What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

RV6a o360 carb issues

P

Pfatt2

Member
I have the o360 a1a engine with the Marvel Schebler 10-3878 carb on an RV6a.

Everything was fine til the other day when at low rpm the engine ran horribly and wanted to quit. It coughed and popped and was clearly not right. At higher rpm it runs better but clearly something is wrong.
Yesterday my mechanic removed the carb and cleaned everything and flew the plane home (to my airport) flawlessly. Ran great.

This morning it's doing the same terrible thing again. A quick look in my logs reveal that this carb was put on (carb #2) only 110 hours ago. THEN, it was rebuilt 10 hours later. And now it needs another!!! My engine has only 1000 hours on it.

Questions.... why would my plane need 2 carbs in only 1000 hours and within 100 hours need 2 more rebuilds??? Absolutely senseless.

Is there a gascolator on these planes and is there not a fuel filter? I could simply find neither (clearly a Flyer - not a builder)

Hellllp?
 
Your carb should have a finger strainer at the inlet where the fuel hose enters the carb. No guarantees. Depending on how far you've dug into your fuel system, you *could* have filters elsewhere too.

As far as carb problems go, if you can start the airplane, see if leaning it helps or hurts. That could tell you that your carburetor float has stuck (or filled with fuel and sunk).

Any additional details you could provide about how the engine ran would probably help, but diagnosing stuff over the internet is difficult.
 
Idle circuit

If it runs smooth above 1500rpms and barely runs below that I’d be looking at the idle circuit. Happened to me after rebuild. Found trash in the idle jet.
 
Pfatt2 said:
I have the o360 a1a engine with the Marvel Schebler 10-3878 carb on an RV6a.

Everything was fine til the other day when at low rpm the engine ran horribly and wanted to quit. It coughed and popped and was clearly not right. At higher rpm it runs better but clearly something is wrong.
Yesterday my mechanic removed the carb and cleaned everything and flew the plane home (to my airport) flawlessly. Ran great.

This morning it's doing the same terrible thing again. A quick look in my logs reveal that this carb was put on (carb #2) only 110 hours ago. THEN, it was rebuilt 10 hours later. And now it needs another!!! My engine has only 1000 hours on it.

Questions.... why would my plane need 2 carbs in only 1000 hours and within 100 hours need 2 more rebuilds??? Absolutely senseless.

Is there a gascolator on these planes and is there not a fuel filter? I could simply find neither (clearly a Flyer - not a builder)

Hellllp?
Click to expand...

These are not normally the signs of a bad carb. A sticking valve would provide you with these issues. Bad ignition wires also.

Leave the carb alone for now, and look for other problems. Make sure your mags are grounded, and when cold, pull the prop through many times looking for a weak or dead cylinder by a sticking valve when the motor is cold.
 
Agree with Gasman....
Another thing to look for is induction leaks......
A small leak is a big percentage of intake at low rpms....less at higher rpms.
 
Carb

A few suggestions to help you narrow it down. Immediately after it happens, assuming you are at home field.

*when it happens leave the throttle alone and enriched/lean. Does one make it worse or better
* snap a photo of your gauges, what is the fuel pressure, rpm,…
*Pull the plugs and read them. Oily, wet, sooted,….
*Open the Fab, is the filter wet with fuel, fuel pooling, strong fuel smell.

I agree with the others, could be a lot of things. Need to try to identify what else is happening during the event and narrow down the fault zones.
 
Pfatt2 said:
I have the o360 a1a engine with the Marvel Schebler 10-3878 carb on an RV6a.

Everything was fine til the other day when at low rpm the engine ran horribly and wanted to quit. It coughed and popped and was clearly not right. At higher rpm it runs better but clearly something is wrong.
Yesterday my mechanic removed the carb and cleaned everything and flew the plane home (to my airport) flawlessly. Ran great.

This morning it's doing the same terrible thing again. A quick look in my logs reveal that this carb was put on (carb #2) only 110 hours ago. THEN, it was rebuilt 10 hours later. And now it needs another!!! My engine has only 1000 hours on it.

