Small engine problem

Discussion in 'Shop Talk' started by Cabin Fever, Nov 19, 2005.

  1. Cabin Fever

    Cabin Fever Life NRA Member since 1976 Supporter

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    Yesterday I started winterizing all of our power equipment. The riding mower would not start. (Briggs, 12.5 HP, I/C, 15 years old). It had spark. I took the carb off and cleaned it from inside out. Still wouldn't start. Sprayed starter fluid into the carb. It ignited and backfired thru the carb! I thought timing problem. So, here's my question, can single cylinder engines be timed? If so, how is it done?

    The other thought I was thinking is that maybe the intake valve was hanging up...but when I had the carb off and turned the engine over, I saw that the valve moved back and forth. I suppose maybe the valve could be carboned up and not fully seating. What do you think?
     
  2. moopups

    moopups In Remembrance

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    About the only way you can affect the timming on single cylinder engines is to change the distance between the points if its not CDI ignition. I have had a backfire in the past shear the aluminum woodruff key that aligns the flywheel to the crank shaft.
     

  3. John Hill

    John Hill Grand Master

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    Cant say for your engine but some four stroke single cylinder engines have a flywheel magneto and if you think about that for a moment you will realise this means two sparks per intake/compression/power/exhaust cycle. The second spark is normally of no consequence and occurs when both valves are open at the end of the exhaust stroke. If your engine is like that the volatile lighter fluid may have ignited and with both valves open you saw it blow into the carb.
     
  4. Ross

    Ross Moderator Staff Member Supporter

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    I'm thinking sheared or partialy sheared key too. When was it run last? Pretty sure the BS I/C used a steel key but they go too. Hate those Techumsie aluminum (designed to fail) keys
     
  5. gilberte

    gilberte Well-Known Member Supporter

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    I think the gas we are buying goes bad (loses volatility?) more quickly these days for some reason. Have you tried new gas with a new plug?
     
  6. agmantoo

    agmantoo agmantoo Supporter

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    The exhaust valve is the more common valve to stick. Have you determined that you have compression? If the engine was running when it last stopped it is unlikely that the timing is off. Sheared keys are typically a result of the blade hitting something immoveable.
     
  7. Cabin Fever

    Cabin Fever Life NRA Member since 1976 Supporter

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    The mower was running fine the last time we used it in October. Since then, it has been sitting in the pole barn waiting for me to winterize it. I think that I'll check the key and the exhaust valve.

    Question: when the coil (magneto) is directly aligned with the magnets on the flywheel, where should the piston be...TDC?
     
  8. John Hill

    John Hill Grand Master

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    The magneto magnets should be aligned with the coil when the spark occurs but this is a very coarse way of judging timing. The instant of opening contact points is the point of firing. If it does not have contact points you need to find how the pickup works, reluctor, Hall effect etc.

    To answer your question directly I would expect the magnets to be exactly over the coils about 15-30 degrees before TDC.

    If the points open (or a pickup opens the primary circuit some electronic way) while the magnets are not near the coil there will be no spark, but there is spark, you know that because it backfired through the carb.

    The spark should occur a few degrees before TDC while both valves are closed, as I mentioned earlier it is conceivable that there may also be a spark at TDC one crank revolution later while both valves are open but we can ignore that one.

    One question, when it fired through the carb did the engine kick backwards or was the 'backfire' more of a lazy sort of 'woompf' of burning lighter fluid?

    I agree the timing may well be out, it might even be due to the flywheel having slipped on the shaft, but I dont think we have proved that yet.
     
  9. Cabin Fever

    Cabin Fever Life NRA Member since 1976 Supporter

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    John, thanks for all of your help. Yes. the backfire was more of a whompf (I know exactly what you're trying to describe). In fact, as the engine turned over, I could see vaporized gasoline shooting out of the air intake end of the carb. So, I figure that the intake valve was open, but on a compression stroke of the piston.

