Too lean at full power?

[gregv]

Hi all

 

I've started the run in process on my 3300 (installed in a J200 airframe) following an early top end overhaul. I have an intermittent problem which looks like fuel starvation, but I can't track it down.

 

In short, the engine surges slightly sometimes at full power. Worst on takeoff (!) and also repeatable occasionally at full power at altitude, though less noticable. not every take off, and not every day. Seems crazy but some days no problem, last week, very noticable and concerning at 200ft, so back in the hangar until I get a better handle on it. EGTs are generally a bit high - wondering if the probes being only 50mm from the exhaust manifold has something to do with it (due to old style pipe).

 

The EGTs are interesting when the engine starts to surge and I don't quite know what to make of what it is telling me. I hope the video shows what I mean

 

 

I have checked :

 

- fuel flow - 50L/hour - seems adequate

 

- no fuel pressure gauge yet - will have soon

 

- fuel lines - replaced and clean from wing tanks through tank under passenger seat to firewall.

 

- new filter

 

- carb has been stripped, cleaned and appears fine

 

- float bowel fuel level 13mm below level of bowl

 

Anyone else seen this behaviour? Feels like a fuel issue - maybe the fuel pressure gauge will point me in the right direction...

 

Greg v

 

 

[Guest rocketdriver]

Hi Greg ..... I had a similar problem except that my engine (non Jab) actually stopped. My pumps were flowing plenty as measured but I think the pressure at the carby inlet was insufficient, leading to a low fuel level in the float chamber. I fixed it by reducing the return line flow (squashed the tube at its outlet inside the fuel tank). Result ... the engine didn't cut after a few seconds at full power and ran a bit cooler in the cylider heads than before ....

 

cheers

 

RD

 

 

[Guest rocketdriver]

just watched the video .... is this a carburetted engine? if so my take is ... temps drop a bit as the fuel mixture riches up a bit at full throttle, then rises due to a falling float chamber level in turn caused by low pressure at the inlet. ... sufficient to keep the chamber full at lower power settings but insufficient to push the fuel into the float chamber fast enough at higher power settings ......

 

Thinks ... do you have inlet filters in the carby entries? Are they clogged? ..... they could be creating too much pressure drop ..... have a look / clean them?

 

cheers again

 

RD

 

 

[Tomo]

Do you climb with the electric pump on?

 

Have you checked the tube that runs from the airbox to the carby, make sure it's not blocked.

 

 

[gregv]

Thanks for the speedy replies!

 

RD

 

This is a standard carburreted J3300, type 94 Bing carb, 255 main jet, standard Jabiru needle with a single circlip slot.

 

I agree, the behaviour seems to indicate fuel starvation at the carb, with the initial richening as the throttle opens, then the EGTs climbing as the mixture leans out due to a dropping float bowl level. No blockage in the carb intake that I saw on stripping the carb, but its coming out this weekend. I've got a replacement 'new' type 94 carb which I'm going to swap out I think.

 

Tomo

 

I've checked the tube back to the airbox and that isn't blocked or kinked. Using the electric fuel pump interestingly seemed to make no difference, both at take off and at full power in flight. The video was shot with the electric fuel pump on, I guess making the 'blockage at carb intake' theory the most likely. I usually climb out with the electric pump on.

 

I'll let you know what I find. I'm sure this is something simple that is just staring me in the face but I can't find it!!

 

I am liking the ability to monitor the CHT / EGT patterns though. The CHT monitoring especially was brilliant for sorting out the baffles over the rear cylinders to get the cooling right.

 

The AerospaceLogic displays are small, tidy and easy to read. Only real downside was the price!

 

Greg v

 

 

[fly_tornado]

The fault you expect to find on the inlet/carb on most engines, is an air leak, which makes your engine run lean.

 

 

[gregv]

The fault you expect to find on the inlet/carb on most engines, is an air leak, which makes your engine run lean.

Agreed, but to have a relatively even spread of elevated EGTs suggests a leak between the carb and the intake plenum chamber. Triple checked the rubber joiner (no cracks, clamps done up tight), and the seal between the intake plenum and the bolted on aluminium 'adapter' is well sealed.

 

 

[frank marriott]

The engine pump is rated at 60 ltrs/hr and the electric pump is also rated at 60 ltrs/hr

 

As Thomo states the recomended procedure is to leave both on to top of climb and in the circuit area.

 

If my memory is correct at 3300rpm the max fuel burn is 39ltrs/hr

 

Either pump should provide more the sufficient fuel

 

Although my input doesen't asnwer your specific question it might give you some food for thought given you state 50ltrs/hr as fuel flow.

 

I will be very interrested in your final fix as so far I have not come across this problem

 

I have had flickering of the fuel pressure light [do mot have a fuel preessure guage] on take off at daylight which is not obvious in better light conditions - but this is only the the sensor required by CASA for LSA registration - a fuel pressure guage would elimate this indicaton.

 

This is not your specific problem so fuel supply seems to be indicated.

 

I only have 300hrs on my 3300 but have never experienced your problem so something is obviously wrong.

 

Frank M

 

 

[pudestcon]

What happens when you give it a bit of choke? Do you get a reduction in EGT? Or nothing, or rough running, or what?

 

 

[gregv]

What happens when you give it a bit of choke? Do you get a reduction in EGT? Or nothing, or rough running, or what?

Little embarrassed to say I didn't try choke, or carb heat - so its on my list when I test it this weekend, all going well.

