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Engine stumble

acam37

Well Known Member
0320/E2G with 30 hrs SMOH. almost every flight it will momentarily stumble for a split second. It can best be described as a (bump). After that it runs fine the rest of the flight. It will only do it once. We changed both mags, wires, and plugs and it still does it. I cant figure out what could be causing this? The plane is going to Sun-N-fun which is 1000 miles one way. I would like to find out if this is a major issue or just an annoyance?
 
0320/E2G with 30 hrs SMOH. almost every flight it will momentarily stumble for a split second. It can best be described as a (bump). After that it runs fine the rest of the flight. It will only do it once. We changed both mags, wires, and plugs and it still does it. I cant figure out what could be causing this? The plane is going to Sun-N-fun which is 1000 miles one way. I would like to find out if this is a major issue or just an annoyance?

Does it do that at a specific point in the flight? Throttle increase, throttle decrease, level off? Is this after 5 minutes of flying or later in flight?
 
0320/E2G with 30 hrs SMOH. almost every flight it will momentarily stumble for a split second. It can best be described as a (bump). After that it runs fine the rest of the flight. It will only do it once.

What conditions for the stumble??

I.E., first started, first leaned, switching tanks, when reducing power??

You need to find a pattern first, IMHO.
 
Stumble

Try switching your carb to a unit with a different 10- #. I had one doing this too.

0320/E2G with 30 hrs SMOH. almost every flight it will momentarily stumble for a split second. It can best be described as a (bump). After that it runs fine the rest of the flight. It will only do it once. We changed both mags, wires, and plugs and it still does it. I cant figure out what could be causing this? The plane is going to Sun-N-fun which is 1000 miles one way. I would like to find out if this is a major issue or just an annoyance?
 
Its random. Sometimes on climbout. Sometimes level flight. At different throttle settings. You never know when. The carb is the only thing that has not been replaced. Its weird.
 
You could try leaving the aux fuel pump on for the whole flight to rule out your pump/fuel delivery. Carb could have debris in bowl, but I would find it quite odd to only present itself once per flight and always once per flight. It also shouldn't present as a bump.

Ignition would have been my fist guess. Did you, by chance, examine and of the gears when you had the mags out? In ar recent post, a guy found a missing tooth on his idler gear. I forget the symptoms he reported, but worth a quick search.

Larry
 
stumble

Had this happen to me for a year 0-320 E2d. Finally got sick of it and cored the carb in for an overhauled one. Hasn't stumbled in 3 months.
 
Does your carb have a pepperbox style discharge nozzle, If not that is your problem. What is the carb P/N? That will tell the tale.
Good Luck,
Mahlon
 
Does your carb have a pepperbox style discharge nozzle, If not that is your problem. What is the carb P/N? That will tell the tale.
Marvel Schebler MA-4SPA model 10-5009
I'm not sure if this has anything to do with my problem, but my co-owner flew it and said that on climb out the #3 EGT alarmed at 1502 the the engine did the stutter and the EGT immediately dropped to 1480 and continued to drop to around 1400. The CHT's were around 350. I will watch the EGT next time and see it repeats.
 
have you thought of doing an exhaust valve SB check? :)
P1020216.jpg
 
David, I'm not sure. I had someone else rebuild my engine when the cylinders were overhauled. Everything internally is new except the piston rods and crank. I even had to buy new accessory gears due to corrosion. I can ask if the tappet clearance was checked. I do know that the lifters are also brand new.
 
Have it modified into or exchanged for a 10-5009N or equivalent and your problem will go away.
Good luck,
Mahlon
Marvel Schebler MA-4SPA model 10-5009
I'm not sure if this has anything to do with my problem, but my co-owner flew it and said that on climb out the #3 EGT alarmed at 1502 the the engine did the stutter and the EGT immediately dropped to 1480 and continued to drop to around 1400. The CHT's were around 350. I will watch the EGT next time and see it repeats.
 
Have it modified into or exchanged for a 10-5009N or equivalent and your problem will go away.
Good luck,
Mahlon

You can buy the pepperbox jets for around $150 and they're relatively easy to replace. There is a part # for a good replacement from Marvel, but I don't have it off hand. I wasn't having your specific problem, but did experience atrocious distribution problems and this replacement helped quite a bit.

Larry
 
So what I am hearing is that I might be having a fuel/air distribution problem that is causing the intermittent stumble? It makes sense. Does anyone have the part numbers I need to change the jet mentioned above?
 
Check your spark plug resistances - all should be < 5kohms. High resistances can lead to flash overs elsewhere in the ignition system causing the stumble you describe.
 
I would try to get more data to see if it is always the #3 or if it is just random. Do you have an engine monitor that can record?
 
Bad Plug

I bet it's a bad plug. High EGT is one plug not firing. The 'bump' is when it starts to fire and then the EGT drops to normal.
 
Mod kit is in SL 1305C from Lycoming

So what I am hearing is that I might be having a fuel/air distribution problem that is causing the intermittent stumble? It makes sense. Does anyone have the part numbers I need to change the jet mentioned above?

