Carb Float Bowl Breather/Overflow

Hi RW,

All the best for the New Year

I would do the test as shown in the MMH on the float needle to seat check.  Be sure that they are sealing correctly.  While your at it confirm they sold you the R floats and not some of the Bing ones.  There are also several versions of the float needle, check the plunger color on the spring, it should be yellow.  (not silver)  There were 3 versions made, silver, the lightest.  There was a black version, the heaviest internal spring.  And the version we should currently use is gold (very light yellow).  This spring is a med tension and least likely to bounce with the 11 to 1 compression pulses on the 100 HP version engines. 

Float valve static test - seem to be sealing as expected.

Float - have replaced OM/New floats, with new R floats, as mentioned earlier - no change to fuel exiting float chamber

Float needle plunger colour - Inspection in situ (bowl removed) think yellow but not sure - do I have to remove the needle to be sure? This is a 2019 built engine - should it not be fitted with the correct needle? The carburettors have just been inspected by Rotax agent - would this not have been part of the inspection?

I'd still check my brass float armature measurement. They may not have done it right. Plus I'd do the pressure test RW suggested or I's just polish the needle valve seat. It's easy to do and I've saved numerous carbs from replacement and over flowing by polishing the needle valve brass seat.

Because of your answers in your post I'd lean on one of these two issues. Both are easy to fix.

Hi Roger,

"..........just polish the needle valve seat"

I think you may have given detailed instructions how to do the above - please direct me to your instructions.

An additional symptome:

I installed the longer & looped, breather tubes. Result - no fuel in catch-can.(I agree - addressing the symptom not the problem)

After a 1.5 hr flight, I removed cowling - right side breather tube, fuel bubbling part way up, the carby side of the tube falling back, before reaching the apex of the loop and repeating many times. Looked to be fuel "boiling" in the float bowl. I have an aluminium radiant head shield, that covers the full length of the carby (inlet manifold to air box attach points) underside and a heat shield on the exhaust manifold below. A light touch of the float bowl found it to be too hot to to keep my finger in position.

The left carby did not have this symptom.

Food for thought,

Two different things may be going on here. You'll need to rule these in or out as the issue. From your description kind of sounds like a heat issue. The carbs do get pretty darn warm under a cowl during a normal run and even hotter when using just a K&N air filter on each carb sucking in all that hot air.  Like running carb heat all the time. Cowl air flow is important. 

One way to maybe rule heat in or out. If it was just the needle valve then if you take the cowl off so the engine is in open air and cooler and then run it up around 3K - 3500 for 10 minutes then the carbs will stay cool and if there is fuel still coming out the vent tube then it is most likely to be either the floats are heavy, the brass float armature is not adjusted correctly or the needle valve or seat is bad. If the cowl is off then the carbs shouldn't overheat and it's a carb issue. If it only happens when the cowl is on and the carbs are hotter then it may be just heat retention. You can use temp test strips to see what the temps are on parts. 

Maybe put in a better heat shield under the carbs to protect from the exhaust pipe under it. Adding a couple NACA vents aimed around the carbs may help or just increase air flow through the cowl. 

Air flow may still be your biggest help?

Roger, sorry to be picky, but...

The part is actually an "independent float arm" as described by Bing.  Rotax calls it a "Float Bracket" in the IPC.  An armature in context of an engine is the electrical winding inside the starter motor that spins.  

Cheers mate