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TOPIC: DT125F spark, gas, compression, timing -shoud but stillwont start

DT125F spark, gas, compression, timing -shoud but stillwont start 05 Feb 2018 08:46 #1

  • dnaj11
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OK. I have this 79 DT125F that did run, then lost spark and was the subject of prior post. Spark required new points - I also replaced the condenser. The timing is spot on with a dial gauge and buzz box. I also found the second piston ring was stuck, so I honed the cylinder and replaced the rings. I have about 130lb compression cold. Piston is oriented correctly. And I had to replace the ignition coil, which failed spec and bench test. And the carburetor is spotless! And yes, we did a leak down test for the crank seals, passed.

I still cant get the darn thing to run.

Two items of weirdness. (1) It seems to fire late as its will sometimes try to fire, and when it does, the fire flames fully out the exhaust port but wont run. I noticed this with the pipe off to make sure it was not clogged, and yes, I know it sounds stupid. With the pipe on, I occasionally get a puff or two but no start. Late spark is just a hypothesis, yet the timing is spot on. The other item of weirdness is (2) I only get spark with a fully charged battery. My reading of the wiring diagram says I should have spark despite a dead battery - as the source coil is wired through the ignition coil, thereby powering the ignition coil. Am I wrong with this part? This model does not have a CD1 but differs from earlier DTs in that it only has one lighting coil.

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DT125F spark, gas, compression, timing -shoud but stillwont start 05 Feb 2018 11:30 #2

  • asco
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Need 12 volt battery fully charged 125

Asco

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YAMA-LAND RESTORATIONS
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MINIBIKESERVICE@AOL.COM CUSTOM MINIBIKE (RALPH) 2002 110cc XR PAX RACING 1971 125cc AT1-C (DOROTHEA) 1971 175cc CT1-C (BRANDY) 1970 250cc DT1-C (MONICA) 1973 100cc Hodaka Combat Rat (ABBY NORMAL) 1969 125cc AT1-M MX (DELILAH)

DT125F spark, gas, compression, timing -shoud but stillwont start 05 Feb 2018 11:49 #3

  • Devin
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The 1969-1976 AT/DT 125 USA models need a good fully charged 12 volt battery to start (unless converted to powerdyno), but 78 and later the 125's didn't need a battery to start. How is the condition of your woodruff key?

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Last Edit: by Devin.

DT125F spark, gas, compression, timing -shoud but stillwont start 05 Feb 2018 11:55 #4

  • MarkT
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ASCO wrote: Need 12 volt battery fully charged 125

Asco


Many other countries had "kick start only" 125's going way back in to the early 1970's where the US enduro 125's were "both kick and electric start" from 1969-1976

If it's an "F" model it's a 1979 and should be a monshock... all DT125's lost the electric start when they went to the monoshock in 1978... and should be 6 volt.

Something has to be wired wrong if you only have spark with a battery on that model.

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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
Last Edit: by MarkT.

DT125F spark, gas, compression, timing -shoud but stillwont start 05 Feb 2018 12:36 #5

  • Carter67350
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Be sure to check the source coil. The black/white wire from the magneto harness to ground should read in the neighborhood of 2.14 ohms. Look for open or very low resistance. Low resistance would indicate an internal short, very common given the age of the machine, the coating on the windings break down with age. The part number of the source coil is 2A6-81312-20-00. I have seen these with internally shorted source coils able to produce spark with the plug removed laying on the head to ground, but won't once installed under compression.

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DT125F spark, gas, compression, timing -shoud but stillwont start 05 Feb 2018 17:17 #6

  • FrankC
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Timing does sound retarded with the flame shooting out the pipe. Check wires with book to see if the coils/and- or wires have been reversed and the harness as well- just a shot in the dark. Could the stator plate have been put in 180 out. Not sure if possible with the screw pattern.

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DT125F spark, gas, compression, timing -shoud but stillwont start 06 Feb 2018 02:58 #7

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Pull the flywheel off & physically check the key, i know you said the timing is right, but sure sounds way off. Crook condenser does funny things with backfiring etc & not starting but trying too, although i see you have that covered. No idea why a fully charged battery helps it spark as isn't related to the magneto. In saying that--some models [but doubt this late a model] had the ignition primary in the mag earthed via the lighting coil. Now [pure speculation & realy don't know what i'm talking about--but] if the rectifier was crook & let 6volts down to the lighting coil & if ignition source coil was earthed through the lighting coil ????, ok ignore all that.
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DT125F spark, gas, compression, timing -shoud but stillwont start 06 Feb 2018 05:15 #8

  • dnaj11
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I'm wondering if I accidently switched the positions of the source and lighting coils under the flywheel. That wouldn't change things electrically but it would screw up the veracity of the spark. I'm gonna check that, then disconnect everything, run my own wire from the mag to the ignition coil.
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DT125F spark, gas, compression, timing -shoud but stillwont start 06 Feb 2018 14:04 #9

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yep just one wire from points condenser source coil where they meet at i suppose condenser & straight up to the plug coil. Have had a little yamy 100 [ag100 farm bike] running ok from the lighting coil weakest voltage output but that's another desperate story of a stuffed source coil & had to get it going lol.

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DT125F spark, gas, compression, timing -shoud but stillwont start 12 Feb 2018 04:22 #10

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Well, the problem was again found to be the ignition coil, which is the second defective coil - both new $10-$15 coils from flea-bay, probably Chinese knock offs. While this coil made spark, it wont run the bike. Swapping it with a known good one from a DT250 and it fired up first kick. So the moral of the story, don't use those cheap Chinese coils.

Which leads me to my next question. The correct Yamaha coil is available but lists for around $100. Does anyone have a better source or replacement? Thanks

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