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Thread: 200 rpm drop at cold mag check; normal drop after warm, but rough; oil temp creeps up in flight

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    200 rpm drop at cold mag check; normal drop after warm, but rough; oil temp creeps up in flight

    Wondering if anyone has seen this issue. Airplane came out of an extensive annual about a month ago. Replaced the rear-mounted oil cooler with new one (identical), new marvel schebler carb (to comply with the venturi AD), new air filter, most of the hoses, a couple spark plugs, and replaced engine mount. Mechanic put fire sleeves on most of the hoses. Compression and engine oil analysis were perfect. Mags checked out good. Everything else looked good and no issues on the initial run-up. As part of the oil change, switched from straight 50W to 15-50 Aeroshell.

    Have flown it a few times since. During most recent flight, I noticed about a 200 rpm drop on the left side during my initial run-up. After letting the engine warm up more, re-checked and it was about a 75-100 rpm drop, but it sounded slightly rougher on the left than right. During flight, I also noticed that the oil temp was creeping toward 200. Normally with the rear mounted cooler, I have to really work the plane to exceed 180 degrees. I chalked that up to the fact that it was a hot day and I was doing takeoff and landing, but afterwards started to wonder if it could be a timing issue.

    During the prior flight (short local site-seeing flight), I also noticed the temp creeping up to around 190 in level flight and, after descending to pattern altitude, I noticed a moment of roughness when I reapplied power. I noticed the same thing yesterday on my third takeoff as I advanced the throttle after back-taxing-- just a moment of roughness. Otherwise, it develops full power and will reach max RPMs in level flight. Maybe it is all related to the hot, humid weather. I don't know.
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    8GCBC's Avatar
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    Vernatherm needs replacing?

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    wireweinie's Avatar
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    Internal mag timing?

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    behindpropellers's Avatar
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    Quote Originally Posted by DanP View Post
    Wondering if anyone has seen this issue. Airplane came out of an extensive annual about a month ago. Replaced the rear-mounted oil cooler with new one (identical), new marvel schebler carb (to comply with the venturi AD), new air filter, most of the hoses, a couple spark plugs, and replaced engine mount. Mechanic put fire sleeves on most of the hoses. Compression and engine oil analysis were perfect. Mags checked out good. Everything else looked good and no issues on the initial run-up. As part of the oil change, switched from straight 50W to 15-50 Aeroshell.

    Have flown it a few times since. During most recent flight, I noticed about a 200 rpm drop on the left side during my initial run-up. After letting the engine warm up more, re-checked and it was about a 75-100 rpm drop, but it sounded slightly rougher on the left than right. During flight, I also noticed that the oil temp was creeping toward 200. Normally with the rear mounted cooler, I have to really work the plane to exceed 180 degrees. I chalked that up to the fact that it was a hot day and I was doing takeoff and landing, but afterwards started to wonder if it could be a timing issue.

    During the prior flight (short local site-seeing flight), I also noticed the temp creeping up to around 190 in level flight and, after descending to pattern altitude, I noticed a moment of roughness when I reapplied power. I noticed the same thing yesterday on my third takeoff as I advanced the throttle after back-taxing-- just a moment of roughness. Otherwise, it develops full power and will reach max RPMs in level flight. Maybe it is all related to the hot, humid weather. I don't know.
    What was the OAT?

    Tim

  5. #5
    cubdrvr's Avatar
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    What engine? Temps on my 150 run 110-120 above OAT routinely.
    Slick mags? Distributor gear?
    "Sometimes a Cigar is just a Cigar"
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  6. #6

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    150 hp with Bendix mags. I don’t recall the exact OAT but it was hot and humid— enough to make a noticeable difference in takeoff, climb and even landing.

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    BC12D-4-85's Avatar
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    Check the rpm rise at idle cutoff (for idle mixture) both just after startup then after the flight....plus the others things suggested.

    Edit: See page 5-5 for procedure. This is to partially address the roughness noted upon application of throttle.....that it's not caused by a too lean or rich air/fuel mixture at partial throttle, upon acceleration, or in cruise flight. Can be function of carb setup or potential induction system leaks that vary with engine temperature for example. The spark stuff is another matter.

    Gary
    Last edited by BC12D-4-85; 06-21-2021 at 07:07 PM.

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    wireweinie's Avatar
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    Read the OP. The high mag drop is on left mag only.

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    I am wondering if it maybe was a combination of unrelated things. As far oil temp, It was very hot and humid when I was flying— I was sweating in the cockpit. Plus I let the oil level dip to 6 quarts (maybe less volume would contribute to heat?), takeoff and landing practice, and switch from 50 weight to 15-50 might i’ll add up to slightly higher oil temps.

