I picked up a '74 DT 125 a couple months ago in really nice original condition. So here is the issue- it seems to be misfiring at mid-range. Here is what I have tried so far:

-Nothing but 91 octane gas

-Cleaned the tank, petcock, in line filter (was very clean to begin with).

-Adjusted mixture screw back and forth

-Tried adjusting the floats +- 1mm around 21mm (didn't make a noticeable difference)

-Tried 3 different spark plugs, tried gap settings between .024 to .030mm

-Tried Star Tron fuel additive from Del Amo Motorsports (old guy at the parts counter recommended it)

-Checked the spark, seemed fine

-Checked compression, @ 130 (very good)

-Checked air filter (was clean)

So far nothing has worked. It runs and idles great at lower rpm, and great at high rpm with full throttle, but everything in between is just sputtering and jerky. Also, it does take a little more effort to start it when it is warm or has been sitting for a couple of days or more, but I've heard this is typical of old Yamahas. Not sure what else to do! Set it on fire? (HELL NO, i'm kidding!) I'm thinking it might be electrical, but I'm not sure how to test it or check. Any help would be greatly appreciated =)

And no I don't have a shop manual. Anyone know where to find one of those? Thanks!

I picked up a '74 DT 125 a couple months ago in really nice original condition. So here is the issue- it seems to be misfiring at mid-range. Here is what I have tried so far:

-Nothing but 91 octane gas

-Cleaned the tank, petcock, in line filter (was very clean to begin with).

-Adjusted mixture screw back and forth

-Tried adjusting the floats +- 1mm around 21mm (didn't make a noticeable difference)

-Tried 3 different spark plugs, tried gap settings between .024 to .030mm

-Tried Star Tron fuel additive from Del Amo Motorsports (old guy at the parts counter recommended it)

-Checked the spark, seemed fine

-Checked compression, @ 130 (very good)

-Checked air filter (was clean)

So far nothing has worked. It runs and idles great at lower rpm, and great at high rpm with full throttle, but everything in between is just sputtering and jerky. Also, it does take a little more effort to start it when it is warm or has been sitting for a couple of days or more, but I've heard this is typical of old Yamahas. Not sure what else to do! Set it on fire? (HELL NO, i'm kidding!) I'm thinking it might be electrical, but I'm not sure how to test it or check. Any help would be greatly appreciated =)

And no I don't have a shop manual. Anyone know where to find one of those? Thanks!

Ok, the one thing I didn't fiddle with was the point gap. It was definitely gapped more than it should have been, but I didn't have the specs to reference from until I acquired a shop manual. Once I set the gap correctly, the bike ran absolutely perfect! Hopefully my issue will help someone else get their bike running great. Now to keep an eye out on it for unusual wear...

Glad to hear you got it worked out. Also good to know that if I have a similar issue to check on that.