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Rough Idle, help needed

P

PCHunt

Well Known Member
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Engine rough idle

O-320-E2D, low compression pistons (150 hp), MA-4SPA carb. 350 hours since field overhaul.

Ran very nicely for about 7 years, then rather suddenly developed a very rough idle.

Diagnosis has shown a very rich mixture at idle, up to about 1300 RPM. At 1300 RPM if I lean aggressively, the RPM rises to about 1550. The engine stumbles irregularly, and produces puffs of black smoke out the exhaust below 1300 RPM. Have removed primer system, and blocked off cylinder ports with solid plugs.

I initially focused on the carburetor. I thought that the idle circuit in the carb might have gotten plugged. So I borrowed a carb to try, and it ran even a bit worse than mine. I have varied the idle mixture screw from fully in, to way out, and no difference in the symptoms!???? :confused:

Here?s the strange thing: I have now tried 3 different carburetors, and the same symptoms show up with all 3 carburetors!

The engine runs progressively smoother at higher power settings, and seems to develop full power. Static RPM is 2150, which is no change from the last few years of flying. If I lean at full power, I can get a 40 to 50 RPM rise, as I apparently lean to best power.

What, other than a carburetor, could cause excessively rich mixture at idle/low RPM?

Mahlon? :eek:
 
I should have mentioned, I replaced all intake gaskets, and checked the intake tube hoses.

But, if there was an intake leak, wouldn't that make the mixture too LEAN?

I have an overly rich symptom. When running at, say 1000 RPM, if I lean quite a bit, the RPM will go up to about 1300. Same with all 3 carbs! :confused:
 
other symptoms?

Pete:

What do your sparkplugs look like?

Wouldn't a very rich mixture, i.e. a fuel issue affecting all cylinders, result in black deposits on all plugs? If the plugs don't look alike, it's another clue that can localize the issue.
 
Fuel pressure has been normal, middle green, at 4 psi.

Will be looking at the plugs today. Thanks, keep these ideas coming my way!!
 
Primer

No one has mentioned checking your primer system if installed..it could be drawing straight through the primer. Since the engine is at low RPM, it couldn't handle the additional fuel well. Cap the feeder line and run it with no primer if so. Easy check.
 
Is this in your RV 6 or is it your 6A?


If in your 6, there could be a chance your float level is slightly too high vs a carb that is set up for a 6A, C-172 or a piper where the engine is level on the ground.

Does leaning at idle smooth it out?
 
Verify fuel pressure

Pete,
I understand why you trust your current fuel pressure gauge, since it's readings haven't changed.

But to verify fuel pressure you would need to hook up a second known good gauge.

Too high a pressure will push by the float needle valve and cause a high fuel level in the carb bowl. This will cause a rich mixture.


Mark

PCHunt said:
Fuel pressure has been normal, middle green, at 4 psi.

Will be looking at the plugs today. Thanks, keep these ideas coming my way!!
Click to expand...
 
Update

Thanks, everyone, for your inputs.

To answer some of the comments:

All plugs, top and bottom, have a uniformly VERY black sooty coating. No oily deposits. The black will wipe off with a swipe of your finger.

Have not done a compression check, or a mag timing check, but that will happen tomorrow! Can't do everything in one day.

Today, 3 very good A&P's, and one with an IA stood by and watched the runup. They are still mystified, as I am.

The fuel pressure has been verified by hooking up a mechanical (Digital/calibrated) gage, which showed a max of 6 psi for both, and either the electric boost pump, or the engine driven pump alone.

The primer has been completely removed from the system and is not a factor.

The engine exhibits the same symptoms on L mag, R mag, and Both mags, with obvious minor changes, at several power settings.

Engine runs great at high power, makes full static RPM, and the mag check at high power is normal.

Still stumped!

All help appreciated. Many thanks! :)
 
Blocked air filter?

I am reluctant to mention this since you probably have been running without the filter already.
 
Yes, test running without the filter installed. Just a bare carburetor. Thanks.

Off to the hangar for more sleuthing! Will be checking compression, plugs, wires, mag timing.
 
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PCHunt said:
Engine rough idle

O-320-E2D, low compression pistons (150 hp), MA-4SPA carb. 350 hours since field overhaul.

