Comment

Comment by Jonathan Porter on August 7, 2010 at 2:02am

Another thought for others - we have had several cases of issues due to deteriorating carb seats. (the rubber flange that the carb sits in). Before flying check for any cracks or de-bonding of the rubber/metal - if it goes in flight it makes little difference for that flight due to continued vacuum - but once they start to go they make a terrible vibration at start up, then the engine wont start, floods and you have a mess. ALWAYS wire lock the carb in place - ALWAYS.... (a friend in Australia saw an aircraft turn into a BBQ due to failure to wirelock the carbs.... ) abd whilst at it the air filters too!

Carb seats are expensive, but due to our conditions we change them every 200 hours.

Comment by Doug MacDonald on August 6, 2010 at 10:34pm

Jonathan, my understanding also was that the enrichener does nothing at high RPM. One of Phil Lockwood's guru says the enrichener actually has a leaning effect at higher RPM. It will give an initial shot of fuel but then make the carb run lean.

The following is my supposition as to why this would be so:

I traced the path of the fuel to the enrichener port. The brass coloured tube in the corner of the carb bowl is the way that the fuel gets up to the rotary enrichener mechanism. This tube has a small vent port that is vented to the top of the float bowl. At higher RPM there would be a vacuum at the vent port. This could have the same effect as having a vacuum on the carb vent line and cause a lean running situation. Essentially, my carb thought it was at altitude so therefor the bellows would not open all the way.

I'm not positive about this but at least it is a plausible explanation for the issues I was seeing. It's working much better regardless.

Comment by Jon Croke on August 6, 2010 at 7:14pm

Glad it was that simple!

Comment by Jonathan Porter on August 6, 2010 at 1:30pm

Glad it worked! However, if you look carefully at the way the Bing on the Rotax is set up, the choke loses any impact at high RPMs (in theory).

We do a full carb clean and check at every 100 hours, then at roughly 500hours change the seals and at 1200hours change the diaphragms... I think it is fair to say that carb overhaul is a key part to successful operations of all engines with carbs - especially if in areas with high humidity and where fuel contamination issues can allow bits of 'poo' into the bowls!