Very poor accelerating xl/xr600 hybrid-Help Please!

Hi All,

I'd be grateful if anyone has any ideas as to whats causing my Xr to run really poorly at higher revs please.

Its basically an 85 xr600 but the bottom end and (i think..) the crank is 85 XL. The top end is later XR as single carb PD8AF. All the top end bits look like they were designed to fit together-ie no bodged bits.

Was running ok then started playing up-eventually i realised that the choke flap wasnt returning so i was running on choke all the time.

 

Now it will backfire at anything over tickover. It does tick over fine tho..

 

Have cleaned carbs. Have cleaned plug-was sooty black. Have pressure tested warm and cold but as i have to manually decomp a bit to get past TDC is that a true test as its not a whole swing of the kickstart is it? I couldnt see how to do this otherwise.. Got 80psi both times. 

 

I couldn't find how to pressure test the cylinder in past posts but it must be there. If it had electric start it would be easy. How do you do it?

 

For this engine should i be using an XR or an XL CDI because they'd have different ignition curves wouldn't they? 

 

If i have a cracked head would i see smoke and wouldn't it be hard to start? 

 

Or even better if someone could just tell me whats wrong that would be great please...

 

Thanks

 

 

It could be a lot of things in a cobbled together bike.  One possibility is that the crank and piston are not matched.  Not all pistons can be used with all cranks and still give a reasonable compression ratio.

 

I don't think the XR or XL CDI would matter; they should both run.  The factory XL600 stators are notorious for going bad intermittantly at the ignition winding.

 

Another possibility is that the cam timing is off.  A quick test for that is to remove the air box and run the engine with the intake side of the carby open to the atmosphere.  If you feel any pressure blowing backward out of the carb, something is very off.

 

And start with a new sparkplug.  A partially cleaned one can give all sorts of running problems, although probably not what you're describing.

Forgot to mention:

 

Arrow stainless pipe, drilled airbox, 68 idle and 160 main jet . bike is run approx 200 ft above sea level. Does that sound right?

thanks again..

 

I'm really hoping that its straightforward (ie cheap..) to fix

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