DT100 - just quit running - looking for insider knowledge and help
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  1. #1
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    DT100 - just quit running - looking for insider knowledge and help

    hey everyone... I've got one of those unsolvable mysteries with one of the bikes, my '76 DT100.

    So i spent a couple dozen hours this winter cleaning it up, running through the elect system, replacing goodies including carb, rectifier, treating tank, plug, etc. A month or 2 ago it started up on a nice late winter day, and i was able to take my 4 year old for a nice slow ride in the field next to our house. we went on a few more of those rides, at it ran great. then 2 weekends ago i was giving the neighbor kid his first motorcycle lesson, and we were riding around the house, it the DT just quit.... like full blown stopped just like i hit the kill switch or turned the key off.

    i checked everything i can think of... there's fuel, i've got spark when the plug is pulled and sitting next to ground while kicking over... i've got ~110 psi compression, and it appears i've got fuel. it's a new carb, and it won't even start on JUST starting fluid... not even an attemp to fire. i don't have much 2-stroke experience, so i don't know all the inside tricks a/o things to look for (i am a pretty savvy mechanic though). i disconnected the kill switch completely from the harness... so that's eliminated as a potential issue (but it wouldn't have spark if the kill was broken).

    one thing i have NOT done is check the timing.... i don't have a dial gauge... is there another way to check it? regardless, i've riden in cars/bikes that have had the timing off a bit, and how this baby just turned off doesn't lead me to think it's a timing issue.

    Anyways, just frustrated here as it's Spring in Iowa i want to ride (well, actually my son wants to ride)! if anyone has any advise i'm all ears. thanks a ton guys.

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  3. #2
    M-J Lifetime Achievement Award
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    How strong is the spark and what condition is the plug....it may be the mix... two strokes can be very temperamental. For it to just suddenly quit usually means electrical...fuel supply/mix usually gives a warning such as spluttering etc.

    Most of my 2 stroke stuff have low speed and high speed screws on the carbs and if I have any problems I usually set both out 1 and 1/2 turns as a base setting...although why this setting may have changed is anybodies guess. Filters anywhere? also what about tank venting?? Timing wise what is the BTDC setting supposed to be and how can you check it?....assuming it has a CDI ignition?.

    Points should have a mark on the alternator rotor that lines up with a mark on the case and points should just be beginning to open....we used to use a cigarette paper between them and when it fell out they were just opening.

    Also dont forget 2 strokes compress the fuel in the crankcase....and this needs to be sealed....often through lack of use the crankshaft seals dry out and crack leaving no compression down below the piston.

    Thats about all I have atm

    Good luck
    Last edited by Aussie Steve; 04-17-2017 at 09:55 PM.
    Smoke me a kipper I'll be home in time for breakfast

  4. #3
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    Thank for the note Aussie Steve!

    I'm not sure how to judge how strong the spark is... any recommendations on how?
    I decomissioned the oil pump, so I'm running pre-mix... i'll think about this and look into it further though.
    i'll check the carb screws.
    no fuel filter; did have air filter but took off for troubleshooting; tank vent was questionable so i took cap off just to be sure.

    interesting points about the crankcase and timing... i'll consider those further also.

    thanks again!

    Quote Originally Posted by Aussie Steve View Post
    How strong is the spark and what condition is the plug....it may be the mix... two strokes can be very temperamental. For it to just suddenly quit usually means electrical...fuel supply/mix usually gives a warning such as spluttering etc.

    Most of my 2 stroke stuff have low speed and high speed screws on the carbs and if I have any problems I usually set both out 1 and 1/2 turns as a base setting...although why this setting may have changed is anybodies guess. Filters anywhere? also what about tank venting?? Timing wise what is the BTDC setting supposed to be and how can you check it?....assuming it has a CDI ignition?.

    Points should have a mark on the alternator rotor that lines up with a mark on the case and points should just be beginning to open....we used to use a cigarette paper between them and when it fell out they were just opening.

    Also dont forget 2 strokes compress the fuel in the crankcase....and this needs to be sealed....often through lack of use the crankshaft seals dry out and crack leaving no compression down below the piston.

    Thats about all I have atm

    Good luck

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  6. #4
    Super Moderator

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    Before you get too carried away, change the plug.
    Just because.
    Don't believe everything that you think.

  7. #5
    Newbie
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    AH! That did it! you guys rock.

    in all seriousness, i did check the plug, it didn't look that bad at all, and it had spark. but as i previously mentioned, i'm not that versed on 2-strokes. So, are these things REALLY THAT finicky when it comes to plugs? obviously, since there's oil in the fuel they'll probably foul more often... and at lower revs probably worse... but i'd estimate i had 1-2 hours on that plug... is that normal?

    really, thanks again guys!

    Quote Originally Posted by Easy Rider View Post
    Before you get too carried away, change the plug.
    Just because.

  8. #6
    M-J Lifetime Achievement Award
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    Spark needs to be fat and bright blue....yellow and weak will fail under compression in most cases...esp with a kick start motor. A plug should last longer than a few hours i would suspect the low speed running has had a detrimental effect...getting them good and hot via a long run helps. Also you can run a blow torch over the plug to burn off the gunk build up.

    A couple of other areas to watch out for on two strokes is carbon build up....exhaust, (muffler which are usually removable to allow burning off of carbon same as the spark plug) and exhaust port. Piston crown can get pretty carboned up too...simple to pull the head off and clean with a soft scraper then polish. Make sure all polish residue is thoroughly removed.
    Last edited by Aussie Steve; 04-18-2017 at 10:30 PM.
    Smoke me a kipper I'll be home in time for breakfast

  9. #7
    Super Moderator

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    Quote Originally Posted by akremzar View Post
    So, are these things REALLY THAT finicky when it comes to plugs?
    Yes. But like all things, plugs sometimes fail too.

    I even had one short internally once.
    The spark test showed it jumping from plug body to ground......but nothing across the gap where it should be.
    Don't believe everything that you think.


 

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