Ran very nicely for about 7 years, then rather suddenly developed a very rough idle.

Diagnosis has shown a very rich mixture at idle, up to about 1300 RPM. At 1300 RPM if I lean aggressively, the RPM rises to about 1550. The engine stumbles irregularly, and produces puffs of black smoke out the exhaust below 1300 RPM. Have removed primer system, and blocked off cylinder ports with solid plugs.

I initially focused on the carburetor. I thought that the idle circuit in the carb might have gotten plugged. So I borrowed a carb to try, and it ran even a bit worse than mine. I have varied the idle mixture screw from fully in, to way out, and no difference in the symptoms!???? :confused:

Here?s the strange thing: I have now tried 3 different carburetors, and the same symptoms show up with all 3 carburetors!

The engine runs progressively smoother at higher power settings, and seems to develop full power. Static RPM is 2150, which is no change from the last few years of flying. If I lean at full power, I can get a 40 to 50 RPM rise, as I apparently lean to best power.

What, other than a carburetor, could cause excessively rich mixture at idle/low RPM?

Mahlon? :eek:
Click to expand...

I am a bit stumped. I would definitely check the timing. An advanced timing will want a leaner mixture and could, but shouldn't, choke on the extra fuel. Also, a very late timing will leave unburnt fuel. However, you should notice a commensurate reduction in power. I struggle to see how it would produce as much soot as you are seeing. Usually the only things outside the carb that cause a rich mixture is a blocked induction system or excessive fuel pressure. Did you confirm that the other 2 carbs ran well on their doner's engine?

Larry
 
Sure you don't have air leak(s) around the induction tube/sump connection?? Have heard of this happening. Apply a bead of RTV to the joint and assure no leakage. Double check your timing??
 
Today's report

Checked compression, 74 and above all four cyls. Checked Mag timing, right on the mark (25 degrees) both mags. Removed rocker covers, and observed valve action, all appeared normal. Several sets of qualified eyes were observing.

Read the troubleshooting guide for MA4-SPA carb, and checked the accelerator pump check valve, passed the test, and the accel pump is functioning normally.


Still stumped.

One of the two trial carbs came off of a running engine that was headed to an overhaul shop. Very clean looking newish carb.

Would the wrong "dash number" carb cause extreme rich mixture?

Keep in mind, all 8 plugs are black with sooty deposit, so all cylinders appear to be equally affected. And while running at 1000 RPM full rich, then with aggressive leaning the engine will gain 250-300RPM. :eek:

And the engine is making full power, full static RPM (2150-2200) at rich mixture, and by leaning a bit, can gain 40 to 50 RPM.

Time to remove the intake pipes and hoses and have a look? But all cylinders are equally affected.

:confused::confused::confused::confused:
 
If the replacement /trial carb came off an AC that has a nose wheel, and yours is a TD, the float level could be too high on your installation. it has most of the symptoms of a float level that is just a touch high. As others have suggested slightly excessive fuel pressure can cause the same excessively rich idle. A float that is only partially fuel logged will do the same.
 
Bypass the manual fuel pump and run the engine on the electric pump only. Also check to see that both tank vents are clear.
 
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F1R said:
If the replacement /trial carb came off an AC that has a nose wheel, and yours is a TD, the float level could be too high on your installation. it has most of the symptoms of a float level that is just a touch high. As others have suggested slightly excessive fuel pressure can cause the same excessively rich idle. A float that is only partially fuel logged will do the same.
Click to expand...

The carb overhaul manual doesn't differentiate between Tail Dragger and Nose Wheel installations. It calls for a 7/32nds drill bit to set the float level. And wouldn't fuel run out of the bottom of the carb with the electric fuel pump running, if the float level was too high? That doesn't happen in my case.

Nose wheel airplane, BTW.

Not arguing, BTW, just trying to have a discussion. You gotta know, I would LOVE to find the problem!!
 
PCHunt said:
The carb overhaul manual doesn't differentiate between Tail Dragger and Nose Wheel installations. It calls for a 7/32nds drill bit to set the float level. And wouldn't fuel run out of the bottom of the carb with the electric fuel pump running, if the float level was too high? That doesn't happen in my case.

