Large Rpm drop with carb heat at run up

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jmarine225

Pre-takeoff checklist
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Jmarine225
I have a piper Cherokee with the Lycomng o320 engine. Long story short but I had it for sale and it went for a prebuy inspection. I recently had the aircraft annual completed with no engine issues. I get the plane back and I have a crack in my spinner and I’m told my carb box has a leak and needs sealant. My first flight after receiving it, there is a huge RPM drop with carb heat during runup, approx 300-500 rpm and it runs rough. Same thing if I apply carb heat at idle. Runs rough nearly stalling. Today’s temp on the ramp was probably in the area of 100f when I was running it. I did t have any of these issues before this prebuy inspection.

The spinner is a different story. Clearly over torqued it divoting the spinner causing a chip in the metal and a crack.
 
Sounds like a collapsing SCAT hose between the heat shroud and carb airbox. Induction flow sucks it shut and the engine starves for air.

Or the mechanic had the SCAT hoses disconnected for some reason and connected the airbox heat hose to the cabin heat valve that was closed, and the carb couldn't get air.

Dumber things have happened.
 
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skyking3286 said:
Idle can create carb ice. Big time.
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But not enough to cause a 300-500 RPM drop with the carb heat. The engine RPM would have been already very poor if it had a large amount of ice. Moreover, the temp on the ramp was 100°F, at the upper limit for carb ice at any power setting, and the engine is a Lycoming, not known for ice at that temperature.
 
Wait, so he signs off your annual THEN alerts you that there are problems that need to be addressed?
 
Can’t understand “has a leak and needs sealant”!

Doesn’t it run 0k otherwise?

Static RPM?
 
Could the "leak"have been hiding a rich mixture and leaning it out with an induction leak? Now it's overly rich and carb heat pushes it over the edge? Spitballing.

When I sold my Cherokee the first prebuy was done by a very nitpicky a&p. When dad pulled his plane out of the hangar in a flat calm day the guy got antsy it wasn't chocked for 5 minutes and did it himself.
 
texasclouds said:
Wait, so he signs off your annual THEN alerts you that there are problems that need to be addressed?
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My mechanic did my annual and was returned with a sign off after some unrelated repairs. The potential buyer had a prebuy inspection done at another shop and that is how it was returned to me.
 
Magman said:
Can’t understand “has a leak and needs sealant”!

Doesn’t it run 0k otherwise?

Static RPM?
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It ran fine before this prebuy inspection. Without car heat it runs fine making Rpm’s up to 2700
 
Upon reviewing the notes from the pre buy inspection it states “carb drain plug leaking. Needs sealant.” I’m not familiar with this as if it would have any effect.
 
Dan Thomas said:
Sounds like a collapsing SCAT hose between the heat shroud and carb airbox. Induction flow sucks it shut and the engine starves for air.

Or the mechanic had the SCAT hoses disconnected for some reason and connected the airbox heat hose to the cabin heat valve that was closed, and the carb couldn't get air.

Dumber things have happened.
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This response makes me think. The Rpm would have a large sudden drop, like it needs air, then go back to normal. Doing this pattern over and over repeating itself.
 
I’ve seen nasty stacks on the lower side of inlet.

Not cracked though. A crack/ leak would cause a large drop

at all rpm’s.
 
It sounds like the pre-buy mechanic damaged your airplane. At the very least he should pay to repair or replace the spinner. I wouldn't let him touch it again if it were mine.
 
Uggh, every seller's nightmare. No way he is going to own up to the cracked spinner. And I'll bet the buyer didn't buy it, did he?
 
Ed Haywood said:
Uggh, every seller's nightmare. No way he is going to own up to the cracked spinner. And I'll bet the buyer didn't buy it, did he?
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Come on man I just need to pull some jugs to check for cam corrosion. My pre buy guy said no problem.

“I wouldn’t buy if I can’t inspect anything I want!”Am I right?

Screw buyers.
 
Does leaning affect it?
I did some instruction in a Tri-pacer with an O-320 that I noticed a pretty large RPM drop when turning on the Carb Heat. Leaning would reduce drop to a more normal level. I assumed the mixture was just set richer than it should have been. Pulling the Carb heat just made it run very rich is what I thought was happening.
Brian
CFIIG/ASEL
 
Yah, raw fuel dripping inside a tightly cowled engine compartment has no effect, until it does.
 
texasclouds said:
Wait, so he signs off your annual THEN alerts you that there are problems that need to be addressed?
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That’s what an annual inspection is. Inspect and report. Repairs are a different work order.
 
Stewartb said:
Yah, raw fuel dripping inside a tightly cowled engine compartment has no effect, until it does.
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It has no effect on the RPM drop. Yes, fuel leaks are serious, though I bet this one is little more than a blue stain around the drain plug.
 
