What's new

912-914 Carbs

ginge

First Solo Pilot
#1
I'm not sure if I've put this in the right place, so if the mods wish to move it please do.
Reading my LAA mag today I found something that may be of interest to the 912 drivers amongst us. It seems that Feb. 2019 Rotax issued a service instruction which among other things seemed to introduce a re-jigged float.
It seems that so far there have been no known problems with the new floats so hopefully getting away from the sinking float problems that have bedeviled many over the last few years. The new one are dubbed "189 floats" . Those that have been affected the service instruction is no 912-032, Running modificationsof the Bing Constant Depression Carbureter, and may be worth a look. Also for you 912 folk it could be well worth reading ,Rotax Service Bulletin SB-912-074, which gives details of what is required to get free replacments for faulty (sinking) floats.
I hope that helps some folks
 

Dave Morton

Student Pilot
#3
A quick fix with the old or old new type floats is to completely dry out and rub down with emery cloth and weigh, it's only a get out of jail fix but does work.
The cost of the floats are extortionate and it seems impossible to source alteratives.
 

Dave Morton

Student Pilot
#5
There was mention of a free replacement scheme, have you tried that.
Luckily the new floats I purchased around 18mths ago are still OK, weighed them around 4mths ago, the quik fix method was used on a skyranger whist on a trip to the Scillies and new floats were purchased when we arrived back but I'll pass on the info regarding replacements, cheers
 

Dave Morton

Student Pilot
#6
After reading the pdf I think the floats affected are of part no 861185 and 861188.
My floats are 861184 but thanks for the heads up Ginge.
 

Aerial

Cross Country Pilot
#7
Dave, I'm pretty sure the floats problem doesn't affect my aircraft but as a matter of wider interest, can you tell me the whys and processes involved with the emery cloth temporary fix please?
 

Dave Morton

Student Pilot
#8
It's simply a case of rubbing the floats with the emery cloth to lighten, can't remember exactly the weight they should be (4g) springs to mind but no need to be that precise if your on your travels.
Removing the float bowls and looking at the floats can reveal if they're sinking or sometimes a rough running engine at low rpm can be an indicator.
The material of the defective floats can absorb fuel making them sink so this is a get you home fix only
 
#9
This lark with 912 floats has been going on for years. It seems every so often Rotax issue a SB concerning problems with floats and previously when they have required changing they cost something like £100 per pair. Maybe this time they have decided it's their error so they should give free replacements. Fortunately my engine isn't in the affected serial number range.

It occurred to me that trying to weigh something as light as these floats is not readily within the capabilities of most of us - I don't know if kitchen scales will have the range, resolution or accuracy to weigh 4 grammes, but decent lab scales would probably be needed. As an alternative to weighing, surely the buoyancy should show whether they are at the correct weight or not. Depending on the weight, the float should sit at a certain point in petrol, so making a mark at the correct point on the float then putting it in a filled chamber should show whether it's correct or not or am I being too simplistic?
 
#10
Not too simplistic at all, the weight is simply what a new correct float should weigh and is a bench mark, actually seeing the floats in situ and ongoing monitoring is as you say the best way of ensuring all is good.
The scales to accurately measure such small quantities are readily available from individuals loitering around street corners easily dentified by their hoodie...
 
#12
Yes, it occurred to me certain small pocket-size scales for illicit substances would seem to be ideal, but whether they or those from fleabay would have the true accuracy, I doubt it.

I also saw on fleabay there are float level-gauges for Rotax carbs fitted in hovercraft and go-carts etc, so why not for 912's?

On the Rotax owners forum there is ongoing consternation about the float problem and from a brief reading of it last night before this server went off-line, there have been from memory 3 or 4 versions of 912 floats issued by Rotax. It mentions the floats should be weighed as pairs and the maximum is 7 grammes per pair. It also mentions that if Rotax replace the floats FOC, the warranty period on the ORIGINAL floats only applies. Another comment seems to suggest Bing (the carb manufacturers) are to blame for the problem, but that's bunkum because certainly in English law your contract would be with the supplier as the carbs are supplied as part of a Rotax engine.

All in all, it's a right shambles and it's about time Rotax were taken to task over it as they're taking the mickey! If their plastic floats aren't up to the job, maybe they should try brass ones that don't absorb fuel irrespective of how much ethanol is in it.
 
#13
I also saw on fleabay there are float level-gauges for Rotax carbs fitted in hovercraft and go-carts etc, so why not for 912's?
The vast majority of floats for Bing carbs are fitted to arms, unlike the ones in the Bing carbs in Rotax engines which float on vertical guides.

If you can post a link, I'm sure our more experienced part scrutineers will eagerly give them the once over...

I have a BB Trya SSDR with a Bing 54, so I'm interested in the long term...
 
#14
The vast majority of floats for Bing carbs are fitted to arms, unlike the ones in the Bing carbs in Rotax engines which float on vertical guides.

If you can post a link, I'm sure our more experienced part scrutineers will eagerly give them the once over...

I have a BB Trya SSDR with a Bing 54, so I'm interested in the long term...
Yes, I found the floats in the Bing carbs on BMW motorbikes use a linked float arrangement. When I refurbished the carbs for my 912 Quantum, I bought most of the identical parts from a local bike shop for a fraction of the price. They couldn't supply the separate floats though, but if they could have I bet they would have been a tenth of the price of ones from Rotax and identical. Maybe parts for your carb are used on something else? I doubt it's unique to the engine on your aircraft.

Which link for what parts?

Here's a link to the thread on the Rotax forum:

https://www.rotax-owner.com/en/912-914-technical-questions/5646-sinking-new-style-carb-floats
 

ginge

First Solo Pilot
#15
(Much clipped) If their plastic floats aren't up to the job, maybe they should try brass ones that don't absorb fuel irrespective of how much ethanol is in it.
One thing mentioned in the article that I refered to suprised me, that was
(clipped from LAA mag. January 2021, Safety Spot) "for reasons that never been properly explained occasionally a carbureter float will absorb fuel and sink. There never seems to be any specific reason, it doesn't seem to relate to the type of fuel used and "time in service" seemed irrelevant"
Before reading that I had also assumed that the villain of the piece was, as so often the dreaded elhanol. It seems that it may not be so. Another intriguing thing was that why only the constant depression carbs as fitted to the 9 series Rotaxes when a very similar, just larger as far as I can see, carb is fitted to a Jab. Also the floats fitted to all the Rotax two strokes appear identical to those in the other Bing carbs, Odd innit.
 
#16
Hi Ginge,

Interesting info. I think think I'm going to give my local BMW motorcycle dealer a call tomorrow to pick their brains about floats and what experiences they've had. Bing carbs have been on BMW bikes for scores of years so I would expect they might have seen such issues.

I'll post what I find out on here.
 

Antoni

Cross Country Pilot
#17
"Drug" scales, even from China are accurate in my experience (airgunning). They might not last long and they might chew their batteries even when off, but according to my accurate 50 gram calibration mass (from work), all of them have been respectable regarding accuracy.

But just think abt how sensitive they are and don't sit on them and expect them to survive. Especially the .01 gram resolution jobs.
 
#20
I dragged my 912 Quantum out today for an anti-det engine run. I checked the float-bowls for water and muck before startup and realised the brass pins on the sides of the floats are just about level with the surface of the fuel. Surely, that should be a quick visual indication of the state of the floats? I'm guessing if they've absorbed fuel, they would sit lower and as I've never had any problems with my floats and they're outside the range of the affected engines, what I've seen is correct.

Maybe learned members could enlighten me?

No, I didn't have to travel to the airfield in case you were wondering😁
 

Top