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ollieduke

Motor hard to start when hot.

ollieduke
14 years ago

Motor is a 18 hp duel Briggs. As long as its running,there is no trouble.(runs good for 3 hours cutting grass,but if you stop it,you have to wait at least 30 minutes before it will start and once it starts it runs good. (its been doing this for 1 year).Since it has no fire when it wont start, naturally the first thing I did was replace the dual coil,but that didn't fix it as it acted the same when hot. While checking for fire,I disconnected one plug wire and accidentally grounded it and it started with only one plug connected,I cut off the engine to replace the loose plug wire and then it wouldn't start as it had no fire. After fiddling with it,(I know this sounds far fetched to you as it did to me,as I was a mechanic years back),but if I ground one of the plugs while it is hot,it will start every time. I tried grounding the motor to frame,in case there was a bad ground. and that did not help starting it. Does anyone have an idea whats causing this problem?


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Comments (24)

  • darg
    14 years ago

    The engine might not be grounded at times from the battery due to a bad ground.

  • tomplum
    14 years ago

    Assuming we're talking about an op twin here. So somehow what is happening is the cd fires the coil when the plug is grounded. Not Mr Wizard here, but the bases I would cover would be: Clean the mount where the coil bolts down. Pull off tins that go around the head to ensure the cooling fins are open. Be darn sure that the ground wire isn't chafing where it enters the shroud. Do this to start. Did you buy an Briggs coil and when?

  • ollieduke
    Original Author
    14 years ago

    Darg, have tried an extra ground when hot,but didn't help,although it does sound like a ground problem.
    Tomplum,is a op twin,did check the ground wire and it looked ok. The coil is a Briggs coil and punched about 2 months ago. Seems like it wasn't rusty where the coil was bolted down,but will check it next time I have the shroud off. Talked to my son who also uses the mower and it has been doing this for at least 2 years. I very seldom had the trouble as I never cut it off till I am finished,but my son has been having trouble a long time as they usually cut it off for a break. We figured it was just the starter getting hot as the moter turned over slower when trying to restart it. But while cutting grass I got a rope caught in the blades and naturally had to cut if off. Since I was in a hurry,I iced down the starter and it spun the motor fast like it normally does and thats when I discovered it had no fire. Seems like if kill wire was shorting,it would also do it while running. I figure the coil is adjusted ok as the mower shop gave me a card like they use to adjust it. I do keep the shroud fairly clean as I blow it out with air often.
    Thank you both for the comeback. My mower shop has no idea what the trouble may be.

  • tomplum
    14 years ago

    I wouldn't assume that any of the above are as they should both be without confirmation that they are. The flywheel magnets shouldn't fail, but might as well know they are strong enough. It would be also in your best interest to verify that there is absolutely no current feeding into the kill wire which will damage the cd in the coil pack. Cranking speed must be sufficient to create the spark. If your twin has been been using oil, grab some headgaskets and decarbonize the top side. So if you end up that you truly don't have spark, the above bases are covered and it is cranking fast enough to fire then ask the dealer to warranty a new coil.

  • darg
    14 years ago

    Check with a volt ohm meter and see if you get a reading on the engine to complete a ground reading to the engine and the frame. If the plug fires when you ground it, sounds like the engine is not grounded good.

    Good luck

  • ollieduke
    Original Author
    14 years ago

    Engine does not use any oil or a very small amount and has plenty of power. Have checked engine with an ohm meter to frame and it reads the same hot or cold.(dead short.) I have also used battery cables and connected them both from the engine to frame and made no difference. Seems like we all agree that it sounds like a grounding problem of some kind is causing the problem, unless I got a new coil with the same problem as my old one. Thanks for all the help. As for now,at least I can start it when its hot by grounding the plug.

  • tomplum
    14 years ago

    From strictly an ignition on standpoint, your engine doesn't care if it's grounded to the frame or battery. The only ground wanted in order to run is that the legs of the coil must share the ground with the spark plug really. Any ground or power that hits the kill terminal is terminal. You really should go through the steps I talked about above. My question regarding oil consumption ultimately is concerning increase compression making it more difficult to crank the engine. Starters are known to get tired on these also. Op twins do not tend to run cool. If you have an older version, the tins have to be removed to effectively clean the fins. Good luck on whatever you decide to do.