Questions.... why would my plane need 2 carbs in only 1000 hours and within 100 hours need 2 more rebuilds??? Absolutely senseless.

Is there a gascolator on these planes and is there not a fuel filter? I could simply find neither (clearly a Flyer - not a builder)

Hellllp?
Click to expand...

Just to be sure, have you checked that the idle mixture screw hasn't fallen out? That happened to me after I backed it out, trying to get the rpm rise during shutdown. Luckily I found it on the ramp -- and the spring too. Symptoms of the missing screw were identical to the ones you describe.
 
thank you

Thank you all for your replies. I have some work to do.

The carb is already out of the plane and will get rebuilt today, so I'll be looking for filters and will clean the daylights out of the thing.

The idle screw is still in place.
Leaning at idle makes no change
All numbers are normal at all rpm's
All plugs look great.
Wires are indeed old, so those will get replaced
I will scope the cylinders while pulling the prop through and see if I can't identify a stuck valve
 
Another thing to check is ignition timing - I have seen Lycomings run great at cruise RPM yet not idle at all because they were mis-timed by 60 degrees (unmarked flywheel put on wrong….) - so do a quick timing check to see if something has shifted/moved.
 
Trouble shooting the carb

What is low RPM?

What is higher RPM?

Is this in the circuit, on the ground or are you on final?

Density altitude?

What happens when you lean it out?

Tim
 
Ironflight said:
Another thing to check is ignition timing - I have seen Lycomings run great at cruise RPM yet not idle at all because they were mis-timed by 60 degrees (unmarked flywheel put on wrong….) - so do a quick timing check to see if something has shifted/moved.
Click to expand...

That was the very first thing we checked . It was right on. Thank you :)
 
Loose carb bowl symptom 101

Did you remove the carb or someone else? That is a textbook loose carb bowl symptom. It will draw air and go lean with no visual indication and requires physically moving it back and forth (look for float bowl to carb body movement). The RV snorkel intake if set-up too tight at the inlet to cowl interface will shake it like crazy on start-up and shut-down as well as power adjustments. It happened on my RV-4, and I've corrected it on a couple others. Mine started stumbling at low power settings and backfiring on final with power pulled back, even shutting off on roll-out. Simply pushing RPMs up to 1000 made it go away. Twice, I re-tightened my safety tab locked carburetor body screws (not the flange -to-sump studs). Hopefully, your not having a re-build again because of this scenario.
 
fixnflyguy said:
Did you remove the carb or someone else? That is a textbook loose carb bowl symptom. It will draw air and go lean with no visual indication and requires physically moving it back and forth (look for float bowl to carb body movement). The RV snorkel intake if set-up too tight at the inlet to cowl interface will shake it like crazy on start-up and shut-down as well as power adjustments. It happened on my RV-4, and I've corrected it on a couple others. Mine started stumbling at low power settings and backfiring on final with power pulled back, even shutting off on roll-out. Simply pushing RPMs up to 1000 made it go away. Twice, I re-tightened my safety tab locked carburetor body screws (not the flange -to-sump studs). Hopefully, your not having a re-build again because of this scenario.
Click to expand...
Hello Bill,

Any sign of dye on the out side to indicate leakage?

Thanks
Tim
 
Pfatt2 said:
Thank you all for your replies. I have some work to do.

The carb is already out of the plane and will get rebuilt today, so I'll be looking for filters and will clean the daylights out of the thing.

The idle screw is still in place.
Leaning at idle makes no change
All numbers are normal at all rpm's
All plugs look great.
Wires are indeed old, so those will get replaced
I will scope the cylinders while pulling the prop through and see if I can't identify a stuck valve
Click to expand...

I would suggest getting them to do the Mooney mod when the carb is rebuilt. The 3878 carb runs very lean at WOT, which is why they came up with the mod. The kit contains a richer main jet and a restrictor for the Economizer cct which provides additional fuel at WOT. Did the mod on my carb which solved my lean WOT issues. Won't solve your partial throttle issue but something to consider.
 