Nose wheel airplane, BTW.

Not arguing, BTW, just trying to have a discussion. You gotta know, I would LOVE to find the problem!!
Click to expand...

One of the more redeeming properties of a carb on an O-320 is that it will run with no fuel pump provided you have a header tank or a high wing AC. Not the case in a low wing RV. However they do not need much pressure. A slightly elevated fuel pressure or a partially compromised float would produce the rich idle symptoms. Since you have tried 3 different carbs.... it highlights the fuel pressure. Unless all 3 carbs had issues.


http://youtu.be/cPaCyv63y_w

I know my own still has the brass floats, perhaps not for long.
 
What would happen if the cam gear had somehow skipped/broke a tooth? Would it produce these symptoms?

Really stretching on this one....
 
too much fuel

When I have seen carburetors dispense too much fuel, it is because:

1) float has a leak in it and sinks
2) too much fuel pressure at float needle valve
3) additional fuel path besides normal jets from leaking seal or crack in the carburetor body, or broken/leaky accelerator pump
4) something causing artificially high velocity/low pressure in the venturi so too much fuel is sucked through the jet (the equivalent of going to higher altitude)
5) some other source of fuel besides the carburetor

switching carburetors would seem to eliminate (1) and (3) and assuming the air filter and alternate air box are removed, also (4).
You haven't suddenly moved to a higher altitude airport, right?

plugging primer lines addresses (5) but are there ANY other paths that could introduce fuel?

Lots of folks focusing on (2) because its probably the most common issue, and it seems like the only one left.

When you swapped carbs, was EVERYTHING downstream of the fuel pump swapped?
Not being familiar with the MA-4 carburetor, so thinking out of the box here:
are there portions of the carburetor system that are not exchanged when you exchange carb bodies? Like if the float chamber were not integral with the carb body and you didn't swap it when you swapped carbs? Or if the accelerator pump was removed from each carb and stayed with the airplane?

Changing cam timing (like a broken gear or a slipped chain) will make the engine run crappy, but I don't think it will act like it is super rich like this. You are getting too much fuel, or not enough air.
 
PCHunt said:
Would the wrong "dash number" carb cause extreme rich mixture?

:confused::confused::confused::confused:
Click to expand...

Not really at Idle and Transition levels. Most of the variations across the model are in the main jetting. At 1300 RPM, you are on the transition circuit. I would not expect any differences here across the dash numbers.

On the subject of TW, Carb floats are not set differently based upon where the wheels are placed. They assume the engine is level and all planes fly at the same level position, regardless of sitting angle. Engines are optimized for their normal condition (in flight) and not their taxi position. Yes, float level will be a bit off in a tail dragger, but it does not significantly impact performance.

Larry
 
scsmith said:
When I have seen carburetors dispense too much fuel, it is because:

1) float has a leak in it and sinks
2) too much fuel pressure at float needle valve
3) additional fuel path besides normal jets from leaking seal or crack in the carburetor body, or broken/leaky accelerator pump
4) something causing artificially high velocity/low pressure in the venturi so too much fuel is sucked through the jet (the equivalent of going to higher altitude)
5) some other source of fuel besides the carburetor

switching carburetors would seem to eliminate (1) and (3) and assuming the air filter and alternate air box are removed, also (4).
You haven't suddenly moved to a higher altitude airport, right?

plugging primer lines addresses (5) but are there ANY other paths that could introduce fuel?

Lots of folks focusing on (2) because its probably the most common issue, and it seems like the only one left.

When you swapped carbs, was EVERYTHING downstream of the fuel pump swapped?
Not being familiar with the MA-4 carburetor, so thinking out of the box here:
are there portions of the carburetor system that are not exchanged when you exchange carb bodies? Like if the float chamber were not integral with the carb body and you didn't swap it when you swapped carbs? Or if the accelerator pump was removed from each carb and stayed with the airplane?

Changing cam timing (like a broken gear or a slipped chain) will make the engine run crappy, but I don't think it will act like it is super rich like this. You are getting too much fuel, or not enough air.
Click to expand...