LW-13761 Carburetor Modification Kit consists of the following parts:
(1)47-828 Nozzle Assembly
(1)16-222 Nozzle Lock Gasket
(1)16-A107 Power Jet Gasket
(1)16-B75 Throttle Body to Bowl Gasket
(4)78-A110 Throttle Body to Bowl Screw Washer
After modification has been accomplished, stamp the carburetor nameplate with the suffix "N"
after the part number 10-5009 or 10-5062 which will then appear as 10-5009-N or 10-5062-N which
is equivalent to current production 10-5135 carburetor.
NOTE: Revision "C" updates Models Affected and adds paragraph after Note.
18078 - This number for Avco Lycoming reference only.
Page 2
 
O-320E2G stumble

this is very common in the Grumman AA5 series - so common it's been referred to as the "Grumman stumble". on a lot of the 4 place 0320 Grumman planes this happens, usually as power is pulled back or a mixture change. On my plane it only would happen once per flight but it was very predictable. My IA who specializes in the Grumman line knew immediately what I was talking about when I brought it up. The fix is to change the jet nozzle to a pepperbox design (many small holes instead of 1 large one).

I had my carb overhauled by Lycon in California to bring it up to current specs and have not had a single stumble since.

here's a snippet from the Grumman Gang Archives describing what was done....

"Still, the 10-5009 and 10-5135
was the approved carburetors that you could use on the O-320-E2G engine
until the 10-5217 was approved in the early 2000's. It incorporates all the
mods that can be done to the 5009 including the "Pepperbox Nozzle" which
helps a lot. If your 5009 has that it will have an "N" stamped on the data
plate (and possibly with an "M" denoting a metal float & a "V" denoting the
one piece venture)you have an updated carburetor commonly called a 10-5009N."

I believe mine was brought up to a 10-5217 configuration. Hope this helps!
 
So, Im not crazy after all. Yes, this engine did come out of a Grumman and the carb does not have the pepper box nozzle. Im glad to hear that my stumble description has happened to other people and there is a fix. I'm going to upgrade my carb andwill post the results.
 
H20

No gascolator, water in the tanks or some kind of contamination. The engine drinks it, the level goes down, mother nature puts more water in the tank, engine drinks it, level goes down, mother nature replaces it, engine drinks it..............got it!
 
The Fuel distribution improvement kit for the10-3878 carb, is called the -m kit. the M would be a suffix to the model number and then it would be a 10-3878-M.
The 10-4164-1 and the 10-5193 carbs, used on the 360's already incorporate that mod but don't have the -m suffix.
Good Luck,
Mahlon
 
Engine Stumble

I appreciate the post, I too have experienced the same situation with my 0320 in it stumbling at various times. Sounds like this pepperbox fit might be the answer. My A & P was also leaning towards a carb situation. Interesting hearing of the Gruman stumble also.
 
Scott, no one has said its a safety to flight issue but we decided not to fly again until we replace the carb with the newer 10-5217 model. I contacted a shop that overhauls carbs and described the issue we were having and they agreed that I needed to exchange the 10-5009 with the newer design. So, our trip to Sun-N-Fun just got cancelled this year.
 
the lack of the pepperbox discharge nozzle and the resultant mhic ups are absolutely, 100% not a safety of flight issue. If it were me, I would go on my trip as scheduled.
Good Luck,
Mahlon
 
Had the same problem

I had an O-320-E2G on my RV-4 in 1981 and it always stumbled at TOC when the power was pulled back. It really got my attention on the first flight! But in hundreds of hours of flying it never missed a beat other than that little burp that I eventually got used to.

Vic
 
Once again, thanks for all the insight to this stumble issue. I hope this helps other people who may encounter this problem. Justin was going to go ahead and take it to Sun-N-Fun, but he/we felt that until we new for 100% certainty that the carb is what is causing the stumble it's not wise to go 1000 miles from home. He flew it in a Sport air race Saturday and it did it 3 times in an hour.
 
Primer lines

Do you have primer lines installed? I once over tightened a primer line fitting and cracked the flare in the copper line causing a similar issue.
 
Do you have primer lines installed? I once over tightened a primer line fitting and cracked the flare in the copper line causing a similar issue.
They are plugged
 
Update

We got a new carb (10-5217) and got to test fly the plane. The engine stumble did not appear, however now the engine doesnt seem to be making full power and has a slight vibration that wasnt there before. The throttle hits the stops so its not rigging problem. It acts like its running too rich. Is there an adjustment we can do to lean it out. The idle mixture screw is turned out about 2-1/2 turns. At cruise with the new carb we are losing almost 15mph.
 
Mike, I dont remember the exact numbers, but When I tried to lean the engine in the air the EGT's spiked to around 1500 so I went back full rich for the remainder of the flight. With the other carb it ran great except for the original problem of intermitant stumble. Will the idle mixture screw on the back cause it to run rich in the air if improperly adjusted. I may have gone too far rich on the idle mixture.
 
