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O320 rough idle

Forgot about this one....my apologies

I was referring to the discharge check valve, you have that covered now.

If the float level is too high you might be getting some discharge out of the nozzle. You could plug the nozzle with a cap, secure it with some tape and see if you get your idle mixture control back.

Have you confirmed your fuel level in the float bowl?
 
O320 rough idle - ongoing problem

Lr172
I'm still struggling with my problem. The engine does not respond to the mixture screw settings. I have tested with 5 turns. I found leaking throttle bushings and I put in a new shaft and new bushings but no change. I tested another, green-tagged carburetor - same thing. There is no rise in RPM when leaning from 1000 RPM. I have a 100 RPM drop at a magneto test at 1700 RPM. It's a bit high I think. I made a compression test and it showed 79 all around which rules out a sticking valve I suppose, or does it? I have put in new intake gaskets and made sure they are installed properly. Replaced the intake hoses with new ones of silicon. It's obvious that the engine is running lean but I can't find the reason. The fuel pressure is steady at 6 PSI. The MAP is 14" at 600 RPM which is too high - should be around 10" they say. Don't know now where to look!
 
Lr172
I'm still struggling with my problem. The engine does not respond to the mixture screw settings. I have tested with 5 turns. I found leaking throttle bushings and I put in a new shaft and new bushings but no change. I tested another, green-tagged carburetor - same thing. There is no rise in RPM when leaning from 1000 RPM. I have a 100 RPM drop at a magneto test at 1700 RPM. It's a bit high I think. I made a compression test and it showed 79 all around which rules out a sticking valve I suppose, or does it? I have put in new intake gaskets and made sure they are installed properly. Replaced the intake hoses with new ones of silicon. It's obvious that the engine is running lean but I can't find the reason. The fuel pressure is steady at 6 PSI. The MAP is 14" at 600 RPM which is too high - should be around 10" they say. Don't know now where to look!

Pls confirm that at 750-800 RPM, you can turn the idle mixture screw all the way in and the engine still runs. Also, are you able to kill the engine by pulling the mixture knob all the way out (ICO). Do you have a Primer system? If so, you will need to disconnect it and repeat the tests; It is possible this is the source of excess fuel.

You cannot adjust idle mixture at 1000 RPM. It must be 800 or less. You may be pulling a bit of fuel from the main jet at 1000 and that will mess with your leaning test. You should be observing MAP while adjusting the mixture screw at 800 RPM. This will be a better indication of what is happening than the RPM rise test. You are looking to get the lowest MAP possible at 800 RPM. make change to mixture screw and observe MAP - Repeat. You will need to re-adjust the throttle back to 800 after each mixture change. You are looking for around 10-11" of MAP at 800. Once you get to the lowest MAP in that range, don't worrry about the RPM rise test.

If 5 turns of the mixture screw does nothing and you are running with the screw all of the way in, you are NOT lean. You are getting fuel from somewhere other than the idle mixture circuit. Several issues inside the carb can do that, but would not expect to see the same problem on two different carbs.

Mag drop tests are all about ignition and have nothing to do with idle mixture. Mixture does affect RPM drop during mag check, but this has nothing to do with the idle mixture.

Larry
 
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I'm test running my O320 and it runs rough at idle and when I throttle back all the way it stops. I tried to adjust the idle rpm with the screw on the carb and I could barely get it to run at 650 RPM and then the rpm fluctuates between 600-700 rpm and it misfires. As for the adjustment screw I think it is screwed in almost as much as it can be (see photo). Does this seem normal? I can't even decide if it is running too rich or too lean. The magnetos are correctly set, the leads and all weather spark plugs are new. Any ideas?


Hans,

Unrelated to the rough idle issue,
I noticed in the original photo that it "appears" that your mixture control pivot is secured by a fiber locknut?
If that is the case, I would consider changing that setup to a castellated nut and drilled bolt. And I don't think replacing the fiber nut with an all-metal locking nut would be appropriate.
Anyway, just my opinion,, I would have someone look at that.

edit:
I just noticed that you are not using a bolt/nut arrangement for the pivot connection. In which case the drilled bolt option won't work. So at the least, I would consider replacing the fiber nut with an all-metal locking nut. Again, just my opinion.

edit #2:
I also noticed that your carb mount nuts are not secured by an internal lockwasher.
My understanding is Lycoming requires (suggests) a lockwasher for those four nuts.

Something else to consider.

