Strange fuel problem on my Cherokee

John221us

John221us

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Sorry, this is long. The engine runs smooth at full throttle. As you pull it back, it runs rough between 2400 and about 2300 RPM or so and then smooths out again. This problem has been with the plane in one form or another, in the two and half years I have had it. My A&P, for some reason, didn't notice it. It seems to only manifest in the air.

On my last annual (april), I had the carbeurator rebuilt. The shop had to send it back three times (bought through Spruce). On the third time, they sent it directly to the Kelly Aerospace (the rebuilder) and they put it on thier bench. The first two times, they couldn't even get the plane past 1,500 RPM with the rebuilt carbs. Well, the problem still existed after the rebuild, but now, you could smooth it out, by leaning it, while in that range (that didn't used to be the case). I took my mechanic up a few weeks ago and showed it to him and he said, Wow! this is a problem and sent the carb to an engine shop he trusts to go through it in detail.

The engine shop had several comments:

1.
  1. Looked at main fuel nozzle at nozzle fuel entry point – looks liked they drilled it out. It has scores and marks…NOT GOOD…
  2. Economizer jet was too large – looks like it was drilled out. About 0.008” too large…Makes for richer carburetor setting.
  3. I installed the float kit in compliance with Marvel Schebler SB2 Rev B – I have had a lot of trouble with the metal floats (both Kelly and Avstar)…I recommend only the Marvel Schebler blue foam floats be installed…
Well, I went up for a maintenance flight with my mechanic today and the problem is still there, only this time, you can't lean smooth. In fact, as you pull back on the throttle, the EGT climbs, like it is too lean, even at full mixture. Also, when you engage carb heat, it starts to run even rougher (detonation?).

Any suggestions?:confused:
 
Now you know why I tell owners not to have their good running carbs rebuilt.

There is no real fix good for your carb.
 
Tom, how much does it cost to retrofit FI to a carbed certified plane? Total cost including all paperwork and A&PII charges. :dunno:
 
Look for vacuum leaks. If you don't find any, look some more. This is a lean condition as you block the intake with the throttle plate. It's over-rich with the mixture pushed all the way in, as you pull the throttle back you reduce the MP dramatically, and the air will come in from the leak. At idle, the engine is running on a different carb circuit(the idle circuit), and it has been field adjusted to compensate for the vacuum leak. Don't just look at the booties and the gaskets. Look at the manifold plenum, thermal barrier between the carb and manifold, all the runners, etc.
 
Tom-D said:
Now you know why I tell owners not to have their good running carbs rebuilt.

There is no real fix good for your carb.
Click to expand...

Hi Tom,
I am sure you are right, but this thing has had the problem ever since I owned it. Is it better to buy a new carb? Can you get them for a '67 Cherokee?
 
docmirror said:
Look for vacuum leaks. If you don't find any, look some more. This is a lean condition as you block the intake with the throttle plate. It's over-rich with the mixture pushed all the way in, as you pull the throttle back you reduce the MP dramatically, and the air will come in from the leak. At idle, the engine is running on a different carb circuit(the idle circuit), and it has been field adjusted to compensate for the vacuum leak. Don't just look at the booties and the gaskets. Look at the manifold plenum, thermal barrier between the carb and manifold, all the runners, etc.
Click to expand...

This seems to make sense. I am not a mechanic, just the guy writing the checks. They have had the carb off at least 4 times. Wouldn't they have seen something like this? I will definitely mention this.
 
Odd - I experienced the same thing on a '80 28-161. It was one out of the three Warriors we have. On climbout, I would pull the throttle back from full to about 2000. Somewhere between 2200 and 2300, there would be a really rough patch that lasted just for a second, kind of a hesitation before it kicked in again.

I squawked it, but we could never identify the issue and I presume it's still happening. I stopped flying the Warriors a few months ago.

If you do find a positive cause, please post it here?
 
John221us said:
Hi Tom,
I am sure you are right, but this thing has had the problem ever since I owned it. Is it better to buy a new carb? Can you get them for a '67 Cherokee?
Click to expand...

You gotta remember the new carbs are made by Kelly aerospace.
 
Geico266 said:
Tom, how much does it cost to retrofit FI to a carbed certified plane? Total cost including all paperwork and A&PII charges. :dunno:
Click to expand...

When you go from carb'ed to fuel injection you will go from 0-320- to IO-320-

read FAR 43-A

2) Powerplant major alterations. The following alterations of a powerplant when not listed in the engine specifications issued by the FAA, are powerplant major alterations.

(i) Conversion of an aircraft engine from one approved model to another, involving any changes in compression ratio, propeller reduction gear, impeller gear ratios or the substitution of major engine parts which requires extensive rework and testing of the engine.

