Trouble with Rotax 912ULS

HI guys - can anyone help solve this problem on my friends engine as per the following story :-

Eurostar Rotax 912ULS – EV97A – BORN Feb 2011  - hours about 360 .hrs

Recent history :This rotax engine (with Slipper Clutch) has done approx. 360 hours since new in 2011 and has an electric fuel pump in series with the mechanical fuel pump.

Owner recently renewed all fuel hoses from firewall forward  using 6 mm bore hoses from Flylight

but 8 mm bore feed from firewall to Mechanical fuel pump March 2022.

Oil tank insulated where feed and recirculation hoses route round the tank, Hoses also insulated as they passed over the top of engine, against engine heat and from hot water feed  to Carb bodies as had been the routing for a long while.

Fuel feed on steady rise on its route to mechanical fuel pump – no ‘high loops’.. Hoses were cut to length using Stanley knife and checked to ensure no stray bits of rubber.  Hoses also flushed out using electric fuel pump before end fitting. Electric fuel pump ran after fitting hoses to check for leaks then engine ran to check integrity. Found OK.

Incident 1 (24^th March) -. Plane flown the 12 minutes to island of Bute, climbing to 2500 amsl over the water. – electric fuel pump on for takeoff and climbout. Flight over was normal. Landed at Bute, and sat for 40 mins, then went for local flight with passenger. Started (without choke, taxiied 400 metres to 27, power checks OK – electric fuel pump on for takeoff and climbout.) 15 mins later into the flight on returning to same runway the engine ‘Stuttered’ akin to a very loud ‘machine gun’ noise. This happened at least twice, lasting 3 to  5 seconds on each occasion, the engine then reverted to running normally without intervention by the pilot. Landed and at the end of the landing roll, the engine stopped as taxied into parking area. Was restarted immediately and taxied as normal to park with no problem.

Engine checked under bonnet – found that the end of the short ‘Drain / Vent hose associated with the mechanical pump was partially ‘pinched’ at its end below the fuel pump where the lower engine cowling had caught it against the top of the water radiator. Rectified, (The vent tube off the pump normally ends, as lie in ‘neutral’ air.) Fuel hoses all checked with electric fuel pump on – all found normal.

Taxied out 400 metres for take off, power checks all OK, electric fuel pump on for takeoff and climb out– all OK. Orbited. runway and check engine running fine / fuel pressure at 0.35 bar and electric fuel pump off

Fuel pressure dropped slightly but engine ran fine. After two orbits set sail back the 12 min flight over the water with electric fuel pump on and flight to landing and taxi at Mayfield farm strip.- all OK.

On ground, fuel hoses checked thoroughly, / fuel filter in cockpit ( glass type dismantleable) dismantled checked and found clean and clear and reassembled. Additional insulation .fitted round oil tank to mitigate heat from oil tank. Carb float bowls removed to check for any debris but found clean / floats were light and seemed OK - . Carbs reassembled and system checked for leaks – none found.. Engine ran and up to full power once heated up - all was OK.

Went for flight, orbiting runway first before flying locally – all went well. Electric fuel pump on for takeoff / climbout and later landing.- but off during flight, with fuel pressure 0.35 bar - all OK.

Incident 2. Few days later, went for a flight 12 mins over to Bute island runway – all fine. Approached Bute and decided to do a practise forced engine failure – approaching runway with electric fuel pump off, throttled back for descent – then did a ‘touch and go’ – On powering up for ‘go around’, switched electric fuel pump on, and power at ¾ throttle. Climbed out and levelled out  at approx. 1500 feet  - just then engine ‘stuttered’ – same ‘machine gun’ noise for a few seconds, then engine smoothed out (electric fuel pump still on). Latter repeated itself - same ‘stutter’ and akin to a machine gun noise again picked up after a few seconds, happening twice more. With electric fuel pump on, all the way, returned to Mayfield farm and landed with no further trouble engine running fine. 

Following days,  - both Carbs removed one at a time and dismantled on clean bench. All jets out and air blown through onto a tissue on each occasion but nothing found banjo connections at inlet to the Carbs through to inlet port to the float operated shutoff inlet valves which had been removed. Fuel float chambers were clean, floats removed and weighed. Port floats (pair) at 4.5 grammes / Starboard (pair) at 5 gms.

When Carbs off, induction rubbers checked for cracks in rubber and found in excellent order.

Carbs then reassembled and mounted. Also ‘Crossroads’ unit where it allows flow to Carbs and recycle line back to fuel tank taken off and blown through onto a tissue – nothing found and recycle hole found clear. Electric fuel pump ran, then switched off, fuel pressure falls to zero as should be expected.

Addendum, for security, fuel was completely drained from fuel tank and replaced with fresh E5 fuel.

(Never has any E10.been used).

Engine run up on a 30 degree upslope on Mayfield’s long runway warmed up and then at full power with only Mechanical fuel pump and then tried with electric pump on.- all ran perfectly.

Owner noted that the idle tick over was fine and was set at 1800 rpm.

Incident 3.-

Saturday 9^th April – engine started immediately / taxied out for flight over to Bute. Power checks all satisfactory, NO Electric fuel pump used this time for take off and climbout.and later landing. - engine ran fine for the 12 minute flight over to Bute. On arrival, engine shutdown - after 20 mins, engine started easily (no choke required) and taxied out 400 metres to takeoff point. Engine power checks satisfactory, again takeoff with NO electric fuel pump on.

5 mins into the return flight to Mayfield, - flying  level at 1700 ft on cruise at 4200 rpm when suddenly engine ‘stuttered’ – noise like a ‘Machine gun’ with slight vibration through the aircraft, then engine reverted to smooth running after a few seconds.  2 mins later it again ‘stuttered’ like a ‘machine gun’ noise and cleared again after a few seconds. 3 mins later it repeated the stutter for a few seconds, then smoothed out again. Throttled advanced a few 100 rpm answered and ran smoothly. On each of these occasions Fuel pressure remained at approx. 0.32 bar and did not alter. Electric fuel pump put on, fuel pressure rose slightly. This pump was kept on for the 6 mins to landing at Mayfield, the engine running smoothly and same when taxiing taxied in.

For interest, after ½ hour, a Carb pneumatic balance check was then undertaken per Rotax video and Carbs balance shown to be fine the engine running smoothly various rpm and balance maintained.

Fuel filter checked and still clear.

Comments :-

1. Incident 1 – second flight of the day after 45 mins after shutdown had passed.

            “     2. – overshoot from practice forced land and go around.

            “     3. – second flight of the day after 20 mins after shutdown had passed.

2. On each occasion the engine ‘stuttered’ sounding akin to a ‘machine gun’ noise. – apparently much worse on incidents 1 & 2. – lesser so on incident 3.

3. Fuel tank had been completely drained and fresh E5 mogas added before incident 3.

Note – ‘WE has always used Mogas.

4. Main fuel filter had been dismantled and cleaned out after incident 1.

5. Carbs dismantled and all jets cleaned and no debris found anywhere after incident 2.

6. Recycle crossroads unit removed and cleaned b4 incident 3 and found clear. 

When incidents occurred – on each occasion the engine ‘stuttered’ definitively akin to 

A metallic ‘machine gun’ noise for several seconds then engine smoothed out, fuel pressure up and steady at all times during incident 3 as witnessed by pilot passenger.

Any leads would be appreciated ... Thanks ... Sandy.