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TOPIC: 1978 dt250 spark gone

1978 dt250 spark gone 07 Sep 2018 20:17 #1

  • kakon864
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Just got the bike the other day and it had good spark. I messed up the points gap because the mark wasn't right and it wouldn't spark for a while and then I re-gapped it a week later and had good spark, but was a slight grinding noise when I took it off by hand and took the plate off and put the plate back on and then it wouldn't spark again. Thought it was points and condenser and replaced them, but still no spark. I tested the resistance on the primary coil (still attached to the plate, points, and condenser and got 1.65, but I couldn't get a reading (infinity) with the coil off the plate by itself. I might have been testing it wrong ( i put negative on the iron bar and the positive on the wire coming off of the coil). I am getting a little voltage from the black/white wire coming from the coil, points, and condenser. Gonna keep messing with the gap for now idk. Any ideas, I hope I don't need a new primary coil. also get slight voltage ( more than primary obviously) from the secondary (top) coil


Now that I think about it probably is the primary coil since it is reading infinity(unless i did it wrong), but idk why it went bad all of the sudden it was sparking fine just two days ago and now nothing, also could improperly gapped points cause super low voltage(not even 1 volt).P.S the top coil is brand new


Update: after setting the points it gave off 1 bright blue spark and then no sparks after. What could that mean?
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1978 dt250 spark gone 07 Sep 2018 20:33 #2

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It would be helpful if you could post pictures of how you are testing and the new point and condenser installation including the wiring on the stator.

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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

1978 dt250 spark gone 07 Sep 2018 21:22 #3

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here are photos on and off bike. would test the coil alone, but I just soldered it to the condenser. It was reading nothing though when I checked resistance with the coil off the plate from the ground to the end you connect to the condenser.
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1978 dt250 spark gone 08 Sep 2018 01:04 #4

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Took the source coil off, doesn't this mean it's bad?
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1978 dt250 spark gone 08 Sep 2018 09:34 #5

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Great pictures!

I think it may be your meter? For one the BAT is showing in the display. I have two of those Centech meters. Mine are incapable of reliably showing the difference of "points open" and "points closed". The method to test the primary coil shown in the service manual is simply to have the points open. When you did that the reading looked about 3.1. With points closed you should have "0". You have 1.2 or so.

My Centech meters read differently from each other and the results are not repeatable when testing a source coil.

I also have another (expensive) digital "autoranging" ohmmeter. It sucks too.

In my experience an old fashioned analog (swinging needle) model with an "R X 1" scale just like Yamaha used. I just don't have any luck with digital meters for the ohm readings on these old bikes.

What reading do you get when you hold both probes together? Should go to zero. Neither of my Centech meters do unless set at a higher scale like 200k. Sometimes the minimum reading with probes touching using the 200 setting is down around 1 like yours with points closed... sometimes it's up around 16 or even 20... usually it likes to settle at around 3 on one and 2 on the other meter. When you're trying to read something that 1.6 ohms, this variance is not acceptable.

It is puzzling why you have an "open" reading with the coil off the bike. I can't see in the picture if your black probe is on the eyelet that the mounting screw holds in place to ground the coil winding? Or just on the metal core? Make sure the probe is firmly touching the ground eyelet... with the screw removed the ground wire might not be touching the iron core.

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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

1978 dt250 spark gone 08 Sep 2018 09:47 #6

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I tried to zoom in on one of your pictures... the arrow points to what I think is the ground eyelet... hard to see.

The circle is an area where sometimes the insulation has been damaged by the spinning points cam inside the flywheel... carefully check all the wires for damage.

Last thing is the points themselves. Clean the contacts with contact cleaner. Cut a white paper uncoated business card into strips and pull the strips through the points until they come out clean. It's amazing how much "black oxide" can be on even a new set of points.

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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

1978 dt250 spark gone 08 Sep 2018 10:53 #7

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I was touching the needle before I took the picture, but getting the same reading(was hard to hold the camera and use both probes). I am getting a reading off it now, I just have to touch the very end on the cable coming off the winding, so I wrapped some 16 gauge around it to get a reading: 3.1 ohms. But the meter gets 1.3 ohms when touching itself. I get a reading when the points are open only when it's touching the coil. when it's just the points wire and black/white wire it's fine. even with the coil attached is it supposed to be infinite?

P.S that wire is fine, I checked it before I put it back on. Cleaning the points a little residue came off, but not much.

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1978 dt250 spark gone 08 Sep 2018 10:57 #8

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Just reread what I said and it is supposed to get a reading with the points open and that reading for the coil. I'm getting 3.0 ohms, but my meter gives off 1.3 ohms. with the points closed I get 1.3, so should I assume that it's 0 and the meter is inaccurate.

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1978 dt250 spark gone 08 Sep 2018 11:29 #9

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Carefully check wiring for the tiniest bare spots. Condensers are known to go bad, today, many of the new ones are suspect. Look for even a tiny whisker of solder or wire grounding condenser terminal to case... or coming close. Soldering needs to be done quickly with a high output iron. Using a small iron can overheat and damage the condenser.

Check wire from stator to coil to make sure it's not broken internally.

Even a little "black" on the points can inhibit spark.

Also note that technically you don't "adjust the points". What you need to do is get the points to open at precisely the right moment by adjusting the points gap... Timing is what you are setting with the point gap. Keep the stator screws in the factory position which is about at the middle of the slots.

Plug cap unscrews from the end of the wire.. Most have an internal resistor and can fail. Remove cap and test with meter and/or test for spark with bare wire.

You said the "top coil is brand new". Keep in mind it could be defective, especially if not OEM Yamaha. Happens a lot and just happened to someone on the forum... new coil from Amazon failed.

So after you've done all that since you're already in there... it's quite common for the problem to be a defective handlebar or key switch. Disconnect both and see if you have a consistent spark.

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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

1978 dt250 spark gone 08 Sep 2018 12:29 #10

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Still no spark. I have the kill switch and the key disconnected. the spark plug is fine (tried three different ones (one from a working outboard)). I have a wire running from the connector to the coil, so the wiring isn't grounding. at 20k on a multimeter, I get 2.56 on the original one and the new one gets 2.13 (top coil). also, I don't think it is the top coil. when I have my test light plugged into the black and white wire it doesn't light up no matter where on the revolution and I double checked the gap. is there a way to test the condenser to see if I got a bad one or if it went bad ( i soldered it fast with a powerful iron).

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