(ii) Changes to the engine by replacing aircraft engine structural parts with parts not supplied by the original manufacturer or parts not specifically approved by the Administrator.

(iii) Installation of an accessory which is not approved for the engine.

(iv) Removal of accessories that are listed as required equipment on the aircraft or engine specification.

(v) Installation of structural parts other than the type of parts approved for the installation.

(vi) Conversions of any sort for the purpose of using fuel of a rating or grade other than that listed in the engine specifications.

Try that route if you like. it takes a new data tag. try to get one. :)
 
John221us said:
Hi Tom,
I am sure you are right, but this thing has had the problem ever since I owned it. Is it better to buy a new carb? Can you get them for a '67 Cherokee?
Click to expand...

Ask the salvage yards if they have a unmolested carb for your aircraft that has metal floats, old style atomizer nozzle, and 2 piece venturi.

If you can find one, send it to me, I'll clean it up and make it work for you.
 
Tom, a friend mentioned to me when discussing the carb fiasco on the phone, that besides the metal floats, and then the problematic plastic floats, there's a third option with a newer plastic material that will basically still float even if you drilled holes in the darn thing.

Any experience with those? (We currently don't have any carb issues with ours thank goodness, but he was having to rebuild an old "good style" carb for his airplane and that's how the topic came up.)
 
Tom-D said:
When you go from carb'ed to fuel injection you will go from 0-320- to IO-320-

read FAR 43-A

2) Powerplant major alterations. The following alterations of a powerplant when not listed in the engine specifications issued by the FAA, are powerplant major alterations.

(i) Conversion of an aircraft engine from one approved model to another, involving any changes in compression ratio, propeller reduction gear, impeller gear ratios or the substitution of major engine parts which requires extensive rework and testing of the engine.

(ii) Changes to the engine by replacing aircraft engine structural parts with parts not supplied by the original manufacturer or parts not specifically approved by the Administrator.

(iii) Installation of an accessory which is not approved for the engine.

(iv) Removal of accessories that are listed as required equipment on the aircraft or engine specification.

(v) Installation of structural parts other than the type of parts approved for the installation.

(vi) Conversions of any sort for the purpose of using fuel of a rating or grade other than that listed in the engine specifications.

Try that route if you like. it takes a new data tag. try to get one. :)
Click to expand...

So being able to put a modern FI system on an old aircraft is not even possible. Wow, just wow.

In the experimental world owners switch to FI with no issues at all. The only limitations are your wallet.
 
Geico266 said:
So being able to put a modern FI system on an old aircraft is not even possible. Wow, just wow.

In the experimental world owners switch to FI with no issues at all. The only limitations are your wallet.
Click to expand...

Incorrect, it's definitely possible. Changing the engine model of your existing engine is one way to do that, but typically not the most efficient way. An STC to modify the engine, and then a second STC to install that modified engine in your airframe, either for one aircraft or for multiple aircraft, depending on your goals, is a more reasonable approach.

There are DERs and DARs that advertise their services in Trade A Plane... I got an estimate for $10,000 in DER/DAR fees, plus $10,000 in parts, plus installation labor. Instead, I sold my Cardinal FG and bought a Cardinal RG for $10,000 more. Today, with 20/20 hindsight, I'd have sought an STC to install the RG FI engine in the FG as a carb icing safety enhancement, and accepted a small reduction in MP redline to stay within the "no more than 10% increase in HP without airframe recertification (spin testing, etc)" FAA guideline.

Paul
 
Geico266 said:
So being able to put a modern FI system on an old aircraft is not even possible. Wow, just wow.

In the experimental world owners switch to FI with no issues at all. The only limitations are your wallet.
Click to expand...

And to add to Paul's explanation, an STC once granted can be used by the holder to perform the change/conversion on other airplanes. My Waco Classic with a Jacobs radial engine has fuel injection with GAMI injectors and electronic ignition replacing the old style battery ignition on the non-magneto side. Both conversions STC'd by Radial Engines in OK. They have the FI on a bunch of Cessna 195s as well.

Ernie
 
To look for an induction leak, a high volume, low pressure air source like the discharge side of a vacuum cleaner (new or well filtered) can be used to pressurize the induction system and then soap bubble test. Hook it up to the inlet side of the carb. The roll joint where the metal tubes going to the individual cylinders are rolled into the engine are a possible leak point. A leak should have less effect at higher power because the flow through the leak goes down as there is less differential pressure at the leak due to higher manifold pressure and there is a greater induction flow rate.
 
