VFR400 NC24 fueling problems (FIXED)

salem1987 · Scooby Slapper Joined: 10 Sep 2013 Karma :

Hi,

Wondering if anybody has any ideas about this as im stumped!

My NC24 was running rough at full throttle and had no power above 9K. This originally turned out to be the choke cable sticking and causing the choke to be 1/4 closed at all times. Rectified this problem and all was well.
After a few days while giving the bike a blast the problem arose again, the choke is wide open and all is good with that and the bike isn't running as bad but still really losing a lot of power at full throttle. Its completely fine at low RPMs and low throttle. Can get to about 70-80mph but thats it. If i keep it in a low gear and open the throttle it stutters and stumbles up to 10-12K. I'm assuming this is still a fueling problem.

Ive checked the vacuum hose on the fuel tap and even temporarily performed the fuel tap bypass mod just to see if it was that, with no success. The tank is also breathing fine and the filter is spotless.

I have not been into the carbs yet and before i do (as it looks like a pretty long/tough job) thought i would ask for some advice!

Cheers

sickpup · Old Timer Joined: 21 Apr 2004 Karma :

When was the last time the plugs were changed?

What condition are the plug caps in?

salem1987 · Scooby Slapper Joined: 10 Sep 2013 Karma :

plugs could may well need changing. Bought the bike a month ago so no idea when they were last changed. They look to be in good order and have all the right colours though.
Plug caps are in good condition as far as i can tell.
Could the plugs be letting me down under load?

Robby · Dirty Old Man Joined: 16 May 2002 Karma :

There are a good few possible causes. Before trying to chase down the problem, you need to get to a good baseline, which means giving it a full service, including replacing items that need addressing due to their age.

This means that along with doing oil + filter, air filter, spark plugs you also need to change HT leads and plug caps, and I would change old perished lines and hoses too.

If the fuel tank is rusty then you need to take care of the rust (https://www.realclassic.co.uk/techfiles/petrol_tank_cleaning_and_rust_removal.html) and clean out the carbs. I think you'll find carb removal to be an awful job on this bike. They will likely need balancing after refitting.

salem1987 · Scooby Slapper Joined: 10 Sep 2013 Karma :

thanks robby, ive just done exactly that today. Carbs were full of gunk, and it was quite "fun" trying to get them back onto the manifolds after cleaning them. One of the main jets was actually black!

Plugs and leads are all sorted now and oil was changed when i first bought the bike.

The bike ran fine for 15 minutes and shocked me with the 400 "power" id been missing out on for the last week.
But all of a sudden at full throttle the symptoms appeared again.

Very boggy and stuttery up to high rpms but completely fine on low to mid throttle.
It reminds me that the problem first appeared when at full throttle, its as if the bike likes it for a few seconds then just decides to say no and never do it again!

thanks for the advice

Jenks · World Chat Champion Joined: 22 May 2006 Karma :

mine did the same. turned out to be the cdi. The plug was full of rubbish and only made full contact sometimes.
____________________
1978 MZ TS 250 1 Supa 5
1992 yamaha fzr1000 exup
1998 aprilia af1 sintesi

salem1987 · Scooby Slapper Joined: 10 Sep 2013 Karma :

thats very interesting jenks. didnt think to check the cdi connection. will have a look now

salem1987 · Scooby Slapper Joined: 10 Sep 2013 Karma :

CDI is all ok. Connections are good and clean. Really confused as to what this could be now...

salem1987 · Scooby Slapper Joined: 10 Sep 2013 Karma :

ok so ive taken the carbs off again to clean them.

I noticed when moving the choke mechanism i can see carb cleaner bubbling out from below the choke plunger on 2 of the carbs.
Does this mean a seals gone in there and it could be sucking air in?

Robby · Dirty Old Man Joined: 16 May 2002 Karma :

I don't know how the choke mechanism works on those, but in principle any seal can perish, and if it does so then it will no longer seal.

You're dealing with 25+ year old seals, which means failure is likely. If you have disturbed them, then failure is even more likely.

The correct way to fix this problem is to properly strip the carbs, get even single component part spotlessly clean, replace any worn parts, then rebuild them to the factory spec.

The easy way is to replace anything that looks obviously knackered and hope.

Unfortunately, only the correct way actually works. It's slow, expensive, and frustrating, but you can find yourself chasing fuelling pixies for ever if you don't do it.

salem1987 · Scooby Slapper Joined: 10 Sep 2013 Karma :

Thanks for the advice robby.

I took the carbs off again and totally stripped them down to pieces. Turns out there is a gauze filter behind the float needle seat. All 4 of them were completely clogged with god knows what. Put it all back together after some serious cleaning and it runs better than ever.

Solved!