hi all, ive just bought a vfr400 nc30 on a j plate,it had been standing for just over a year all the fuel had been drained out and i had put fresh fuel in,so i turned her over and she fired up straight away on the choke, she idles fine low revs ok but she wont rev out when you go into higher revs she struggles and vibrates while backfireing, so i checked all spark plugs and gap thet were fine so with all 4 out i plugged them into the coils and turned her over spark was good on all, so i checked carbs they looked good inside not not dirty mixture screws were 2 1/4 turns out,its got a new battery on it and keeps charge as genny is good its getting fuel to carbs but cant understand why she wont rev clearly,its got a full stainless aftermarket exhaust system on but dont sound like a v4,ive checked near enough everything and fuses,if some one could shed some light on this id be very gratefull..

Howdo,

My 10 p is that you need to plug in the speedo and that the bike is showing the 5.5k cutout when the CDI cannot see the speedo. Either fuse for instruments has gone, or speedo not plugged in.


Welcome to the site

hi,thanks for the reply checked all fuses and are good speedo is plugged in but need 2 check its ok,but i dident think it would stop the bike revving clearly,the engine has only done 12 thousand miles.

The bikes have a limp mode that makes it run on 2 cylinders at 5.5k revs. I'd be surprised if this is not the problem by the sound of it.

can you rev it to 5.4k revs and get it to trigger the limp mode just to make sure?

how do you get it to trigger limp mode? all 4 pipes are hot, it wil rev 2 the red but struggles and dont sound rite.

As CMSCJ1 said, if you try rev it above 5500rpm it will drop two cylinders if the speedo has a faulty speed sensor or is disconnected. Under 5500rpm it should run happily on 4 cylinders.

ant75 wrote:how do you get it to trigger limp mode? all 4 pipes are hot, it wil rev 2 the red but struggles and dont sound rite.

Start the bike and let it warm up a bit.

Rev it upto 9 - 10k rpm, dont' rev the engine over this amount it's bad for the engine.

Turn off engine
Disconnect the speedo plug (back of the speedo)

Try test again.

If it revs worse then it's not the speedo.

If it revs the same your speedo sounds faulty.

As been said, the speedo limits the bike to 180km/h (112mph)

If you disconnect it, above 5500rpm the bike only runs on two cylinders

thanks for the help i wil try this now,fingers crossed.

checked speedo all plugged in and cable not broken,still wont rev clearly could it b the regulator? as some1 said it can go 2 ways, 2 many volts blow fuses and light bulbs and fry battery,or not put enough volts in meaning the bike wont rev??

ant75 wrote:checked speedo all plugged in and cable not broken,still wont rev clearly could it b the regulator? as some1 said it can go 2 ways, 2 many volts blow fuses and light bulbs and fry battery,or not put enough volts in meaning the bike wont rev??

Thats not what I said do.

I said rev it up with the speedo pluggged in, turn the engine off and disconnect the speedo plug and rev it up again.

If it revs much worse with the speedo disconnected then you know there isn't a problem with the speedo board.

If it revs the same with the speedo disconnected then it's likely that theres a speedo board problem.

Just becuase it's plugged in doesn't mean it works.

Once you test this by seeing if having it disconnected makes the bike worse or not you can move onto checking other things