Graueradler said:
To look for an induction leak, a high volume, low pressure air source like the discharge side of a vacuum cleaner (new or well filtered) can be used to pressurize the induction system and then soap bubble test. Hook it up to the inlet side of the carb. The roll joint where the metal tubes going to the individual cylinders are rolled into the engine are a possible leak point. A leak should have less effect at higher power because the flow through the leak goes down as there is less differential pressure at the leak due to higher manifold pressure and there is a greater induction flow rate.
Click to expand...

It is a very specific range, but vacuum leak seems to make sense.
 
You CAN uy a new carb made by the "new" Marvel Schebler company. We have installed some with no problems. I will go +1 on a thorough check of the induction plenum and tubes. Also be sure the primer is not leaking if you haven;t already done that. Charlie Melot Zephyr Engines
 
charlie said:
You CAN uy a new carb made by the "new" Marvel Schebler company. We have installed some with no problems. I will go +1 on a thorough check of the induction plenum and tubes. Also be sure the primer is not leaking if you haven;t already done that. Charlie Melot Zephyr Engines
Click to expand...

Thanks,
I will talk to my mechanic. I don't know if has checked that, yet. They did replace a tube, when they had the carb off.
 
PaulMillner said:
Incorrect, it's definitely possible. Changing the engine model of your existing engine is one way to do that, but typically not the most efficient way. An STC to modify the engine, and then a second STC to install that modified engine in your airframe, either for one aircraft or for multiple aircraft, depending on your goals, is a more reasonable approach.

There are DERs and DARs that advertise their services in Trade A Plane... I got an estimate for $10,000 in DER/DAR fees, plus $10,000 in parts, plus installation labor. Instead, I sold my Cardinal FG and bought a Cardinal RG for $10,000 more. Today, with 20/20 hindsight, I'd have sought an STC to install the RG FI engine in the FG as a carb icing safety enhancement, and accepted a small reduction in MP redline to stay within the "no more than 10% increase in HP without airframe recertification (spin testing, etc)" FAA guideline.

Paul
Click to expand...

ebetancourt said:
And to add to Paul's explanation, an STC once granted can be used by the holder to perform the change/conversion on other airplanes. My Waco Classic with a Jacobs radial engine has fuel injection with GAMI injectors and electronic ignition replacing the old style battery ignition on the non-magneto side. Both conversions STC'd by Radial Engines in OK. They have the FI on a bunch of Cessna 195s as well.

Ernie
Click to expand...

For $2,500 you can add FI to any experimental.
 
The Marvel Scheblers have a bunch of quirks that got a lot worse as the years went by and the carb line changed hands and quality gradually got worse. The ADs that forced the venturi and nozzle changes added to the problems. As Tom says, an old, non-modified carb is probably the best bet.

A couple of things I've found with those carbs:

1. Missing accelerator pump check ball springs. That allows the manifold pressure to suck extra fuel out of the pump nozzle, and it seems to be the worst as the throttle is closing a little. The nozzle is off to one side of the carb throat, so the fuel gets more into one or two cylinders and makes them too rich and the engine roughens.

2. Bad atomization of the fuel as it leaves that stupid crossdrilled nozzle. It flies past a wide-open throttle plate OK and gets divided more or less equally between the four intake runners, but close the throttle a bit and it strikes the throttle plate and gets deflected toward the front intakes and makes the front cylinders rich. Close it a bit more and the turbulence behind the plate evens things out again. On one 172, I found that max RPM on climbout was with the throttle not quite wide open, where that lumpy fuel got beat into smaller bits by the plate without being deflected too badly.

I sure like my 66-year-old Stromberg carb on my A-65. So smooth, so reliable.

In the case of aircraft carbs, they really don't build them like they used to.

Dan
 
There are lots of yellow tag carbs on shelves. Have your mechanic arrange to rent one for a flight test.
I am confident that the rental carb will behave weird also because you have an induction leak.
Have the mechanic spray carb cleaner at each joint on the induction tubing with the engine running, when you hit the leak the engine will surge.

My most recent one turned up under a padded clamp around the induction tube for routing the primer tube and the EGT wire.. There was a hairline crack on the backside of the tube completely covered by the clamp and not visible. I could just feel it with a fingernail is how I found it - mechanic clean missed it and wanted me to replace the carb for 1200 bucks.
 
charlie said:
You CAN uy a new carb made by the "new" Marvel Schebler company. We have installed some with no problems. I will go +1 on a thorough check of the induction plenum and tubes. Also be sure the primer is not leaking if you haven;t already done that. Charlie Melot Zephyr Engines
Click to expand...

I am definitely going to chase this down, but is only a small range of RPMs. Is this an airworthy issue? I have self grounded for the last several weeks, while we chase it. Should I find the problem before I fly it again?
 
Well, they found a small induction leak, but it did not solve the problem. They also did a compression check and found a bad cylinder (not the problem).
We fixed one minor induction leak at cylinder #4, found by pressure testing entire system.