  • walt2002
    14 years ago

    "The engine might not be grounded at times from the battery due to a bad ground."

    "Check with a volt ohm meter and see if you get a reading on the engine to complete a ground reading to the engine and the frame. If the plug fires when you ground it, sounds like the engine is not grounded good. "

    "Have checked engine with an ohm meter to frame and it reads the same hot or cold.(dead short.)"

    "From strictly an ignition on standpoint, your engine doesn't care if it's grounded to the frame or battery."

    Tomplum above is absolutely correct. EVEN if you checked with a VOM, it wouldn't tell you anything much. Just because there is continuity does not indicate the connection is capable of carrying sufficient amperage to run the starter for instance. Slow cranking as Tomplum also indicates, may not be producing enough spark. Flywheel must spin at least 800 rpm to give proper CDI operation.

    You might remove both plugs when the engine is failing to start and see it it now fires both plugs while out. B&S does not recommend repeated nor prolonged firing with one plug disconnected, I don't know why.

    You can also try jumping from a known good battery with a GOOD set of jumper cables directly to the starter when failing to start. Also as Tomplum said, the starters are taxed to max on these engines.

    Carbon build up is a very common cause for difficulty in starting this series of engine but that would not account for a lack of spark except again, causing slow cranking. Note that compression is typically higher when the engine is warmed up thus naturally cranking harder and magnifying any border line condition.

    Walt Conner

  • ollieduke
    Original Author
    14 years ago

    May have not made my self clear,the starter spins the engine ok. When its hot,it spins it slower(I figured old starter),if I let it sit for 5 minutes,it then spins the engine normal but I have no spark,until I let is set for another 15 minutes,or ground one of the plugs. After reading your posts,I realize I haven't run a jumper from the battery to the engine.only from the frame to the engine. So when it warms up here,will do that.
    Thanks again for the help,having a good board like this really helps and reminds me that I aint as smart as I think I am.

  • mownie
    14 years ago

    tomplum and walt are precisely right. This engine does not use any external battery power for creating a spark, the ignition spark is created by the magneto system. The magneto is self powered by a permanent magnet in the flywheel. This is not a "battery and coil ignition system".
    I don't exactly know the reason Briggs says not to run these engines with one plug wire disconnected, but my suspicion is based on the fact that this engine uses a single magneto coil to fire BOTH plugs simultaneously, and therefore, this coil was engineered to be capable of generating a very hefty spark (under normal conditions). Running with one plug wire disconnected would mean that all the spark energy would have to discharge through just one plug instead of two. This extra energy would then try to locate an alternate path to ground. If that alternate path to ground just happened to be a small fissure or crack in the insulation of the spark plug cable and touching a metal surface, the spark energy would tunnel through to ground. A short time of doing this would cause the crack in the insulation to become a permanent path to ground (as the rubber gets burnt to create carbon, which is a pretty fair conductor of electricity), and then the spark would rather take the easy way to ground instead of jumping the spark plug gaps. I believe the problem you are having is related to too much compression when hot, causing the spark to be suppressed at the plugs. Even though you say "Engine does not use any oil or a very small amount and has plenty of power", there could still be enough build-up of carbon and ash in the combustion chamber surfaces that will have the effect of raising the compression of the engine. This means your aged starter is less able to spin up to make good spark, and that weak spark simply gets suppressed by the higher compression. Clean up time.

  • ollieduke
    Original Author
    14 years ago

    Mownie,so if it is its like you said,would shorting out one plug,give more spark to the other plug? Another point,that you might be right is if you noticed in my first post,I said it would start when hot if I disconnected one plug wire,which could give more spark to the other plug. When its running,it does have a very hot spark. Is there any way to clean the carbon off the heads other than taking off the heads? The mower is getting too old to spend much time on and as from my other post,am starting to look for a new one. In the old days,we ran Cascite thru motors to clean them out. Carbon can be quite a problem,as 20 years ago,I had a mower motor that just stopped and lost all compression and was caused be a big piece of carbon trapped between the valve and seat. Had forgot that it doesn't need voltage to excite the coil. Thanks for comeback.