Regards,
 
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Pls confirm that at 750-800 RPM, you can turn the idle mixture screw all the way in and the engine still runs. Also, are you able to kill the engine by pulling the mixture knob all the way out (ICO). Do you have a Primer system? If so, you will need to disconnect it and repeat the tests; It is possible this is the source of excess fuel.

You cannot adjust idle mixture at 1000 RPM. It must be 800 or less. You may be pulling a bit of fuel from the main jet at 1000 and that will mess with your leaning test. You should be observing MAP while adjusting the mixture screw at 800 RPM. This will be a better indication of what is happening than the RPM rise test. You are looking to get the lowest MAP possible at 800 RPM. make change to mixture screw and observe MAP - Repeat. You will need to re-adjust the throttle back to 800 after each mixture change. You are looking for around 10-11" of MAP at 800. Once you get to the lowest MAP in that range, don't worrry about the RPM rise test.

If 5 turns of the mixture screw does nothing and you are running with the screw all of the way in, you are NOT lean. You are getting fuel from somewhere other than the idle mixture circuit. Several issues inside the carb can do that, but would not expect to see the same problem on two different carbs.

Mag drop tests are all about ignition and have nothing to do with idle mixture. Mixture does affect RPM drop during mag check, but this has nothing to do with the idle mixture.

Larry


Larry
I'll do the testing in the coming weekend.
You said the mag drop is an ignition issue. I have Slick 4370/4371 mags. I had the 4370 tested in a aviation mechanics workshop and it had a good spark down to 500 RPM. We did not test the 4371 because the hazzle of having to remove the impulse coupling and I thought that if one is OK so is the other. I have replaced one coil, changed both distributor gears and set the E-gap. Could there still be something with the magnets?
 
edit #2:
I also noticed that your carb mount nuts are not secured by an internal lockwasher.
My understanding is Lycoming requires (suggests) a lockwasher for those four nuts.

He is using double nuts, the ultimate approach for vibration resistance. When torqued properly, this is far more resistant to vibration loosening than a star lock washer. I had one nut loosen with the star washer on the servo and moved to double nuts.

Larry
 
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Larry
I'll do the testing in the coming weekend.
You said the mag drop is an ignition issue. I have Slick 4370/4371 mags. I had the 4370 tested in a aviation mechanics workshop and it had a good spark down to 500 RPM. We did not test the 4371 because the hazzle of having to remove the impulse coupling and I thought that if one is OK so is the other. I have replaced one coil, changed both distributor gears and set the E-gap. Could there still be something with the magnets?

Ignition is generally unrelated to the problem you are having. I think it is best to do the idle mixture testing via MAP readings first. Ignition should have no bearing on your ability to adjust idle mixture. If your ignition was having issues, it should also show difficulty in starting and exhibit symptoms at all RPM ranges, not just idle. Having one freshly overhauled/tested ignition is enough to initially rule out ignition in the rough idle diagnostics.

A MAP of 14" at 800 RPM is a good indication that something in the mixture being delivered is at issue, assuming everything is running well at other RPMs.
 
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Hans,

Unrelated to the rough idle issue,
I noticed in the original photo that it "appears" that your mixture control pivot is secured by a fiber locknut?
If that is the case, I would consider changing that setup to a castellated nut and drilled bolt. And I don't think replacing the fiber nut with an all-metal locking nut would be appropriate.
Anyway, just my opinion,, I would have someone look at that.

edit:
I just noticed that you are not using a bolt/nut arrangement for the pivot connection. In which case the drilled bolt option won't work. So at the least, I would consider replacing the fiber nut with an all-metal locking nut. Again, just my opinion.

Regards,

The rod ends he is using are a ball / socket arrangement and freely articulate. Just like with a Heim, there is no rotation of the threaded end going through the arms.
 
My point was t do much with the locknut, as it is with the type of locknut.
I prefer to use all-metal locknuts, firewall forward.
 
He is using double nuts, the ultimate approach for vibration resistance. When torqued properly, this is far more resistant to vibration loosening than a star lock washer. I had one nut loosen with the star washer on the servo and moved to double nuts.

Larry
Interesting,
I dig some looking and in the latest Lycoming parts manual for an O320-D1A, the callout is as it is here:
Plain washer, plain nut, pal nut

However in the parts manual for an O320-E2D, it's back to:
Plain washer, internal lockwasher, plain nut

Go figure.
Since I have enough threads available, I am using both. Probably an overkill,,,
 
Pls confirm that at 750-800 RPM, you can turn the idle mixture screw all the way in and the engine still runs .........

