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1978 DT125MX starting problems

  • ecurbrenots
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1978 DT125MX starting problems was created by ecurbrenots

I am a 67 old with experience in old British 4 strokes with Lucas magnetos (whn good they give a really good spark) but I have limited experience with  2 strokes.  I was riding my DT the other day on the woods after a few miles of Green lanes when i stalled it,  it would not restart although the starting had been a first kick affair until then, this resulted in a long push home.

I have only got it to start twice since then on a bump start and once by the kickstart when it fired up then died.

This is a CDi model,  and the spark did appear a little weak so i changed the coil,  better spark  but still no start. 

Cleaned the carb and blowed through with air line but still no start,  checked the reed valve one petal not sealing fully fully, letting in light but not horrendus so i put it back still no start, changed plug no difference.

I have connected the ignition circuit with direct wiring so i think i have eliminated wiring problems.  I have also pulled the flywheel off  a bit of moisture inside which i dried out but keyway is good and key intact.

I am now  perplexed as what to try next, could the reed valve cause this problem,  i will add that the plug is not getting wet even after many kicks,  it has very good compression and the fuel is clean and new.
 

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02 Sep 2021 09:53 #1

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Replied by Sneezles61 on topic 1978 DT125MX starting problems

If theres spark... The fly wheel hasn't shifted... the plug is dry... I'm going with fuel system... 
Sneezles61
02 Sep 2021 11:11 #2

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Replied by ecurbrenots on topic 1978 DT125MX starting problems

I tend to agree with you, however the float chamber is filling with fuel and i am pretty certain the jets and galleries in the carb are clear.  This is what made me wonder about the reed valves.
02 Sep 2021 11:16 #3

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Replied by RT325 on topic 1978 DT125MX starting problems

Was going to say crank key but seeing that's ok then its blown out a crank seal either left or right. If right it would've disappeared in a cloud of gearbox oil smoke although maybe not if an instant stop rather than seal coming out on an angle & letting oil get sucked in until it popped fully out & stopped. Pull the mag & check the left side first. CDI spark never looks like a spark from my B33--when i had one 55 years ago!! but if it sparks it should run.

Definitely CDI? as the ordinary DT125 "78 still ran points but the 175 had CDi so i guess the MX used the 175 CDI mag or similar. If you had No spark i'd suspect the stator source coil--but you have spark. Back to crank seals out.
02 Sep 2021 16:06 #4

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Replied by ecurbrenots on topic 1978 DT125MX starting problems

Still no joy,  it is a CDI ignition which is right for French models  for 1978.

Removed magneto and stator plate left hand seal in place and no signs of any leakage passed the seal. I cannot bring myself yet to strip the engine and check the right side seal.  When I put my hand over the carb intake and kick it over it appears to suck and my hand gets wet with fuel although it also appears to push back a bit as well possibly due to the poorly  seated reed valve..

No sign of it even trying to fire up even if i put petrol down the plug hole.  Could a duff CDI give me spark but not when under compression or at the right time. Sorry but starting to clutch at straws a bit

Whats best to try next thinking of a crankcase pressure test which should show up any major air leak including crankcase seals or ? 

Any suggestions apart from putting it back in the barn and leaving it.
03 Sep 2021 06:25 #5

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Replied by alnarv on topic 1978 DT125MX starting problems

The spark plug requires a higher voltage to jump the gap under compression and in a fuel air mixture than in open air. A quick test is to open up the gap on a spare plug (say 5-6mm) and see if it still sparks. If not, you don't have enough voltage to fire in the engine. If it does, your ignition is not the problem.
alnarv
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03 Sep 2021 06:51 #6

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Replied by ecurbrenots on topic 1978 DT125MX starting problems

Just tried that with a plug set to a 3mm gap, still a good spark, how can i check if it is sparking at the right time or dont CDI units go wrong in that way.  That was the biggest gap i could get after that the spark jumped  to the side as that was closer than the electrode
Last edit: 03 Sep 2021 07:26 by ecurbrenots.
03 Sep 2021 07:23 #7

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Replied by RT325 on topic 1978 DT125MX starting problems

If its sucking good then don't worry about the right side seal. More often than not they'll blow out during kick starting through having a belly full of flooded fuel with nowhere to go. Reed valves don't seat as such & can see air past them after years of flexing but still going good as get blown shut each down stroke. I've had then running with pieces missing although not sure if one petal broke off complete. If it happened with revs on it'd probably still keep going but lean & gaspy & not start next time. Not a low flying duck sucked in the airbox inlet. Have seen an old steel based seats collapse onto the airbox inlet on a bike at work & only showed up when sitting on it, stupid carry on finding it initially. Could close the plug to nearly shut but i dunno, frustrating if it sparks & won't go. Clutching at straws but could a lump of carbon have blocked the baffle after hitting a good bump. Had that happen to my AG100 but didn't stop it just strangled it to 50cc power. You said crank key is still in one piece.
Re-reading i see you say it pushed back a bit on the inlet but they probably do a little. Easy too check for a broken reed just to be sure. Had early YZ reeds break which didn't do the motor any good but probably about when Boyesen reeds arrived as were digestible. Run out of ideas. Can you ohms test the source coil. You say 1978 which i can't find a stator pic but do you mean "87?. Here's a link i randomly stumbled on googling. I fell asleep by page two haha but i've gotta come back to see if he got it going. So link might help--or not.
yamahaclub.com/forums/topic/21466-dt-mx-wiring-help/
Last edit: 03 Sep 2021 07:56 by RT325.
03 Sep 2021 07:26 #8

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Replied by ecurbrenots on topic 1978 DT125MX starting problems

Definately 1978 2AG motor  the CDI and  stator is the same as the DT175 of same year, this model was sold in France as a DT125MX. 

 Just trying to work out my next steps,  i am wired exactly as per the diagrams. 

More head scratching.
  
03 Sep 2021 12:18 #9

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Replied by RT325 on topic 1978 DT125MX starting problems

Might pay to remove the clutch cover to eyeball the crank seal. if still in place it wont be that. There will be a retainer plate but it can still be forced out & bend the plate in extreme cases or just force out at an angle as still half retained, they need two plates & there's a threaded hole for the case spreader or call it crank pusher outerer lol where second plate could mount. Running out of options. damn it. Wish i wasn't half a world away in nz. Worked in the yamaha shop through those models & i know everything--yeah rite, "i know B-- all" or it'd be sorted in an instant. Talking timing, there will be marks on the flywheel--maybe 3 close together but longer in the middle one. Even if one mark find the casting mark on the backplate @ about 2mm before tdc. Then strobe it if you can with someone 'slave' turning the rear wheel in gear. But first verify tdc or that 2mm before on the piston is really 2mm on the flywheel so ya know that if the strobe shows the marks ok then you also know the piston is in the right place. In woffling about 'that' the only way it can be wrong is if the crank key is broken. Did that on my mower the other week. 4th time hitting a rock, so running out of corners on a pair of alloy head steady plates to cut a corner off & make a key lol.
Talking reeds, with carb off you can get a finger in there & feel each reed so you'll know if one is missing.
Just thinking, if its the right crank seal & you pressurize the trans through the breather "& not much pressure" you'll hear it come out the inlet with the carb off & even with the reed block still in place i think.. Or plug out & piston down about half way & hear it out the plug hole or exhaust, something like that. If a bit of pressure holds or resists like expanding the breather hose then won't be the seal out, just don't blow too hard.
Last edit: 03 Sep 2021 16:57 by RT325.
03 Sep 2021 16:46 #10

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