912 Carb mount rubbers.

[Guest Maj Millard]

About a month ago during a 50 hrly oil change on my 912uls (450 hrs TT) I took the opportunity to fit a new Pierburg electric boost pump,to the Lightwing. During the 19 hrs I've flown since then, I have had the previosly perfect-running engine, miss about four times in flight. It would just run rough for a few seconds, and I would whack on the boost pump, pull carb heat, and switch back to both tanks if I was on one.

 

This seemed to take care of it, and I put the first couple down to carb ice, or a bad load of fuel.

 

Last Tuesday I flew a friend from Townsville to Atherton to look at a Savannah, and it did it once on the way up, and once on the way back, just for a few seconds each time, usually at altitude in cruise, but enough all the same to put one into ALERT mode !....(all good practise).

 

Today on my usual commute home (1 hr), it ran just perfect as it always has. However during my usual warm up today, I noticed that the throttle didn't want to stay where it usually does, at 2000 rpm or a little less. It wanted to advance up to about 2500 or so. Hmm....strange ??. After landing at home base there was a smell of fuel as I was taxying up to the hangar. Right after shutdown I pulled the cowl off, and spotted the problem straight away. One carb mount rubber had split, and that carb was sitting at a bit of an angle, which had probabily been the reason the throttle stop had changed a bit. I was planning to pull the cowls off this weekend for a look anyway, thinking my previous fuel pump installation, or fuel line routing was at fault.

 

During the last 100 hrly, at 400 hrs, I had removed the mount rubbers for a good check and clean, and they look fine, so they were refitted.

 

I bought this aircraft at 300 hrs, and it has always been hangared and really well maintained. As it is a 2000 model I can only assume that the carb mount rubbers are nine years old, and Rotax does recommend replacement at five years. I will be following this recommendation to the letter in future, and I do think it is the way to go after todays find.

 

I had been watching those rubbers closely, it's obviosly not enough, and I almost got caught. Two new ones are on the way....................................................

 

 

[hihosland]

If the screw clamp that secures the rubbers to the carb body is too tight the rubbers will split at quite low hours. Mine only lasted 100 hrs.

 

More recent ones come from Rotax with a collar that limits how tight the clamp can be done up thus ensuring that the rubbers have best chance of delivering their design life.

 

Davidh

 

 

[Guest Maj Millard]

So correct there Hihosland, the new limiting collar is a great idea that works, and hopefully will see this potentially dangerous scenero dissappear in the future...........................