Engine Stumble source?

[nrpetersen]

I'm having fuel delivery problems with my 85 hp J-4A. It allows only the cowl tank for takeoff, but has a Marvel Schebler carb to tolerate the low fuel head in accordance with the C-85 TCDS. I'm certain it is starving out when it stumbles badly (fades out for a couple of seconds - at least) shortly after liftoff with a half full tank after a few T&Gs, running on 2/3 MoGas and 1/3 100LL.

Might it be fuel line percolation/vapor lock? or is it due to the initial acceleration of the aircraft where the tank is behind the engine? Some have even said all Pipers do that (!).

I'm certain it isn't contamination, the fuel selector valve detent system, or the fuel venting, or even a partially blocked line. It seems to happen after everything warms up thoroughly and after a taxi-back. (I still am having high cylinder head temp (400 degF max) although the oil temp has come down to about 180 degF.)

There have been three such aborted takeoffs. Fortunately all have been on a long enough runway to set it back down. The cowl tank is about half full, and the aircraft is at gross weight. The fuel line has a 45 deg el coming out of the gascolator and a 90 deg el going into the carb. The fuel line system is only 3/8" nominally. Doing a full flow test, I get about 15 gph with an open line at the carb inlet level.

Next is to instrument the carb fuel inlet with a thermistor to see if there is a heat soak/hot fuel issue at the carb fuel inlet. From that, possibly adding insulation to all of the fuel system that gets exposed to under cowl temperatures.

Surely others have had a similar problem. Maybe some of you have done some instrumentation to find out what's goes on. I'd like to hear any similar experiences, and any observations on the effects of under cowl temperatures.

[cub builder]

I suspect your fuel flow is inadequate. Have you checked the carb inlet filter? If so, then it sounds like you need to do a fuel flow test with the header tank low on fuel and the mains up on a platform to simulate your climb angle. Look up the max fuel flow for your engine at sea level. Minimum fuel flow with the tank low on fuel and the nose up in a climb angle would be 150% of the max rated fuel burn for your engine. Try your fuel flow test by flowing the fuel through the carb inlet and measure what comes out of the carb drain. If the fuel supply can't get enough fuel into the carb float bowl, then flow through an open line is meaningless. An additional player could possibly be a low float level in the carb.

I'd find it hard to believe a J-4 is heat soaking the gascolator or fuel lines. I've spent a lot of time in a J-4 running Mogas and have never seen a hint of a miss due to heat.

-CubBuilder

[nrpetersen]

Good idea to use the carb bowl drain flow. Carb inlet filter etc is whistle-clean. Where can I find the full throttle fuel flow on the C-85? the 150% number is also good to see. THX!

[Bill Greenwood]

All Pipers definitely do not do that, not my J3 with a C 90 in it and no other Cub or Supercub I have flown.

I would not be surprised if the float level is low, but an easier thing to do would drain out the fuel tank and fill it all the way up with only avgas, just to be sure. Then do a full power run up on the ground with the plane securely tied down. If that is ok then try to fly and climb out pretty flat, not nose high. See if it will run full throttle that way. Make sure the vent to the fuel tank is open.

I don't really see the point of using mogas in a Cub. Yes, you can, I did once in mine and it ran pretty much the same. That was before ethanol was commonly in gasoline. But a Cub burns about 4 gal an hour, how much money can you save anyway? I'd rather be sure about the fuel and be able to focus on what else might be the problem.

If the engine should quit in the air, and after you glide safely back to the runway with the engine out, pull it over to the side and remove the plugs and look at them. You then will then know if the cut out was a lean condition from no fuel or not enough fuel or maybe something else like ingniton. If the plugs on the other hand are black and sooty, a rich condition, maybe the carb heat is coming on.

You don't say if this is a new plane to you? If not and it ran well in the past, what could have changed?

[Bill Berson]

My Grob G109 has an AD that requires a steel heat shield over the gascolater, which is on the firewall.
But I don't see how this applies to a 60+ year old airplane.
Are you using ethanol gas?

[rv8bldr]

The C-85 overhaul manual states the engine burns 42.5 - 45.5 lbs/hr ( 7 - 7.6 gals/hr) at max power so you should see at least 10.5 gals/hr for the test.

I couldn't find an operators manual on line so I used the overhaul manual. I assume it is accurate...

[rwanttaja]

Looking at Harry Fenton's write-ups, he makes this comment about stumbling on takeoff: "Another possibility is that the holes where the throttle shaft passes through the carb body are worn. If too much air is pulled through the holes, then the mixture will lean out and the carb will stumble until more fuel is available. "

Take a look at Harry's page at http://www.bowersflybaby.com/tech/fenton.htm

Drop Harry an email as well...address is on the page.

Ron Wanttaja

[nrpetersen]

FWIW - the carb is new as I had to replace the old Bendix w a Marvel $chebler. I am certain it is a starve out stumble - it is a classic fade away thing that only seems to happen after everything is thoroughly heat soaked. My '41 J4A is a fully cowled engine & an 85 has never been in this particular airframe. It has a metal prop that pulls very well too. THX for the ideas guys...........

[WLIU]

There are some carburetors on some airplanes that have their float height set differently depending on whether they are fed from a fuselage tank or a wing tank. Is it possible that the carburetor in question was set up for the pressures of a wing tank and needs to be adjusted? The overhaul instructions for the carburetor might say something about this.

Best of luck,

Wes

[nrpetersen]

There are some carburetors on some airplanes that have their float height set differently depending on whether they are fed from a fuselage tank or a wing tank. Is it possible that the carburetor in question was set up for the pressures of a wing tank and needs to be adjusted? The overhaul instructions for the carburetor might say something about this.

Wes - I'll look into this. THX

Another thought - I think the vent for the float bowl is ported to the air intake which I have sealed tightly to the cowl front. This means fuel dribbling into the bowl has to push against the "pitot" air pressure, which is about 3-4" of water at 60 mph air speed. The fuel tank vent on the other hand is simply vented to approx static pressure. There is no pressurization of the fuel tank from a "pitot" tube on the fuel cap like there is on some aircraft. That's giving up a few inches of possible fuel head.

Many don't recognize the effects of float bowl vent.