vfr nc21 only firing on three

[deno98]

Hi all new member and new to this have bought a vfr 400 nc21 87 runs but only fires on three when i go to hi revs it sometimes kicks back in not sure which cylinder isnt firing as just got bike also back firing person i bought off says he has cleaned and balanced carbs changed plugs put new coils on new battery but still no joy also has crome down pipes and noticed a black oily stuff were the four goes in to one all help would be appreciated thanks

[arsey30]

Black oil suggests piston/valve seal trouble.
All the usual checks/replacements have been done, so first establish which cylinder has failed.
Run the engine at fast tickover for a while and check ex pipe heat to find the cold one, then concentrate on that cylinder.
Run and blip throttle to high revs to see if that pipe heats up.
This will help to prove carb or ignition fault.
Check that plug condition [oily?]

[deno98]

thanks arsey hope its no the piston/valve but got bike cheep so don't mind spending abit of money on her.
its defintly back left when im sat on bike
also its an nc24 soz
will get plug out next and check that
read something about putting a table spoon of oil down pot to test compression do you recommend this and also thanks for fast replie

[arsey30]

Check to see if that cylinder is working at higher revs.
If you put oil into the bore when checking compression, it will indicate if piston or valves are leaking.
If compression is greater when oiled, then piston/rings/cylinder leaking.

It could be a broken ring, but look at the plug condition first.

Carbs are the biggest source of poor running, engine problems the least, but as your bike is fairly old the mileage may be high, although NC30s with 70,000m are still running well.

Do a compression check on the 2 rear cylinders first to compare readings [fronts more difficult to access]

[deno98]

thanks for help arsey think i have fixed it firing on all four again things i did

first checked plug which was black no oil and had a spark
then cheked vacuum pipe from tap had a split in it so cut end off and reattached
then checked oil which was like water and stinking of petrol
drained old oil which was aprox 3.2 liters way to much
put new oil in
then fired up and away she went happy days
still goin to get new plugs tomoz tho just for peace of mind thanks for your help

[deno98]

also no black oil on down pipes now to result still blew smoke on way home from uncles but cleared up mostly by the time i got home think it was burning wot was in the pipe :)

[arsey30]

Petrol in oil suggets a carb is over flowing, which is why 1 cylinder failed .

Both the tap diaphragm and 1 carb needle valve is leaking.

Turn the tap on, there should be no fuel flow, but if the engine ran OK with the vacuum hose split, I reckon fuel will flow.
Find which carb needle valve is leaking, remove carbs and fill them on the bench and look for overflow.
Drain contents and check for equal contents in each bowl.

[deno98]

do you rely think so
the bike had been sat up like
but will get on with checks you said and get back to u
thank again for all your help

[arsey30]

It sounds like a very slight leak that will not be obvious if the bike is used regularly.
If left standing for a while, then the volume leaking and overflowing may be considerable.

[Neosophist]

Erm.

I dont' think that much fuel in the oil would be becuase it's standing.

The petrol tap is vaccumn operated so only works when the engine running even if tap is set to on no fuel will flow.

3.2 litres of oil is over half a litre too much.

Check the carbs for leaking!