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Continuing missing F-20 engine saga

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Pete L

12-08-2003 10:22:10




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Guys - - Some frequent readers have seen my pleas for help this past summer on my F-20. After all efforts to make this engine run smoothly, I still have not stumbled into the problem. The missing problem comes when the engine is running spark advanced and little or no load, at any speed. Spark retarded, it runs smooth. Spark advanced and under a heavy load, it runs great. Have verified valve and spark timing, swapped carburators and magnetos to rule them out, checked for vacuum leaks, new rings, ground & set valves to .020" hot, plugs & wires, and a bunch of other typical stuff. Still no effect. Several of us are baffled. I did notice the other day that each time the engine misses under an advanced spark, a light popping sound is heard at the intake of the carburator. There has never been any serious back-firing issue. Is my problem in the head ? What correlation does this popping sound have to the engine missing? Your thoughts please..... ..

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Jonathan Mirgon

12-09-2003 02:40:23




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 Re: Continuing missing F-20 engine saga in reply to Pete L, 12-08-2003 10:22:10  
I had the same problem on y (a) and it turned out to be valvlash adjustment that was extremely loose on one cylinder, I readjusted all of the valves and this corrected my problem.



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grg1938

12-08-2003 22:03:36




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 Re: Continuing missing F-20 engine saga in reply to Pete L, 12-08-2003 10:22:10  
Try a set of Autolite 3077 sparkplugs. It helped my F20 alot with interminant miss problem.



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Nebraska Cowman

12-08-2003 15:57:13




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 Re: Continuing missing F-20 engine saga in reply to Pete L, 12-08-2003 10:22:10  
erratic firing under no-load is real common on F20s. I believe it is caused by the spark jumping on the rotor distibutor block. I take them out and sand flat to get rid of the groove where the brushes run. The brass conductor tab is quite thick and you can remove a lot of material without damaging it.



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Bill Smith

12-08-2003 11:38:52




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 Re: Continuing missing F-20 engine saga in reply to Pete L, 12-08-2003 10:22:10  
A popping noise at the intake sounds to me like a valve or seat problem in the head. Kind of baffling though, since you say you ground valves. Did you really take the time and do a good valve job as far as making sure the valves, seats, and guides were all good and make sure the valves seated properly when you put it together? If you got a single valve problem, the mis should be restricted to that cylinder but may not mis every time. If you got multiple valve problems, the mis could vary among the cylinders with valve problems. Popping noise from exhaust or intake, is a pretty good indication of a valve problem. I'm not saying that is your problem, but would be my first guess.

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Pete L

12-12-2003 07:22:18




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 Re: Re: Continuing missing F-20 engine saga in reply to Bill Smith, 12-08-2003 11:38:52  
Thanks for the input thus far guys. I do have had new Champion, then new Autolite plug in it with no impact on the issue, and swapping mags with a good running F-12 has ruled out the mag. I did use an induction type timing light to check the fire on all 4 wires, and regardless of mag setting, I was getting a very repeatable spark thru the wire on all cylinders. I am thinking that the head will have to come off and have someone else look at the valves, though I still haven't figured out the connection on why it runs pretty smooth while running retarded, and randomly missing 30-40% of the time on all 4 cylinders. The missing that is occurring has all the indications of either not enough compression, or simply no gas in the cylinder to fire on. No change in exhaust smoke color is apparent during the missing (indicating no wasted gas going up the stack). Maybe someone can shed some light here. It's pretty dark where I am. Thanx guys ..... ...

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Bill Smith

12-12-2003 18:10:47




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 Re: Re: Re: Continuing missing F-20 engine saga in reply to Pete L, 12-12-2003 07:22:18  
After reading your re-post here, now you got me second guessing. Do you know for a fact that it is randomly missing on all 4 cylinders? Which would lead me to beleive the problem might be on the intake end of things (carb problem or govenor out of adjustment) since you have eliminated mag trouble. Valve problems generally only affect the cylinders with the valve problems (unlikely to be all 4 cylinders unless you ground the valves at the wrong angle, or all the seats are bad, or the valves that you ground should of been rejected or something of that nature). Back to the popping noise you are hearing. A bad valve (say an intake valve) is burnt (has a hole and does not seat completely all the way around) will make a light popping noise that can be heard out the intake (especially if air breather pipe disconnected). Is what you are slightly hearing is that cylinder actually firing on that bad valve. The cylinder may still have enough compression to fire the piston but the popping noise will be there on every fire on that cylinder witch means the noise will be timed to that cylinder and as fast as the engine is running. This noise is almost identicle to the noise you can make by opening your lips while trying to suck air in through your mouth, or whipping mashed potatoes (not a loud backfire noise). Same thing will happen on exhaust valve, only the noise will come out the exhaust instead of intake. A backfire (out the exhaust) only happens when cylinder doesn't fire, blows unburnt fuel mixture out the exhaust, and is then ignited by hot exhaust parts before it gets out the exhaust system. If I was you, I would double check timing to make darn sure it is correct (since problem appears with spark adjustment). I would then rule out any carb or governor problems before taking the head off simply because those problems are more easily fixed in the process of elimination for figuring out what is wrong. You ussually notice problems with govenor adjustment with throttle adjustment. I remember how baffling my F-20 was. I finally got it running good. Be sure to use the process of elimination. My trouble was, I had about three things wrong and was trying to pin point the problem. I am not sure I helped you any but I hope I did.

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Pete L

12-14-2003 20:16:44




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 Re: Re: Re: Re: Continuing missing F-20 engine sag in reply to Bill Smith, 12-12-2003 18:10:47  
Bill - - Thanks for the input. I do know that my spark timing is right, as I time it off the presence of the piston. I am getting a spark thru the wire, as I've checked it with an inductive timing light and get a very rythmic light, both advanced and retarded. Had a couple different sets of plugs (both Champion and now AC). All plugs appear the same when removed for inspection after several hours on the engine. I have disconnected the govenor from the carb and held the butterfly rock solid, (not a good thing to do while the engine is running....thought the engine was going to take off to the moon till I got my hands on the butterfly link) and different speeds. No effect on the missing problem. I think I have ruled out the actual carb and mag, as both were swapped out from good running tractors with no impact. I just can't figure out what changes within the engine as the spark advances that would make the engine miss. The cylinders that do fire are firing more smoothly, as an advanced running engine should. Its just that some cylinders decide to randomly rest while the spark is advanced. What's your thoughts now ? Thanx ! !

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