'67 M20F runs rough...then smooth...then rough...then smooth....

[CajunMooneyguy]

Ok guy......heres a real puzzle for all you gear heads out there!! I have a 1967 M20F, IO-360 fuel injected 200hp.  two weeks ago on my way to Tenn from La, after 2 hour of smooth running .... the engine started running rough with a subtle drop in rpm (by the sound of it) ....not "stuck valve rough"....but a noticable vibration. It would do this for 8 or 9 seconds......then run smooth like nothing happened.   after a minute or so, the cycle would repeat. Also....while it was running rough....at the same time, the cabin would fill with an acrid exhaust smell. almost like a exhaust/raw fuel/running rich smell.   SO......of course i set it down asap. On the way down I tried troubleshooting to gather info for the mechanic. I tried both mags, tried full rich and max leaning, boost pump on, swich tanks, various power settings, and Nothing made a difference!  All of the readings/pressures/temps, on the EDM 711 engine anylizer were perfect, no changes, heater was off.   The ONLY abnormal (besides the smell and rough running) thing was....while running rough for the 8-9 seconds....I would get a slight "RISE" in fuel pressure from 25.5 psi to 27 psi! then back to 25.5 when running smooth.  Once safely on the ground, I did run ups and checked everything again. Still did it, even at idle!! and sitting there in the sunlight i could actually see the white smoke/exhaust coming from the firewall while it would run rough.  The lineman said he could see a "mist" coming from the engine breather tube when running rough.  And it did in fact puke more oil on the gear door than usual.  OK....so the mechanic didnt have a clue.  I was stuck in Louisiana and he and the plane in Savanna Tn for a week.. He put a new set of plugs, new engine/mechanical fuel pump, cleaned the injectors and line, checked every thing for leaks, compression test, cleaned fuel filters, and basically, checked every thing he could and said it all checked out great!  So, a week and $1,200 later i pick it up and head to La. Ran smooth and solid like i was used to.....for a little over an hour, then started doing the exact same thing!!!!  AARRGGGHHH!!!!   So, I landed in Greenwood Miss yesterday, and there it sits!  HELP!!!!  Has ANYBODY out there had this same issue and what was it??!  The old man behind the counter at the FBO told me his buddy had a very simular problem and it turned out to be badly pitted points in the mags!  Makes sense? Please help if you can! Throw it out there. Ill try anything! Thanks!!

[meddesign]

just going to throw some stuff out there,  Bad fuel distribution spider, partially clogged injector... shows up as a miss at cruse when you try to lean at cruse, (or maybe a miss in cruse)  leaky fuel hoses, cracked exhaust. to go back to the first issue, as your spring in you spider or inside of spider corrodes the particles, partially clog the injector, cause one cylinder to run very lean and hot and some of that oil on all of our engines burns off for the smell and smoke.   The fact that your engine analyzer, shows a rise of pressure could be a link, I did not have full fuel flow at takeoff with a partially clogged injector, but I do not have a multi-point analyzer, I read it on the Kelly site under fuel system trouble shooting.  The leaky fuel hose/injector line would not show up anywhere on the computer, but would cause the smoke.  You can run on the ground with the miss then check pipes to see which is the cool one, use water pistol, however your engine computer should show it, should show any miss which cylinder.  Let us know what you find.

 

Don 70E

[N201MKTurbo]

It sounds like something is pressurizing your crankcase. Crankcase pressure will change the performance of the mechanical pump a little bit. The NA pump has the top of the diaphragm vented to the crankcase, the turbo pump has the top vented to reference pressure.

 

I had a broken piston once that produced similar symptoms except it was continuous not intermittent. In my case the piston land between the oil ring and the second ring about half way around the piston had broken loose. The piece was riding along and didn't cause any additional damage. This burned/blew all the oil out of the engine until the prop lost control. Had to do an off airport landing.

 

In my case, all the cylinders still showed good compression.

 

I would start troubleshooting this by removing the dip stick and have the mechanic hold his hand over the dipstick tube with the engine running and see if there is a change in crankcase pressure when the problem occurs.

[takair]

Don has some good ideas above. Was thinking injector myself. Would also check flame tubes in exhaust to be sure they are not obstructing the exhaust. Back pressure could throw off mixture, increase cylinder pressure and perhaps crankcase pressure. A broken ring could cause extra pressure in the crank case as well. Compression check may or may not show this. I would think that if this were a mag it would show up during a mag check while the problem was occurring.

[Seth]

See if there is any debris in the fuel injctors.  Good luck.  I had a 67 F that twice wound up with debris in the fuel injectors which caused a rough running engine.  In both cases, it occured as I was leaning after climb - or when I noticed the rough running part. 

 

-Seth

[Cruiser]

sounds like a mechanical fuel pump problem. 

[Shadrach]

sounds like a mechanical fuel pump problem. 

 

 

I was thinking the same, but a 9 sec drop in RPM almost has to be prop/pressure related.