lower RPM flat spot

My XJ has a flat spot at lower RPM's but above 3500 it runs very well. A mechanic who has looked at my rubber vacuum diaphrams said that replacing them will make it run much better. The diaphrams look ok to me.

Does someone else have an opinion about these?

Has someone replaced these and noticed an improvement at lower RPMs?

 

on my 750, there wasn't a strong pull from lower RPMs, but after i installed the dyno jet kit and adjusted the needle height, it pulled a lot harder from idle to 4000ish. I guess these bikes were tuned to be lean at those RPMs for EPA reasons, which lowered the performance. But besides that there could be an air leak in the system somewhere, or maybe the idle mixture is incorrect. Oh, and also check the spark plugs. Good luck.

 

Might want to make sure the pilot circuit is clean and the mixture screws adjusted.

Which XJ. The 650 doesn't produce much torque until 3000RPM. I assume the others are similar.

 

It is an 86 XJ700S. I would be especially curious if someone has replace the rubber diaphrams and seen an improvement.

 

Also, is there any way of telling if the pilot circuit is clean?

 

I seriously doubt it's the diaphragms. Sounds like your problem goes away when they come into play. Unless they have holes or tears in them you should be ok.