Yes mag black & back/white are all related so long as it ends up at the secondary coil. I thought black/white was kill switch but no worries it'll only be killing the primary from the mag regardless. Whatever color from the points/condenser/source coil must end up at the top coil but you obviously have that if sparking off & on. I do wonder if you're testing the source still connected so getting a false reading or getting the secondary primary added on plus condenser. Getting a bit outa my depth but can usually fox my way through. Trouble 'is' the amount of crook source coils i've been caught out by i could count on one hand over 50 years but crook cdi source coils i'd need more hands but [to me] the difference in say 350 ohms for a cdi source like my 465 or 490 yamy is pretty clear cut if it's down tosay 100 ohms where on a points source it has me doubting my tests, like--it 1.65 is good then is 1.85 or 1.35 no good--i don't know. There's a farm yamy AG100 around here somewhere running off the lowest lighting coil wire i cleverly [joke] made work. Maybe it didn't last but was good at the time & had a crook source coil but i wasn''t 100% convinced--well was but couldn't 100% prove it but substitution proved it. Ended up snipping the source across to the condenser & points the n trying all the lighting wires till i went for the lowest i tucked it into the black at the secondary coil so it fed back down to points etc. Never saw it again but pity the next poor bugger that has to fox it lol. was just a [i've go no money] job but my kids want to ride it!!. Sorry for woffling on.

It was probably me that posted that... I was going by the Yamaha parts list saying the flywheel is the same from 74-78. I looked at the 1978 service manual supplement (in Tech Library) and it shows the F1T 350 for a 78 so you're probably okay...

Have you checked the serial number to make sure of the year? Many bikes are registered wrong and the date on the steering head sticker is usually the problem... anything made after about June or July is usually the following year model. So a 8/77 date would be a 1978 model year bike.

It is definitely a '77, but I am not sure if it is an early or late '77. I had to do a crash course in serial/identification numbers when I bought it. The ad had the bike labeled as a '79, but the serial number (stamped on the neck/motor) is coded as '77. The sticker on the neck is illegible.

I checked the points timing this evening with a homebrew test light. They seem to start opening at exactly 3.2mm BTDC.

I have 3 three ignition coils and could not get any of them to spark the sparkplug tonight through the harness or when using a jumper wire. I am picking up just over 6 volts at the end of the black/white wire at the ignition coil when kicking the motor over, which I expected to see as the 'spark signal'. I ground off a little paint on the frame and cleaned the metallic mating surfaces with steel wool in hopes of cleaning up a poor ground at the coil. It did not help.

Meter on AC volts? 6 sounds low... Have you tried cleaning the points real good?

If not, I'd "guess" the most likely culprit is the condenser.

Yes i don't know--but just wondering Mark? anyone--does the condenser store a high charge like higher than 6 volts even though its a 6 volt system, so does it store more like accumulating it then Bam!!--it lets it go to the top coil when points open. So using that theory [warped theory] does the source coil need to put out more than 6 volts if you were to test the coil only, which means you'd have to cut the wire so points & condenser are taken out of it. If ya desperate then snip the coil wire & tuck a lighting coil wire in the black up top for an experiment. Too high voltage & points will look like an arc welder so less is better & will still be more than it's meant to be. I think but might be wrong but wonder if the source coil is wound in a way to put out max voltage within a certain range regardless of how hard you can rev it. Lighting coil possibly just keeps climbing over max required with revs.

Meter is on AC volt. I am still using my digital VOM. Perhaps it doesn't react quick enough to record the maximum electric pulse, but it was consistent with results. I think I have the points clean. I have used a combination of contact cleaner, fine sandpaper and compressed air (to blow out any partials).

Are you familiar with a homebrew condenser test? I see the service manual calls for an electro tester to test the capacitance in µF.

Hi 87 Warrrior, I also own a 1977 Yamaha dt250 and have just fixed a similiar problem I had on my bike, intermittenent spark. Are you still having the problem or did you fix it. I may be able to give you some advice.

This is an old one thread, but maybe by posting here, might bring out some others that may have seen this exact same situation?

I was brought a very low mileage, really unmolested 11/77 DT250 that has been sitting forever, but with this exact same issue at the beginning of this thread. The ground wire from the battery, leading away, then both directions into the main harness insulation melted, and also the ground wire off the regulator mount. Melted into other wires. Obviously this is why this particular bike was parked, and doing a cursory search on the web only turned up this thread. In resurrecting this thread, has anyone else come across this situation with this era DT? 
 

 

That's odd. It's like it's direct shorted to a positve close by. But if the fuse was in place it should blow first. Unless it shorted nearer the battery before the fuse. How far up the loom does the melt go, or just what you show. Sorry i'm useless. Is the main loom earth attatched up at the coil mount. Or is 'that' the main loom earth. Or is that added to be sure the regulator is well earthed [looks original]. Is it a total black its plugged into or does it have a white stripe. I'm out of ideas. Battery fitted back to front--but i think fuse would blow first. Maybe not the battery in it now. Wouldn't stop it running as anything related to battery isn't required as has its own magneto. But if anything black from the magneto loom has been earthed it would short out the spark side.