JaB 3300 CUTTING OUT ON TAKE OFF

[Chatterton]

Engine is momentarily - one or two seconds at a time - cutting out on climb out at about 100 feet - i.e. after several seconds. I back off power and it clears after 30 seconds or so and then runs normally, but it is a long 30 seconds ! Only happens on initial take off, not on a go around. Full throttle runs before take off are normal and I can't replicate the problem on the ground.
New manual pump fitted, and now electric back up with pressure regulator.
Any advice / bright ideas, please ? Rob Chat.

[daleandee]

Engine is momentarily - one or two seconds at a time - cutting out on climb out at about 100 feet - i.e. after several seconds. I back off power and it clears after 30 seconds or so and then runs normally, but it is a long 30 seconds ! Only happens on initial take off, not on a go around. Full throttle runs before take off are normal and I can't replicate the problem on the ground.
New manual pump fitted, and now electric back up with pressure regulator.
Any advice / bright ideas, please ? Rob Chat.

Unfortunately all the smart people are at AV so I'll chime in and see if I can give some sage advice.

My best advice is not to fly the thing until you get it fixed. I know you said the problem won't appear on the ground but flying an aircraft with a known deficiency is not only illegal, it is unwise (read stupid) and could get you killed. Not trying to be harsh but rather to keep you alive. Your report that it doesn't do it on a go-around suggest that it is likely a problem related to too much heat under the cowling.

What carb do you have? If it is an Aerocarb (now called an Aero Injector) they are world famous (as are most all slide type carbs) for stumbles and burps because of vapor lock and air bubbles in the fuel line that go through the carb instead of out a vent as on a float bowl carb. Some pilots live with these burps but personally I wouldn't and I don't. The vehicle I drive to the airport on the ground don't do that so it is not acceptable to me that my airplane do it.

Heat shielding all the fuel lines & the gascolator (some suggest to remove it but I disagree) and adding a cooling blast tube to the gascolator can help. Rerouting fuel lines away from heat sources can also help.

Also be aware that if you are using a slide carb they do not like much fuel pressure and/or ram air induction.

Perhaps you can give some more details as to what the CHT's, EGT's, and fuel flow look like before, during, and after these events. What is your fuel pressure reading?

Keep in mind that this is occurring with the nose up while climbing. Have you done a gravity fuel flow test and made certain that there are no obstructions to fuel flow at high angles of attack?

Hope you get this resolved because there is not much worse than flying a plane you can't trust. Been there, got the T-shirt & the mug ...

[BRS]

Does your avionics have an engine monitor recording?
It would be interesting to take a close look at the relationship between things like RPM and engine cut-out; or FF & cut-out. For example does it cut every time you reach certain engine conditions. What is your max recorded fuel flow? Could it be you are leaning out as the aircraft accelerates after the initial lift-off?

For the longest time on my Revmaster (w/ AeroInjector) I was getting bad stumbling on take-off that would clear half way down the runway. Got to practice my abort proceedures. In the end it turned out to be that I was just running too lean. Now, my full rich, full throttle setting is rather rich. The engine behiaves very well. I do have have to lean after the initial climb else I'll be burning one or two extra gph. Which is a lot on an 85hp engine. Oh, btw, this would mostly be evident on the first flight of the day.

[Chatterton]

Fully agree Daleandee. Am a devout coward and have grounded the plane already, hence my post. Problem is not being able to replicate it on the ground. Carb is standard Bing for the engine. Don't think it is a heat problem on initial take off as would be hotter under the cowl by the time I got to a go around. Fuel pressure 3 pounds and flow is good. T's an p's all ok, but don't have a flow meter for use in the air. Pulling about 2900 on climb out.
Have added info requested by the other poster. Will try raising the front wheel to take off angle and see if that shows up anything.

Thank you both for the i input.

[rbarber]

I have seen similar issues caused by float bowl fuel level issues in other carbureted aircraft. May be worth an investigation to see if the carb is getting flooded or starved during the climb (nose high) phase. The Jabiru aircraft that the engine and carb are designed around does not climb at the same angle as a Sonex by a noticable amount. Try a shallow climb? Put the carb on a bench and see if the float opens at a steep angle? Etc.

R.

[sonex892.]

Is this test flying a new aircraft or a new problem on a flying aircraft? Have you called Jabiru to run it past them?

It sounds like it could fuel mixture issues to me.

Smooth filtered air entering the bing is crucial.
On my first Jabiru powered aircraft, a Sonerai, the first flight was nearly a disaster with the engine spluttering after getting airborne. The engine had checked out ok on the ground before first flight. I had used scat tubing to connect the air filter to the carb and that messed up the mixture metering. The sense port for the mixture is in the inlet throat of the bing. I redisigned the air inlet arrangement and relocated the airfilter and problem solved.

Do you have the balance tube connected from the carb to the clean air section of the filter?
I accidently left this tube disconnected one day after maintenance and I found out just how important it is. It caused the engine to stop on climbout. After pulling carb heat the tube flopping around in the cowl then balanced with the warm air and it came to life.

I have seen using an airfilter directly to the carb cause problems. I asked Jabiru Australia about using this method and they advised against it. But I have seen others use it and seem to be ok. This would still need the balance tube is fitted.

As mentioned earlier it could be the float level. That is pretty easy to check.

[DCASonex]

In addition to all the other good comments, if you are using A Bing carb and the problem is more common in hot weather, direct a small cooling air jet to the Jabiru fuel pump. Made a huge difference in stability of mine, and had a comment back then from someone in the Southwest that he could not fly without that. Also, if using an inline auxiliary electric boost pump, check the wiring to that. If you have not already noticed, when you shut that down in flight, fuel pressure will drop precipitously, due to the auxiliary pump having compressed the diaphragm of the mechanical pump, but recovers after a few seconds. If your auxiliary pump were to cut out briefly for some reason right at moment of maximum fuel flow requirement, could see the result you are seeing.

David A.

[Murray Parr]

You could have a fuel flow problem as the fuel load would be greater on first take off. Given that it clears after backing off the throttle suggests that the fuel system gets a bit of time to refill the float bowl. Also, check the float level is high enough and as others have suggested, make sure the needle and seat can still open on high angle of attack.

[peter anson]

Lots of good suggestions here so I'll just mention one that I experienced. I have the standard mechanical fuel pump on the engine plus an electric back-up. I tend to leave the electric pump on all the time but in the process of trying to track down another problem I turned the electric pump off. The engine ran fine at cruise power but as soon as I tried full power it cut out, indicating a faulty mechanical pump. When I inspected the pump I found a couple of small cracks in the operating arm so that it had bent slightly reducing the stroke of the pump, so it was supplying enough fuel for cruise but not enough for full power. OK, you have fitted a new mechanical pump so that's not your problem but when I received my new pump from Jabiru it also came with a new cam follower pin that is about 1.5 mm longer than the old pin. Seems like not enough pump stroke might have been a bit of a problem.

Peter

[Chatterton]

Many thanks for the various contributions and suggestions. Will keep looking and report back when, hopefully, I've sorted the problem.