I agree with everything here and struggle to come up with alternate theories. I thought about cam timing as well, but it just won't create the symptoms, as described. particularly the fact that it runs normally at full power.

I would look carefully at the mixture linkage. Have you tried aggressive leaning?

Have you completely removed the air box assembly or just the filter? There is a hole in the top (top points to the ground in a vert sump) of the carb that admits atmospheric pressure to the idle and transition circuit. This hole must be unobstructed or you will get a very rich idle and transition A/F ratio.

Have you observed your manifold pressure (i.e. vacuum) during the excessively rich condition? Do you know the base line readings when normal? Is so, please advise here.

I continue to contend that leaks in the induction system cannot create a rich condition. That is a classic lean condition

Larry
 
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See embedded comments in Dark Red

scsmith said:
When I have seen carburetors dispense too much fuel, it is because:

1) float has a leak in it and sinks
True. But on 3 different carbs? And wouldn't fuel drip out the bottom of the carb with fuel pressure if the float had sunk? Not sure.
2) too much fuel pressure at float needle valve
We have measured the fuel pressure with a digital pressure gage, which confirmed the normal reading.
3) additional fuel path besides normal jets from leaking seal or crack in the carburetor body, or broken/leaky accelerator pump
Three different carbs.
4) something causing artificially high velocity/low pressure in the venturi so too much fuel is sucked through the jet (the equivalent of going to higher altitude)Again, 3 different carbs
5) some other source of fuel besides the carburetor[COLOR="DarkRed]We thought of that, but can't come up with another source. All 4 cylinders appear to be equally affected, as witnessed by the uniform very dark sooty deposits on all the spark plugs. So the issue is common to all cylinders.[/COLOR]

switching carburetors would seem to eliminate (1) and (3) and assuming the air filter and alternate air box are removed, also (4). [COLOR="DarkRed"]Concur[/COLOR]
You haven't suddenly moved to a higher altitude airport, right?Have not, still at the same airport.

plugging primer lines addresses (5) but are there ANY other paths that could introduce fuel?If there are, several of us are not aware of them. We wondered what a defective (leaking) engine-driven pump would do?

Lots of folks focusing on (2) because its probably the most common issue, and it seems like the only one left. As mentioned, checked the fuel pressure with a calibrated digital gauge. But it seems to be the only common issue.

When you swapped carbs, was EVERYTHING downstream of the fuel pump swapped?
Not being familiar with the MA-4 carburetor, so thinking out of the box here:
are there portions of the carburetor system that are not exchanged when you exchange carb bodies? Like if the float chamber were not integral with the carb body and you didn't swap it when you swapped carbs? Or if the accelerator pump was removed from each carb and stayed with the airplane? All the "parts" that you mention are integral to the MA4-SPA carburetor, and are renewed with each carb change.

Changing cam timing (like a broken gear or a slipped chain) will make the engine run crappy, but I don't think it will act like it is super rich like this. You are getting too much fuel, or not enough air. Concur. But still stumped.
Click to expand...

After several hours with several qualified local observers, one of them finally ventured: "Maybe you really did get 3 bad carburetors in a row!" Seems nearly impossible.

Plan to remove all intake tubes and inspect. And borrow 8 different spark plugs, before putting yet another carburetor on the engine.

Anybody know what would be the effect of a crack in the sump where the intake goes through the oil sump? Could oil be sucked into the intake and make these symptoms occur?

Grasping at straws.............

And thanks for your thoughtful post, Steve.
 
lr172 posts:

Have you completely removed the air box assembly or just the filter? There is a hole in the top (top points to the ground in a vert sump) of the carb that admits atmospheric pressure to the idle and transition circuit. This hole must be unobstructed or you will get a very rich idle and transition A/F ratio.

Have you observed your manifold pressure (i.e. vacuum) during the excessively rich condition? Do you know the base line readings when normal? Is so, please advise here.

-------------------------------------------------------------------

Completely removed all cowling, airbox, filter. Running on bare carb only. You bring up a good point about the hole that sends ambient pressure to the top of the float chamber. But, still, on 3 carbs???

No manifold pressure gauge installed on this Fixed Pitch Prop equipped aircraft.

