Carb Heat and RPM Drop

Carb Heat and RPM Drop

<HTML>When I apply carb heat on my C150-H the RPM drops 200 on the ramp and 200-400 at 7500 feet. Is this normal? The engine typically runs rough at runup until I lean it out and run it full lthrottle for a minute. The engine log refers to a "R&R" on the carb in 1992 when the needle and floats were replaced. I'm a new pilot and this is my first plane and I sure would hate the thing to give up the ghost on final.</HTML>

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Re: Carb Heat and RPM Drop

<HTML>William,
Naturally RPM drop when carb heat is applied is normal,I USED to have rough engine on runup until I got the Autogas STC...NOW....This is NOT meant to open the old can of worms that has been the topic of many many discussions on this site,(big ole grin to Grorge)but Ive had almost no mag drop at all since using the autofuel,and the engine actually runs smoother...back to carb heat...Typically the warmer the air entering the carb throat,the richer the engine will run,the richer it runs,the less heat you have going to the muffler shrouds,and thusly the less heat available for carb heat when you need it most,I suspect that the rpm drop youre seeing is relatively normal,as a matter of fact Id be more worried about the LACK of rpm drop with carb-heat applied.
Concerning the rough engine on run-up,Id take a good look at the plugs(especially the bottom ones) and wires..and of course if all else fails,the STC is a buck a HP,and for my situation it was the best 100.00 Ive spent on the bird...Tailwinds</HTML>

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Re: Carb Heat and RPM Drop

<HTML>Thanks Alan.</HTML>

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Re: Carb Heat and RPM Drop

<HTML>If your engine clears up after a full throttle run at runup, then the autogas (no lead) will not solve your problem.  Lead fouling will be reduced with autogas (in direct proportion to increased long-term maintenance increases [grin]) but your full throttle runup solution is indicative of lower plugs suffering from oil fouling.
Oil fouling is due to worn valve-guides allowing the oil contained in the rocker-box to drain down the valve stem and into the cylinder while your engine is sitting.  The oil pools in your lower plugs and prevents them from firing until the oil is burned away.
  Lead fouling, which some people experience from operating excessively rich on AvGas can only be removed by mechanically cleaning them after removing them from the engine.  It can be prevented by using Alcor TCP, or by aggressively leaning including and especially during ground operations.</HTML>

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Re: Carb Heat and RPM Drop

<HTML>Since lead fouling can only be removed by mechanical means, it is not indicated in your situation.</HTML>

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Re: Carb Heat and RPM Drop

<HTML>Uh oh, worn guides and oil pooling around lower plugs? The motor is 35 hours into a complete major and all of my plugs appear to be on top of the cylinders. What gives?</HTML>

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Re: Carb Heat and RPM Drop

<HTML>Your engine has plugs on both top and bottom of your cylinders.  A new engine that has excessive clearance on it's valve guides would behave this way, although another explanation COULD be (not likely) that the rings are not yet seated and/or the oil control ring is improperly installed. 
  Next time your engine sits overnight, pull your lower plugs BEFORE starting and see if any are fouled or excessivley wet with oil.   Proceed according to that discovery.</HTML>

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Re: Carb Heat and RPM Drop

<HTML>William,
Listen to George,he KNOWS his stuff..Also,Id check to see if Mineral Aviation oil was run the first 25 hours of operation.This is suggested in my OHC manual,and I believe is an industry standard.
According to the manual youre supposed to run straight mineral oil in youre engine for the first 25 hours of operation after changing a major assembly(such as a jug) this practice is supposed to help the rings set in the cylinders....I dont know if it will have any effect on valve guides,but Id suspect it would.After the initial 25 hours,then run regular Aviation oil.....Howdy george.....Is it there yet?</HTML>

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Re: Carb Heat and RPM Drop

<HTML>gentlemen... i've just stumbled into this forum and it seems to be a goldmine. i am a student pilot and a new owner of a 150e (1964). had a situation a few hours ago where the tach would not exceed 1700 on a takeoff run. the takeoff was aborted and, after discussion with my mechanic, we chalked it up to sticky valve (although #4 cylinder was repaired a few hours ago with new exhaust valve, guide, and valve seats).  most recently in a pre-flight runup i could not get the engine to run smoothly at a steady rpm for mag and carb heat check. my instructor called the flight off. i was wondering about sticky valve again when i read an article on carb icing, even on the ground taxiing. now it seems to me as though i might have had a carb icing problem (high humidity and 70 degrees on the field). in the meantime i'll throw some marvel mystery oil in the crankcase (or aviation equivalent) to fend off sticky valves, and play with the carb heat and fuel mixture if condition reappears. by the by, i'm running mineral oil to break in the new cylinder...  thoughts?</HTML>

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Re: Carb Heat and RPM Drop

<HTML>Thanks guys,

I'm still running mineral oil at 35 hours. I'll swap in the detergent stuff soon and inspect the cold plugs before I begin and check the compression. The motor is not THAT ratty at runup and runs the same on each mag during the check (does that rule out the oil on the lower plugs theory?). Being a new pilot I almost never lean the motor during flight though I have started pulling the mixture out an inch when taxing to the runup area. Most of my flights are at 3500 or less. During the prepurchase inspection the compression was in the mid to high 70's on each cylinder though one 'hissed' down. Everyone including my mechanic attributed that to unseated rings or carbon buildup. Looks like I might have to redue a jug. The cylinders are new Milliniums.</HTML>

