Engine Stumble source?

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.

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

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!

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?

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?

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...

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

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...........

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

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Originally Posted by 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.

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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.

Norm, when you do the flow test, place the tail in a ditch. This will simulate the take off attitude. CHEERS don

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Originally Posted by dpellegreno

Norm, when you do the flow test, place the tail in a ditch. This will simulate the take off attitude. CHEERS don

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(Norm formerly of EAA, is my older brother)

More info. At a three point attitude & a 60% full cowl tank, I get only 8.6 gph out of the carb drain. I instrumented the gascolator housing with a thermistor and found that the temperature would peak at 122 degF on climbout.

With the cowl tank now refilled up to 80% full, the EGT suggests the mixture is adequately rich on takeoff as there is about a 100 degF reduction from peak. After a few landings and taxi backs, the EGT was noticeably higher on climb out - but no stumble.

All data was taken on a day of about 78 degF air temp.

My initial conclusion is to retain the instrumentation, insulate the gascolator, and try more takeoffs to see if the rising EGTs present itself again.

I'm glad I didn't know the fuel temp was reaching 122 DegF :eek:

This might be of interest.

We had an incident recently where an Ercoupe engine failed on takeoff.
The owner told me that mechanics had inadvertently reversed the fuel line/vent or return line during maintenance.
It would run fine on the ground, but not enough fuel flow for a full power climb.

I should add that I'm also going to try adding a pitot tube to the cowl fuel tank vent, now that I've got an indication using the EGT for showing impending fuel flow starvation. That should add another 3 to 4 inches to the effective fuel head at 60 mph airspeed.

Another trick under consideration is to replace the fuel line end fittings and hose with ends that have a smooth sweep instead of the present -6 JIC 45 deg el at the gascolator, and 90 deg el at the carb fuel inlet. Theoretically it promises only about 0.6 inches of fuel head improvement in total though at 8.6 GPH.

There must be other J4s with an 85 hp engine. How do they do it?

There are no Tees in the fuel tank outlet - but that is a very interesting consideration. THX

Update - Insulation with pipe wrap reduced the gascolator temps to a little over 100 degF. I also added a 1/4 dia pitot to the fuel vent at the same time & found that the EGTs now show no evidence of fuel starvation at full power. I have to get the fuel level down some more to verify that one or more of these fixes work tho.

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!

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

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

A quick update on my J4A fuel supply with a C-85 and cowl tank.

I added a 1/4 dia forward facing pitot tube to the fuel cap using epoxy, added an aluminum doghouse around the firewall-mounted gascolator. I also added fire sleeving to the fuel line between the gascolator and the carb inlet. This dropped the max temperature rise (over outside ambient) of the gascolator bowl from 60 degF to about 10 degF. All three were done at the same time so I have no way to know the comparative effects of either modification. The carb is a new Marvel Scheibler, and the above fuel flow tests were done at ~ 1/3 full cowl tank.

No engine interruptions have been encountered since, although a lean roughness has been noted at moderate throttle on a cool dry day, verified by it smoothing out w carb heat. Before that I had leaned out the idle adjustment substantially to minimize exhaust soot - probably too much. The EGT does seem to run higher than I'd like on climb out yet. Still using ~ 1/3 100LL 2/3 Mogas.

I think the gascolator is properly plumbed but will definitely verify this.

From Harry Zeisloff's comments many years ago, he said vapor lock is initiated by more than simply a few bubbles - he compared it to more like letting soda pop or champagne fizz off.

"You can never know too much about fuel and fuel systems."

Perhaps you should consider replacing the big 85 with an A65, as it was designed for. The fuel cap ram air seems innocuous enough, but if you have some overly anal, greedy, by the book IA who knows old Pipers perform your annual, he'll ground her for it...unless you've paid him handsomely to fill out and send in a FAA Form 337 for it.

Quote:

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Originally Posted by nrpetersen

A quick update on my J4A fuel supply with a C-85 and cowl tank.

I added a 1/4 dia forward facing pitot tube to the fuel cap using epoxy, added an aluminum doghouse around the firewall-mounted gascolator. I also added fire sleeving to the fuel line between the gascolator and the carb inlet. This dropped the max temperature rise (over outside ambient) of the gascolator bowl from 60 degF to about 10 degF. All three were done at the same time so I have no way to know the comparative effects of either modification. The carb is a new Marvel Scheibler, and the above fuel flow tests were done at ~ 1/3 full cowl tank.

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After rereading this thread, it reminds me of an issue I have run across twice in the last year or so since this thread originated. Since your J-4 now has a C-85 and MS Carb, I assume the fuel line between the gascolator and carb was replaced. Check to make sure the fuel line between the gascolator and carb doesn't loop up high enough to be higher than the bottom of the tank header. If you get just a bubble of air in a fuel line configured like that, it will stop or significantly hinder the fuel flow. Most recent case was on a T-craft with an identical configuration to yours. When they made up a new fuel line to the MS Carb, it was a bit long so looped upward just enough to create an intermittent problem with fuel flow when the header tank was below 50%. A ram air to pressurize the tank would have worked as a workaround. However the fix was to simply lower the height of the fuel line between the gascolator and carb so all of the fuel line was below the height of the bottom of the tank.

-Cub Builder

The C-85 really puts new life in the old girl. I can't imaging the original J4s having a 65hp (even some with 55 hp) with full old electric (6 V yet). I wonder what the empty weight that was assigned by Piper was, although I have a metal prop. I ended up with an empty weight of 920 lbs.

I agree that the fuel tank pitot isn't factory original, but from pictures there seems to be a lot of precedence (sp?) in other J-4s as well as other airplanes. If a fussy IA objected, I'd point to the almost non-existent factory service manual, the unfinished Piper assembly documentation with no revision control, and then make him fly it around the patch with low fuel. It sorta came under the guide of don't ask, don't tell..... For certain, I wouldn't ride along.:)

My -6 JIC fuel line is only 14 inches long and has only a downward loop to it with a 45 deg fitting on the gascolator and a 90 deg ell on the carb. It would be better if it was a -8 fitting and hose etc but there are so many points in the system where things are 1/4 inch NPT so it isn't practical to convert.