R

RV6ARoger

Member
Maybe one of you guys has some insight as to what this might be. Flying back from Oshkosh, I took off and the engine burped or stumbled for a couple of cycles. It then was OK, but it did it for about a second on the flight home about 4 times. Now, when I take off, it will stumble for a couple of rotations then clean itself up. Here is what has been checked already. (O-360 A1D, Hartzel CS, Carb.)
1. Checked fuel for water. All new fuel in tanks and system Checked flow.
2. Carb was rebuilt by a reputable rebuilder about 50 hrs before this happened.
3. Installed a P-Mag on the left side and had about 3 hours on it before this happened for the first time. Got with the P-Mag people went through their checks with them.
4. Had the right mag rebuilt. It needed that anyway.
5. Checked all plug wires for continuity and had them megered.
6. Checked all fittings, wires, mag switch, etc. etc.
7. Run up is normal not much drop on either system.
8. Tied plane down and did a full RPM run-up. It won't do it tied down, but shoot down the runway, and will scare you to death either on the ground, or shortly after breaking ground it will stumble. Doesn't do it much in cruise, but will burp every 5 minutes or so for a couple of prop revs.

Things I am considering and I am having a good A & P look at it.
1. Could I have a sticking valve? Do you guys think that might cause the condition?
2. Any other ideas of what to check? I am about out of ideas of things to look at. Any ideas, no matter how crazy will be seriously considered. That is how desperate I am.
 
Valve wobble test

RV6ARoger said:
.....

Things I am considering and I am having a good A & P look at it.
1. Could I have a sticking valve? Do you guys think that might cause the condition?
2. Any other ideas of what to check? I am about out of ideas of things to look at. Any ideas, no matter how crazy will be seriously considered. That is how desperate I am.
Click to expand...

Perform the Lycoming SB-388 Valve wobble test.

http://www.lycoming.com/support/publications/service-bulletins/pdfs/SB388C.pdf

gil A
 
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Even though you checked for water this is my most likely suspect. I had a similar problem for a long time and apparently my sumps were not at the lowest poin when on an un even floor. It is humid here and when I would let the plane sit for a couple of days with half tans I would get condensate that didn't completrely drain when on an un even surface.

Sump your tanks on a level surface and fill them then sump again. Also get your fbo to fill a small glass jar before they pump any other fuel one morning.

2nd thing to look at is your fuel pump affected by a not to distant past AD.

3rd if you have an AFP FI system check the small screen at the fuel inlet to the throttle body.

Good luck.
 
Sounds like an ignition problem to me. Most of your recent work was on the mags, suspect that area first.

Does the Pmag advance timing over the Mag? If it does then, are your plugs aircraft or auto plugs?...Have you cleaned them and pressure checked them? Try that 1st. If it makes any improvement you are on the right track. It's cheap, easy, and you probabbly need to do it anyway. ;)

A new / rebuilt "hotter" mag may now be producing a hotter spark that is shorting somewhere in / on a bad / weak plug. Just a guess.

Don't get frustrated, thats when mistakes are made. If you can't figure it out time to call in an expert and let them fix it. Good luck! Be careful!
 
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I would be interested in hearing a better description of "burp". I spoke a bunch about a repeatable momentary drop in rpm that I had in past threads that was tracked down to ignition (p-mags).
 
We had it too....

..on My buddy's -4..and yeah, it'll make you sit up! I happened to be flying it. We checked for water and there were a few drops on each side. We also pulled the carburetor drain plug and found a drop or two...they probably plugged the jet until they either passed or moved.

Regards,
 
Once I had a mag problem that was a bit harder to detect. Bad condenser. It would work fine while cold/cool and the run up would be perfect. However once the engine became hotter it would fail and fail the mag. When it cooled down the connection would again be made and the engine would run fine.

However, I join some of the others in suspecting water. Running down the runway also moves any drops of water that may be in the tank and might move them to the outlet casing a "burp" at the engine.

I'm assuming "burp" means a definite momentary power loss and "shuddering" engine which goes away within 5 seconds but as you say catches your attention.

Your carb was also worked on recently. A float problem could give similar symptoms but in my experience it has been more on power reduction rather than going to full power.

Whatever it is --- don't press your luck too far as it might be something that results in a more serious situation one day. I had some "burping" in my Maule which I thought might be carb ice only to have the engine fail one day completely abeam the runway --- carb float stuck.

Good luck!
 
RV6ARoger said:
Now, when I take off, it will stumble for a couple of rotations then clean itself up. (snip)

It won't do it tied down, but shoot down the runway, and will scare you to death either on the ground, or shortly after breaking ground it will stumble.

(snip)
2. Any other ideas of what to check? I am about out of ideas of things to look at. Any ideas, no matter how crazy will be seriously considered. .
Click to expand...

Crazy I can do ;-)

How is your induction set up? Have you looked for induction leaks?.

If it does it consistently, you can test various changes, as long as you have a long enough runway to comfortably abort. Try taking off with just the mag or just the Pmag (obviously with enough remaining runway for a comfortable landing). Try removing your airbox or filter. Try a really slow acceleration.