    As far as roughness on the left mag, the 200 rpm drop disappeared after letting the engine run a few more minutes, though there was a bit of lingering roughness. Not terrible, but just not as smooth as the other side.

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    Richgj3's Avatar
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    200 rpm drop is usually a fouled plug. Easy to check and eliminate.
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    aviatoraf's Avatar
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    I’ve seen similar behavior with a spark plug lead that was barely finger tight, never got snugged down, and loosened later on due to vibration.
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    BC12D-4-85's Avatar
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    If loose leads are suspected disable the squelch on your com radio and listen for unusual static. Leads that are loose can make a popping noise coinciding with the spark. They rise and fall in frequency with rpms.

    Gary

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    Thanks for that information! I appreciate the suggestions!

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    coil
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    Damm, if it was Slick mags that would be easy. I take it you do not have 4 cylinder EGT/CHT to spot a bad cylinder (gabe can you read this). As others have said could be a plug. Could be mag timing/mag points/ect. High oil temp usually means hight CHT that happens with advanced mag timing so that has to be checked. Check the left mag and see if moves by hand just for giggles then check timing.
    DENNY
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    skywagon8a's Avatar
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    Quote Originally Posted by Richgj3 View Post
    200 rpm drop is usually a fouled plug. Easy to check and eliminate.
    This. Pull the two bottom plugs connected to the left mag when the engine is cold after sitting overnight. One of them is likely fouled with oil. 200 rpm is indicative of one plug being fouled with oil. The reason it is better after running for a while could be the oil in the bad plug is burned out. If you have a multi-probe EGT, look for the cold cylinder while running on the left mag. That is the one.

    Changing to the thinner 15-50 could be the cause of a little more oil leaking past a valve guide while the engine is sitting.

    You could also swap the plugs top to bottom on the same cylinders. Does the rpm drop switch to the other mag?? If yes, it's definitely a plug.
    N1PA
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    55-PA18A's Avatar
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    Check the resistance of your plugs when you pull to check for fouling. See past threads on this issue. In original post, you mention several plugs were changed. Why? What brand and type of plugs are you using?

    I like to see 190 F oil temp. Any idea of accuracy of oil temp gauge?

  18. #18

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    The existing plugs are Champion. To be honest, I had not looked close at the new ones. I should have asked what the issue was with the old plugs, but at the time it seemed like a minor detail given the long list of work done during the annual. The plane does not have EGT/CHT-- it is a barebones Super Cub, with the same instrument panel layout that it had in 1950. Not sure on the accuracy of the oil temp gauge.

    I was a little concerned, given the amount of work done (new engine mount, new oil cooler, couple spark plugs, new hoses, air filter and carb replaced, etc), that the timing had been knocked off. Hoping it is just a lose connection!
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  19. #19
    skywagon8a's Avatar
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    Quote Originally Posted by DanP View Post
    I was a little concerned, given the amount of work done .................... that the timing had been knocked off. Hoping it is just a lose connection!
    If the timing had been knocked off (as you put it), the rpm drop would not have changed when the engine had been run for a while. It would still be the same. The same with the loose connection.
    N1PA
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  20. #20
    aktango58's Avatar
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    A couple of key indications: You changed oil to 15-50... been a few discussions on this subject, and I will not try to rehash that other than to say that multi viscosity oils run additives that are smaller molecules than the strait weight- which allows for oil leakage out of smaller holes than you previously saw. That could easily lead to fouled plug issues.

    Higher temps on the oil can easily be attributed to running the multi-grade, as the oil is overall thinner, allowing it to heat up more.

    The stumble in advancing the throttle sounds like carb ice maybe. Hot, humid, low power to higher power then clears quickly- easily could be ice. Are you using carb heat at the lower rpms?

    I would check all the plug wires also. I had that issue once after annual. Sometimes things happen.
    I don't know where you've been me lad, but I see you won first Prize!
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    Who made the hoses? We’re they checked for any obstructions before they were installed. Was the magneto timing checked or reset. Idle mixture only effects engine at or near idle. Is mixture arm going to full rich and lean. How many hours on the magnetos are they due or over due for a 500hr inspection? This tends to be ignored allot of the time, it’s a good idea if there old and have allot of hours on them.
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  22. #22

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    I just went thru this with a 300 rpm drop on one side. Moved the plugs around with no change in that mag drop on the same mag. So we replaced the mags. Also found a broken ignition wire where it goes thru a hanger. Changed the harness and now I have 50 and 75 rpm drop and all is good. We did find oil in one of the mags most likely a bad seal.
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