So, I've got a real rough running O-300D in a C172. I've been running car gas and the last time I flew it in November, it ran fine. At annual, in January, we discovered the fuel was contaminated, it had more crap in it than I have ever seen in the 20 years I've owned it. We flushed the system real well and fired it up, but it ran real rough. I figured what ever was in the tanks was in the carb too, so we yarded that of and sent it to specialty shop to have it evaluated. The guy said there we some gunk in it, but he'd see worse. He cleaned it up real good and made some repair to the accelerator pump. We put it back on today and still rough. The mags check out and the six probe EGT shows all the jugs nearly the same temp. What happens is it just won't take power worth a damn. If the throttle is advanced at with any pace the thing will sputter and sometimes quit. It seems to make full power, just wont take any power bursts. Anyone ever had such problems? Any help is appreciated. Thanks in advance.

Either you still have crap in the fuel, or your carb guy messed something up with the accelerator pump. What kind of "speciality shop" did it go to?

Gump

RMAA in Mills Wy. Tomorrow we are going to swap out the carb with one from another O-300 and see what happens. Hopefully it will run correctly and we know what the hell to go after.

My 182 was running rough a few months back and it was a loose intake tube. It ran rough at low RPM but fine at higher power. Found a clamp that had worked loose. Easy fix but it could have been several other things so I got lucky.

Your gascolator should be full of crud too, no? Clean and gapped plugs??

If your getting the same temperature out of all your cylinders I would think that it is still a fuel problem since it is effecting all the cylinders equally. I have had both a stuck valve and an intake leak. Both caused rough running but one of the cylinders was a lot cooler in both cases because it was not firing like the rest. Have you checked the compression? It has to be Ignition, compression or fuel.

Check to see if you have good fuel flow, maybe the line is plugged and it's fuel starvation. If it is, start with the tank selector, on /off selector. Probably the smallest area of flow in the system. Could be plugged up.

The way you describe the roughness/sputter/quit with the advancing of the throttle, makes it sound like the accelerator pump.

But, my first 170 had an 0300-D which gave me a similar problem once, ran rough at low power settings (pattern, approach) and would quit as I flared to land. Turned out the primer system had a o-ring go bad allowing air to enter the fuel system. We put a fresh o-ring on the plunger right at the handle shaft, dressed it up with fuel lube, and good as new.

If your not having luck with any other remedies, you might give it a shot.

Good luck,

Tom

Yes the plugs been cleaned and gapped, the entire fuel system flushed with fresh 100LL. I have a quick drain in the belly, just down stream from the fuel selector. We ran lots for fresh gas out of that. New fuel line from strainer to carb. Strainer all cleaned. Compressions checked. The number one cylinder was a bit low, 69, and coming out the exhaust. That is our next suspect, but like I said, the jug is as warm as the rest of them. We worked on the primer and I'll find out about the o-ring. Could be? We looked close at the intake manifold today and didn't see anything bad there. Last annual, we replaced the rubber hoses and clamps on the intake. I still think it is the carb. I'll let you know tomorrow, after we make the switch.

69 is Ok on compression. I replaced one on an O-300 that was 62 and I couldn't tell any difference. The exhaust valve might be sticking. My O-200 did that and it just wouldn't rev up at all without popping and sputtering. The valve finally stuck open. I sent it off to be fixed and the guy told me the valve guide was all carboned up. I had been leasing my plane to a local student and I don't think he was leaning it right.

Try pulling the wires off one cylinder at a time and see if you can find out which one is the problem. If they all do the same thing, go back to the fuel system. The fuel flow might be enough to pour from a fuel sump but that flow wouldn't sustain the engine at full throttle. I would pull off the fuel line right at the carb and make sure it comes pouring out. That's easy to check. Could still be the carb too, People do make mistakes.

You can do a lot of diagnosis yourself, just pay the mechanic to fix it. Might save you some money.

Does it run OK for a while then start cutting out? If it does, check the vents on your tanks. Might have got some insects in there and causing a vacuum after it runs for a while.

When you check the fuel at the carb, let it run for a while. If the vent is clogged it will run good for a minute then start to drizzle out. Open the fuel cap and see if it starts running again.

If you find one cylinder that is the problem, take off that valve cover and watch the valves as you turn the engine (MAGS OFF< KEY OUT!!!!). If it is a sticky valve you Might be able to see it. If the valves and compression are good look at the intake for leaks. If that is OK it's time to check out the wires, plugs and mags.

I had the same thig happen on a long cross country trip at a fuel stop. My flexable fuel line at the carburetor was all swelled up and restricting flow to the carb. !! Replaced the flex hose and the problem went away !!!

Test ran today with the borrowed carb. Ran like a champ. Carb going back tomorrow.

Gump said it right, sputters on acceleration, no fuel being fed by acceleration pump or allowing air to get in around throttle shaft. Swapping carbs proves it.

OK, throwing money at this thing doesn't seem to solve the problem. Sent the carb back to RMAA to have it re-inspected to the tune of over $100.00. First we put the borrowed carb on and ran it. Ran fine. Then we put the original carb on that has been to the shop twice and ran it. Started out what seemed like fine, but after a few minutes it started getting rough. Low RPM was ok, but at about 1700 it was rough as hell and didn't really improve with more RPM. So just to check for sure, put the borrowed carb back on and check it again. Runs fine. So the guy at RMAA says he can't find anything wrong with it, but it don't run right. He has disassembled and cleaned and blown out twice. There must still be something stuck in one of the passage ways? Any ideas of how to clean it differently or what to look for? This is starting to piss me off!

You need the carb guy to really check this out. Just blowing it out won't cut it. When I sold my 170 the carb had a problem which I advised the new owner about. It ran too rich at full throttle. The new owner determined to get to the bottom of this ( he was an an a&P) and took it to someplace around Minneapolis that had a flow bench. Yep, it was too rich, but why. After several tries to fix it. ( I had tried too), they determined it had the wrong float bowl on it. He ended up springing for a new carb.
What I am trying to say is, although they look simple, they can sometimes really baffle you.
Could be a little passage plugged, could be a passage corroded througn into another passage. could be quite a few things. Wrong parts not so likely on yours if it worked properly before.

One way to fix it :

http://www.aircraftspruce.com/menus/ep/ ... elsch.html

Finally got it all figured out. After having the guy at RMAA in WY look at the carb twice and it still didn't run right, I decided it was time to throw in the towel and get an exchange carb. At that point, we decided to open it up and see what the hell was the problem. This carb had the white plastic float and it doesn't take a very close look to see two spots on it that are suspect. I cannot believe this guy missed this! Obviously, he never did check the float for leaks by dipping it in hot water. When you do, bubbles come out of both these places! So we installed a new brass float and are back in business!
I have been running Autogas- 91 octane, but I have never checked it for alcohol. This could be damage from that? I don't know. I will check for alcohol before using autogas again.

Are any of you guys familiar with the venturi issue on the MA4-5?? If memory serves me they started with a one piece plastic body venturi then changed it to a two piece unit. This one caused several problems that were worse than what they were trying to cure so after someone in the field proved the failure they went back to the one piece.

These things are very easily damaged by even the smallest carb fire that you may not even notice happening. This has little to do with your issues with the float just another example of goofy things that you run into.

Glad you got it fixed.

Jaerl wrote:One way to fix it :

http://www.aircraftspruce.com/menus/ep/ ... elsch.html

When I was a kid we could buy the same damn Marvel-Schebler carb for a tractor for $11 at the feed store as I recall. Amazing.

Gump

How damn old ARE you?