My Jabiru (sn - 590 approx) dies if I advance throttle too quickly (ground running), I have to go very slowly with the throttle and tease it passed the point where it would cut out and then it will rev almost to max. I've renewed everything (filter, hoses, jets, floats, diaphragm, inlet rubbers, inlet gaskets, re-sealed plenum etc) on induction suspecting air leaks but no luck. I've fitted a second carb - same symptoms... So I gave up looking for air leaks, removed air filter, put a cone filter direct on carb and its just about perfect... Stock air filter is brand new and Jab part.
So - No filter or air box attached = fine, full RPM and stable. Cone filter directly on carb = fine. Air box attached without filter inserted = goes to full RPM then hesitates and drops back approx 200RPM. With filter in air box= stalls /dies on anything but gentlest throttle advance.... tried with and without crankcase breath hose attached, no difference. Tried with and without Aux fuel pump running, no difference. I have stripped and checked the entire ignition system and replaced one loose rotor arm.
Any advice? Does it depend upon Ram air into the airbox to achieve correct breathing?
A bit more info, MJ is 135 and NJ is 2.68. I moved the needle position to 3rd from top to enrichen and it now revs cleanly with full induction system attached (airbox, filter etc), unfortunately though it will not pass 2490RPM on a static test. I have tried different MJ and NJ sizes but best I have seen is 2500RPM with 135MJ and 2.75NJ. Prop is GT and is the finest pitch they supply (brand new). Checked for induction leaks, checked float levels etc.
Hi Hobson,
The fact that a cone filter fixed it makes it likely that there's something squiffy in the air box system.
There is a fair amount of info in the airframe technical manuals and the engine installation manuals talking about the induction air box, things like;
- Making sure the hose between air box and carby is smooth with no or minimum sharp corners
- Checking the sense tube is in a suitable location
- Checking that the sense ports at the inlet to the carby are not being blocked or getting excess turbulence from incoming air
- Checking for sharp corners or other messy geometry in the air box which is causing turbulence or air flow restriction
Airframes with not much room between the firewall and the carby inlets can be problematic in this area... the symptoms you're describing makes me think that there's something going on in the air box that's delivering turbulent air to the carby or wrongly pressurising the sense hose.
Has the aircraft always had the issue or is this a new thing? What type of airframe?
Best Regards,
Doug.
Hobson,
We need to talk and exchange ideas!!?
I presently have almost identical issues when advancing the throttle quickly. Only difference seems to be that disconnecting the air filter system on my aircraft (Arion Lightning) makes no difference. Like yourself I have spent many frustrating hours looking for air leaks and found nothing (yet). Here is the current entry in my Maintenance log:
Engine stumble on opening throttle quickly
No problem if throttle opened slowly
Ignition System:
Occurs with left magneto, right magneto or both
Spark plugs replaced
HT leads and distributer rotor checked, all OK
Fuel System:
Occurs with left and right tank
Electric pump makes no difference
Hacman disconnected, no difference
Air filter disconnected, no difference
No obvious air leaks
Complete Carburettor change, no difference
Inlet manifold spider able to hold vacuum (22lbs)
Inlet manifold rubbers loose and oiled from oil inhibitor system. ?source of air leak
Leak down test excellent (all < 5%)
Manifolds pressurised to 20 psi with no obvious leak
Tried advancing throttle with choke applied
Still stumbles but black smoke only from left side of motor. ?Suggests air leak left side of motor
Inspecting Savvy Analysis records and seems to be a problem with No 3 shortly after New Engine was installed. EGT on number 3 is significantly lower than others especially under load.
Replace manifold gaskets, induction runners, “o” rings in manifold
Sorry something happened before I finished posting, so......
Am about to replace intake manifold flange gaskets and the 6 "o" rings in the in intake plenum (when I can get parts from Jabiru tomorrow!!)
Will post outcome to this site when done (probably by the end of this week). In the meantime if you think talking over the issue would help then give me a call 0405181122
Geoff
