flat spot

[vfrgav]

Up till about 7000rpm, my nc30 is flat and unresponsive when warm. When cold and just started, it responds A LOT better and pulls harder from idle right up through the rev range. jetting is 118/120 and std filter/exhaust. I had the carbs apart before i put it on the road for the 1st time and installed an extra .5mm washer under the needle to give 1mm washer total. Ive obviously got a rich condition so would it be safe to say removing the additional washers should improve my fuelling and help the unresponsive and stumbling 0-7000 rev range. Now i no these are rev happy bike but this should be better.

thanks

[The Hamster]

The bikes always have a flat spot between 5-8'000rpm from the factory (Depends between bike). I managed to remove the flat spot completely on mine by drilling the carb slide air bleed holes out from 2mm to 2.5mm, and then just upped the jetting to increase power and performance.

The jetting can vary a lot between bikes, some like to run rich some lean, nobody can explain it, they just do. So its diofficult to accurately say what your bike needs.

[banoffee]

sounds like quite serious mains for stock exhaust?

[alexwitham]

118/120 are fine, im running a totally standard '89 nc30 on those jets and it runs sweet as a nut. My dyno graph's show this jetting to be more or less bang on.
As hamster rightly said, 99% of NC's have a notorious 'flat-spot' anywhere from 4500rpm-7500, but u've got to expect this. After all, its only a 400.
However, if its totally 'gutless' under 7000rpm, then it could be anything from your home made modification, to the bikes way of telling you ''I NEED A SERVICE' lol.
Best of luck on getting it sorted.

[J.B.]

The flat spot is notorious - one very experienced V4 tuner once told me to forget about getting rid of it. If the rest of your bike is standard then I'd say remove the additional washers you installed under the needles and return the carbs to standard also. Unless your changing the way the engine breathes - air box, filters or exhaust etc. - there's no point changing the fueling. Have you checked the valve clearances?

[vfrgav]

its not totally gutless but i think it could and will be better hopefully after i try a few changes. just a pity its a pain to do the changes . im used to the punch of a crf450 on the dirt so maybe im just expecting a bit much. The stock jets are 110 allround, i think they are a bit small but might try them after ive tried removing the additional washers.
Ill post my results
thanks for the replies

[banoffee]

have just fitted Dynojet 116 fronts and 118 rears on my '89 with a race can. seems pretty much spot on. shame the stock can isn't so easy to swap out and back in to test

[The Hamster]

Lets put some facts down:
JETS
Japanese Spec Bikes should have 116F-118R jets in.
UK Spec Bikes Normally have 118F-120R Jets in.
Both bikes normally run 35 Idle Jets all round.

NEEDLES
Im not sure of the difference between Jap and UK needles, but Japanese needles usually have a 0.5mm Washer underneath them, whereas the UK spec ones have no washers underneath them.

SLIDES
The Japanese Slides have only one Air Bleed hole, 2mm in size at the base of them and they are made of Plastic.
The UK Slides have two Air Bleed holes in the bottom of them and are made from metal.









SO now that is known:

I would definetley say your bike feels gutless because the jets are way too small. I have never known a VFR400 to run 110 Main jets? Ive never known one to run under 116 actually!.

Try putting the bike back to completely standard and remove any extra washewrs you have put in and see how it goes then. Obviously it wont help the notorius flat spot, but below and above that, it should run fine?

To put it into perspective, I modified my Carbs. (Japanese Spec). I went from 116F-118R Jets to 125F 127.5R and the bike gained almost 6bhp progresssively along the range. We modified the Fuel tap to give constant feed as it wasnt supplying enough at top end, and we also Drilled out the Single Air Bleed hole on the slide from 2mm to 2.5mm to increase throttle response. The Needles were kept standard and with just the one 0.5mm washer underneath them and the bike now pulls like a train from 3'000rpm right up to 12'500rpm where it hits peak power and starts to drop off. The flat spot has completely dissapeared!

[CRM]

Lets put some facts down:

I have never known a VFR400 to run 110 Main jets?

early 89 models did as standard.
also jap spec bikes didnt come with washers under the needles, thats a mod people do after in an effort to cure the 7k flatspot.

[CMSMJ1]

1989 bikes are 110
1990 onwards are 115F and 118R
Japanese style

We'll not talk about jetting and power for a bike this time.