Larry


Larry
I measured the MAP at 750 rpm at various turns of the mixture screw. I could pull the mixture knob to ICO anytime and the engine stopped. I don't have a primer system. Here are the values at turns of mixture screw from closed to open. I stopped at 5 turns.
Turns / MAP
0 / 15
1 / 14
2 / 13.7
3 / 13.9
4 / 14.3
5 / 13.8
 
Larry
I measured the MAP at 750 rpm at various turns of the mixture screw. I could pull the mixture knob to ICO anytime and the engine stopped. I don't have a primer system. Here are the values at turns of mixture screw from closed to open. I stopped at 5 turns.
Turns / MAP
0 / 15
1 / 14
2 / 13.7
3 / 13.9
4 / 14.3
5 / 13.8

While I have never tried this on an MS carb, you should NOT have a smooth running engine at 750 RPM with the idle screw turned all the way in. The fact that a pull to ICO shuts the engine down means that all of the fuel to run the engine is being provided through the carb. I would definately repeat the test with the FAB removed. A plugged filter or other issues could be causing problems in the bleed side of the idle and main circuits. The most troubling part of your results are the 15" at 0 turns and this is best explained by either a blockage in the filter or high fuel pressure.

You are getting a peak at 2 -3 turns out, which is pretty typical for an MS on a 320. However, I would expect larger jumps in MAP per turn of the screw and, of course, would expect much lower MAP readings. Does your MAP gauge read close to ambient pressure before startup? Have you confirmed there is no leak in the hose/line? For now, leave the screw 2.5 turns out.

I would suggest a problem in the idle circuit or possibly pulling fuel from the main jet. However, the likelihood of seeing this problem on two different carbs with one being freshly overhauled by a pro makes this an unlikely scenario. I am assuming this data was taken from the freshly overhauled carb.

Another possibility is high fuel pressure. That carb can only restrain about 6-7 PSI of fuel pressure before the fuel begins to overfill the bowl. This usually results in most problems at idle and off idle transitions. Do you have a hesitation when accelerating from idle? At some point, we should confirm the accuracy of fuel pressure reading. AC style fuel pumps can and do fail by delivering too much pressure. I had this happen on the chevy small block in my boat. IN that case, I could see the fuel overflowing from the bowl vents, however if yours is only marginally high it may impact idle without overflowing. Have you seen any blue staining around the carb?

With your MAP readings, I would not suspect high fuel pressure, except that it could explain why you get 15" at 0 turns out.

If were trust the rebuilt carb, we need to start looking for a leak somewhere in the path between the carb and each of the four intake bosses. You can get a can of starting fluid and start spraying it everywhere on the intake path (don't spray at the main opening of the carb) while the engine is idling. Any change in sound or RPM, means you just sprayed on an area with a leak. Don't forget that the cylinder heads themselves can crack, as can the sump and tubes, so spray everywhere the air/fuel goes to the cylinder. Intake leaks are most problematic at idle and low RPMs, so this does fit your symptoms and would explain the high MAP readings at idle.

The irregular nature of the MAP readings could be explained by an intake leak. However, they can also be explained by carb problems as well as problems in the valve train.

Larry
 
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While I have never tried this on an MS carb, you should NOT have a smooth running engine at 750 RPM with the idle screw turned all the way in. The fact that a pull to ICO shuts the engine down means that all of the fuel to run the engine is being provided through the carb. I would definately repeat the test with the FAB removed. A plugged filter or other issues could be causing problems in the bleed side of the idle and main circuits. The most troubling part of your results are the 15" at 0 turns and this is best explained by either a blockage in the filter or high fuel pressure.

Larry


The MAP gauge shows 30 with engine off wich is correct (29.9 at standard atmophere). The data taken are from my "old" carb. When the the other carb showed the same values I switched back to my old carb thinking carb was not the problem. The green-tagged carb is not freshly overhauled. It was overhauled in 1987 and has been put on a shelf until a bought it.
I uploaded a log file on the SavvyAnalysis site and I can see that 6.5 PSI is the highest fuel pressure, 6 being the normal. I removed the oil pan some weeks ago and checked the sump and tubes for leaks and didn't find any. I will try the test with the FAB removed.
 
I uploaded a log file on the SavvyAnalysis site and I can see that 6.5 PSI is the highest fuel pressure, 6 being the normal. I removed the oil pan some weeks ago and checked the sump and tubes for leaks and didn't find any. I will try the test with the FAB removed.