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Re: Carb Heat and RPM Drop

<HTML>The old adage not to lean an engine below 5,000 feet was done to keep flight training simple for the high numbers of military pilots in WW-II.  Those guys would be in B-24's/B-17's etc in less than 200 hours, so a lot of shortcuts were taken to simplify their training.  (The more sophisticated airplanes they would ultimately end up in had auto-lean fuel controls and/or flight engineers who would take care of engine leaning, if necessary, or they would be taught the correct proceedure as part of their transition training.)   Since that time, for several decades after the war, civilian pilots were taught the same old proven methodology,....despite the fact that the process is wrong!   
  It is correct to lean at any cruising altitude, even if at sea-level!  Or while taxiing!
Just remember to go full rich for takeoff and intitial climb. 
  It sounds as if there is a possibility that your engine is running excessively rich.  Carbon fouling can also short out a spark plug's electrodes, yet will clear up with a high power run prior to takeoff.    If your plugs aren't oil soaked when cold,....then try aggressive leaning during runup to see it that cures the problem.  Also try a static full power run with it full rich and then observe how much increase is attainable as you lean the engine out at static full power.  If it's more than 200 RPM you might ask you mechanic to confirm the carburetor is correct and correctly set, and the intakes are not obstructed, and the carb heat system is fully shutting off.</HTML>

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Re: Carb Heat and RPM Drop

<HTML>George,

Over the weekend my A&P and I pulled the plugs on my cold engine which had not been started for 7 days. All of the plugs were dark grey except for the bottom number 1 (pilots right and rear) which was dark but not wet, and number 2 (pilots left and rear) which was very dark with slight wetness on the face and first first couple of threads. We tested the plug and it produced normal spark. The compression on number 2 was 78/80 cold. The airplanes tie-down is on a slant so the number 2 and 4 cylinders are angled down such that I loose upwards of 6 gallons through the left wing vent after a topoff. My mechanic leaned the carb slightly and switched the top and bottom plugs on cylinder 2. He says due to the angle of the motor that a very small quantity of oil went past the ring and onto the plug. He says the excellent compression rules out any serious problem with the cylinder. We changed the oil and filters. The old oil and filters were very clean without the slightest indication of particles. The motor uses almost no oil at 40 hours on the rebuild. We switched from mineral to synthetic. I'm hoping everything is fine but in the wee hours of the night I fear a defective cyclinder, contamination by a disgruntled paint shop owner, contaminated AeroShell oil, defective ring, etc.</HTML>

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Re: Carb Heat and RPM Drop

<HTML>George,

One other piece of forensics on the motor - it was run only 4 hours from the rebuild in 1995 until I bought it last year. Perhaps there is some corrosion in the number 2 cylinder due to prolonged exposure to air through an open valve. Unfortuately, we did not have the bore scope with us Saturday. Thanks for you and your peers input. If you do not think it is wise to keep running the motor - please speak up.</HTML>

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Re: Carb Heat and RPM Drop

<HTML>I presume it runs up to rated static RPM and otherwise operated fine (other than this carb-heat thing)?  If so, i'd not be afraid to run it at all.  Your carb heat drops are not all that far off.</HTML>

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Re: Carb Heat and RPM Drop

<HTML>Also keep in mind that there may be other reasons for excessive carb-heat drop.  This last annual inspection my mechanic was about to close my cowling back up at the end of the inspection.  For reasons not completely clear to me, I decided to remove some "scat" hose in the carb-heat system, just to inspect it's interior for the condition of the supportive wire-coils. (Carb heat worked normally.)  What I found was a shock.  A mud-dauber's nest the size of a plum inside the hose!  It was on the inlet hose to the heater-muff so likely wouldn't have caused any serious problem, ....but what if it'd been on the muff exit?  Or the inlet hose to the air-box?
  A Piper lost all engine power shortly after takeoff at a nearby airport I'd visited recently.  It was only 5 hours after it's annual inspection.  During my visit the local mx shop was being grilled by the FAA inspectors.   It was due to a mud dauber's nest in the carb airbox. It had been built in the 10 days between the inspection and the crash-flight.  Fortunately no one was hurt.  But the airplane was damaged in the subsequent emergency landing only 1/2 mile off the departure end of the runway.
  The mx shop was cleared because they'd obviously inspected the airbox because of a signed-off rebuild of the box due to cracking.  The pilot was criticized for "in-complete and faulty pre-flight" despite the fact that even the FAA couldn't determine the cause until the airbox was removed from the airplane.  There was no way that pilot could have inspected that airbox interior prior to flight.  I guess they just had to blame someone.</HTML>

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Re: Carb Heat and RPM Drop

<HTML>I suspect that the large carb heat RPM drop was due to a slightly rich carb  - the hot air had enrichened an already too rich mixture causing the excessive drop. Leaning the carb appears to have solved the problem.

As for the slightly wet bottom plug - I think it caught a drip that ran down the steeply inclined cyclinder from the rings. Given the quality of the rebuild and the 78/80 compression, I think at most I have a ring that is not totally seated at 40 hours.

At any rate, I hope I have correctly concluded from all your input that it is ok to run the motor.</HTML>

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