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

[43-29080]

Old thread, I know, but new member here.

I had a problem identical to this one with my continental A-65 which plagued me for almost a year. Turned out that the gascolator had been mounted backwards while remaking a leaky fuel line connection to it found during the annual inspection.

There should be no place anywhere in the fuel system for small bubbles in the fuel lines to form big ones, which can cause blockages with symptoms identical to vapor lock. Small bubbles should just flow up after forming, right into the fuel tank. This cannot happen with the gascolator reversed. Fuel should be flowing into the glass or aluminum bowl through the large central hole in the top of the gascolator, not the smaller hole above the screen which is outlet only. Remove your gascolator from the firewall bracket and blow through the two holes to determine which is which.

No one could figure this problem out, and it resulted in the replacement or overhaul of numerous components inside of the cowling including the carb, plugs, magnetos, harness, fuel lines, mag switch, and P leads, among other things. The good news is now the old Cub runs perfectly (as it did before) and everything under the cowlings gleams like new pennies! A gent who had been wrenching on Cubs for 45 years mentioned this possibility, and the problem was finally fixed in 30 minutes. the joy is now back in flying the old Cub!

[1600vw]

What I find interesting about this thread, no one mentioned to do minimum fuel tests to see what the minimum fuel flow is. Doing fuel flow test with full tanks tells nothing, well it does tell something but is only part of the test. Imagine taking off from your homebase or field and flying somewhere else. You go ahead and take off for home knowing you have enough fuel to make it back with what is in the tank but now your flow is reduced and you have an engine out on takeoff. Why because you did not do a minimum flow test.

If someone mentioned this I missed it.

Tony

[1600vw]

I am getting ready to do flow test on a new to me airplane. It has hundreds of hrs flying, but I put a new tank in her. I changed nothing but the tank. I am still doing a flow test, minimum and max fuel flow test. I need to know what the least amount of fuel I can fly with. I will then post the info in the airplane.

Tony