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Ignition spark brain teaser

Discussion in 'Mechanic and Repair' started by rockytopp, Aug 12, 2007.

  1. rockytopp

    rockytopp LawnSite Member
    Messages: 120

    Hey Gang.
    I know I should have the ID.#, how ever there are no plates left on the engine , how ever its a MTD. built, with a 21 HP. Briggs, ohv, Vtwin Cast Iron. Please bear with me.
    I have spark,fuel (elec.sol. on carb) every thing is fine as far as it goes,.
    The only way it will run is to unhook one coil, makes no diffrence,either one, as long as one is unhooked it will start and run fine, except it is running about one third throttle, if I touch the ground off of coil to engine ground it will idle down and run as it should ,, however when I ground the kill wire to engine ground there is a spark coming thru the kill wire, if I hook it back to the coil when running it dies,,
    now there is no ground left any where on the engine, I have unhooked the kill wire, carb seloniod plug and unhooked it from the carb so that all that is left is just the wire from the plug to the coils, I have made a new kill wire for the coils as the first one showed up as haveing a break in it inside the shielding.
    All safetys are bypassed but that should not have bearing on just the engine turning over and running.
    For the life of me I cannot figure out how there is spark comeing out of the kill wire and how it makes the engine idle down, I can unhook bothe coils and it will still run one third throttle unless I ground one of the wires to the block or some such ground... I have put a meter on all the grounds and hot wires
    and they show to be good and not have a short in them,,, that is just the carb ,, kill wire, as I must hook up the selonoid for carb to operate... I am going nuts,,, Oh yes I am checking spark with plugs out, so that it don't start every time I turn it over to check it.But as stateted above if I hook up both coils I lose spark. and I have tried new coils to make sure there is not a short inside of one of them...Help.............
    thanks for any and all.
  2. fixer67

    fixer67 LawnSite Silver Member
    Messages: 2,098

    If it is the set up I am thinking about there are two diodes in the kill wiring harness to keep the two coils from inter-fearing with each other. One or both diodes are bad. You are supposed to get spark out of the kill wire when you ground it to the engine that is how you kill the engine. The kill wire has about 1/10th the voltage as the main plug. When you ground the kill wire you take away the coils ability to build the higher voltage seen at the plug. I do not mean to be mean but from your post you need to get someone else to do this work for you before you get hurt or do some damage.
  3. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029


    Check your old kill wire lead for small diodes, One of these goes out it will cause all these haywire ignition problems you are having. This could explain the open circuit test you got. I can't recall how this kill system works because it's been around 8 yrs. since I fooled around with one.

    Since you have no model numbers the part number for a new kill harness is 692319.


    Good Luck
  4. smokediver

    smokediver LawnSite Member
    Messages: 59

    Hey guys, nice site btw.
    I've got something similar going on with a 23 hp vanguard that's been retroed to a gravely ZTR.
    It has eaten 2 sets of plugs in less than 3 hours of operation, both sets started firing erratically through the insulators instead of out the end and it spits and sputters and pops and smells rich. The plugs are black. Been through the carb and can't find a darn thing wrong. The only adjustment is for idle mixture and speed.
    3 times that it wouldn't start I removed a plug (just one) to check for fire, and when I turned it over it started on the other cylinder. I clipped one of the kill wires but it made no difference. I assumed if either diode was bad, separating the 2 wires would solve the problem, right?
    Sometimes it will run almost smooth without a load, but if I engage the blade, or even try to drive it the sputtering starts again.
    Almost seems like a timing issue, but there's nothing to adjust. Checked the flywheel key, straight. And the coils are firmly mounted.
    Checked everywhere for loose wires and bad switches, nothing.
    Do vanguards have a hotter coil or something that requires special plugs??
    Any suggestions would be greatly appreciated.
  5. rockytopp

    rockytopp LawnSite Member
    Messages: 120

    Thanks guys, and oh yes Fixer67, I have been working on small engines since 1959 and I do know the function of a kill or ground wire, if you read close you will see that is not what I was refering to. I do attemp to keep up on new machines but with some of the new stuff one is bound to miss some things that are important. But as stated I have not in all my years saw a problem of this type. How ever thanks for your input.
    I think perhaps Restrorob has come up with perhaps some of the problem, I thank you for this and will pick one up or order which ever be the case tomorow and report back as to the results.
    Thanks Rockytopp
  6. fixer67

    fixer67 LawnSite Silver Member
    Messages: 2,098

    Sorry about that. It just sounds different that is all. I guess if all else fails you could just a double pole kill switch and run a separate kill wire to each coil. I know what you mean about the new machines, just as you learn something they go and change it.
  7. Bill Kapaun

    Bill Kapaun LawnSite Senior Member
    Messages: 944

    Skokediver- I'd try a different brand of spark plugs.
    What are you gapping them at?
  8. smokediver

    smokediver LawnSite Member
    Messages: 59

    Gapped them at .030.
    Actually, I have tried a different brand. The first were champion, and suspicious in my opinion, but the second were NGK. I picked up a set of autolite's today, but haven't installed them yet.
    I just suspect some type of ignition problem that's toasting them so rapidly.
    Question..Is the hall sensor incorporated into the coil?
    And could it be defective and triggering spark at the wrong time somehow?
    I need to look again, but seems the plugs were firing quite rapidly for starter speed. This would cause premature ignition, and all sorts of timing problems.
    The whole ignition system is so darn simple, it has to be either the plugs or the coils doesn't it??
  9. smokediver

    smokediver LawnSite Member
    Messages: 59

    Installed the new plugs and removed the air filter and it runs pretty well. Still sputters a little on initial startup, and a little when a load is first applied. But that's what it did before when I changed the plugs. If I put the factory filter on, it loads up and runs rich. I replaced it with a piece of oiled foam and it leaned out. The filter is new and clean, but something's not right there. Is there more than one filter available? Would one be less restrictive??
    I've only run it about 30 minutes on these plugs, so I'm curious to see how long they last....
    Could a coil fire but still be defective, and is there any way to check it other than ohm it out for shorts?
  10. Bill Kapaun

    Bill Kapaun LawnSite Senior Member
    Messages: 944

    Did you check the part# of the filter vs what's listed in the owners manual/IPL?
    I read on another forum where the "counter guy" gave the customer what APPEARED to be the correct air filter for a Briggs, but wasn't. I can't remember if it was Vanguard, Intec or??? but it was a "larger" engine.

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