  • walt2002
    14 years ago

    "if you noticed in my first post,I said it would start when hot if I disconnected one plug wire,which could give more spark to the other plug."

    FROM your first post - "While checking for fire,I disconnected one plug wire and accidentally grounded it"

    "but if I ground one of the plugs while it is hot,it will start every time."

    Not exactly the same and no, grounding the one plug wire would not give more spark to the other plug. I think you are missing something here. By the way, when was the last time you put in new spark plugs? AND just how are you determining it does not have fire until 15 minutes after it is shut off hot and how are you determining it has good fire once running? The latter goes back to my remark that the CDI requires 800 rpm min. to work properly.

    "Is there any way to clean the carbon off the heads other than taking off the heads?"

    There are still products sold to do this, one popular one presently is SeaFoam.

    Walt Conner

  • ollieduke
    Original Author
    14 years ago

    New plugs every season and originally took out one plug and connected it with plug wire and grounded plug on motor. Engine turned even faster with one plug not not being screwed in head and no fire thru plug,but then as I was finishing the test ,the plug lost its ground and the silly thing tried to start. I checked the fire by putting a ground close to plug(while engine was running) and it was a nice spark jump.When I am trying to start it hot,usually have battery charger hooked up or using jumpers on truck battery,so its turning fast. Its been doing it for so long,we have learned to live with it.

  • walt2002
    14 years ago

    For some years, I specialized in rebuilding this series of engine. I have never seen nor heard of such a scenario as you are describing.

    When I REALLY want to know if there is fire in a plug wire, I stick a Philips screw driver in the plug boot, grasp the shank in my hand, rest a finger on a bare spot of the head or block and hit the starter.

    One time after a somewhat similar experience with an engine, I held the spark plug up to the light and discovered a very, very fine carbon filament running between the electrodes, about 1/4 the size of a hair. After cleaning plug, the engine would start. Problem, wrong heat range of plug for the engine. As plug cooled, filament cooled and broke. I believe RJ12 is the recommended plug now for this engine.

    I have seen similar situations with this series engine but plug fire was present in these.

    IF all circumstances are as previously outlined, only suggestion I have is try another CDI OR try your CDI on another engine. Seems extremely unlikely that two CDI units, your old one and the new one, would have the same flaw.

    Walt Conner

  • nitrojc
    14 years ago

    HI GUYS, Nitrojc here, haven't been on this site for a year. Was reviewing the Hot hard starting condition, and didn't find anything said about " Valve adjustments."
    I have the twin B/S 18.5 HP, and have noted that the small battery's output is marginal at best. This battery was replaced with one of the Max Amperage that I could find ... I believe it was 235 CCA, and still this isn't enough Juice for a hot engine.
    I have another 1980 Monkey Wards tractor 11hp Syncro-balanced B/S engine that still runs fine. However, 15 years ago I went to a small car battery. I mounted the battery directly over the engine, and this has been the Fix/Cure for any starting condition, at any temperature.
    Some day I may have to fabricate in a car battery on the newer (1998) Model tractor, But there's No room under the hood ! FOLKS, JUST MY TAKE ON THE ISSUES. NITROJC.

  • walt2002
    14 years ago

    "HI GUYS, Nitrojc here, haven't been on this site for a year. Was reviewing the Hot hard starting condition, and didn't find anything said about " Valve adjustments."

    Welcome back.

    It is very rarely that this model engine needs valve adjustment and that usually after someone has flubbed a "valve job". He is complaining about lack of spark which is curious to me given his scenario.

    Hard cranking, starting, is a common complaint with this engine and I have a list of possible causes but it hasn't been requested.

