[mikehallam]

Last week I flew my Rotax 912, 80 hp Rans S6--116 for 5 hours - in three stages - and recorded the regular 13 3/4 litres/hour at a modest 2,600 rpm & 88 air mph.

A few days later I made my return trip but was astounded that the hourly rate had shot up by at least 2 l/h, yet temp's and pressures all looked good & the engine ran smootly.

Since then I've check all fuel piping for security & sign of leaks, ditto the gascolator and run it cowling off too. Still no evidence.
Today after carefully dipping the wing tanks I did the 35 minute run, Sussex to Sandown, then back & the consumption remains far too high at approx 16 3/4 l/h.
All I can imagine, if not losing fuel overboard, is a very rich carb, but it continues to start & run very nicely.

So..... Could anyone produce some experience or concrete ideas to assist my search, please ?

[Charles Hunt]

Plugs out, see if one bank is running noticeably richer than the other? Leaking float on one side leading to rich running?

[mikehallam]

Excellent thought, it had crossed my mind & may have been next , it will now definitely be done; thank you.

Plus any more ideas out there ?

[G-JWTP]

Look on the bright side, you could have an IO540.

G-JWTP

[GrahamB]

Look on the bright side, you could have an IO540.

That would be one hell of an S6.

[PeteM]

Have you complied with the float level or weight check? Quite possible one of the floats ahs absorbed fuel - richening the mixture

[cockney steve]

Fuel pump diaphragm? Is your oil level increasing? AIUI, the pump is cam-driven......It could be a possibility, that, as this engine is dry-sump, any fuel passed by a perforated diaphragm, is flashing-off in the crankcase and being ejected through the breather. (assumes an atmospheric, not a closed -circuit breather.

Relative to "normal" consumption, this loss is a small proportion and would probably have little effect on running.

[Miscellaneous]

Last week I flew my Rotax 912, 80 hp Rans S6--116 for 5 hours - in three stages - and recorded the regular 13 3/4 litres/hour at a modest 2,600 rpm & 88 air mph.

I'm confused here Mike, can I assume 4600rpm and the 2600rpm being an error?

My first instinct would be to run the engine faster for a comparison, I'd consider running a 912 at 4600rpm bordering on being too slow.

[TheFarmer]

He must mean 4600.

2600 is pretty much idle.

[Charliesixtysix]

Float(s)

[ivor.phillips]

I would never run my rotax 912/4 below 4800rpm because you will get a reduced fuel consumption but to the decrement of the reduction gearbox , so what you save fuel wise you will put back in gearbox overhaul,
There are many service bulletins on the Bing carburettors so check the serial numbers and comply with any that are applicable,

[mikehallam]

All very helpful & will be done ( it was 4,600 rpm outbound and 4,800 back).
Mr. Rans own manual indicates 4,800 cruise, but it was very hot over central France and reduced rpm were my way of keeping temperatures not too far above 100C.

[Shoestring Flyer]

Have you adjusted or changed the propeller pitch angles recently? I assume it is fixed pitch, ground adjustable?
Maybe worth checking if the pitch angle of the blades are all correct and the same.

[Boxkite]

Last week I flew my Rotax 912, 80 hp Rans S6--116 for 5 hours - in three stages - and recorded the regular 13 3/4 litres/hour at a modest 2,600 rpm & 88 air mph.

I'm confused here Mike, can I assume 4600rpm and the 2600rpm being an error?

My first instinct would be to run the engine faster for a comparison, I'd consider running a 912 at 4600rpm bordering on being too slow.

And an S6-116 should cruise happily at 105mph. Maybe he's talking prop speed?

[Miscellaneous]

@Boxkite see above, Mike acknowledges it should have bee 4600rpm and the 2600rpm was an error.

At what engine speed will the Rans do 105mph?

@mikehallam I don't know that a single 35min is a sufficiently long flight to accurately determine fuel consumption of 16.75 ltrs per hour? It does seem consumption has gone up and some investigation is necessary (I'll be interested in hearing the outcome), but I wouldn't be convinced on a single flight.