New member of the 'Sinker Club' (carb floats)

[Piper J3]

Originally Posted by mcems

Maybe being just a little past the acceptable level would explain the intermittent nature of the problem.

Any progress to report?

I think setting float level on low end of spec is a good experiment. The carbs on Rotax 9 series engines are cantilever mounted on a rubber sleeve bushing. What makes matters worse is the heavy air filter at the un-supported end. A mickey-mouse extension spring attached to the top of carb is used to counter the overhung weight.

I think the fuel inside the float bowl is very un-settled on an aircraft engine and especially so on the Rotax 912. Turbulence can throw the fuel in every direction making float level control challenging. At cruise power setting float level is not so critical because fuel flow is much higher. Engine runs smoother at cruise speeds as well and carbs don?t dance around so much.

Every 912 I have seen does a dance at lower engine speeds. Imagine how floats are trying to set correct level when carbs are jumping around. So, if floats are wiggling around and level is trending high, this is condition for rich mixture and/or float bowl overflow. The condition is worsened because fuel flow for combustion is low at slower engine speeds.

[rvbuilder2002]

Originally Posted by Piper J3

A mickey-mouse extension spring attached to the top of carb is used to counter the overhung weight.

The spring was added to the engine design early on, not to support the filter weight, but to change the natural resonant frequency of the carb / engine interaction which caused a failure of the carb. mounts.
A different / more durable carb mount was introduced later but they left the spring.

[mcems]

so here is the update

After doing all of following.

1.comlpete carb overhaul both sides both and both fuel pumps new

2. complete removal of coils testing with ohm meter both hot and cold and test all pick up coils the same way
(when doing this i found one of the coils was rubbing/chaffing bad on the motormount .it actually wore through the powdercoat down to the metal motor mount is OK)

I thought oh boy I found the problem. No to be.Same issue returned.

Here is the interesting part. With the top cowl off and cool air flowing over the top of engine and not retaining heat I could not re create the problem.

Put the cowl back on and problem returned.for sure heat related.

I ask myself the question what else is on top of the engine that heat is getting to. THE FUEL SYSTEM. I am thinking vapor lock ??.

Sure enough drained all the fuel and put in new 93e10. problem solved

3 weeks of trouble shooting and all is well.
Now why wouldnt I check that first you ask. Well I have had 93 no ethanol premium deliver to the farm here for 10 years and always added stabil to all the 300 gal delivery. Never had a problem in 10 years and 1000 hours.

I always thought thats why my 1000 hour floats are still good less that 6g

All the things I replaced that I mentioned above needed to be done anyway due to 1000 hours so not all wasted.

The only thing I dont like is running 93e10. (CarbFloats)I will have to find another supplier for premium 93 with no ethanol. I think last delivery was bad stuff.

Thanks for all your replys

Brad Stiefvater

[Piper J3]

Interesting. Auto fuel does not store well even with stabilizer added. Octane rating takes a hit. Running fresh 93E10 is Rotax recommendation.

I wouldn't worry about floats - its pretty much out of our control. Take a spare set of carb floats and tools in the airplane for emergency change-out.

Thanks for report - glad things worked out.

Stay safe...

[John-G]

Well I have had 93 no ethanol premium deliver to the farm here for 10 years and always added stabil to all the 300 gal delivery. Never had a problem in 10 years and 1000 hours.

Brad,

Your experience sounds to me as though it may very well have been a heat related vapor lock issue. In our area, the summer blend fuel has a mandated RVP (Reid Vapor Pressure) not to exceed 9 psi between June 1 and September 15 ... the winter blend has a much higher RVP meaning it evaporates much easier. (Also of note ... the winter blend RVP can fluctuate wildly depending on the local temperatures at the time it is refined).

So I'm wondering ... is the 93 non-ethanol fuel you purchase for your farm subject to the EPA's winter/summer blend mandates? If so, you may may have been typically purchasing fuels during the summer period and hence having no issues with running it in your Rotax engine ... however, your last purchase may have been a winter blend, which would certainly explain the issue you were experiencing now that the outside temperatures are becoming more summer like.

Anyway, glad to hear changing fuel solved your issue.

Happy flying,