The International Cessna^® 170 Association

After a 2 hour flight, the bottom left side of the airplane was black with carbon. I pulled out all 12 spark plugs and the two plugs on cylinder 6 were completely black. All the other plugs were clean. Compression 78/80. Removed the cylinder cleaned the piston and ring grooves, the cylinder, and checked the valve seats and bushings. All were within tolerance limits. Reassembled the engine and after half an our of flight the engine ran rough and lost some power. I made it back to the airport and checked all 12 plugs. Again all but the two on cylinder 6 were clean. The two plugs on 6 were completely fouled with carbon and the intake valve had lots of oil. How can this oil get to the intake of just cylinder 6? The valve is tight with only .0005" between bushing and valve. (the overhaul manual for the O300D Continental engine specifies max .001"). Good compression on all cylinders #1 76/80, #2 74/80, #3 70/80, #4 72/80, #5 72/80, #6 78/80. Could it be a crack on the cylinder? I turned the cylinder over and filled the inside with gasoline (valves installed but no springs) After 2 hours there was no leak. Also a crack will show on the compression test 78/80. Before I go ahed and replace what looks like a good cylinder I would like to find a logical reason for my problem.
Any ideas on how to solve this puzzle would be greatly appreciated.

Thanks
Frank, N2800C Cessna 170B

Were the rings gummed up with carbon when you pulled the cylinder?

The first time I pulled out cylinder #6 the groves were full of carbon. With everything cleaned, and after half our of flight, removed the cylinder, the 2 plugs were black with carbon but the grooves on piston were clean. Lots of oil by the intake valve.

Plug wires bad? Swap plugs #1's for #6's and try again

After cleaning the plugs have you tried doing a mag drop check? Whats the plug lead condition ? Plus i once saw a similar thing on a engine [lycoming] GO540,and what was causing it was a primer line was allowing fuel to leak into the cylinder.

You said "after a two hour flight". Was that the first time it happened or has it been getting worse over time? It makes a difference in trying to troubleshoot the problem.

Oil in the intake valve area and no valve leaks sounds like the valve guide has failed. Carbon in the rings on a cylinder that has good compression sounds like the oil ring has failed. Carbon on the plugs indicates a mixture that is excessively rich for some reason in that one cylinder.

There could be a crack hiding somewhere.

You will figure this out I am sure because your approach is very good and you have eliminated many things. The oil you are seeing on the intake just might be your best clue even though the tolerance appears very good on the valve guide. Remember that the 0-300 and C-145 cylinders typically run much richer on a carbureted setup due to the short manifold design to those cylinders being a more rich mixture, so seeing black plugs isn't uncommon. Do you have an EGT setup like an EI? That can help if you have one. #1 & #6 typically run 200 degf colder than the rest at cruise, at least on mine. Please let us know when you nail this down as it will help others. Also, I recently had an exhaust valve leak on #4 (hearing it on a compression test) even though the compression was very good. Relapped the valve and solved the problem.

Tank's for all the help I am getting from you. I will keep you posted on my progress towards finding the problem as I follow the different suggestions. I swapped the plugs, ran the engine on land without cowling for 15 minutes. First idle then 1000 rpm, 1700 for mag check getting rpm Dropp of 50 on R and 75 on L. Again the two plugs on cylinder 6 were full of carbon and all others clean.

Francisco Linares wrote:Tank's for all the help I am getting from you. I will keep you posted on my progress towards finding the problem as I follow the different suggestions. I swapped the plugs, ran the engine on land without cowling for 15 minutes. First idle then 1000 rpm, 1700 for mag check getting rpm Dropp of 50 on R and 75 on L. Again the two plugs on cylinder 6 were full of carbon and all others clean.

Do you have good crosshatch pattern on cylinder wall or does it look very smooth with very little crosshatch ? Or do the walls have a smooth brownish color (glazed wall) ?

After eliminating a faulty cylinder by swapping 5 & 6 with results similar to the ones previously described, I re-gaped spar plugs to .022 and re-adjusted the carb. air-gas mixture needle to 2 1/2 turns from closed. Previously the spar-plug gap was .016 and the air-gas mix needle was 3 1/2 turns from closed. Ran the engine on land for 10 minutes. Mag check Dropp 50 on R and 75 on L at 1700 rpm. Full throttle 2,400 rpm static. Pulled spar plugs from cylinders 5 & 6 and they were clean.
The oil found previously on cylinder 6 came from my A&P applying excess oil to the rings at the moment of re-install.
I will fly the airplane this coming Saturday and check all spar plugs again.

After discarding everything else, I replaced the two sparkplug cables on cylinder 5 now on the left at the position of cylinder 6 . Before, I did not have replacement cables or ways to test them. The problem was solved! No more sparkplug fouling or carbon on the belly of the airplane on the left side.
The confusing part is that on mag checks, the engine always had rpm drops within tolerance. The wire to the lower sparkplug showed signs of arching inside the protector and probably happened at high rpm. The wire to the upper sparkplug didn't show any signs of arching.
Your comments gave me clues to solve the problem by elimination. Thank you very much for all the help. I can provide additional information and pictures if somebody wants them. On my next anual I will replace all of the 12 wires with new ones.
I am in Panama, Central America and N2800C is based at MPDA airport in David about 200 miles west of the Panamá Canal close to Costa Rica.
Email: flinares@cwpanama.net