In cruise, try running for 10 minutes at a time on just one side of the ignition to see if it can be isolated.

Good luck.

Have you checked the resistance of the plug wires for the Pmag? They're cheap enough it might be worth making up an extra set and throwing in a set of new plugs (The good ones are almost $2 apiece though...).
 
Carb or Ignition

It's one of the two. I would re-install the mag you ditched for the p-mag and see if it clears up. Next I would check the carb.
 
What P/N carburetor do you have?
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
 
Thanks, guys for the info.

To answer a previous question the carb is a MA 4-5. I spent over $600 about 50 hrs ago having it completely rebuilt.

From all of the comments and suggestions, I need to revisit the fuel issue. I don't know what else to check, though. We completely drained the tanks, blew out the lines, checked all filters, drained everything. But, this will be revisited. I have a great A & P in our area coming over to look at it when the weather gets a little better. Pretty darn cold in the unheated hangar this time of year.

I am going to concentrate on the Valve issue and the P-Mag for right now.

As soon as I have a resolution, I will post it. I am sure all of you will be interested to find out exactly what the "smoking gun" was with this. That way, we can put it into our mental databases for future reference.

Again, thanks, guys.
 
engine burp

We had the same problem and it turned out to be the carb float adjustment. This was on a freshly overhauled carb just like yours. Might have it checked out.
 
Burp

Check the screws holding the main part of the carb to the top flange that's bolted to the sump. An IA mechanic friend of mine has found a rash to these screws loose on recent rebuilds. Some so loose that there was a sizeable gap in the parting surfaces with fuel leaking out.
 
Our 15 SMOH 0200 in the C150 had a stumble/burp, and it only happend when it got up to operating temp. The initial run up was OK, departure was OK, and then at about 100-200' it would stumble and shake like a wet hound and you would POOP your pants.:p

We checked the mags, plugs, fuel, ignition harness, installed a new carb, checked the valves, and finally found a seized lifter on the #4 cylinder exhaust valve.

It's worth checking...
 
SB-388

vansrv8 said:
Our 15 SMOH 0200 in the C150 had a stumble/burp, and it only happend when it got up to operating temp. The initial run up was OK, departure was OK, and then at about 100-200' it would stumble and shake like a wet hound and you would POOP your pants.:p

We checked the mags, plugs, fuel, ignition harness, installed a new carb, checked the valves, and finally found a seized lifter on the #4 cylinder exhaust valve.

It's worth checking...
Click to expand...


Yep... and the SB-388 I referenced previously requires that the lifters be removed and cleaned...:)

It is worth doing - even though it cost me a top overhaul on my Tiger....:)

gil A
 
RV6ARoger said:
To answer a previous question the carb is a MA 4-5. I spent over $600 about 50 hrs ago having it completely rebuilt.
Click to expand...
MA-4-5 is the model number not the P/N. If your P/N is 10-3878 you might want to consider modifying it to a 10-3878M or trying another carb that is so modified. Your symptoms pretty much match the symptoms associated with needing that mod on an O-360 with a plain 10-3878 carb. Might not be the issue, but your situation sure matches the symptoms.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
 
Figured out the problem. Maybe it will help someone else.

I told everyone that once I figured out what was going on with with engine burping, I would let everyone know what I found. After much research and angst, I think we have the situation resolved.

1. It appeared to be the fuel pump. The pump would not pump about 1/2 of the stroke. the shaft had to be moved about 1/2 way through its stroke before it actually moved anything inside it. We surmised that at low RPM, it could pump enough fuel to keep things running OK. At high RPM, we think the spring would not return the arm adequately to allow for full strokes on the pump. IT must have been marginal, since it would supply fuel most of the time at high RPM, just not all of the time. The problem was pretty noticeable when I got a new pump and compared its operation to the old one.

2. Another issue-- when I removed the fuel pump from the accessory case, and went to put the new pump back in, I could not find the pump rod. This rod takes the left mag gear shaft, which has a cam on it and pushes the pump arm up and down. Some @#$%$## person forgot to put the rod in properly during the overhaul of the engine and just shoved the rod up in backwards. There is a ball on the top end that rides on the cam, and this ball must have been pushing the pump arm, with the flat end of the rod working on the cam. Had to dissassemble the entire back case, sump, carb, etc. to get the rod out and put it in right. Checked everything and it is OK on the back end. This overhaul was done by a well know overhaul facility, whose name will remain nameless here, but just beware. Even certified houses make mistakes. I have a bunch of hours on the engine now, and it runs like a champ. Fuel pressure is almost 7 psi. IT is nice to go flying without worrying about the engine running right! Roger.
 
Electric Pump

Interesting that you had the condition on take-off with (I presume) the boost pump on.

I always thought the boost pump would run things properly with an inoperative engine pump.... perhaps the pressure pulses as the engine pump went intermittent did something to the carb...

Glad that you fixed it. Even if you don't publicly name the "well know overhaul facility" you should call them and let them know what happened... perhaps it can help improve their inspection process.

gil A
 
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