NC30 Main jet sizes

[mikey65]

Hi i have been stripping down the carbs to clean them. i have a standard nc30 vfr400 K 1989 I have jet the mains jets frounts are 120 backs are 122 is this ok seems like its the spec for a UK spec bike but its an import. Will these jets be ok or do i need to replace with 118's all round.

[The Novice]

viewtopic.php?f=24&t=14524

should help.

[mikey65]

Thanks for that I used that link when removing the carbs I am trying to find out because my bike is standard how will it run if i replace the 120 122's jets alreadyin it back to 110's how will it run????

[Morespeedvicar]

Was it running ok before with the big jets in? Has it got a race pipe on? I'd try it with them, and if its a bit rich try 118/120. You putting a new air filter in while its in bits? Hopefully?

[mikey65]

The bike is standard in every way. Just seems odd the jets are not standard seemed to run ok with them in.

[mikey65]

would it run better with 110's in and what difference would it make????

[Neosophist]

The bike is standard in every way. Just seems odd the jets are not standard seemed to run ok with them in.

Its probably have a full or half system on it in the past and been re-jetted to suit.

Many many owners keep all the stock parts and then re-convert it to stock before selling it, for some reason the carbs are almost always left with the later jets in.

Now fortuantly I can offer you some advice as I have an 89 that used to run 110s all around with stock system but I fitted an aftermarket half system.

Providing it isnt super lean or super rich the bike will just not feel quite as sharp and crisp when running.

Here are my findings.

I found that even for the standard system the 110s are a bit lean, you can get a nicer ride from the bike by increasing the jets..


I first changed to 115/118 setup and tried the stock and aftermarket systems.

Bike feels much better and faster, pull is nicer and harder through the gears.

Next I went larger, 118 / 120 setup and once again tried both systems.

The stock system feels overly rich, the bike no longer pulls as well or as hard as the smaller size.
The aftermarket half system pulls as crisp as before, maybe slightly better so stoped here.

120s or larger are too rich for the stock system, the bike doesnt run as crisp. Only with a half/ full system are 120s ok.

Id recommend 115/118 combo for your bike, this is also a good setup if you ever whip a new can on, dont need to mess about as it pulls really good and crisp.

[mikey65]

cheers I will try that then. 115 118