3rd opinion wanted from engine smarty pants
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3rd opinion wanted from engine smarty pants
Got a nagging issue with the Maule since the engine tear down inspection and new prop after the prop strike, Here goes:
When I do a mag check it runs rough on the left and really rough on the right, a lot of farting and popping and shaking and outside the tolerances for rpm drop, with a 300rpm drop on the right mag like the plugs are fowled, I go to 2300 rpm and lean it out and run it for a minute and it clears. If however I taxi for a minute (with it leaned) then do another mag check I get the same problem, I took it back to the shop that done the engine tear down and they said it was running rich and set the carb a bit leaner, this did not clear the problem, I took it to a shop at my local field they said when they removed the bottom plug from cylinder 1 a bunch of avgas ran out. They also said it was set to lean, the adjusted the mixture removed cleaned and tested the plugs and said the engine run up and mag check was fine. I get Into the plane taxi up to the runway and the same **** starts happening, I do a high rpm run up at lean to clear the problem then take off, the engine runs fine in the air and switching mags checks out ok. However as soon as I land and taxi for a minute then try do a mag check the same **** starts with all the popping and farting and rough running. I note that on the right mag the number 1 cylinder drops while the rest raise while doing a mag test. I asked the shop if they rotated the plugs and they said they all checked out so there was no need.
Add this problem to the new prop "surging" on take off and I'm starting to get pissed off here.
Is it possible that the plugs are actually ok as the shop said but the mag has an intermittent problem during run ups but works fine while flying..
Could the prop surging on take of be related to this mag/ plugs issue?
Could it be the primer cable was reinstalled wrong?
Edit to add: if this was on Scotland I'd have the bugger in the hangar and remove, rotate, and clean the plugs myself but I'm here in LA with no facilities or tools. And 2 different shops don't seem to think I have a problem
When I do a mag check it runs rough on the left and really rough on the right, a lot of farting and popping and shaking and outside the tolerances for rpm drop, with a 300rpm drop on the right mag like the plugs are fowled, I go to 2300 rpm and lean it out and run it for a minute and it clears. If however I taxi for a minute (with it leaned) then do another mag check I get the same problem, I took it back to the shop that done the engine tear down and they said it was running rich and set the carb a bit leaner, this did not clear the problem, I took it to a shop at my local field they said when they removed the bottom plug from cylinder 1 a bunch of avgas ran out. They also said it was set to lean, the adjusted the mixture removed cleaned and tested the plugs and said the engine run up and mag check was fine. I get Into the plane taxi up to the runway and the same **** starts happening, I do a high rpm run up at lean to clear the problem then take off, the engine runs fine in the air and switching mags checks out ok. However as soon as I land and taxi for a minute then try do a mag check the same **** starts with all the popping and farting and rough running. I note that on the right mag the number 1 cylinder drops while the rest raise while doing a mag test. I asked the shop if they rotated the plugs and they said they all checked out so there was no need.
Add this problem to the new prop "surging" on take off and I'm starting to get pissed off here.
Is it possible that the plugs are actually ok as the shop said but the mag has an intermittent problem during run ups but works fine while flying..
Could the prop surging on take of be related to this mag/ plugs issue?
Could it be the primer cable was reinstalled wrong?
Edit to add: if this was on Scotland I'd have the bugger in the hangar and remove, rotate, and clean the plugs myself but I'm here in LA with no facilities or tools. And 2 different shops don't seem to think I have a problem
Sounds like a leaking needle valve in the carburettor. Not wide open, just leaking.
At higher revolutions the engine can burn the fuel faster than it leaks, but not at idle, where the engine will be partly flooded.
At higher revolutions the engine can burn the fuel faster than it leaks, but not at idle, where the engine will be partly flooded.
Once bought a RV6 which had the same problem. The answer was found to be the
float setting ; the engine had been in a nose wheel a/c and when fitted to a tailwheel
configuration the carb flooded on rough ground. In the air it was fine.
I assume you have a proper Maule and not the learner driver type !
I would suggest blocking the a/c and lifting the tail into flying attitude and performing an
engine run.
We slightly adjusted the tab on the float that operated the shut off valve ; this cured the
problem instantly.
float setting ; the engine had been in a nose wheel a/c and when fitted to a tailwheel
configuration the carb flooded on rough ground. In the air it was fine.
I assume you have a proper Maule and not the learner driver type !
I would suggest blocking the a/c and lifting the tail into flying attitude and performing an
engine run.
We slightly adjusted the tab on the float that operated the shut off valve ; this cured the
problem instantly.
Last edited by NearlyStol; 18th Sep 2016 at 20:24.
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Done a bit more troubleshooting moved the plug from No 1 cylinder lower to no.3 upper and the problem stayed with the cylinder (no1) . The plug that came out of the problem cylinder was black and appears oily, is it possible as the engine has just had a tear down inspection and rebuild that the rings for cylinder number 1 are not seating correctly and are letting the combustion past at low rpm. The plug for cylinder 1 was cleaned and now it's fouled up again within one hour of flight.
I think you might be right about the rings not seating. Give it a good hard run or two. If that doesn't work you might need to remove that cylinder and have it honed. Sometimes the bore will glaze and the rings don't seat.
I have heard of drastic measures like putting brasso into the cylinder and giving it a run to bed in the rings.
You need to flush the engine and put in fresh oil afterwards.
I have heard of drastic measures like putting brasso into the cylinder and giving it a run to bed in the rings.
You need to flush the engine and put in fresh oil afterwards.
