flat spot

ok, now that i've got a new clutch in my 85 xj700n and it actually grabs i can get a good feel for the bike. i've got a wicked flat spot from about 4500 to 7000rpm. up until 4500 the bike pulls very hard soon as it hits that it's almost like it just sortof quits accellerating. then at 7k it comes alive again and begins pulling like it's going out of style. any ideas?

thanks

 

Do you have any mods on the intake or exhaust?
Flatspots are usually caused by fuel/air ratios. On mine, I had to adjust the needles up a little bit, and I've gotten rid of my flat spot, which was round 4k. That's where the needle jets really kick in. It idled fine, and ran fine just off idle, but it had a definite flat spot around 4k.

If you've put a different filter setup on your bike, this is common. If everything is stock - I have no idea what your issue may be.

Good luck

 

intake is all stock but the exhaust is a 4-1 setup. not sure what brand though. i've only tore the carbs apart on my vstar apart. are the needles on a diaphram setup? if so then it's like the ones i've done before. how do you "adjust" them up? by shimming them? or do they have different clip heights? then again maybe i have no clue what i'm talking about. lol
thanks

 

Yeah, the carbs should have a diaphragm/needle setup.

I don't know if your particular year is the same, but the stock needles on my xj were not adjustable. People put a washer under it in order to "shim" it. So, I guess the answer is "yes". You can shim them. A 4-1 exhaust will effect your mixture. It shouldn't be huge, but if you are really in tune with your ride, you will definitely notice. I would suggest cleaning your carbs too, if you've never had them down. Getting varnish out of there will make your bike run more efficiently anyway. You may want to do that BEFORE you adjust the needles. A little gum in the jets can definitely make you run lean in places.

In order to adjust the needles, you just pull the tops off the carbs, lift the diaphragm out (be careful, it's old), and use an allen wrench to gently loosen the nylon nut that holds the needle in place. Pull out needle. Insert thin washer (unless your needles have clips). Then reverse the procedure. Good luck!

 

just as i guessed. sounds like they're very similar to the ones i messed with in the past. the more i think about it though i don't think it's a carb issue. it doesn't seem to have to do with throttle position but rather exact rpm's. in theory if i did need to add a shim then at wide open throttle it should be exactly same from say 4k on up. and that also wouldn't jive with the fact the bike comes alive again at 7k. from my experience in the past the needles control most of the fuel in the mid-throttle range. and if my main jets were too small it would stay on it's face in the higher rpms and not come back. almost like i possibly have a coil issue. any ideas?