Help - Old XJ750 running rough

[alanhugs]

Hi there,

I bought an old (1982) XJ750 a few months back, which I have been fixing up. She has got nearly 80000km on the clock.

Currently I have problems with the way she is running:

She starts ok, idles a little unevenly, but rides away ok.

As she warms up, literally after 2 or 3km, she develops a "flat spot" at lowish revs (2000 to 2500 rpm). This gets worse and worse as she warms up, till (after 8 or 9km) I have very little power at anything under 3000 to 35000rpm, making slow riding almost impossible. She feels really flat as I try and accelerate, and then a surge of power as the revs gets over 3000rpm. She sounds fine at higher revs. And if I stall her I can not get her to restart.

Oh and she didn't have this problem when I got her, she had other things wrong with her, but was not doing this...

I changed oil, air filter and plugs when I bought her. The new plugs have some carbon now, so guess she runs a bit rich.

I suspected the carbs, so took them out and cleaned out the float bowls. Haven't balanced the carbs yet (as I don't have vacume guages).

If anyone else has had these symptoms, or knows how to go about solving it, I would be very grateful.

Thank you from a newbie on this site,

Alan.

[RXSrider]

It could be the float levels set too high causing a rich mixture, coils cutting out when hot, poor battery (alternator giving better charge at higher revs-stronger spark) or dodgy plugleads.

[Dave C]

What you are describing sounds like typical running rich which could also account for the rough idle. Also balancing the carbs on these bikes can make a world of difference to the idle and general smooth running and quietness of the motor.

[Airhead]

Hi Alan, I agree with the previous comments about running rich, but would suggest you check the needles and emulsion tubes for wear, unfortunately its a carb strip down job though!