gas smell in aerocarb

[tx_swordguy]

Guys , Getting a gas smell inside the cockpit of a sonex jab 3300 with an aerocarb. Can't seem to locate a leak. Is there somewhere on the aerocarb that is prone to leaking? If enging is shut off and fuel is open it leaks at the carb, but read that is normal so don't really see how to check it that way. It shows up mostly when I go to idle and either drop the nose or approach to land. Side note that may or may not be involved. My vent line goes horizontally out of the tank and goes to the side and exits below the plane. it has a bit of a kink in it and I am going to replace it. It doesn't seem to affect starting or power (although I am chasing down higher EGT on full throttle) by working on the carb needle. It doesn't bog down or stumble on full throttle (which I would expect if it won't vent enough), just getting EGT above 1350 if I don't throttle back to about 2900 rpm. (OAT is 40-50 elevation 900') Summer time I occasionally see high egt but mostly see rapid CHT temps go above 310 on climb. Although I maybe throttling back for the CHT before the EGT has a chance to spike during the 100 degree heat. Just seems odd that I smell the gas at a time I go from a higher usage to a lower usage.
Mark

[peter anson]

Possibly a leaking gas cap. If the tank is more than about 2/3 full the fuel will slosh into the filler neck on descent. Do you have a sealed dam around the filler? To check for a leak just pick up the tail while the tank is full.

Peter

[wlarson861]

Every time I have smelled gas in flight it was a loose fuel cap. I have the earlier snap cap that some times needs to be adjusted. I now as far as pre-flight try to twist the cap after it is locked down. If it can turn it will leak.

[DCASonex]

Loose gas cap will do it, but so will a tank fitting if old style with molded in fittings rather than the oops fittings now used. This can happen at any time, even a year or two after first flight. Cycling hot to cold environment seems to be a major cause.

David A.

[builderflyer]

Mark,

If you are smelling fuel when lowering the nose with a near fuel gas tank, my vote us for a loose gas cap. Regarding your EGT experience, I've been experiencing the same phenomenon for 14 years now........high EGTS on back 2 cylinders at full throttle during climbout. I suspect it is related to the short distance between the the Aerocarb needle and the back 2 cylinders, and inadequate mixing of fuel and air which becomes more evident at their location during full throttle operations. If you come up with a solution for this, I'd love to know what you have done.

Art,,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,Jabiru 3300 #261

[sonex1374]

Even with a filler neck catch pan sealed to the firewall and a vent tube that exits the cowl at the bottom of the airplane it is still possible to smell some gas fumes when the tank is full and the nose is lowered (especially on approach to landing). The gas sloshes forward in the tank, exits thru the vent tube, and vaporizes immediately outside the airplane. This vapor has a way of finding entry into the fresh air vents, or gaps in the canopy or wing roots. For me, it never happens unless the tank is very full (15+ gal) and I get the nose down low.

As for fuel leaking out the AeroCarb/AeroInjector when parked, this could happen even with the mixture in the full cutoff position (albeit very slowly). If the mixture is cracked even a bit that can be enough to drip noticeably (you might need to adjust your control cable to ensure it closes completely if this continues to be a problem). This is the reason Sonex recommends to always turn off the ball valve attached to the main tank whenever parked and not running.

EGT's above 1350 at high rpm (2900+) are common with Jabiru 3300s. It's partly due to the high power being produced, and that power needs sufficient fuel flow, but it's also due to the engine design. High rpm causes the valves to open/close more rapidly, and that sends more unburnt fuel out the tailpipe.

Jeff

[tx_swordguy]

Thanks for the ideas guys. I have had the plane since April of 2018 and the gas smell has only come about within the last month or so. I have one of the rubber compression type gas caps that squeezes the rubber out to seal against the tank cap by pushing a lever down. I have another one perhaps I will try that. It just seems odd that I have noticed it now so readily as soon as I pull power back. I have the vents shut off but if I open the vents it doesn't seem as bad. I guess I just need to keep looking lol.

[LarryEWaiex121]

Agree with Art, 100%.
I think the matter is a non-event as Art stated. I experience same higher temps on 5 and 6 when the engine is cold. When its fully up to temp, my egt's, like in cruise actually are slightly lower than 2,3 and 4.
As long as your not going crazy lean on the rear cylinders, I'd not loose much sleep over it. On initial power reduction after takeoff it will also come down nicely. I only use full rpm for first 1,000 foot and then cruise climb at either 2800 or 2850 if I'm really needing to get up and over some tall hills.

Larry
Waiex121YX, Camit 3300, Dynon Skyview, 675 hrs

[tx_swordguy]

Update on the gas smell. After looking at all the fuel lines and moving them around trying to tighten, I looked at the gas cap like was suggested. My lever/sqeezed rubber gas cap was slightly loose and I could wiggle it a bit. I found that (unknown to me) the lever that sqeezes the rubber up against the fuel inlet is screwed onto the bottom plate. It had loosened up enough to allow wiggling so I tightened it up and now it seals tight. I have not flown yet but feel VERY confident that was my fuel smell source. I just hope I did not cause actual leaks in messing with the fuel lines lol. Thanks for the suggestions guys
Mark

[GordonTurner]

Just returned from Australia where i visited Peter Anson, who makes among many other great products an IMPROVED FUEL CAP. Not flying yet but bought thenfuel cap, it is a work of Engineering art for those of us with the thermos bottle fuel tanks...

http://www.ansoneng.com/

Gordon