Vintage Dirt Bike Q & A

Hi i was wondering , I have a nice original Mt250, the engine runs strong, does not lack any power, but if i try to rev it out it when it gets to about 5000 revs it starts to flutter, the bike has vey good low down power, it accelerates well but i have to change before it hits 5000 rpm, if i hame in a high gear it will rev higher if I let the revs build up a bit slower.
One thing i notice is the exhaust gets very oily at the outlet and it pelts out more smoke at the high revs.
The auto lube is in adjustment to factoy specs.
Other than this it rides and runs well, i just cant do a racing or quick accelerate start , hope someone could supply me some info on this do notice it improves slightly when it has warmed up.
regards Steve

I would clean the carb with special attention at the main jet. If that fails look for an air leak in the motor.

When you mention crank seals, is it possable for the gearbox oil getting sucked in to the engine or does the gas leaks the other way/
i looked in the manual, one crank seal on the left hand side looks easy to replace, but the other side looks like a major job, or is there a cheat to that ?
thanks for the firts reply regards steve.
The main jet in the carb is ok...............

yes its possible for trans fluid to be sucked in the motor from the clutch side. Remember the crankcase seals are not the only place for it to leak, it can leak around the carb manifold, the cases, cylinder and case, cylinder head, spark plug etc. My advice is to preform a leak down test. If you don't have a tester (motion pro) you need to take it to a shop.

A couple of ideas before I would consider replacing crank seals. At high RPM, the loss of power could be from point float. A weak spring on the points, although rare, can cause all sorts of havoc. A check of the troubleshooting section in the Clymer manual points to either the points, or too large a main jet, as the primary causes for the loss of power at higher RPMs. The points are a little harder to get to on a MT, so tend to be neglected more on this model. Slight changes in the point gap and timing made a big difference on a CR, but may not have as big an impact on a stock MT.

The autolube pump should be providing more oil at higher RPM. But, if the cable is in good condition, and adjusted properly when the throttle is completely closed, and repeated twists of the throttle show that the marks continue to align properly, that is rarely a problem. The pump could be temporarily eliminated, and the line plugged though, on the off chance that too much oil is being pumped at higher RPM. If the smoke and dribble go away with a 32:1 premix, that could be an issue.

I just got offered a complete MT250 basket case for $50, and was sitting here debating on picking it up and taking on another project bike this winter. I'd really like to know how this turns out.

_________________
Steve Amling

1976 CR 250M
1980 Yamaha DT100G
1980 XR 200
1981 CSR650

Steve, IMHO get the MT.

I know they use steel rims and not a lot of the parts are at the level of a CR. Thing is except for weight and hp they handle like a CR since most of the geometry is the close. Of the dual sports of that era, they are my favorite 2 stroke.

There were better street bikes, there were better dirt bikes, just for being something you could ride to the store during the week then take off the lights and run hard in the dirt on the weekend, they were the best.

Rick