How often does it show 6.5. 6 is the END of the normal range (4-5 is normal) and would expect some leaking past the needle / seat by 7 PSI or less in some cases. THis leakage past the seat will raise the fuel level in the bowl and will cause a univerally rich mixture. I would encourage you to put a mechanical pressure gauge on the engine to confirm this. You can also do a test. Run the engine at 750 RPM and then try to get the MAP down to 11 or 12 via leaning with the red knob. If you are able to do that, you likely have an issue with the fluid level in the bowl, most likely caused by high fuel pressure or possibly debris in the needle/seat (unlikely with an o/h'ed carb).

Larry
 
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How often does it show 6.5. 6 is the END of the normal range (4-5 is normal) and would expect some leaking past the needle / seat by 7 PSI or less in some cases.

Larry


Here are the MAP and fuel pressure readings.






 
Pls confirm that at 750-800 RPM, you can turn the idle mixture screw all the way in and the engine still runs. Also, are you able to kill the engine by pulling the mixture knob all the way out (ICO). Do you have a Primer system? If so, you will need to disconnect it and repeat the tests; It is possible this is the source of excess fuel.

You cannot adjust idle mixture at 1000 RPM. It must be 800 or less. You may be pulling a bit of fuel from the main jet at 1000 and that will mess with your leaning test. You should be observing MAP while adjusting the mixture screw at 800 RPM. This will be a better indication of what is happening than the RPM rise test. You are looking to get the lowest MAP possible at 800 RPM. make change to mixture screw and observe MAP - Repeat. You will need to re-adjust the throttle back to 800 after each mixture change. You are looking for around 10-11" of MAP at 800. Once you get to the lowest MAP in that range, don't worrry about the RPM rise test.

If 5 turns of the mixture screw does nothing and you are running with the screw all of the way in, you are NOT lean. You are getting fuel from somewhere other than the idle mixture circuit. Several issues inside the carb can do that, but would not expect to see the same problem on two different carbs.

Mag drop tests are all about ignition and have nothing to do with idle mixture. Mixture does affect RPM drop during mag check, but this has nothing to do with the idle mixture.

Larry


My RV7 friend said I should test the carb fuel level so I did. It was 1/4" high so I bent the float tabs slightly and got the fuel level right. The tapered end of the mixture screw was a bit worn so I substituted the screw from the other carb. That screw looked new. I tested today again, turned the mixture screw all in and the engine ran as before. That stops fuel from entering the top hole but there are three more idle holes where fuel can come in and they cannot be adjusted (if I understand it correctly). What are the issues inside the carb that can cause this? I tried to lean it from 800 rpm but it did not change the MAP at all.
 
O320 rough idle - End of story

Pls confirm that at 750-800 RPM, you can turn the idle mixture screw all the way in and the engine still runs. Also, are you able to kill the engine by pulling the mixture knob all the way out (ICO). Do you have a Primer system? If so, you will need to disconnect it and repeat the tests; It is possible this is the source of excess fuel.

You cannot adjust idle mixture at 1000 RPM. It must be 800 or less. You may be pulling a bit of fuel from the main jet at 1000 and that will mess with your leaning test. You should be observing MAP while adjusting the mixture screw at 800 RPM. This will be a better indication of what is happening than the RPM rise test. You are looking to get the lowest MAP possible at 800 RPM. make change to mixture screw and observe MAP - Repeat. You will need to re-adjust the throttle back to 800 after each mixture change. You are looking for around 10-11" of MAP at 800. Once you get to the lowest MAP in that range, don't worrry about the RPM rise test.

If 5 turns of the mixture screw does nothing and you are running with the screw all of the way in, you are NOT lean. You are getting fuel from somewhere other than the idle mixture circuit. Several issues inside the carb can do that, but would not expect to see the same problem on two different carbs.

Mag drop tests are all about ignition and have nothing to do with idle mixture. Mixture does affect RPM drop during mag check, but this has nothing to do with the idle mixture.

Larry


I had two RV friends volunteer to have a look at my problem. We tested for leaks and found a few very small ones. I didn't think they'd matter but they insisted on fixing them so we did that, replacing a few gaskets. After setting the idle RPM at 700 the engine runs fairly well and at 800 rpm I now have MAP of 13.2 which is marginally better than before. The initial problem is still there but I don't see what I can do about it. I've done everything and more. I did a few high-speed ground runs today and I hope to be flying shortly. End of story!
 
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