Carb

The idle mixture will not affect cruise power settings. You appear to have learned a very expensive lesson about Marvel carbs. I am very happy with the antique many times superceded carb on my 0 320 and have no intention of changing it. It does run a bit lean and if you yank the power back quickly it will stumble.
There are literally thousands of older Marvel carbs doing just fine on homebuilts.
This has been discussed many many times yet it keeps coming up: the EGT reading on a normally aspirated engine is meaningless. Early EGT gages did not have numbers, only lines. Lean until the EGT peaks, then richen 50 or 100 degrees.
For years we ran the Lycomings without EGT gages and in many cases without CHT gages. It was extremely rare to not make TBO.
Below 75% power Lycomings position is that you cannot hurt the engine with the mixture control.
 
Idle adjustment has no effect above about 12-1400 rpm. The carb change, to the pepperbox style nozzle, should produce a slight increase in richness, besides eliminating the stumble, compared to your other carb. maybe 25 degrees egt wise richer then before. Roughness is indicative of a cylinder partially misfiring, it is difficult for a carb to cause this at power unless it is extremely rich or lean and causing one cylinder to slightly misfire. If leaning at power doesn't remove the roughness its not to rich. Try cracking the carb heat to artificially enrichen the mixture to see if that smooth's it out. If either test fixes it, you need a different carb under warranty. There are no power adjustments, to the carb that you can make, other then float level and nozzle size. Both of which should be correct out of the box on a fresh carb.
Good Luck,
Mahlon
 
Rv_8 flap tape

Do not see what to use or where to install tape on flaps or to Inside of wing. Words of wisdom needed. Thanks
 
Sounds like it may be way too lean. Do you know if the previous carb was ever modified to increase the fuel flow? It seems like the vast majority of O-320s installed on RVs are running way too lean. A carb 10- setting that works fine on an O-320 in a C-172 or Cherokee 140 will be way too lean on an RV due to less restrictive exhaust and intake systems. The only way to know for sure is to go fly and record the EGT temp differential from full rich to peak.
 
I have an RV 4 with a 0360 that has a stumble when I apply go around power from an idle. Just after I bought the plane on my very first over shoot from a botched landing I almost xxxx my self.

I had my mags rebuilt and that help immensely but there is a stumble that is still present. My carb also seems to run on the lean side.

I agree with Mike as I hear that a lot of the RVs seem to run leaner with a bit of a hesitation over a factory built AC I'm assuming some or all of it could be contributed to our more efficient induction system and exhaust (straight pipes)
 
Mike, I flew it again and paid attention to my EGT's. At 2500 rpm the EGT was around 1460. As soon as I cracked the mixture to lean the EGT's immediately spiked to over 1500. Back to full rich and they dropped back down. I get rid of one problem only to create another. I will do a search on lean carbs and call the shop that I got the carb from. Its under warranty so I dont need to be doing anything to void it.
 
Mike, I flew it again and paid attention to my EGT's. At 2500 rpm the EGT was around 1460. As soon as I cracked the mixture to lean the EGT's immediately spiked to over 1500. Back to full rich and they dropped back down. I get rid of one problem only to create another. I will do a search on lean carbs and call the shop that I got the carb from. Its under warranty so I dont need to be doing anything to void it.

Definitely sounds lean but you need to lean slowly and find peak EGT. Without knowing full rich and peak EGT numbers it will be hard for the carb shop to know what fuel flow setting you need. If you have fuel flow indication, those numbers would help to determine carb set up as well.
 
Mike, I flew it again and paid attention to my EGT's. At 2500 rpm the EGT was around 1460. As soon as I cracked the mixture to lean the EGT's immediately spiked to over 1500. Back to full rich and they dropped back down. I get rid of one problem only to create another. I will do a search on lean carbs and call the shop that I got the carb from. Its under warranty so I dont need to be doing anything to void it.

The richness leanness of a engine carb combination is indicated by the spread from full rich to peak Egt, at lower altitudes, in cruise configuration, not by the absolute value of the egt numbers or the amount you can move the mixture control. So, if you have a spread of 75-100 degrees of egt from full rich to peak, in cruise power at say 2- 3000 asl than you are not too lean. In another words, you are flying at 75 to 100 degrees rich of peak when full rich, which is 100 % acceptable in my book. Most 320's will yield somewhere from 50 to 100 degrees of spread, cruise power at lower altitudes. It will get richer as you go higher, so if it is Ok at 2-3000asl then it is good at any altitude.
Good Luck,
Mahlon
 
Original problem solved

I am happy to report that after 2-1/2 hours with the new carb, there hasn't been one stumble. So thanks for all the advice and I hope this thread will be helpful to others who may happen to have a similar problem. We drilled out the main jet with a #38 bit and that seemed to help considerably with the lean conditions.
 
Stumble

Thanks all for solving my problem. I'm flying a 9A with a O320-E2D with new cylinders and a freshly overhauled 10-5009 carb. It stumbled on the first flight, really got my attention. I now have 150 hrs on it and hundreds of flights, it has stumbled every single flight. From the info in this thread, I purchased and installed a pepper box nozzle, converting it to a 10-5009N. I've flown it 6 times since and it has not stumbled once. Thanks again for the great thread.
 
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