The Zenith 912 FWF kit comes with a coolant overflow bottle that looks to me like a medical irrigation bottle, but the published installation instruction (and the Homebuilt Help video) clearly depict this is the bottle to be used. This bottle conflicts with the Rotax engine installation manual instructions and depictions. Examples include no accommodations for a vent, feed hose from the expansion tank is located on top of the bottle vs. the Rotax depictions where it is on the bottom, and the Rotax Caution that there must always be some coolant in the overflow bottle as no air should be allowed to enter the cooling system concerns me as with a top fed overflow bottle it is not possible to avoid air in the system unless the bottle and feed line were completely filled with coolant, defeating the functionality of the bottle. Is this the overflow bottle you are using and are you experiencing any issues with it?

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Your analysis seems sound to me. I wonder if you are supposed to use the top fitting as the overflow outlet and you are supposed to install another fitting in the bottom as the inlet. I would not install an overflow bottle with only one fitting on the top for the exact reasons you state. Note that you can get a very nice bottle from Rotax. You can find the part number in the illustrated parts manual.

look at the bottle , the top entry point ends up on the bottom of the reservoir, just like a medical irrigation bottle  works. venting is either from leak at the threads or a small hole at the top. attachment to the firewall is easily done with a'' bicicle cage''

.Sometimes  the simple solution is the best.  450 hrs with this set up and no problems


Thanks Harmen, did you drill a vent hole at the top?

The bottle that comes with the FWF package works just fine , just follow the instructions. Harmen van der Velden is correct.

Thanks Jerry.  The only written instructions I can find are the ones in the Rotax Installation Manual and they specify a 2.5mm vent in the bottle is required (there was a SB about this for older Rotax OEM bottles).  It appears the FWF irrigation bottle works, so I will use it but I think I’ll drill a vent in the cap.

The instructions that come with the fwf kit say to drill a hole, it is even underlined!


LIB.  I found your posted photo in the old bed mount instructions.  I have the new ring mount so those were the instructions I am using, and the ring mount instructions don’t have this portion of the bottle mount. 



I thought the rotax fwf was completely plug and play. How is it going? I have been away for a while, haven't been able to check on this site. Charlie

Hi Charlie

Progress is occurring albeit at a slower pace than I’d like. The FWF kit contains the major components one needs but there’s much it does not contain in recognition that every builder tailors this phase of the build to their particular wants. At this stage one must pause and complete a detailed design of their electric and fuel systems to include placement of every item, routing of cables and hoses, etc. and there are constraints and delays simply due to a lack of my understanding, experience, or location (or sometimes lack) of information.  But I just keep showing up and having at it every weekend!

As always you’re welcome to come visit and see the build. 


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