Thanks,
 
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Here is my stretch. The fuel pump is leaking fuel into the crankcase and the fumes are getting into the cylinders at high vacuum ? not noticeable at higher RPMs. Either by an induction leak in the sump or via the rings.
 
Another wild guess

Any chance an exhaust hangar is vibrating the throttle cable / accelerator pump linkage? Perhaps just the throttle cable itself ?
 
PCHunt said:
lr172 posts:

Have you completely removed the air box assembly or just the filter? There is a hole in the top (top points to the ground in a vert sump) of the carb that admits atmospheric pressure to the idle and transition circuit. This hole must be unobstructed or you will get a very rich idle and transition A/F ratio.

Have you observed your manifold pressure (i.e. vacuum) during the excessively rich condition? Do you know the base line readings when normal? Is so, please advise here.

-------------------------------------------------------------------

Completely removed all cowling, airbox, filter. Running on bare carb only. You bring up a good point about the hole that sends ambient pressure to the top of the float chamber. But, still, on 3 carbs???

No manifold pressure gauge installed on this Fixed Pitch Prop equipped aircraft.

Thanks,
Click to expand...

PChunt,

You'll find all of these suggestions a bit on the extreme, but that is only because we're all stumped. There is no good reason why you are running rich outside the carb itself. In your desperation, I would be looking at something that could be causing this that would exist in each of three carbs. I agree that it seems unlikely that all three carbs have the same condition, but we are running out of alternate theories.
 
fuel pump leaks and that vent hole

I thought about the possibility of engine-driven fuel pump leaking too. It would dump fuel into the accessory case and sump, and would dilute the oil. So you would notice the thinned oil, and you would be able to smell it at the dipstick hole. It would be pretty hard to suck enough vapor into the cylinders from the sump to affect the mixture. If the fuel pressure is normal, it suggests the pump must be working -- a bad leak and it probably wouldnt maintain pressure.

But just in case, can you bypass the engine-driven pump and try running from just the boost pump? Or try running from a small tank hanging over the plane from the hangar roof or a ladder or something, to just gravity feed?
If it still floods with gravity feed, we KNOW its not the fuel pressure.

The vent hole that must be open.....is it possible that a gasket can be reversed or something like that could seal that hole so that it would do the same thing with all three carbs? Or does the gasket come off with the air box and that vent hole is visible? Spiders built webs in all three vent holes?

Here is a potentially useful test: With engine off, run the boost pump for a reasonable period of time. If the float chamber is functioning normally, nothing much should happen. If the needle valve is stuck open, the float chamber should overflow and fuel will drip out. Again it seems odd/unlikely that all three carbs would have the problem suddenly.

Is there anything else that could cause the air pressure in the carburetor internals to be different from normal atmospheric pressure? A vacuum line that gets connected? A vent line that runs somewhere that all three carbs have been connected to?

Something has to be causing the fuel level to be too high in the chamber that supplies fuel to the idle jets.
 
First, I want to thank everyone for their thoughtful and considered responses.

Here is an update, and my plans for the next week or so.

Today I checked the following:

Plugs, check for resistance, all 8 were 2500 ohms or less. then they were all tested in a Champion plug "Bomb" tester, and all sparked well. This was before I cleaned them and gapped them. The gaps were OK.

Ignition wires, checked with an Eastern brand high voltage wire tester. All 8 wires passed the test.

Removed all intake tubes, inspected the oil sump intake passages, all seemed normal.

Mag timing checked, spot on at 25 degrees.

THE PLAN:

I have taken my carb apart, at the big seam. Inspected, found nothing obvious. Removed old metal (brass) floats and needle valve. Tested floats in boiling water, no air bubbles. Ordered new float/valve kit to upgrade to epoxy floats. The vertical tube that sends ambient pressure to the float chamber was clear.

Will reassemble carb, and try it on my (hopefully) good friend's RV-9A, while trying his carb on my plane. (Shhh! He doesn't know this yet!) :p

That should settle it. The problem will either follow my carb, or it won't.

Will let you know in a couple of weeks.

Again, THANKS to all.
\
PS: Have run the boost pump (45-5 psi) at length, and no leakage from the carb.

