My 72' 250 WR has a mysterious illness

[DPete]

Every now and then it will run away ( no throttle control), usually if I miss a shift. Throttle seems to work fine, I can hear the slide hit the stop. It is rich at idle but runs clean thru the rest of the rpm range, plug is chocolate brown. It has a Mikuni, I will take it apart and check float level, slide, or something loose in the carb. Maybe the bottom end is flooding I don't know. Any suggestions? It's new to me so I don't know what the jetting is until I get it apart.

 

[DPete]

Had the carb apart, was real clean, blew out jets anyway, took intake manifold off can't see any problem there, adapter boot to Mikuni is OK, I had a 70' Yamaha DT1 that did the same thing, blamed it on to much oil.

 

[Crashaholic]

What exactly do you mean by "it will run away"? To me that sounds like it revs up without opening the throttle. If that's the case then theres probably an air leak or the carb is intermittently running out of fuel, i.e. its experiencing a lean condition at the moment of revving.

 

[DPete]

Yes reving without throttle, kill switch has no effect whenit happens, must be sucking air someplace, runs good under load, carb is not running out of gas as it runs fine going up a hill. Supposed to have recent crank seals. The head bolts were not tight, I re torqued them when I bought it, looked like the head had been seeping. I Guess maybe pull the head and cylinder to check base gasket next.

 

[justintendo]

Yes reving without throttle, kill switch has no effect whenit happens, must be sucking air someplace, runs good under load, carb is not running out of gas as it runs fine going up a hill. Supposed to have recent crank seals. The head bolts were not tight, I re torqued them when I bought it, looked like the head had been seeping. I Guess maybe pull the head and cylinder to check base gasket next.

do not ssume anything is good on the motor until you verify it...that is a cardinal rule with used bikes..
base gasket could be bad..or crank seal. even if seals are new they could be damaged or installed wrong.
when you have carb apart write down all jets, for future reference..float height? slide size?
right now the bike is in a dangerous condition, both to you and itself. running away means its getting high combustion heat..too much oil isnt even close..
the only way to "check base gasket" and everything else is to do a leak down test.

 

[DPete]

OK thanks, I'll gather up the stuff to do it, never done one before but I watched a YouTube vid

 

[Crashaholic]

I agree, a leak down test is the best place to start plus verifying that the timing is correct.

 

[stormer254]

Before doing all that it is worth checking the spark arrestor, they can get blocked and the back pressure can cause the fault you described.

 

[wrx]

that just screams air leak.

 

[ruwfo]

that just screams air leak. I'd 2nd that ****************************************

 

[ajcmbrown]

Third that.....

 

[Bigbill]

Try spraying some ether near the carb manifold, cylinder base gasket. Check the head bolts.

 

[DPete]

I think I got this solved, looks like it was sucking air between the aluminum intake manifold and the Mikuni adapter. I put a sheet of 320 sandpaper on a flat table and surfaced the manifold. So far I believe it's air tight.

 

[suprize]

that will do it. my 400 has been running on slightly at idle. tightening the manifold bolts fixes that apparently