Rotax Parts Supply Problems

[danny_galaga]

By and large talk of baffles etc is academic on many Rotax installs since they seem to be doing fine 'naked'. I have however seen at least one set up with fibreglass shrouding similar to the tinware on the big boy air cooled engines. I could have sworn it was an Aeroprakt Foxbat but I'm not seeing that when looking on the line.

 

Ibob, my engine has the same sensor locations. Initially I had wired the front sensor as that seemed easier to wire at the time. But it was suggested to me that will always show cooler and so it's better to hook up to the rear sensor.

 

Now I'm thinking I may emulate bluesadventures set up and hook them both up to a switch 🙂

Edited December 25, 2023 by danny_galaga

[skippydiesel]

2 hours ago, RFguy said:

............I'd also like to see some blast air cooling around the carb bowls since that sit above the exhuast headers (or some heat shields )  Skippy - are you taking care of any overflow from the carbies in a good fashion ? floats since, or seats come out, carb floods, fire ensues.

RFguy, 

 

I have just put my Sonex back in the air, after many months of modification work, mainly to the cooling system. During this period I sent my carburettors to Flood/Wal to see investigate the ongoing small, float bowl breather, fuel leak. 

Wal diagnosed faulty float bowl valve seats. He replaced the seats (something other Rotax experts believed impossible/not done). He also checked the float bowl level, etc and gave the carby's a clean bill of health.

Unfortunatly, within the first flight hour, I caught about 50mm fuel in the temporary breather catch can. Now about  about 3 Hobbs  hours later, I am recording about 25 mm/hr discharged from the float breathers. Don't know why the system is sealing better after a few hours or if it will continue to improve . I have advised Wall - yet to get a response.

FYI - The engine runs beautifully - possible but unlikly causes: tailwheel slope, engine movement at shut down, "boiling" of the fuel in the float bowl, after shut down??????????

 

On the cooling of the carburettor/float bowls;

With the changes to my cooling system, the undercowl temperature has gone down about 10 C to 30-33 C / OAT 18-20 C, which I may try & improve/reduce. To put this in context, my last 912 ULS aircraft had a fairly consistent undercowl temp of Ambient +10C and non ducted carb air inlets - ran just fine.

To reduce radiant heat to the carbies, I have a home made partial head shield on each rear exhaust and I have an aluminium heat shield/deflector below each carb. No idea if they work (to reduce heat impact on the carbs) but like the idea.

The carbs are fed fresh/ambient air, through a "horn" from the right side cowling inlet, into a home made airbox (copy of Rotax design but may not be as sophisticated).

[turboplanner]

52 minutes ago, skippydiesel said:

Wal diagnosed faulty float bowl valve seats. He replaced the seats (something other Rotax experts believed impossible/not done). He also checked the float bowl level, etc and gave the carby's a clean bill of health.

Unfortunatly, within the first flight hour, I caught about 50mm fuel in the temporary breather catch can. Now about  about 3 Hobbs  hours later, I am recording about 25 mm/hr discharged from the float breathers. Don't know why the system is sealing better after a few hours or if it will continue to improve . I have advised Wall - yet to get a response.

 

Check the tank height distance above the float bowl fuel height vs other aircraft with the same engine. Too much pressure will alow leakage which will get less as the seat wears in.

[RFguy]

Sean, just FYI...... There have been a few Jabiru 230's pump the engine bay with fuel following the float valve seat coming out when subject to huge vibration bumping when the carb was touching the firewall. In J230s , depending on the rubbers and length of the mount tubes, the carby can be millimeters or closer to the firewall, (some are closer than others) and contact can occur, and this is known to dislodge the float seat  (pressed in) and the engine bay fills with fuel, as does the airbox wquite a mess and at least one fire and loss of aircraft

Just FYI- may not be anything to do with your kit

-Glen.

Edited December 25, 2023 by RFguy

[skippydiesel]

On 25/12/2023 at 2:55 PM, turboplanner said:

Check the tank height distance above the float bowl fuel height vs other aircraft with the same engine. Too much pressure will alow leakage which will get less as the seat wears in.

I haven't measured the Max/Min fuel level as it relates to he carburettors however below is a photo - the main tank is directly behind the, yet to be painted (grey) cowling,  same shape at about the same orientation. The engine is of course below the cowling. By my estimate the carburettors would be about mid way on the tanks vertical orientation - so not much positive head, when tank full - dropping to minus head, as tank nears empty. The mechanical pump delivers a fairly constant 3 psi fuel pressure, well inside the Rotax Max of 5 psi. The Boost pump lifts the the pressure to about 4.5 psi (Note: pressure numbers from memory would have to check notes for greater accuracy).

 

I hope you are correct about the "get less as the seat wears in" - this did not happen within the first 50 hrs of engine running.