    BTW, I don't think that there are any points on this engine. If there were, where would they be?
     
  10. John Hill

    John Hill Grand Master

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    OK Cabin, I feel that what happened with the backfire does not really tell us anything about the timing!

    I dont know your engine at all but you did mention magneto. An old style magneto would have points that you would usually get at through a cut out in the flywheel or a backing plate. However a modern magneto might not have points at all, I guess it is possible for someone to design a magneto where the movement of the magnets past the coil somehow signals the high voltage spark to be generated. Otherwise there will be some sort of electronic sensor on either the crankshaft or the camshaft.

    What I would do is take the spark plug out so the engine is easy to turn then put a small bolt, nail or whatever in the plug lead so I could hold that in my hand. Then I would gently turn the engine until I felt the spark. Hopefully by turning the flywheel very slowly I would only get a tickle but enough for me to see exactly where the flywheel was when I got it. If I did this and found the spark occuring at anywhere from about 30 to 0 degrees before TDC I would say the timing was correct. Like I say, thats what I would do and I will fully understand if you do not decide to do the same!

    Fuel should not blow out of the intake unless the engine is turned backwards or the inlet valve is stuck, possible I suppose as it gets a lot colder than the exhaust valve and might have a gummed up valve stem. A thorougly blocked up exhaust system might be another cause but it would have to be really, really blocked. Of course if the inlet valve is stuck open you would not get any compression.

    Something I have been told, and I have no reason to doubt it, is that 95% of all ignition problems are in the carburetor!
     
  11. Ross

    Ross Moderator Staff Member Supporter

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    Pretty sure the I/C has points, it might be electronic....... mines never needed much truth be told. I would think you'd have to pull the flywheel to see them. I'll try to get out and have a look.
     
  12. skinner

    skinner Active Member

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    Points type coils have two large square lugs , made of layers of sheet metal, spaced off the flywheel, abouth tha thickness of a business card.

    Electronic coils have the same apperance, but with a short lug between them, about the size of a pencil eraser.

    My experience with with B.S from 15 years ago was all electronic.
    (Son & I built 5 hp B.S. & raced go-carts)

    My 78 16 hp Simplicity had points. Eliminated them with an ADAM chip.Still works great.

    If the magnets on the flywheel or the contact side of the coil get rusty, it closes the air gap. If that is the case, sand the rust off , blow dust out with air hose.

    Maybe that will help.
    skinner
     
  13. Columbia,SC.

    Columbia,SC. Thats MR. Redneck to you

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    sand the flywheel and magnito, loosen the mag and take the top off of a carton of cigarettes to set the gap between them. wrap the carton top around the flywheel and loosen the magnito screws, they will slap against the flywheel, (be sure that you have it turned to the magnito) then tighten. pull out the cig. carton paper and mulch leaves! I hope, this helped.
    Good luck, Columbia,SC.
     
  14. Cabin Fever

    Cabin Fever Life NRA Member since 1976 Supporter

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    I am guilty of not filling you in on the outcome of my problem (Heck, I complain when others don't do this and here I am doing the same thing!)

    I gave up on the problem and brought the mower into the same engine repair shop in town. The guy said the problems were many (1) carboned up valves, scored piston walls, leaky "read cracked" crankcase. He could get it running for $250 but couldn't say how long the fix would last. So, since the riding mower was a cheapy (Lawnboy 12.5hp), was 15 years old, and had become a money pit for the last couple of years (new belts, new spindle, new coil, several weld jobs on the mower deck, etc), and it leaked oil a lot.....I gave it up.

    I'm looking at a new Cub Cadet hydrostatic with a Kohler twin 23hp engine and a 46" deck.
     
  15. John Hill

    John Hill Grand Master

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    Thanks for putting us in the picture!
     
  16. Whip Hussmann

    Whip Hussmann Well-Known Member

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    Had a problem with a motor that gave me the same symtoms.Had an expert tell me to change the spark plug. I did and it ran fine . Still does.