vfr400 nc24 rich and revs hang

[Persicus]

Hi all,

Recently replaced the carb rubbers, stripped down carbs, cleand and reset everything to "standard" ie 6.8mm float, 2 turns idle screw, and synced carbs using method told to me by Rick Oliver.

The bike is running pretty rich and the revs seem to hang at about 3000rpm then slowly drop to idle 1400rpm. have checked for any air leaks but none present.

When out on the road she pulls lovely (only slight flat spot between 5-6000ish rpm) and will happily go round to redline on load.

Its just this hanging revs which is bugging the hell out of me!

Any ideas much appreciated.

Cheers

[Neosophist]

Hanging revs is very typical of a lean condition usually caused by an air-leak.

Are the carbs in the rubbers peroprly and all clamped up?

Are your carb balance screws of vaccum pipe leaking air?

You can try and find the issue by spraying around the carb rubbers and other sources of vaccum leak with brake cleaner whiel the engine is idling.

if your revs change its becuase the bike has sucked in the brake fluid and altered the mixture.

carbs can be a pain to seat right and often have small leakage areas.

it's noticed on return to idle as when the throttle butterflies are shut the vaccum is the highest and this sucks excess air in from aroudn the leaky joint, at open throttle the vaccum effect is less and most of the air comes through the butterflies.

[Persicus]

Carbs seem to be seated ok, and no signs of any air leaks.
You say this is a typical sign of a being lean yet the bike runs very rich ie, very sooty in the end can.
it gets worse when warm.

[Neosophist]

You might have both issues.

A lean idle hang caused by an excess air-leak and rich running caused by either float levels (stuck floats too), jets, pilot screw, bad air filter.

did you try spraying aroud the rubbers when its hanging with carb cleaner?

an overly rich idle circuit will cause the revs to drop off and fall below idle speed (maybe even stall) as the bike gets over-loaded with fuel..

[Persicus]

yep trying spraying around rubbers, no effect.
all the floats were set to 6.8mm, all other jets etc cleaned out etc.
the air filter is new (well a modified one but still new)
then a/f idle screws are set to 2 turns out.

have had the carbs on and off so many times now to try and figure this out i can do it with my eyes closed!

what i dont understand is the fact it will easily rev through to redline underload so the surely the setup cant be far off?

[magg]

What mods to the air filter?

[Neosophist]

I see.. It's hard to diagnose something without seeing it right.

You said its rich but maybe its not? some soot would be normal, does it smell excessively of fuel in the exhaust system?

Air filter mods can cause problems but since you said its pulling ok id focus on one issue at a time.. hanging idle inst usually an air filter issue.

You could get it dyno / gas tested to check the mixture but for now lets just focus on the revs hanging

You say you reset the pilot screws? They are 2 turns out right, not 2 half turns? (ie 1 full turn), i've come across it in the past where people have counted half a turn as a full turn and had a lean idle that way... dot on the screwdriver handle helps you to see where you started from.

[Persicus]

it does smell of fuel from the exhaust, and the plugs are black as you like!

yep defo 2 turns out, i double checked them

[Persicus]

air filter mods are only its using the same original housing, but using a car air filter cut down to same size, air filter flute count ie folds exactly the same and depth, width etc exactly the same.

[Persicus]

further update.......after not starting a couple of days, it back fires through exhaust on startup, too much fuel i would assume, nice bang then sits at idle smooth as you like (no choke at all!). took rear plugs out and again black as hell.
twist throttle ans revs sweet, 2 minutes late twist throttle and revs hang, blip throttle and she drops, checked cables to any kinks or too tight ut all ok as well.
wish i had bought a new one now lol.