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Engine roughness for a couple of seconds

kiwicubber

PATRON
Te Kowhai, New Zealand
Its winter now in New Zealand with daytime temps in the 10- 15 degree C range. I have an O-200 that in the circuit at about 800 - 1000' after takeoff runs rough for 1 or 2 seconds then runs fine. Often it only does it for the second or third circuit then not again for the rest of the session, or even the rest of the day. sometimes it only does it for one flight in the week, other times 5 or 6 times per flight. Never does it if I takeoff and cruise for an hour. Had the same problem a few years ago and figured out that the W100+ oil I had just changed to was the problem, the engine then had 100 hours since overhaul. I read a thread on another forum that recommended not using W100+ oil on O-200 engines with ECI cylinders. Thats what I had. The thought was sticking valves. I changed back to W 80 and didn't have the problem again. Now we can't get W80 so I have used W100 for about 3 years with no problem. The engine is now 1250 hours since overhaul 8 years ago.
What things should I be checking to find the problem?

Any suggestions gratefully received,

cheers Bill
 
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Remove the drain plug from the carburetor bowl and let a good flow of gas flush out the carburetor. Sometimes a mall amount of water and/or dirt gathers in the bottom of the bowl allowing a small portion of it to get sucked into the engine at full power. A good flush sometime clears the issue.
 
Sticking exhaust valve. Tough to find without a 4 cyl egt. Even then, hard to isolate. My Legend Cub did this for years. I never could catch it in the act. Using TCP and MMO seemed to help. Worse comes to worse you can do the “rope trick” and remove the valve springs and see if any valves are tight in the guides. I finally did this on my O-300 (same cylinders) to solve valve sticking intermittently. But it was worse than you describe so it was easy to see the culprit on EGT. But doing all the cylinders showed two with very tight exhaust guides. I also put new springs on since it was already apart. The whole thing only took a couple of hours.
 
Your engine time is about when small Continentals can start to show cylinder issues. Lead, carbon, whatever gets to the rings and valve guides. I like to put 2oz of marvel mystery oil in 5 gallons of fuel. Makes me happy and might reduce rough running. The Shell Plus oils contain elixirs that are supposed to support good engine service.

Gary
 
Are you running leaded fuel in it? Maybe worth some marvel mystery oil and a switch to auto fuel or some unleaded alternative. Does sound like sticking valve.

Multi weight oil too. Phillips, its the only non-synthetic multi weight oil so should scavenge lead better than aero shell. Theres a reason that lawnmowers are the only thing we run straight weight oil in. I dont know that the oil change would help but I dont think it could hurt.

Im not a mechanic btw, so maybe file my thoughts away in the “i read it on the internet” category.


Sent from my iPhone using SuperCub.Org
 
Are you running leaded fuel in it? Maybe worth some marvel mystery oil and a switch to auto fuel or some unleaded alternative. Does sound like sticking valve.

Multi weight oil too. Phillips, its the only non-synthetic multi weight oil so should scavenge lead better than aero shell. Theres a reason that lawnmowers are the only thing we run straight weight oil in. I dont know that the oil change would help but I dont think it could hurt.

Im not a mechanic btw, so maybe file my thoughts away in the “i read it on the internet” category.


Sent from my iPhone using SuperCub.Org

Running 100LL. ��
 
Remove the drain plug from the carburetor bowl and let a good flow of gas flush out the carburetor. Sometimes a mall amount of water and/or dirt gathers in the bottom of the bowl allowing a small portion of it to get sucked into the engine at full power. A good flush sometime clears the issue.

Even better, catch what you drain in a clear container

Glenn
 
About a year ago during an annual, we drained my C-85 carb into a container like Glenn recommends. Small black specks were noted. Backing upstream we removed my carb inlet finger screen. It was surrounded by black deposits yet flowed. Cleaned out, the float needle then failed to seal the carb (debris loosened from the inlet?). Then the accelerator pump quit. About $950 later the carb was rebuilt. It always ran ok, but wasn't in good shape. Nothing was found in the main fuel filter or tanks.

Gary
 
I had this exact same problem in a C90 a few years ago. Turned out to be sticking exhaust valves. It progressively got worse until I finally pulled valve covers off, dropped valves into the cylinders, and used a flex hone to clean the valve guides up. Use mechanical fingers to get your valves poled back up into the guides. Really, not too bad of a job.
 
I have had success with sticky valves by squirting Kroil on the stem and letting it sit for a while. This stuff is great for unsticking metal parts. https://www.kroil.com/product/kroil-original-penetrant/
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The former owner of our local engine shop told customers to diagnose sticking valves by adding MMO to the oil. If the problem stopped? It was a valve and you can decide what course of action to take. A half quart should be plenty in a small engine but I know guys who rin a quart in Lycoming 320s.
 
Sure sounds like a sticky exhaust valve to me. You can add a cup of MMO to the oil and see if it clears up. I suggest leaning aggressively as well. Or you can pull the keepers and springs, then push the valve into the cylinder and run an 11/32 reamer down the guide to clean it out. This task is a lot more detailed than I'm going to write up here, so look for notes on "rope trick" or reaming valve guides on a small Continental. For me, it's about a 2 hour task depending on whether things go right or I run into weird issues.

FWIW, I run a cup of MMO in the oil on all my engines, and add about 40 oz per 100 gallons of fuel.

-Cub Builder
 
Thanks for the replies. How long should MMO, Avblend or any other additive take to work?

cheers Bill
 
Elixirs applied directly to the valves or pistons/rings is quickest. Coat when warm not hot (the solvents flash off too quick), soak a day or two while exercising the crankshaft by hand back and forth, then drain and fly. Some advice says on valve stems it can force the valves away from any remaining deposits into the guide. Removing the cylinder is best as the rings and valves can be directly inspected. But, being who we are there's nothing wrong with Cub Builder's advice and going flying. I use acetone and ATF (cheap to mix up some) if Kroil not available.

Gary
 
Sure sounds like a sticky exhaust valve to me. You can add a cup of MMO to the oil and see if it clears up. I suggest leaning aggressively as well. Or you can pull the keepers and springs, then push the valve into the cylinder and run an 11/32 reamer down the guide to clean it out.....

I had trouble with sticky exhaust valves with the O-300 in my old C170 years ago,
ended up doing the rope trick on all 6 cylinders & reaming all the exhaust guides.
An 11/32 reamer sounds too small... I used a .437".
I'm pretty sure the O-200 uses the same size valves as an O-300.
 
If you can get some Kroil squirted and worked into the valve stems. it will dissolve the restrictive material. No need for any disassembly. That stuff is amazing. Let it sit and do it's thing.
 
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