Just completed ground run:

2300 rpm max static-egt looks good-down from the peak yellow needle by at least 100 degrees.

Start retarding throttle-2250-2275 egt begins rising-everytime-about 2200 running rough, with egt above the peak needle. Worst running / high egt at 2100 indicated, start advancing throttle rpm rises, egt drops and smooths out about 2250-2275.
Click to expand...

 
My old Cherokee 180C used to do the same thing. If you jammed the power on too quickly, it would sound like it was fuel starved for a second, and then come right back. Similarly if you retarded power too quickly. It was always in the same RPM range that it happened. I never tried going REALLY slowly to find out exactly where it happened.
 
SkyHog said:
My old Cherokee 180C used to do the same thing. If you jammed the power on too quickly, it would sound like it was fuel starved for a second, and then come right back. Similarly if you retarded power too quickly. It was always in the same RPM range that it happened. I never tried going REALLY slowly to find out exactly where it happened.
Click to expand...

The first problem you mentioned, firewalling the throttle too fast, is in the POH. You are supposed to take 2 seconds to depress the throttle. There is referral to an SI I found which sounds similar and says you should reverse the wiring harness. http://www.generalaviationnews.com/2008/01/25/what-to-do-about-a-‘slightly’-rough-running-engine/
 
bflynn said:
Odd - I experienced the same thing on a '80 28-161. It was one out of the three Warriors we have. On climbout, I would pull the throttle back from full to about 2000. Somewhere between 2200 and 2300, there would be a really rough patch that lasted just for a second, kind of a hesitation before it kicked in again.

I squawked it, but we could never identify the issue and I presume it's still happening. I stopped flying the Warriors a few months ago.

If you do find a positive cause, please post it here?
Click to expand...

Which one was that?
 
SkyHog said:
My old Cherokee 180C used to do the same thing. If you jammed the power on too quickly, it would sound like it was fuel starved for a second, and then come right back. Similarly if you retarded power too quickly. It was always in the same RPM range that it happened. I never tried going REALLY slowly to find out exactly where it happened.
Click to expand...

Old four stroke carbureated motors do not have the best/quickest throttle response. That is normal i'd say.
 
Dr. O said:
There are lots of yellow tag carbs on shelves. Have your mechanic arrange to rent one for a flight test.
I am confident that the rental carb will behave weird also because you have an induction leak.
Have the mechanic spray carb cleaner at each joint on the induction tubing with the engine running, when you hit the leak the engine will surge.

My most recent one turned up under a padded clamp around the induction tube for routing the primer tube and the EGT wire.. There was a hairline crack on the backside of the tube completely covered by the clamp and not visible. I could just feel it with a fingernail is how I found it - mechanic clean missed it and wanted me to replace the carb for 1200 bucks.
Click to expand...

Ok, the new cylinder is in and that did not improve the problem (I didn't think it would) and my A&P just installed a loaner carb and the problem did not change. He swears they have completely checked for induction leaks.
 
Also, the engine was last rebuilt in 1977 and has about 800 hours since then. I have personally put about 250 hours on it in the last 2 1/2 years. I know this is an old engine, but what does that really mean as far as reliability?
 
dell30rb said:
Which one was that?
Click to expand...


80A. Take off and stay in the pattern. When you throttle back on reaching altitude, there's very small hiccup in the engine - when I say small, I mean small, it lasts a fraction fo a second. It's almost something you feel rather than hear. But it is consistent.

We talked with the A&P about it, but we can't pin it down to a serious problem, so for now it's just a quirk of a 30 year old plane.
 
Funny I was just thinking about this. Runout mag bearings shifting in the case, will hold their position until the pressure of constant RPM is removed, then will shift miss, until constant RPM is re-established.
 
bbchien said:
Funny I was just thinking about this. Runout mag bearings shifting in the case, will hold their position until the pressure of constant RPM is removed, then will shift miss, until constant RPM is re-established.
Click to expand...
Yes they will act erratic but they will do it all the time every time.

This sounds like the typical mag switch, it goes bad, you wiggle it, it works fine until you wiggle it again.

and the bare "P"lead is a good catch, but did it cure the problem?
 
A weak or fractured point spring can develop an harmonic "chatter" (misfire) at certian RPM's depending on the distance between the weak spot and the anchor point.

Chris
 
Tom-D said:
Yes they will act erratic but they will do it all the time every time.

This sounds like the typical mag switch, it goes bad, you wiggle it, it works fine until you wiggle it again.

and the bare "P"lead is a good catch, but did it cure the problem?
Click to expand...
...I think John (OP) reports it consistently.....just thinkin'....
 
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