i just put an e-31 head with the triple mikuni carbs on my 73 240Z it backfires a little and each time i switch through my gears the rpms raise to about 4000 until i go on to the next gear. does anyone know the cause of this problem.
First thing is does it backfire through the muffler or the carbs. If it is through the carbs it is likely your idle jets (too small) though I would need more info to give you any other advice.
As far as the fast idle between gear changes sounds almost like the return spring for your throttles is not strong enough. Do not rely on the return springs on the throttle shafts themselves they are not strong enough. Try a spring from a point on your valve cover to your linkages.
These are two VERY simple suggestions. Please respond with more descriptions so I can get a better idea of whats going on.
It does backfire from the carbs.The acceleration is when i'm shifting through the gears. I'll be taking off from first gear as soon as I step on the clutch to change into second and so on the rpms rise high and takes a while to come down to the respectable time to change gears but it goes in anyways but it burns away a little clutch. I hope this helps
Back to the backfiring again ;-) If it is under light throttle look at getting bigger idle jets (if thats what you call them. I use Webers which are similar so I'm not positive on your terminalogy.) If its backfiring (sort of sounds like sneezing) only in the higher rpm then you may want to try using a richer main jet or experiment with air correctors which will also affect the higher rpm's. Does the power "feel' like it hits a flatspot? if so what rpm? Air correctors can really help with that aspect.
Have you got a good manual? if you can't find one for the Mikuni's try getting one for Webers. The trouble shooting tips and tuning tips will be quite transferable to your application. Mikunis are basically Weber authorized improved replicas. Like the SU's on the 240Z which are actually Hitachi's which purchased the rights from Skinner Union.
If this still doesn't help please reply again and let me know where your at.