Re: Connection of the Float Chamber Venting Lines to the Rotax Airbox
by HKK » 2 weeks ago
Hello Michael,
1. The vent hoses should not be made of PVC. As has already been written, PVC becomes hard and for a wile it sits no longer firmly on the nipple of the float chamber.
2. Viton or Tygon (permanent materials) are the first choice but very expensive, I use simple silicone hoses - permanent also..
3. The position where the hoses end should be a flow- and pressure-neutral zone - according to ROTAX - but see also #8.
4. The vent hoses of both carburetors should end at the same pressure-neutral zone. I have joined my hoses with a T-piece so that the same pressure is in both float chambers.
5. The position in the engine compartment where the joined vent hoses end is important, I spent a long time looking for a good neutral point/zone.
6. The pressure in the float chambers is crucial for the mixture, with lower pressure you can lean the mixture and vice versa, that's why the position of the float chamber ventilation is so important.
7. My experiments showed that changing the end of the float chamber ventilation has a significant influence. when changing from a pressure-neutral point to an airbox/air collector, the engine stopped immediately at idle, the lower pressure in the air collector leaned the mixture too much. You then have to readjust the idle mixture.
8) I don't want to criticize the ROTAX recommendation to connect the float chamber ventilation to the airbox, but there is no neutral zone in the airbox. There may be advantages to connecting to the airbox because the mixture will generally be somewhat leaner (less consumption but possibly temperature problems???) - please ask the experts for feedback on this.
9. The length of the ventilation hoses was not relevant, at least for me, whether shorter or longer, I did not recognize any effect.
Greetings
Re: Connection of the Float Chamber Venting Lines to the Rotax Airbox
by Michael Tucker » 2 weeks ago
Hi HKK,
Thanks for your suggestions and comments. I tend to follow the Rotax manuals unless I am convinced (after much research!) that there is a better way. I would like to see other readers' feedback on your items #7 and #8 above. I do have silicone hose, but my understanding is that it is not recommended for fuel/fuel vapor hoses.
Mike
Re: Connection of the Float Chamber Venting Lines to the Rotax Airbox
by Sean Griffin » 2 weeks ago
Hi Michael,
Whatever tube/hose you use will have a service lifespan. The tubing should always be of fuel resistant/compatible material, as will be continually exposed to petrol fumes and even raw fuel. The PVC (& similar) will have a short service life, likly to go hard relativly quickly. The Tygon/Viton etc are likly to do at least the 5 + year span - nothing should be assumed to be "permanent" - check condition at every service, replace as necessary.
For the length & size of tubing you will need, the cost is minimal. You may be able to get what you want through your local mower/chainsaw shop.
The bulk of HKK' comments have little relation to the use of an airbox, which should delivering identical air pressure to both carburettor intakes.
The Float Bowl Vent hoses must, for best carburettor performance, terminate in the SAME AIR PRESSSURE (or as close as possible) as the carburettor intakes pressure. I do not know what is meant by a "neutral zone" in this context .
When an air box is used, the Vent lines are, by design, terminating in the same air pressure as the carburettor air intakes.
IF not using an airbox, go with the Rotax recommended location (tuck the tube into the bowl clip) OR go through a prosses similar to that of HKK's OR as some pilots have done - terminate the hose on the surface of the filters or within the filters (I have seen photos of the tube being routed into the end of the conical filters).
NOTE: The ID of the hose needs to be such that it is tight on the nipple/barb. Tygon ID 3mm is very elastic/stretchy, so will usually accomodate a 4mm OD barb. If in doubt, get a small test length (30-40mm) and try it for size.
😈
Re: Connection of the Float Chamber Venting Lines to the Rotax Airbox
by Michael Tucker » 2 weeks ago
Hi Sean,
I have a Rotax air box, so I followed the installation manual and connected the carb vent lines to the nipples on the air box. I used Viton rubber hose. I agree that all hoses (regardless of material) need to be inspected and replaced based on condition.
Thanks,
Mike
Re: Connection of the Float Chamber Venting Lines to the Rotax Airbox
by HKK » 2 weeks ago
Hi Michael and Sean,
on the subject of pressure neutral zone see "Installation Manual 912 Series Ed3 R0 dated Jan. 2021 73-00-00 page 18" in the blue ATTENTION box it says "...Pressure differences between intake pressure and pressure in the carburetor
chambers may lead to engine malfunction due to incorrect fuel supply. The float chamber venting lines have to be routed into a ram-air and vacuum free zone or into the GENUINE ROTAX® airbox,..."
A ram-air and vacuum free zone = pressure neutral zone.
Even more important is the carburetor synchronization, the position of the venting hoses is only a small but important side issue.
I have summarized this just for myself. Do not copy, because not approved by Rotax. The Rotax instructions only make sense to me if they include an explanation. In many cases, the Rotax instructions lack the why and a deeper understanding of the physics - Rotax has this, of course, but it is not explained. This is also not possible in some cases, otherwise the manuals would be thicker than the Bible. Not that the Rotax instructions are wrong, but if I don't understand something, I go in search of the why, only then do I act accordingly.
So don't copy, it's all nonsense and dangerous " https://hkk-edv.de/content/Carburator-Synchronisation.pdf " .
On your own risk read the section "The topic of a pressure-neutral area".
Greetings
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