No matter. It’s unacceptable and noted during inspection. Relevant to the carb heat? No. I wouldn’t worry about the carb heat drop myself, just that carb heat works, and his apparently does.
 
jmarine225 said:
The potential buyer had a prebuy inspection done at another shop and that is how it was returned to me.
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jmarine225 said:
It ran fine before this prebuy inspection. Without car heat it runs fine making Rpm’s up to 2700
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William Pete Hodges said:
It sounds like the pre-buy mechanic damaged your airplane. At the very least he should pay to repair or replace the spinner. I wouldn't let him touch it again if it were mine.
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In that an engine shouldn't run rough / get close to dying when carb heat is applied, yes, there is a problem.

Mr. Hodges I believe has the answer. I would take it back to my A&P, tell him what is happening, and have it fixed. The Pre Buy guy didn't fix anything - supposedly - to it must be that he didn't put everything back correctly and/or he inadvertently broke something.

Could be anything. Because carb heat bypasses the air filter, maybe something as simple as a rag left in the carb heat ducting. Hope it's that simple.
 
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brcase said:
Does leaning affect it?
I did some instruction in a Tri-pacer with an O-320 that I noticed a pretty large RPM drop when turning on the Carb Heat. Leaning would reduce drop to a more normal level. I assumed the mixture was just set richer than it should have been. Pulling the Carb heat just made it run very rich is what I thought was happening.
Brian
CFIIG/ASEL
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I was back at the plane yesterday and tested this out. Leaned it out with carb heat on and it runs fine, with the expected carb heat drop.

Also as far as the “leak” the mechanic noted. There isn’t one drop of fuel on the inside of the cowling, the ground or in the carb drain plug. The plug is clearly visible and is safety tied with no bluing around it either.
 
jmarine225 said:
I was back at the plane yesterday and tested this out. Leaned it out with carb heat on and it runs fine, with the expected carb heat drop.
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But it didn't used to do that, right? Red flag. Lycoming carb heat systems are deliberately weak, since that hot sump mitigates much of the carb cooling that causes ice.

It needs looking at.
 
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jmarine225 said:
I was back at the plane yesterday and tested this out. Leaned it out with carb heat on and it runs fine, with the expected carb heat drop.

Also as far as the “leak” the mechanic noted. There isn’t one drop of fuel on the inside of the cowling, the ground or in the carb drain plug. The plug is clearly visible and is safety tied with no bluing around it either.
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well at least you now know it is running probably richer than it should be. The trick now is to figure you why? What changed? Is a mixture adjustment working it's way loose? I don't know anything about adjusting aircraft carburetors.
let me know what you figure out.
While I had recommended to tri-pacer owner that he get it checked out, I kind of doubt he ever did. Not even sure he is currently flying the airplane. But if you figure it out I will let him know what you find.

Brian
 
What's the rpm gain while ground leaning? Post annual one time had a +400 rpm gain.
 
Racerx said:
What's the rpm gain while ground leaning? Post annual one time had a +400 rpm gain.
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If I very slightly lean it it stops the engine from running rough during run up with carb heat on. When it stops running rough the drop is approximately 100-150 rpm with carb heat on. I mean very slightly. Like pulling the mixture maybe half inch at most.
 
My bet is that the pre buy A&P broke something in your carb heat system. Probably a block in the ducting / system not opening all the way that is acting like a partial choke, causing over rich.
 
It’s worth checking the Primer as it can present some of these symptoms.

So even with the excessive Carb Heat drop the aircraft was flown?

You might be “Lucky” to find a collapsing heat duct in this manner too.
 
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Magman said:
It’s worth checking the Primer as it can present some of these symptoms.

So even with the excessive Carb Heat drop the aircraft was flown?

You might be “Lucky” to find a collapsing heat duct in this manner too.
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Yes I’ve flown it as it runs completely normal without the carb heat. Visually checking the outside the ductile everything looks fine. Again, a very slightly lean of the mixture and it’s back to normal.
 
jmarine225 said:
Visually checking the outside the ductile everything looks fine.
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That doesn't tell you whether a SCAT hose is collapsing or not.

SCAT is thin fabric impregnated with silicone, internally reinforced with coiled music wire and string around the outside between the wire coils to keep things together. When those hoses get old, the wire (and string) can break loose from the silicone's grip and leave sections of the hose unsupported. It can look good, but when air is drawn through it, it collapses and restricts airflow, causing vacuum that collapses it further. Shut the engine down and it all pops back out and looks fine.
 
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Check the cable for slippage. Could be that simple.
 
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