Hi Mark,
I hear what you're saying too.
You clearly know carb issues better than I do. Especially the two cycle.
And unless you turn out to be the guy who gets hurt BECAUSE someone did preventative e maintainence and did it wrong (some NTSB findings on fatal crashes HAVe turned out that way..really) I support your better safe than sorry stance.
There are many things I have done and will do on our plane in that spirit.
And I DO value reading when rotax says about when to do what... If for no other reason that whenRotax says rebuild system X at 600 hours it gives me confidence that it's very unlikely to need attention any SOONER that that . ;)
That said, although I would be very unhappy to try to limp to a landing on one carb, it's still not in the class of risk of, say, ignoring a rattle in the reduction gearbox, a fuel line clearly showing its age,etc.
And as for "Stick to what the manufacturers recommend, they are speaking from experience.".....
Yes Rotax has brlllliant careful engineers looking at this stuff making these calls and recommendations. But Remember they are also speaking from a liability stance: Tha is the attitude that "we'll never be sued for telling people they have to service the carbs every 200 hours even though
We believe it,s extremely unlikely that's needed in less Tha 1000 hours...but let even one of our 30,000 owners claim he got hurt because his accident might have bee prevented by rebuild at 200 and we could be in a multimillion dollar lawsiuit." And not meaning to be cynical ... There IS a nice profit margin in selling complete rubber seal/gasket/parts kits if you can get 30,000+ owners to replace every single rubber part every 5 years despite the fact that some are under use,temperature, environment, and total hours that age those components differently.... And the risks of deterioration of some rubber parts would not be high risk.
I just don,t think there's anyway of getting around the reality that "an ounce of prevention is worth around or cure" and "better safe than sorry" AND "if it ain't't broke don,t fix it" all have I validity and need constant judgement calls...even in the face of Rotax guidelines.
The two carbs aren't there as a back up. If one had a problem the chances are you won't be limping to your landing site, your engine will shake it's self to pieces. It won't run on one carb.
If you don't know how to do something on the engine, don't do it yourself or bury your head in the sand and say
" well it will probably be OK, it is just Rotax trying to cover their arse or make some more money out of me",
get someone who knows what they are doing to sort it.
You owe it to your passengers to be responsible and to listen and do what people with a lot more experience and understanding recommend i.e.Rotax.
Through a little experience, I am with "correct preventative maintenance" all the way.
By the way.... Mind telling us approx what the each Bing carb rebuild kit cost you when you did yours?
I think I may remember seeing each was over $400. Surprising as I'm pretty sure last time I did a rebuild of a carb (was a two barrel one on a auto) the kit was under $40. The large difference in price , IIRR, was puzzling. Did you have to use a full kit or just selected parts??
There are different carb rebuild kits from different Rotax distributors. They unfortunately are not standardized. You may get a kit of just "O" rings for around $40 from one or a kit with everything inside the carb (more or less) for around $350. These kits are for two carbs. A good complete kit from CPS with everything except the floats is $255. There is nothing in writing in the Rotax manual for a mandatory carb overhaul, just the rubber parts. I am a believer in carb overhauls at some point. I do them on a regular basis in my shop that are sent from all over the US. The overhaul doesn't take long, is straight forward and just needs a little attention to detail to make the carb brand new again.Carb parts are in motion inside and do wear. "O" rings do crack and split and get flattened out and then can leak (#1 issue). Metal tolerances on moving parts do wear. If you let carbs sit for long periods of time with fuel in them they will start to clog especially the very tiny offices in the idle jet system.
I just did a set yesterday where someone had a bowl gasket leak and used silicone to try and seal it instead of replacing the bowl gasket. That silicone migrated throughout the carb and caused restriction issues. Never use silicone around your fuel system, it is not compatible.
If you call a distributor and order a carb rebuild kit ask what is included. Kits are a nice way to go, but if you needed just a certain part it can be ordered that way.
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
You wrote --
"I just did a set yesterday where someone had a bowl gasket leak and used silicone to try and seal it instead of replacing the bowl gasket. That silicone migrated throughout the carb and caused restriction issues. Never use silicone around your fuel system, it is not compatible."
That's an interesting coincidence.
A month ago we pulled the bowls on our carbs for routine check. Found the same issue you described from previous owner having used clear silicon caulk (or maybe it was gasket sealer) apparently to address the issue of a cracked bowl gasket on one side instead of replacing the gasket.
I'm sure he thought his fix had worked OK because the bowl was not leaking.
In our case the silicon had not yet caused problems but it was very clear that small gobs of the silicon were at serious risk of breaking free and floating elsewhere in the carb causing problems.
FWIW, IMHO putting a tiny bit .... very thin layer .... of silicon _grease_ on of surface of gasket (helps to get gasket stay seated in groove while seating bowl and presumably better seal) is acceptable. (Not as a substitute for replacing a gasket.) Your thoughts on that?
Also curious if you have any opinion of the third party silicon rubber bowl gaskets, which some claim are superior to the standard Bing cork ones?
...................
Contrary to the impression my exchange with Mark may have left, I too am a believer in as you say carb overhauls at some point. More so obviously on an aircraft than, say, an ATV.
Just don't fully buy into the idea that anyone who doesn't do everything Rotax recommends and do it on the Rotax recommended tach hours is irresponsible and incurring significant or necessarily any risk.
Do I take it right that you aren't hard core about "do it every 200 hours" and more likely to make your call on a case by case basis?
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I assume that, unlike whe just checking the bowls and r&r bowl gaskets, the carbs have be completely removed from the engine, and tha in turn means we'd expect to have to do a complete mechanical and dynamic carb sync after reinstalling them. Right?
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