Greetings,

I have a '74 DT250 that had an injector pump seal failure and leaked oil into the crankcase.  This engine does not have a drain plug for the crankcase.  The engine is fresh-built, carb too with real Mikuni jets,  Good used pump installed and adjusted as per the factory book.  New points/condenser and timed per the book.

The engine starts easily and idles nicely - but when I wind it up, it begins to blubber and stumble around 4k and won;t rev past 5k or so.   The porcelane tip on the plug is nicely colored but the whole end is wet with clear oil.  My assumption is that there is a pool of oil in the bottom of the case.  As the revs rise, the increasing rush of intake charge picks up more and more oil from the pool until the charge will not ignite.

I will be grateful for all experienced opinions and practical advice.  At the moment, I can't see any way out of removing the motor, removing the top end, and turning it upside down overnight.

Sincerely,

Chris Finlayson
Existential Motorcycles
Alexander NC 28701

In the past I have attached a drill (holding a correct sized socket) to the nut on the end of the crank and, spark plug out, spun it clockwise. A protective piece of cloth or such like should be placed between the plug hole and the frame and tank!

Just bring the revs up & down slowly & increasing gradually until it revving clean up top & it'll be well cleaned out.
The only way the pump can leak into the crank is if it has 'bum fluff' under the miniature ball & spring on the outlet union heading to the cylinder & gravity takes over & in time lets oil from the tank through.
Not a biggy in regular use but if you can get the union out & be very careful off the ball & fine spring--just check it out. Some unions are a press fit & older bikes had bolt in unions.
#16 & 17 www.yamahapartshouse.com/oemparts/a/yam/...209bc7849ee/oil-pump

To my above post--i see you have another pump so i hope its ok in the area i mentioned.
Regarding limited revs 99% of the time the exhaust is the issue & in your case the tail muffler.
Within that it has a spark arrestor which gathers carbon.
Burn it out REALLY BURN IT out.
I but mine in the log fire.
The complete exhaust could be compromised but i'd start as i mentioned.
There's a carb Oring that leaks & causes the side mounted main jet to get bypassed & essentially goes like a super large main jet & all rich.
#4 on partzilla for the DT250A. It seals the bowl over the needle jet/jet setter so it then must suck via the main jet on the side.
Link below.
www.partzilla.com/catalog/yamaha/motorcy...74/dt250a/carburetor

Spark arrestor & logfire burn link below.
photos.app.goo.gl/Kw9qFn4hKyVbUfPx9
photos.app.goo.gl/uoYVboCJRTssZXPU6
Here's my flooded up DT1 "from sitting".
Goes on for a bit so drag it along if ya want. 
Bike & rider should be permanently parked up haha.
Should be admonished for revving the tripe out of the poor old girl.
Goes like a rocket with an MX piston & 360 carb.

That was a while ago--i'm Real old now!!

As the old saying goes, I have a feeling you're barking up the wrong tree.

A video posted to youtube or someplace and linked here would be helpful.

Member DEET is the expert on rebuilding oil pumps.

Keep an eye on your transmission oil level, especially if you didn't use the hard-to-find original SW-type crank seals.  

Oh, and first thing I'd try is a new spark plug gapped at 0.020".  

NGK has pretty much discontinued non-resistor plugs. And a lot of counterfeit junk out there if buying online.   Autolite 4054 replaces B8ES and is available at most auto parts stores like AutoZone, O'Reilly's, etc.  

"bum fluff" you made me laugh, thanks

I hope it's as simple as that lol. I've seen very small bits of fluff looking strands in the spring. So only take a strand of something as fine as hair to keep the ball off the seat in that union.