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Piston ring gaps sometimes line up and allow leaks, but never the same cylinder twice, helluva coincidence if they did. And they would sort themselves out after a hard run.
I would pull that cylinder off and have a look. It's not impossible to refit the thing with a ring missing altogether, or cracked while fitting it. No disrespect to your maintenance crew but it is possible.
If the plugs are fouled up after an hour it's definitely not behaving as it should, rings, valves, valve guides/seals.
I would pull that cylinder off and have a look. It's not impossible to refit the thing with a ring missing altogether, or cracked while fitting it. No disrespect to your maintenance crew but it is possible.
If the plugs are fouled up after an hour it's definitely not behaving as it should, rings, valves, valve guides/seals.
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Why not just have a compression test done?
Another thing worth investigating is the carburettor float. It was mentioned earlier that it may have been set up for a nosewheel aircraft during overhaul.
It may also have become unseated during storage. I had a similar situation after rebuild of the carb on my J3. Engine ran like a sack of spuds.
Engine shop suggested tapping the float bowl reasonably lightly with a mallet. Fixed the problem.
Why not just have a compression test done?
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This is my view on such things, for what it's worth.
Barrels when new are honed, that is they have a pattern of sharp 'scratches' in the bore. New rings are slightly overlarge and are sprung into the barrel but that deformation means that the curve of the ring cannot be exactly circular so sealing contact with the barrel will not be perfect around the ring. During break-in the sharp honing of the bore will abrade the rings to conformity while at the same time that sharp honing will be dulled by the rings so the ability of the honing to wear the rings will quite quickly be lost. Post break-in the rings should be perfectly lapped to the bores.
Disrupt that established setup, as with a prop strike inspection and it might be impossible to get everything back to just how it was. And while the honing will still be visible in the bores and able to hold an oil film it will no longer be sharp and abrasive, therefore no longer able to lap the rings to the bores in their new relationship. Sealing between the rings and bores will be less than perfect resulting in blow-by, high crankcase pressure and oil loss.
The situation may be further complicated by one cylinder having routinely run hotter than the others so that it might not be truly circular. That would exacerbate the problem of getting rings back into their previous alignment.
I suspect this maybe what you are now having to contend with so my opinion is that the bores may have to be honed, new rings fitted and the engine broken in again.
Perhaps one way to check if all this is likely is to measure the crankcase pressure, engine running, but for that to be meaningful you'd need to have some sort of 'normal' reference to evaluate against.
Barrels when new are honed, that is they have a pattern of sharp 'scratches' in the bore. New rings are slightly overlarge and are sprung into the barrel but that deformation means that the curve of the ring cannot be exactly circular so sealing contact with the barrel will not be perfect around the ring. During break-in the sharp honing of the bore will abrade the rings to conformity while at the same time that sharp honing will be dulled by the rings so the ability of the honing to wear the rings will quite quickly be lost. Post break-in the rings should be perfectly lapped to the bores.
Disrupt that established setup, as with a prop strike inspection and it might be impossible to get everything back to just how it was. And while the honing will still be visible in the bores and able to hold an oil film it will no longer be sharp and abrasive, therefore no longer able to lap the rings to the bores in their new relationship. Sealing between the rings and bores will be less than perfect resulting in blow-by, high crankcase pressure and oil loss.
The situation may be further complicated by one cylinder having routinely run hotter than the others so that it might not be truly circular. That would exacerbate the problem of getting rings back into their previous alignment.
I suspect this maybe what you are now having to contend with so my opinion is that the bores may have to be honed, new rings fitted and the engine broken in again.
Perhaps one way to check if all this is likely is to measure the crankcase pressure, engine running, but for that to be meaningful you'd need to have some sort of 'normal' reference to evaluate against.
I would think that the blackened plug could be indicating a rich mixture rather than oil fouling. If it's sooty then it means too much fuel, only if it's wet and oily does it mean oil problems.
If you have sufficient oiling to wet a plug, then your exhaust gasses will be smokey.
However, maybe you are taxiing with the engine too slow. At low rpm the cylinders suck harder against the closed throttle. So there is more suction to pull oil past the rings and/or valve guides.
.
If you have sufficient oiling to wet a plug, then your exhaust gasses will be smokey.
However, maybe you are taxiing with the engine too slow. At low rpm the cylinders suck harder against the closed throttle. So there is more suction to pull oil past the rings and/or valve guides.
.
Last edited by scifi; 21st Sep 2016 at 08:19.
Rough running and prop surging
For the rough running check the primer seals, if they have a slight leak it will bleed extra fuel into whichever cyl has the primer line (#1?) Causing it to run rich. This would also explain the fuel running out of the plug hole after it pools on shut down.
For the prop surging - if you have an MT govenor check the serial number - early ones had a surging problem which MT should fix. Otherwise the prop internal fine pitch stop may be set wrong. This is not uncommon on new props. You can check this by winding the govenor fine pitch stop right out and doing a full power run to check the max rpm. Be carefull you do not overspeed the engine! On hartzel the fine stop can be adjusted on wing. On MT it will need to go back to the prop shop.
For the prop surging - if you have an MT govenor check the serial number - early ones had a surging problem which MT should fix. Otherwise the prop internal fine pitch stop may be set wrong. This is not uncommon on new props. You can check this by winding the govenor fine pitch stop right out and doing a full power run to check the max rpm. Be carefull you do not overspeed the engine! On hartzel the fine stop can be adjusted on wing. On MT it will need to go back to the prop shop.
