Thanks guys, sadly the ‘78 DT250 was a bust. Turns out that the ‘79 was the first model with electronic ignition and the ‘78 runs points. Thought about buying it anyway and seeing if I could convert mine to points, but as it turns out, the seller couldn’t get a spark out of the ‘78 anyway, so I saved myself a 150 mile round trip, as there wasn’t much else on his bike that I could have used.
Pretty sure it’s not crank seals as it’d be blowing smoke as it sucked in the gearbox oil, and now, with the oil pump bled and primed, it hardly blows any smoke at all. I tried putting the choke on as I was riding along, and it immediately tried to die, so doubt that’s the issue. I will pull the carb and the reed valve housing to check the reed valve and ensure there are no air leaks, but, surprisingly, all the rubbers are still nice and soft, unlike the ones on a CB750 that go rock hard after a few years.
I’m leaning towards a suspect CDI box. I’ve been reading lots of posts on various DT sites, and the common theme is that a duff CDI box will still allow a DT to start and run, but the “advance” mechanism stops working. Mine starts first or second kick every time, but starts to miss and fart after around 4K RPM, or about 50 MPH. It’s fine around town when I’m just piddling along with traffic, but cranking on the throttle in 5th gear will do nothing, until I back it off, which could be an incorrect float level, or a bad CDI box.
I’m tempted to buy a new CDI box, there doesn’t appear to be any new OEM ones around, but a company in Britain makes a “New improved” version for not a lot of money, so if playing around with the carb and reed block bears no fruit, I might have to order one. I’ll have another play with it after work tonight. I doubt that it’s a timing issue, the nut holding the flywheel on the magneto doesn’t look like it’s ever had a spanner on it, and it doesn’t ping, or kick back, so unless it’s seriously retarded (can we still say that?) I don’t think that’s the problem.