Hi everyone, I have a 92 NC30 ive just finished re building and it has a flat spot above 7K revs. The carbs have been cleaned out, float heights checked and balanced using vacum gauges. The bike starts and idles perfect but when riding as soon as you hit about 7k revs it seems to hold back and splutter slightly. If you only use about half throttle its fine but when you open it up the problem starts.
Any help or suggestions would be greatly appreciated

Common problem on these bikes, usually just related to fuelling.

- what air filter are you using?
- standard airbox setup?
- what sized main jets?
- do you have a washer fitted under each carb needle?
- rubber flap fitted in front of the airbox?

1 or more of the above will be responsible, get back with this info.

Are you riding this or revving it on the stands?

I hope it is an easy problem to solve then lol. It is standard airbox with a K&N filter fitted and rubber flap is fitted. It has standard jets in it and does have one standard washer on the needle. One thing to mention is someone has cut the rubber tubes on the carbs that stick up into the airbox.
Also the problem is when riding the bike on the road.

crighton7 wrote:I hope it is an easy problem to solve then lol. It is standard airbox with a K&N filter fitted and rubber flap is fitted. It has standard jets in it and does have one standard washer on the needle. One thing to mention is someone has cut the rubber tubes on the carbs that stick up into the airbox.

Easy enough to fix mate, these bikes are just a bit fussy when it comes to induction system changes. Common problem when air filters and boxes are modified, and owners remove the rubber flap etc.

I personally would be fitting a standard Honda air filter back in, jet appropriately (if needed) and it'll run better. Aftermarket air filters are more pain than gain in my humble opinion - they might liberate 1 hp or so but ruin the midrange power/torque delivery.

Cutting the velocity stacks wasn't a wise move by the previous owner - I'd plan on fitting some standard ones - though this on its own isn't responsible for your flatspot problem. A set should be easy enough to come by if you stick a wanted ad up in the classifieds.

Other thing to consider - you say you have standard sized jets, but do you have a stock exhaust system to match?

Ok cheers for that i have ordered a standard air filter and will let you know how i get on.
It also has an aftermarket can on it, would it have to be re jetted to suit this?

I have now replaced the k&n with a standard air filter and fitted a set of secondhand velocity tubes but the problem is still there.
Would the carbs need re jetted because there is an aftermarket end can on it? If so any suggestions on size to try?
I also read raising the needle height might help but dont know if it would make a difference or not.

Do you know what your current carb settings are?

Everything on the carbs is standard, airbox, filter, jets, needle setting, pilot screw 2 turns out. The only non standard part on the bike is an aftermarket can.

You may need to increase the main jets for the can, I'd try 118F/120R first