    Walt Conner

  • nitrojc
    14 years ago

    Hello Walt, I am Officially requesting the "Possible Causes list." I believe this would be bennificial to All with the same symptoms "Hard cranking, and Starting condition." Thanks, from All of us, Nitrojc.

  • walt2002
    14 years ago

    I am hesitant to post it here because of length but will this one time, send direct after this. This is from previous post.

    While difficulty in cranking is a fairly common complaint with these engines the problem is very seldom due to valves needing adjusted unless someone has just done a valve job and did it poorly. This is an "L" head or "flat head" engine, the valve adjusting problem is common to Over Head Valve engines. The starter is taxed to its' limit when all else is ideal. Adjusting the valves of this engine is a major undertaking, not one I would try to give you a step by step on but requiring a Service Manual.
    First I would be sure you have a good battery, that all battery connections are good, clean at the battery AND where the cables attach to the solenoid and to ground. I would check the starter output shaft where it exits the end plate/mounting bracket. Is there excessive side to side play? The top bushing tends to wear egg shaped and allow the armature to drag on the field magnets. Some times a field magnet will come unglued from the starter housing and drag on the armature. Putting 3 - 4 drops of oil on the shaft where it exits may help as the bushing may be dry. Also the brushed could be worn, bad, but not likely.
    A fairly common cause of hard cranking for these engines is carbon build up in the combustion chambers. This increases the compression ratio and thus the cranking effort required. The head can be removed and the carbon manually removed but if the engine will start, I would use a few tanks of fuel with SeaFoam mixed in it according to instructions on the can. This should remove carbon build up and save the expense of new head gaskets and the time involved.
    I have had people tell me that too small a battery cables caused excess resistance and the starter does not get enough amperage. It is possible the starter solenoid is at fault but not likely. Using an old screw driver to jump between the two large posts of the starter solenoid will help tell if this is the problem. Do not do this timidly or you will get a lot of sparks, stab it like you mean it.
    It is also possible the starter its' self is weak perhaps from repeated attempts to make it turn a resistant engine. The starter should never be engaged for more than a few seconds at a time with rest (cool down) between. Excessive heat causes the windings to loose resistance and thus power.
    Good luck,
    Walt Conner

  • ollieduke
    Original Author
    14 years ago

    Thanks for all the come backs. Let me answer some of the questions.1 The motor is not hard to start when it has fire to plugs.2 The starter spins the motor the same when cold as when hot which is plenty fast. 3 When its hot and you stop it,you have to wait from 20 to 30 minutes for it to cool,as it has no fire,but when its hot and wont start,if you ground one of the plugs,it will start up and run until you turn it off. Its been doing this for over one year and has gotten no better or worse. I wonder if when its hot and wont start,maybe by shorting the plug it excites the system some way. Actually the heading is wrong as it is not hard to start when hot,It WONT start when hot.(except by shorting a plug to ground)

  • tomplum
    14 years ago

    What steps have you taken since posting?

  • ewalk
    14 years ago

    Ollie : De-carbonizing solution other than fuel treatments , you may wise to carefully spray a fine mist of H20 through
    the carburetor if ambient temps are warm. Back in the Day we successfully reduced(Internal Steam Clean) carbon deposits in Diesels and Gasoline engines in the Lumbering Sector where equipment idled continuously causing carbon knock issues. Fogging the engine carefully while at high idle may assist , while also utiizing a Top End Fuel Treatment long term...Ty !

  • esemilio
    14 years ago

    ollieduke - have you solved the problem, I am curious as to the solution.

  • ollieduke
    Original Author
    14 years ago

    Haven't addressed the problem,as its been too cold and have the winter lazies. Most likely wont do anything as long as I can start it by grounding the plug,plus I never stop it until I am finished cutting the yard. Also it is getting very near replacement time. I want to thank everyone for all the help on this weird problem. This is a very good board and have learned a lot from reading all the posts.

  • stockace
    12 years ago

    ollieduke,I have an 18.5 B&S I had the same problem you have. I took the coil off and cleaned the rust off and the head where the coil bolts on (problem solved) hope this helps billy.cannon@att.net let me know

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