 

Edited December 26, 2023 by skippydiesel

[IBob]

Skippy, I can see the problem right there: someone has made off with your front wheel.......(

[turboplanner]

10 minutes ago, skippydiesel said:

I haven't measured the Max/Min fuel level as it relates to he carburettors however below is a photo - the main tank is directly behind the, yet to be painted (grey) cowling,  same shape at about the same orientation. The engine is of course below the cowling. By my estimate the carburettors would be about mid way on the tanks vertical orientation - so not much positive head, when tank full - dropping to minus head, as tank nears empty. The mechanical pump delivers a fairly constant 3 psi fuel pressure, well inside the Rotax Max of 5 psi. The Boost pump lifts the the pressure to about 4.5 psi (Note: pressure numbers from memory would have to check notes for greater accuracy).

You have to meet the rotax 5 psi max so pumpwise that looks OK.

 

However, the higher the head the more the gravity flow pressure on top of the pump pressure.

If that horizontal line along the fuselage/bottom of the Canopy is level in flight, you measure up from the line to the top of the tank and down (if the carby is below) from the line to the split of the bowl from the carby and that's your head. Compare that with the head that Rotax specify (or if they don't have it the head on a Jabiru etc. That will tell you if you're running a higher head pressure than others.

 

If the bowl is overflowing, it's likely to be in the hangar over time.

 

10 minutes ago, skippydiesel said:

I hope you are correct about the "get less as the seat wears in" - this did not happen within the first 50 hrs of engine running.

That only applies if there is an initial weep on assembly of a new carb or after new seats.

 

All of this presupposes you set the float valve to the Rotax recommended fuel height in the bowl. On Mikunis I used to very carefully unscrew the full bowl and then measure down from the top machines face of the bowl with a vernier. I managed to spill more fuel than I measured so I bought and extra bowl drilled a hole and fitted a nipple at the botto and ran a piece of clear plastic tube up the side and could measure accurate fuel height with the engine in place.

 

[skippydiesel]

Lots of thought provoking comments - my thanks!

 

Of course the lack of a nose wheel makes float bowl fuel level checking a tad more difficult - have to place tail wheel on stand each time. I am hoping that Flood/Wall has done a good job and checked/set the float height to Rotax specifications.

 

I stand to be corrected, as I view the carb's, the float bowl vent, is on the up side, when my aircraft is on the ground ie a tail wheel configuration, should be less prone to ejecting fuel.

[IBob]

Strictly speaking, that aint a float bowl vent....it's not for venting, it's for pressure balancing in the float bowl....but I'm sure we've had this conversation already........)

[facthunter]

Yes. If the end of it is in the wrong place pressure wise, the mixture will be affected. Just another little idiosyncrasy to deal with. .Nev

[skippydiesel]

17 minutes ago, IBob said:

Strictly speaking, that aint a float bowl vent....it's not for venting, it's for pressure balancing in the float bowl....but I'm sure we've had this conversation already........)

Intended or not, the vent performs both functions ie pressure balancing & venting of fuel.

In my aircrafts case, there is a small amount of fuel being vented - no idea why or when.

When the engine has been ground run (cowling off) during pre test flight run ups/carb balance/etc, I have not seen any fuel from the vents. Bear in mind that the longest ground run of this nature has only been about 20 minutes or so. Also ground runs (max vibration) have not usually had the engine at Max rpm for very long, compared with a take-off/climbout scenario.

In theory, the float valve should be keeping the fuel level well below the vent. I wonder could fuel "boiling" from radiant/convection heating, after landing/ at shutdown be the problem.

Others have speculated that inflight ram air  and or prop pulses, pressurising the air box may have something to do with the spillage but then the float bowl vent tubes are plumbed into the airbox, which should result in the same pressures ("balancing") within each and no leakage.

[facthunter]

IF there's any "boiling" in the float chamber the density of the liquid reduces and the floats will sink and let more fuel in. Nev

[skippydiesel]

2 hours ago, facthunter said:

IF there's any "boiling" in the float chamber the density of the liquid reduces and the floats will sink and let more fuel in. Nev

Well there you go - could be the problem.

 

How to prove one way or tuther? If it is happening it is most likely at shut down - minimal air flow max conductive/radiant heat from engine exhaust system. Should it be so, how to stop it happening (already have pretty comprehensive  radiant heat shielding)???

[danny_galaga]

You may have shown it before, but what sort of heat shields have you got between the carby bowl and the exhaust?

My plane hasn't flown yet, but I suspect my heat shields, which are just stainless steel will pretty much only work well while flying- it's directly absorbing heat from the exhaust and then moving air is taking that away from the shield. I have a feeling my carbs are going to to get quite toasty once landed and parked. If so I'll probably try and attach something to it to insulate better.