So my 1977 dt250 has been running really poorly and now not at all. When i can get it started it just shoots out so much smoke its ridiculous, it fouls plugs in a couple of seconds. I have ensured that the main and pilot jet are at stock (150 main, 50 pilot). I thought maybe the oil pump could be pumping too much oil in so I removed it and plugged the hole and filled the tank with premix and it did not make a difference. It has 130psi of compression.

I am starting to think that my crank seals are bad and it is sucking in oil from the crank case. To Inspect I removed the cylinder tonight. The cylinder measured at 70.00mm which is within range. However the cylinder walls are smooth and the piston looks a bit scored on the sides as you can see in the included images. At this point I am just trying to get some suggestions as to how I could Isolate the problem further.

Just to sumarize I think that there are two possible culprits, the seals being bad or possibly the choke plunger is bad and is constantly making the bike run rich. 

Thank you for any help or insight.

 

 

Crank seal is in the clutch cover--RH crank seal that is & only one related to smoking the village out. Not a usual thing to happen on a 250 or 400 but deffinitely a possibility. For it to be as bad as you're describing it'd need to be blown out of the case & would need to bend the retaining plate but possible for sure if there's been either an explosion in the crankcase-- but unlikely--or its got a case full of fuel from a carb &/or petcock fault & just pushed it out with pressure of kicking & full of fuel, in which case she'd be 'well' flooded up to start. Maybe seal is just old & given up being tight in the case. That's my very long winded guess lol. Other guess, although not smoke related--if the points [think that year still had points] if they're closed down in the gap through fiber heal wear it'll knock the power right back & eventualy not run at all when they give up opening.

Also, I've never seen a choke plunger 'not work'. I've seen the dust-dirt seal on top sit above rather than sitting in place & held the plunger off the seat at where the lever arm hooks on to it..

Thanks for the response. The bike also kicks back a lot. and incredibly hard. So hard infact that when it was running it snapped a kicker in half and then stripped the teeth out when i welded it back together. You think that could be an ignition issue? Also I'm not sure if you could see the images of the pistons before in my post but I think i fixed it. Does it look too far gone with the scoring on the sides?

if you have the engine out, have you tried turning it upside down?
if nothing comes out, then the oil is getting in during operation....if oil pours out, then perhaps the crankcase just got flooded with oil from a leaky pump over a long period of time eg. sitting for 20+ years somewhere before you got it?

I would def check the timing, but that onto itself doesn't generate smoke, just hard to start or kicks IMO

Piston looks pretty nice...if the cylinder is nice and smooth then can clean it up with 400 and put back in

Thanks for the suggestions. It was running when I got it honestly pretty fine. and it just started to get worse and worse. The first issue was that the guy told me he removed the oil injection system which he didnt. So i was running premix and the injection at the same time. After a while the bike started to really run like trash so I figured that out and switched to gas and the injection system which fixed the problem for a short period of time. Which birngs us to where we are now. My first thought was that it might be the injection but I removed it and ran just premix and it didnt solve anything. 

try taking the silencer off, maybe all that oil over time has clogged the silencer up badly.

cheers
les.

looking back at post #4 "kicks back a lot" i'd say its conclusive & broken off the flywheel key. That's my guess for the day & easy to eliminate or fix if that's the case & if you have a flywheel puller.

I'm sorry, what do you think has broken off? the flywheel? I do have a puller so I can go do that.

The woodruff key that locates flywheel to the crank which in turn determines the ignition timing. No way should it kick back like you say, & break things. Similar could happen if the point gap is miles to wide but probably wouldn't spark anyway if that wide. Cheers. ps, gotta be tightened very tight to avoid that happening. Only my guess so could be wrong & unrelated to severe smoke.