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1972 Honda XL250 Wont start after its hot or has been running.

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Just got my 1972 Honda XL250 back up and running after a fresh cylinder, piston and rings. I replaced the intake gasket and have a new Chinese 1979 carb all hooked up. 

Bike usually starts first kick when cold but after I run it a little bit it will either shut down when getting low RPMs when taking off (like I stalled it but it just wants to die), or ill turn it off and go to kick it over 20 seconds later but it wont have it unless its been sitting a couple minutes or so. Choke or no choke, i usually take the plug out and its dry then put it back in and it fires right up. 
Carb issue? 

I have another little carb without a return cable but I like using the new one I bought because it has a return cable. 

I tried turning the carb air/fuel mixture screw a turn or two in both directions and running it like that but no difference, really.

 

I did die on a mountain and took the plug out the other day and it seemed to maybe have a weak looking spark but it was sunny out and I wasnt sure. 

 

have at me!

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Max,

Here are some random thoughts about your problem.

Check the fuel flow from the gas tank, may not be getting enough fuel.  May be something floating around in the tank that only blocks the fuel now and then.  Could even be some kind of internal problem with the China carb.  Stranger things have happened like this before!!

Since the only thing you did that is not usually done is add a new carb, that could be what you need to be suspicious of. Does the new carb mount into the original rubber carb mount?  This insulates the carb from engine heat which can cause a variety of uncommon problems. Maybe if you put your original carb back on the bike and try it to see if you still have the same problem, then you can eliminate the new carb as the source of the problem.

Return cables on the XL 250/350 are not really necessary,  most carbs on single cylinder machines don't have them. I've done it both ways. It is nice if you want to slow the bike a little more on a steep down hill by forcing the carb slide closed a little more (the idle screw is not on the slide).

It is probably not a fuel mixture issue ( low speed adjustment, jetting, or needle position).

Could also be the points heating up on their pivot shaft and sticking and then working OK when cooled down or something with the ignition coil and condenser.  Can be difficult to troubleshoot.

This is not a common problem with these bikes.  Hopefully some other members can share their knowledge on this problem.

Mark

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