UL912 Intermiddent engine failure

Hi Kevin

The ignition switch is actually not a switch but two each on/off switches. I'll take a look at the GND point, it could be a root cause. 

I had this happen several times a few years ago.  Here is a video: https://youtu.be/bsK483N6cM0

Reducing the throttle from full open (5600 rpm) to about 95% (5400 rpm) seemed to fix the problem, and so did turning on the electric fuel priming pump.  After this video I resolved to leave the electric pump running for the duration of each tow,

The problem has not recurred since the 5-year service (4 years ago now) at which we replaced the fuel pump AND fitted a vent tube to the fuel pump vent port.  I saw in other posts that backpressure on the vent was suspected as a cause of reduced fuel delivery pressure.  The vent tube outlet is directed vertically down into clean airflow. 

This is really strange as we have the same issue.

Here’s what we have done so far:-

Checked fuel tank is venting.

Fuel drained and replaced with new fuel.

All pipes replaced with correct diameter hosing.

Pumped Fuel volume checked and fine.

Fuel Filter changed

Fuel Pump Changed

Carbs refurbished, new floats, seals, gaskets etc.

New carb inlet rubbers fitted

Air filters cleaned and oiled (not excessively)

Fuel cross flow pipe junction fitting replaced.

Fuel pressure is good and well within prescribed limits.

Carbs Balanced

Engine runs fine on each mag.

Aircraft chocked and run at varying rpm for 15 minutes.

Engine coughs lasts only a second or two at most then continues to run smoothly.

Any further suggestion will be very much appreciated.?

Hi Mel

I'll let you know if we pinpoint the problem.

(Reposting, the first attempt did not work (maybe a link to YouTube is not allowed?). Apologies in case this reply appears twice)

I had the same issue a few years ago. It happened several times and was caught on video (search for 'Fuel Starvation While Towing a Glider' on YouTube).

It only happened at full throttle (5600 rpm) with nose high attitude. The problem was always quickly fixed by immediate action to lower the nose, reduce throttle to give about 5000 rpm and turn on the electric booster pump (used for priming the fuel line).  I made it a habit then to keep the booster pump on for the whole tow (rather than just the first 500'), and that prevented it recurring. 

The problem has not happened again since we replaced the fuel pump 4 years ago as part of the 5-year rubber replacement service. At that time we also fitted a hose to route the vent line from the fuel pump down into clean airflow.  I read on-line somewhere that backpressure on the pump vent can cause a reduction in delivery pressure.