VFR 400 Runs on 3 cyclinders after 10k rpm

[mattevans58]

Just took my VFR for a spin and was having a blast, decided to pin it and I got to about 4th gear 12k rpm and all the power dropped and twisting the throttle further didnt increase the rpms it struggled and you could hear it was running on 3. I pulled over, the revs went to idle and the other cylinder fired up again and off I went again. I havent taken it to bits yet because its nearly 11pm so will tomorrow if I get the chance but at first I thought it was the fuel filter restricting the amount of fuel and killing a cyclinder but that didn't really make sense. When it fires up, its fires on all 4, can rev it in neutral and its fine but when I pin the throttle it goes blur.

Carbs are balanced and jetted perfectly.

I'm presuming it's plugs, leads or carbs but any experience?

And another thing, if it something that can only be checked when I go for it then thats a bugger....

[redeem ouzzer]

In all gears or just 4th upwards?

[Old Git Racing]

Start with the basics and put new plugs in, check plug leads at both ends for good connection and check plug caps are securely seated.

OGR
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2022 Tiger Sport 660 2019 Street Twin 2003 K3 gixxer 1000 1998 Srad 600 track hack

[Ariel Badger]

I would suggest you have a blocked main jet on that pot.
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Bikers make great organ donors, get 115 on your licence today.

[.Bishbash.]

Anything like This?!?!?!?
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Current bike - GSX-R1000 k8

[smallfrowne]

Ah man, bummer. Had some issues on mine and they were jetted and balanced fine Only way to do it is to go through things logically.

I've had issues with a coil pack, identified by swapping them over, the issue moved with it from front cylinders to back. But that was only running on 2cyl that time.

I've had a brass float seat come loose a few turns in one of the carbs before. It acted in a similar way to as you described. With one pot out it ran fair enough until you wound it on, and then didn't quite rev out, getting to ~12ish and then blrrrrrrr. I'm guessing it wasn't letting the float needle open up fully so fuel didn't flow fast enough to keep up. Having sat a while at idle filling up it briefly ran fine again. But blocked jets might do similar (as mentioned), or something else float related like being stuck/blocked, not set quite right...

I'm also having an intermittent issue where the cdi is hitting the rev limiter before the tacho reaches redline orrr the tacho is reading too low. Either way it cleared up after turning the ignition off and on again a few times. But I've not heard of this problem before, pretty unlikely.

Is it restricted still? Can't remember what speed 12k is in 4th. Cdi will cut 2 cylinders at ~110mph.

I'd guess a fuelling issue. But arguably it's easier to check plugs, leads and coils before you whip the carbs off.
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'90 VFR750; '89 NC30; '95 DR650; '89 CD250U; ~'82 CG125; ~C90

[mattevans58]

[mattevans58]

[mattevans58]

[.Bishbash.]

[Jefr0]

My friend had the same problem.

There is a electronic speed restriction in the speedo. At a certain speed (like your 4th gear 12k revs) it would cut power to restrict it.

I'd suggest it's this. Changing plugs/coils etc could be avoided if you try 12k+ in the first 3 gears and see if it's fine and not just a speed restriction.
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Rides: Honda XR 125 ('03) Honda CBR 600F ('91), Ducati 916 Biposto ('95)

[mattevans58]

[/quote]

That was a former bike I did up....I grew many a grey hair due to that bike.

OK, start with the plugs and HT leads(as suggested) Swapping them over etc, if they are all fine, then have a go at the carbs and give the jets a good clean out. When were the carbs balanced and jetted??? Recently? And when did the problem start?[/quote]

All the plugs and leads are all fastened in correctly, all sparking. Stripped the bike this evening and the carbs are being given to a friend of mine who is a fantastic carb cleaner, they come back looking spanking new inside and out. Told him the details and he said he'd make sure they're in pristine condition for me so and get them rebalanced when they're put back on the bike. All for £20! So that'll rule out another problem. I can order new plugs and leads if needs be but I'd really rather not

[mattevans58]

[bikertomm]

This might be a really stupid suggestion, I don't really know.

When my CB600 started mis-firing it was a spark plug - even though it was new, the housing it was sitting in had come slightly loose which allowed the plug to bounce all over the shop causing intermittent firing (usually high in the revs because of the vibrations!)

May be worth a quick check. Solved with some rubber sealant
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07' Honda Hornet now full powaah! My guide on performing an oil change!

[Fladdem]

At work, we had a dropped cylinder and misfire caused by a plug too. We'd put new plugs in as part of the service, but one must have been damaged internally so we couldn't tell. We never suspected because they were brand new, after eliminating lots of other things, we thought about swapping plugs and the misfired went with the plug. Weird.
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Current:1991 Honda MT50 (Soon to be a H100/MTX/MT5 hybrid), 1976 Honda Cub C70, 2005 Honda Varadero 125, 1993 Yamaha TTR250 Open Enduro , 2010 Road Legal Stomp YX140, 1994 Honda CRM 250 MK III, 1999 Cagiva Mito 125, 1992 Honda CB400 Super Four, Stomp T4 230, 1984 Honda H100s, 2009 Sym XS125K
Past:2003 Aprilia RS125, 1982 Kawasaki GPZ550(FREE BIKE!)
I'm having more fun than a well-oiled midget.

[mattevans58]

by the sounds of it then I need some new plugs! Will get some new HT leads on top because they're only £36 on top so, when they're delivered and fitted i'll let you all know how it went!

[craigs23]

I had a similar thing on my Superfour, where the bike felt like it wouldn't exceed past 7k in top. Ended up being the Japanese 180kph restrictor in the speedo was faulty, and essentially restricting the bike earlier than 180kph. Fitting a new resistor in the loom to remove the restriction corrected the issue.

[mattevans58]

Upon some more reading, I read somewhere that the imported Japanese NC30's had a speedo restriction at 113mph by their laws. Once youd reach 113 the restriction would drop 2 cyclinders and youd have to slow down again to get them firing (sounds just like my issue). To overcome this you either fit a a MPH speedo or use a speedo translater from kph to mph....and I JUST so had an NC30 speedo converter laying in my toolbox. Will post back tomorrow if the issue is still there.

[Bikeless]

[mattevans58]

Update -

My carbs is still of at a friends being cleaned, and rejetted for a new exhaust, also putting Rick O's carb needles in to reduce the midrange flat spot NC30's have. The bike is to be put in storage in the next month or so as I start my Marine training soon. Oil's been changed and flushed, new oil filter, air filter, throttle and clutch cables, new brake pads, plugs all round, new HT leads all round, thorough clean up and awaiting the carbs to be put back on and balanced then I shall have to see if the issue has gone, although I am 99% sure it's the speedo restriction.

Thanks all again, I would never had known about the speedo restriction if someone hadn't have mentioned it, cheers!