Help urgently needed.

[WarMachine]

Hey guy's, I've had this major issue on my motorbike for a while now, and nobody seems to be able to tell me what exactly is wrong, I'm hoping one of you will know.

I have a VFR400k, that recieves so much love, regularly serviced, etc etc..


When I ride the following happens, I pull away in first, hit second then third, without any sort of hiccup, smooth as silk no matter how hard I twist the throttle.

However, as soon as I hit 4th gear, it immediately bogs down, bike looses all power and then it seems to come right further up the rev range, I then hit 5th gear and the whole gear is just one big bogging and spluttering experience and have to go back down to 4th... I can not tell you if it feels like either spark or fuel related, I just don't know. it just boggs, feels like i've twisted the throttle closed when it's open, exhaust also sounds hollow when it does this.

It seems to get gradually worse the hotter she gets, but even if I have sat in peak hour traffic for a while, 1st, 2nd and third gear is always fine.

So to eliminate a few things, I have got a brand new chain and sprocket kit, well lubricated. My tank can breathe and breathe well. my vacuum hose that goes to the tank is brand new. when I remove my tank and put it over a bucket and open the tap, the fuel runs out beautifully both in the on position and RES. My stator has recently been rewired and voltages are right. my carbs has been serviced and balanced. Bike doesn't smoke. Has new spark plugs.

PLEASE HELP!!!!!

[mrrowley87]

Perhaps an air feed issue? High speed meaning odd pressure in the air box or at the carbs?

[dobbslc]

You need to start with all the basics that are easy and free to test.
Test the coils if you haven't already as they can break down when they get hotter, and higher gears may put more load on the ignition so it's ok in 1,2 & 3 but starts to die in 4th.
Also test the ignition pick-up coil as these do fail intermittently.
Test the above when hot and cold if possible to rule out an intermittent temperature related fault.
Check all the block connectors and wiring around the bike and dismantle and clean all the earth / ground points.
Check or override the side stand and clutch switches if you still have them, these cause problems on an old bike.
Check and clean the kill switch, use proper switch cleaner and dry it afterwards as any tiny copper filings can cause a short if they are floating in WD40 etc. ( I spent days with this one on my old RD125lc).
It sounds more electrical than carbs as if you say it rides fine in 1,2,&3 at low and high revs etc they sound ok.
Have you tried opening the fuel cap when it's hot on a ride? Just in case there is a blockage in the tank vents.
Hope some of this helps.
Let us know how you get on.

[Neosophist]

i assume you are sucking on the vaccumn pipe for the fuel to flow when you did this test?

When did the bike start doing this?

[WarMachine]

You need to start with all the basics that are easy and free to test.
Test the coils if you haven't already as they can break down when they get hotter, and higher gears may put more load on the ignition so it's ok in 1,2 & 3 but starts to die in 4th.
Also test the ignition pick-up coil as these do fail intermittently.
Test the above when hot and cold if possible to rule out an intermittent temperature related fault.
Check all the block connectors and wiring around the bike and dismantle and clean all the earth / ground points.
Check or override the side stand and clutch switches if you still have them, these cause problems on an old bike.
Check and clean the kill switch, use proper switch cleaner and dry it afterwards as any tiny copper filings can cause a short if they are floating in WD40 etc. ( I spent days with this one on my old RD125lc).
It sounds more electrical than carbs as if you say it rides fine in 1,2,&3 at low and high revs etc they sound ok.
Have you tried opening the fuel cap when it's hot on a ride? Just in case there is a blockage in the tank vents.
Hope some of this helps.
Let us know how you get on.

How would I go about testing the coils??

Where does the ignition pickup sit?

I don't have a stand switch, but I do have a clutch switch, can this be bypassed?

kill switch and tank vents are perfect.

[WarMachine]

i assume you are sucking on the vaccumn pipe for the fuel to flow when you did this test?

When did the bike start doing this?

No I ain't sucking on the vacuum pipe, it just flows when I switch the tap to the on position, and trickles when it's on Res.

Could this cause an issue like that?? I would think that a constant flow of fuel would be what you need.


It's been doing it for months...

[dobbslc]

Your fuel tap is not right, it shouldn't flow anything on "on and pri" without a vacuum.
You might have knackered you plugs by flooding the engine, easy to do on small Honda plugs.
For the coils you will need a multimeter to test them, cheap on eBay or go to maplins.

[WarMachine]

Your fuel tap is not right, it shouldn't flow anything on "on and pri" without a vacuum.
You might have knackered you plugs by flooding the engine, easy to do on small Honda plugs.
For the coils you will need a multimeter to test them, cheap on eBay or go to maplins.

Can the fuel tap be repaired, or do I need to go ebay hunting?

I have a multimeter, but I don't know what to test for or where on the actual coil to test for what exactly.




Thanks for all the advice so far guy's.

[Neosophist]

You are getting some good advice but a lot of it is geared towards NC30s.

The NC24 has two igntiion pick up coils and 4 normal coils.. any one of these cuold be breaking down causing issue.

Also electrical system could be a problem.

Without being there this really is one of those things that could be caused by almost anything.

Anyhow.

Your fuel tap is shot for a start. You need to rebuild it. The NC21/NC24 tap can be rebuilt easily. The vaccum hose sucks a diaphram open on the tap to allow fuel to flow, only when the engine is cranking.

Id drain the tank and get the tap off and opened up and see whats going on.. somebody might have bodged it before.

IF the rubber diaphram is split this can be cuasing you issue too.

Get that sorted out before proceeding.

[WarMachine]

Guy's, I managed to sort this issue out.... after months of struggling.

I did the HRC mod on the fuel tap, and I drilled 2x 6mm holes under the fuel filler cap and Bob's your uncle..

Smooth as silk... I'm happier than a pig in shit.