy or ridged cabling.  The thinner cable slides easily and smoothly through the throttle cable sheath and allows the carb spring to do its job. Those carbs are easy to sync and stay that way every run up of the rpm. 

Brett,

You say that you are returning fuel to the gascolator.  If that is in fact your set up, you are simply recirculating hot fuel.  The fuel return line should return fuel to a fuel tank, where the heat it picked up from the engine dissipates.

Fuel needs to vaporize in order to burn properly, but it should vaporize in the combustion chamber/intake manifold, and not before.  Fuel boiling in the lines can cause vapor locks, and fuel boiling in the float bowls can cause flooding.

If you don't have one, install a fuel return line from the fuel distributor back to the fuel tank.

Richard W