Have not tested or inspected the engine driven pump. It has two diaphragms, IIRC, and if the first one (upstream) leaks, the leakage should come out of the 'witness" drain before going into the engine. Both diaphragms would have to go bad to get fuel into the sump. Correct me if I'm wrong, please.

I also have not removed the mags for bench testing. All that may come later if my PLAN fails.
 
PCHunt
I had a similar sounding issue on my 68 Bonanza. Turns out it was the engine driven pump. Sent it out for repair and the shop said the diaphragm was "like a potato chip". I'm assuming they meant not nice and flat but all wavy (stretched out) and stuff. Will be reinstalling this weekend.
Good luck!
Jeff
 
I do not claim to be an expert on fuel pumps. However, the above post made me think about the mechanical fuel pump. I highly recommend bypassing the mechanical pump as lifeofreiley suggested early in this thread.
Suppose some failure in the mechanical pump is aerating the fuel. The aerated fuel would cause the floats to drop on any carb. The richening effect of the lower floats might be less noticable at high power settings.

A non-running test with the boost pump could show nothing wrong since the mechanical would not be running.

To others that have more experience with Lyc fuel pumps, Is there a failure mode of the pump that could aerate the fuel?
 
rzbill said:
I do not claim to be an expert on fuel pumps. However, the above post made me think about the mechanical fuel pump. I highly recommend bypassing the mechanical pump as lifeofreiley suggested early in this thread.
Suppose some failure in the mechanical pump is aerating the fuel. The aerated fuel would cause the floats to drop on any carb. The richening effect of the lower floats might be less noticable at high power settings.

A non-running test with the boost pump could show nothing wrong since the mechanical would not be running.

To others that have more experience with Lyc fuel pumps, Is there a failure mode of the pump that could aerate the fuel?
Click to expand...

a good thought. However, I believe that aerated fuel is only an issue when trapped/contained in sealed and otherwise air tight vessels, such as fuel lines. Carb bowls are vented to the atmosphere and aerated fuel will quickly give up it's it's air, just as aerated water in a glass will.

Aerated fuel is a major problem for fuel injected setups (fully sealed system), but not carbs (vented system).

Larry
 
UPDATE

UPDATE

Today I installed yet another carburetor, this time one from my good friend's plane that ran perfectly yesterday, and my engine still runs badly at 1300 RPM and below. So it is NOT the carb.

My next step will be to bypass the engine driven pump as suggested. After that, I will again be out of ideas. After an 8 minute run mostly at low RPM, a look at one plug showed dry black uniform deposits. And an aggressive leaning of the mixture resulted in a 250 RPM gain from 1000 up to 1250.

Hopefully will be able to report back Saturday night

And THANKS to all for your help! Any and all ideas are greatly appreciated!

And, if there were a crack in the portion of the intake that passes through the sump, what would the symptoms be?? Anyone ever had that issue?

What ignition problems could cause these symptoms?

:confused::eek::confused:
 
PCHunt said:
What ignition problems could cause these symptoms?

:confused::eek::confused:
Click to expand...

Put a strobe light on each mag and verify timing running at different rpm's. ( below and above 1300) I'm thinking impulse mag may not be happy. Doing a timing check with engine running can sometimes show what a non running check won't.
 
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If there is an intake crack in your sump I would think you would see blue smoke during start up at a minimum. You would likely see oil draining down into the intake plenum/carb as well.

Edit
Looking back at your earlier posts it looks like you have verified your ignition system. A running timing check can't hurt tho...
Run it with the boost pump and see if it runs better.
 
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Major Update!!

Well, today was an eye-opener.

A friend at the airport who has been helping me troubleshoot had noticed that I had been running all the carbs bare, i.e.: no airbox and filter installed. He wondered if the prop blast could be upsetting the airflow into the carb enough to cause the symptoms.

We both didn't think so, but it was an easy experiment, so I bolted up the airbox, with newly cleaned and oiled filter. This is on another friend's known good carburetor.

Well, it fixed the problem on his carb!!! All ran well, and I was elated.

But........ stay tuned, for the REST of the story!! (With apologies to Paul Harvey.)