RV7 To Go said:
I would suggest getting them to do the Mooney mod when the carb is rebuilt. The 3878 carb runs very lean at WOT, which is why they came up with the mod. The kit contains a richer main jet and a restrictor for the Economizer cct which provides additional fuel at WOT. Did the mod on my carb which solved my lean WOT issues. Won't solve your partial throttle issue but something to consider.
Click to expand...

I totally agree.
This single change made the biggest influence on reducing CHT’s on my O-360 powered airplane during climbs at WOT.
 
Staining

There may be a stain if it has seeped for a while, but once it loosens, things happen quick. Mine had light stains, but it's jot something you can see easily without moving the snorkle side to side .
 
2 carbs plus an overhaul and a tear apart within 100 hours should tell you that the carb is not likely at fault here. Given the symptoms, my first guess is a fuel pump that is intermittently failing high. Meaning too much pressure. When this happens, the excess pressure over comes the needle/seat and the bowl overfills and makes the carb run rich (coughing and sputtering). It would be worse at idle and get a bit better under higher power, where the percentage of excess fuel is a bit less and more tolerable. You can confirm this if anyone saw black or dark grey smoke coming from the tailpipe while this was happening.

coughing and sputtering is a pretty classic symptom for over rich. Lean condition produces different symptoms.
 
Last edited:
RV7 To Go said:
I would suggest getting them to do the Mooney mod when the carb is rebuilt. The 3878 carb runs very lean at WOT, which is why they came up with the mod. The kit contains a richer main jet and a restrictor for the Economizer cct which provides additional fuel at WOT. Did the mod on my carb which solved my lean WOT issues. Won't solve your partial throttle issue but something to consider.
Click to expand...

OP's issue is most obvious at idle and the main jet is not even flowing at idle RPMs. Not saying this is a bad idea, but it can't possible fix the primary issue of coughing and stumbling at idle.
 
No mention has been made of this so I'll jump in and start another episode of "The Primer Wars"! :D

If a Lycoming has primer lines and nozzles in place it is possible for the engine to draw fuel through the priming system. This would result in an too-rich condition rather than too-lean. The net result as perceived from the pilot's perch is the same - an engine belching and farting and generally not at all happy.

I discovered this early in the ground runs of my O-360 which has priming fuel connected to three of the four injector port bosses on the cylinder heads. I had failed to ensure the Essex manual primer was fully locked in the closed position. Operator error resulted in a very rich-running engine.
 
lr172 said:
OP's issue is most obvious at idle and the main jet is not even flowing at idle RPMs. Not saying this is a bad idea, but it can't possible fix the primary issue of coughing and stumbling at idle.
Click to expand...

I am assuming the recommendation was made based on the carburetor 10 – number that the OP specified. I agree that wide-open throttle mixture is not likely related to his specified problem. If he did end up exchanging or overhauling the carb, it would make a lot of sense to do the Mooney mod because the engine is probably running lean at WOT whether the OP knows it or not.
 
Canadian_JOY said:
No mention has been made of this so I'll jump in and start another episode of "The Primer Wars"! :D

If a Lycoming has primer lines and nozzles in place it is possible for the engine to draw fuel through the priming system. This would result in an too-rich condition rather than too-lean. The net result as perceived from the pilot's perch is the same - an engine belching and farting and generally not at all happy.

I discovered this early in the ground runs of my O-360 which has priming fuel connected to three of the four injector port bosses on the cylinder heads. I had failed to ensure the Essex manual primer was fully locked in the closed position. Operator error resulted in a very rich-running engine.
Click to expand...

good speculation that matches the symptoms. Definitely worth investigating.
 
rvbuilder2002 said:
I am assuming the recommendation was made based on the carburetor 10 – number that the OP specified. I agree that wide-open throttle mixture is not likely related to his specified problem. If he did end up exchanging or overhauling the carb, it would make a lot of sense to do the Mooney mod because the engine is probably running lean at WOT whether the OP knows it or not.
Click to expand...

I agree that if it had previously shown signs of being lean at WOT, this is a good opportunity to make the change. Just didn't want the OP to think it could be the cause of the reported problem
 
You must log in or register to reply here.
Back
Top