 

Not sure if the choke circuit would make much difference at full power on this carb design?

 

 

[fly_tornado]

Take a squirty bottle of soapie water as well when you head out to the air strip and give the intake manifold a coating. Assuming its an airleak, it can't be that big.

 

 

[fly_tornado]

squirt it on as a foam

 

 

[jetjr]

Interesting I have a J200 which does a similar thing, mainly cyl 2 but 5 does it too

 

I did have it cough at WOT but lifted carb level and it was OK, should check I think 13mm might be too low and this will definitely do it.

 

 

[Jabiru Phil]

Main jet seems a wee small. I upped mine on Jabiru advice to I think 2.85 or 2.95 (sorry the maintenace log is in the plane)

 

Are you getting more revs than previous?

 

Have you changed the prop?

 

 

[JabSP6]

Gregv

 

This is a bit of a long shot but i did hear that early on some of the bing carbies fitted to 6 cylinder engines had an intermittant problem with fuel starvation at full power.

 

The root cause was the size of the opening in the needle/seat. The 4 cylinders didn't require quite as much flow as the 6 cylinder so was never an issue until they fitted the 4 cylinder needle/Seat to the 6 cylinder. They ended up checking the size of the opening and found by replacing the original needle/seat with a slightly bigger seat which allowed the fuel bowl to refill slightly quicker the problem was solved. It also came up as an intermittant hesitation/missfire and only happened in certain conditions, ie temp humidity etc. I don't want to lead you up the wrong path here but may be worth a look. Not sure what the original size should be but an example would be something like 2mm hole in the seat - 4 cylinder and 2.4 mm hole in the seat - 6 cylinder. This is not the right size but Jabiru would be able to give you the correct size hole. It was something that one of the blokes at Jab told me to check when i first bought my SP6 so they know about it.

 

Andrew

 

 

[jetjr]

This is true, I had mine upgraded and it was a very difficult task. Jabiru indicated is was a very unlikely problem.

 

Parts are just a simple seat but getting it out can require special tools and heat

 

I now have whole new engine and still have the same EGT problem

 

There is an old SB on this but I had to request it from Jabiru

 

 

[gregv]

GregvThis is a bit of a long shot but i did hear that early on some of the bing carbies fitted to 6 cylinder engines had an intermittant problem with fuel starvation at full power.

The root cause was the size of the opening in the needle/seat. .................... Not sure what the original size should be but an example would be something like 2mm hole in the seat - 4 cylinder and 2.4 mm hole in the seat - 6 cylinder. This is not the right size but Jabiru would be able to give you the correct size hole. It was something that one of the blokes at Jab told me to check when i first bought my SP6 so they know about it.

 

Andrew

Thanks for the suggestion - the behaviour I have seen could reflect a restriction at that point so definitely worth knowing about. Engine is a solid lifter 3300A, #391. The needle jet is 2.85. Main jet is 2.55. Idle jet is 0.45. Std 'Economy' needle, post the problematic 'economy' kit I believe.

 

I have a 'new' carb from a 3300A sn approx 1500ish on the way, so will probably try a straight swap first and see what we get.

 

Again, thanks for all the tips guys, much appreciated.

 

Greg v

 

 

[deadstick]

Float level would be my first check, then check the cobra head has not covered the density sense port on the carb inlet, also check to make sure the ram air flap at the top of your air box is not stuck closed.

 

 

[gregv]

Some feedback. Again, thanks for all the suggestions from more knowledgable board members than myself.

 

I stumbled across a 'new' second hand Bing carb for $300 (off a 3300A serial 1525, idle 35, needle 285, main 255). Bought it, fitted it - and problem solved.

 

No loss of power at full throttle, EGTs are close enough to balanced between left and right bank, and they STAY down between 660-690°C at full throttle, fuel flow about 36-7L/hr.

 

As soon as time allows, I'll strip the old carb again and check the inlet size into the float bowl. It MUST be the problem...

 

Greg v

 

 

[jetjr]

Ill be very keen to see what you find, ive gone throught the same sort of problem and I DID change the inlet with nil result

 

I have heard of another where there was a hairline crack in carb body letting in air at WOT

 

 

[bushpilot]

The fault you expect to find on the inlet/carb on most engines, is an air leak, which makes your engine run lean.

Agree with this.. Check the sealing of the connections of the inlet pipes to the heads and especially the seal out of the bottom plenum..

 

 

[JabSP6]

Greg V

 

Now that you have sorted the problem by swapping Carbies did you strip dowm the old carby and find the cause of the intermittant problem at WOT?

 

Andrew

 

 

[gregv]

Greg VNow that you have sorted the problem by swapping Carbies did you strip dowm the old carby and find the cause of the intermittant problem at WOT?

Andrew

Sorry about the slow reply. We stripped the old carb down and went over it carefully. The behaviour we were seeing suggested insufficient flow into the float chamber at full power, despite a normal result from fuel flow testing (tail down, using the electric pump) through the float valve. About 55L/min from memory.

 

Have to take a closer look at the diameter of the float chamber inlet and any potential restriction next. Will post it when I get a chance - hoping for next week.

 

Greg v

 

 

[gregv]

Missed the important bit above - all parts looked to be in good condition when we stripped the old carb down, all appear to be adjusted appropriately, correct float level (13mm from top of bowl).

 

 

[Modest Pilot]

Check the brass plug below the butterfly valve. The one you use for the manifold pressure gauge.