weird running issues

[thisischris]

Hello,

I have a 1992 Jap VFR400. I have a couple of weird issues.
First: it runs great cold, when warmed up it seems to misfire here and there.
Second; I've read that they don't like to start without the airbox installed. This bike starts no problem without the airbox choked or not.
Third; With the bike running and airbox removed- if i cover the fwd right trumpet with my hand, the bike slowly stalls, cover the fwd left trumpet the bike stalls fast, cover the aft left trumpet the bike almost stalls and finds idle, cover the aft right trumpet and the bike rev very high.

I removed the carbs today and opened them up and they look very clean. I did notice if I look down the cylinders the valves look dirty- not oil covered but carboned up.

Could this all be carb adjustment issues?

[Cammo]

With the bike running and airbox removed- if i cover the fwd right trumpet with my hand, the bike slowly stalls, cover the fwd left trumpet the bike stalls fast, cover the aft left trumpet the bike almost stalls and finds idle, cover the aft right trumpet and the bike rev very high.

The rear right cylinder is responsible for opening the fuel tap (via a pipe on the intake under the carb). When you cover this intake all carbs will run out of fuel (some faster than others) - it's revving high because you just leaned out the carbs.... .

FWIW most nc30's will run without the airbox, but it's not a great idea because most people remove the airbox to diagnose running issues. Running without an airbox will make it run poorer and harder to diagnose the issue in my opinion, and if it's running really poorly it probably won't run at all without it.

For your issue I would firstly suspect the fuel tap vacuum line, when old ones get hot they can collapse and intermittently block off fuel flow. Change it for a new one if in doubt, it runs to the bottom of the fuel tap.

After that i would suspect the electrical system - reg/rec, gennerator and coils. Get hold of a multimeter and give it a good checking. Guide below:

http://www.400greybike.com/files/Chargi ... Checks.doc

I wouldn't really suspect the carbs judging by your bike's symptoms, but if you've got as far as opening them you should check that all parts are in the right places and take note of jet sizes. Guide below:

viewtopic.php?f=24&t=14524&start=0

Carbon build up on the intake valves is normal, you should see the inside of a diesel manifold if you want a real shock!

[thisischris]

Thank you very much Cammo for the two write ups. I replaced the fuel tap vacuum tube and it was not at fault. I will meter my electrical system and confirm carb jets.

[BigDonk]

Cammo
on the charging checks is this the same for the NC24?

[Neosophist]

Yep

[BigDonk]

daft question that one