On a second run only 10 minutes later, I had difficulty starting the engine, and upon performing a mag check, found that the Left mag was totally DEAD!

I am now surmising that the original rough running at low RPM might have been a mag issue, and that the subsequent troubleshooting done with a bare carb led us off the track!

(I think I have a headache!!)

Stay tuned for the conclusion.
 
I've had stories like that before. Drives you crazy, doesn't it?! I never run and engine unfiltered on the ground. Too much chance of sucking sand into a formerly perfectly good engine.
 
I knew that impulse was the issue. Sooner or later it was going to fail.
Glad the issue presented itself, what a relief you must feel.
 
Success!

Success!

Finally, a solution! Thanks to all for the help.

Bottom Line: It turned out that the left mag was in the process of failing, which was causing the rough idle. Fortunately for me, it completely failed in the end, forcing me to get it looked at.

However, in trying to troubleshoot the issue, when I took the upper and lower cowl, airbox, and filter off, (thinking I was eliminating those as issues,) what I REALLY did was add in another problem, which masked the original issue.

Running the engine with a bare carb, (meaning no airbox and filter,) turned out to be a very bad idea. Apparently the airflow into the carb gets disturbed, and that?s what was causing all the rich running at low RPM.

I was even warned about that possibility in a PM from a true engine expert, who occasionally frequents this forum. So, how did I ?ignore? his warning?? I ?reasoned?, or at least thought I did, that the engine was running badly with the cowl and airbox installed, with little difference after removing them.

I shoulda listened!! Would have saved me a month of down time chasing the wrong issue!!

And on day one or day two, one of my local airport experts said: ?Sounds like an ignition issue to me?..?

And another friend said, ?Take the mags off and have them checked.?

Again, shoulda listened!! :eek:

So, after having a 500-hour inspection and repair on the left mag, the plane is running normally. The mag needed a new cam, and a new carbon brush. Apparently the cam had worn irregularly, and was causing the points to open improperly.

Listen to your friends, especially those more experienced than you!!!

And, again, thanks to all who responded. :)
 
Solution?

PCHunt said:
UPDATE

Today I installed yet another carburetor, this time one from my good friend's plane that ran perfectly yesterday, and my engine still runs badly at 1300 RPM and below. So it is NOT the carb.

My next step will be to bypass the engine driven pump as suggested. After that, I will again be out of ideas. After an 8 minute run mostly at low RPM, a look at one plug showed dry black uniform deposits. And an aggressive leaning of the mixture resulted in a 250 RPM gain from 1000 up to 1250.

Hopefully will be able to report back Saturday night

And THANKS to all for your help! Any and all ideas are greatly appreciated!

And, if there were a crack in the portion of the intake that passes through the sump, what would the symptoms be?? Anyone ever had that issue?

What ignition problems could cause these symptoms?

:confused::eek::confused:
Click to expand...


How did you solve the problem?
 
PCHunt said:
Checked compression, 74 and above all four cyls. Checked Mag timing, right on the mark (25 degrees) both mags. Removed rocker covers, and observed valve action, all appeared normal. Several sets of qualified eyes were observing.

Read the troubleshooting guide for MA4-SPA carb, and checked the accelerator pump check valve, passed the test, and the accel pump is functioning normally.


Still stumped.

One of the two trial carbs came off of a running engine that was headed to an overhaul shop. Very clean looking newish carb.

Would the wrong "dash number" carb cause extreme rich mixture?

Keep in mind, all 8 plugs are black with sooty deposit, so all cylinders appear to be equally affected. And while running at 1000 RPM full rich, then with aggressive leaning the engine will gain 250-300RPM. :eek:

And the engine is making full power, full static RPM (2150-2200) at rich mixture, and by leaning a bit, can gain 40 to 50 RPM.

Time to remove the intake pipes and hoses and have a look? But all cylinders are equally affected.

:confused::confused::confused::confused:
Click to expand...


My engine has the same symptoms. Did you solve your problem?
 
Hans, in the upper right corner of each posting is it's number. This post will be #50. So just go back a few posts until you see #41, and you will see the solution if you read through to the end.

If this doesn't work, let me know, and I will re-post.

Cheers!
 
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