VFR 400 (NC24) Engine flat spot???

[Bishbash]

I did a VFR 400 up for my woman and I have ridden it twice, now, it will be OK for her to ride, but I want to get it perfect for her, so here is the issue;

I have bought new plugs, swapped the HT leads, and it runs on all 4 cylinders and I cleaned the carbs when I fitted them.

The bike will idle perfectly and rev to the limit and probably beyond. The issue resides in high gears and top speed, If I thrash the bike from stand still it will accelerate beautifully up until about 90mph then it does not want to go any further. I have tried slipping the clutch to get it to do something, but it doesn't, it just hits that flat spot and stays there.

I have accelerated gently and the same issue is there. It feels like it is restricted. I am pretty sure it isn't, but I will pull the carbs out to check to see if it has a kit in there. But am dubious to do so as it takes a while to due to the carbs.

Has anyone got any idea why it is doing this? ANY IDEA! I cannot figure it out.

Thanks


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My current bikes are - YZF-R1 & GS 500 E

Trackday videos - https://www.youtube.com/user/bishbash13

[P.]

Might not help, but my GS did that, but at around 80mph in 5th.. it kind of just not bogged down, but refused to pull harder.

Did a plug chop, was wet and apparently was over fuelling like mad.

[Frost]

So it won't go past 90 in any gear? or does it struggle at a certain number of revs? or just fail to pull higher gears?

Might well be restricted in some way.

[Bishbash]

Yeah I am 80% sure it wont pull past 90mph in any gear, I will check it this weekend, in fact if I don't figure it out, I will stick my camera on there and post the link, might help solve the issue. You might be able to hear something I don't.
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My current bikes are - YZF-R1 & GS 500 E

Trackday videos - https://www.youtube.com/user/bishbash13

[MarJay]

Check all of the breathers are OK and not kinked or trapped under the tank. The Airbox breather is a particular pain as I recall.

My NC21 wouldn't go past about 90 when it was 33bhp restricted though, the carbs are a bitch to remove. You have to make sure that you get them squarely on the rubbers before pushing them down, and you most defintely must loosen the clips which hold them on or you'll just pinch them when you try to put them back.
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British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[Bishbash]

The breathers should be OK due to it running through all the other gears,but I will check them.

That's interesting to know about your NC21 not going over 90 when restricted. Makes me think it's got some washers in there now.
____________________
My current bikes are - YZF-R1 & GS 500 E

Trackday videos - https://www.youtube.com/user/bishbash13

[Azonicben]

Has it got kph or mph clocks?

I know on the Nc29(cbr400) they are restricted for the japanese market, a mechanical speedo converter takes away the restriction. The restriction kicks in at 180kph ~ 110mph of course the converter makes the clocks read in mph so the bike never reaches 180kph on the dial as its 180mph with the converter.

cheapos on eBay===> https://cgi.ebay.co.uk/mechanical-kph-mph-speedo-converter-honda-yamaha-suzuki_W0QQitemZ120520228310QQcmdZViewItemQQptZUK_Motorcycle_Parts?hash=item1c0f90bdd6

https://cgi.ebay.co.uk/KPH-KM-H-MPH-KILOMETRE-MECHANICAL-CONVERTER-MOTRAX_W0QQitemZ370322860990QQcmdZViewItemQQptZUK_Motorcycle_Parts?hash=item5638f66bbe

This may work, depends whether the vfrs are restricted in the same way, a light bit of googling suggests they may be


EDIT:

[Robby]

NC24s don't have an electric speed limiter like on the NC30. They just limit their top speed by being shit.

You should be able to check for restrictors by getting the tank and airbox lid off, opening up to full throttle and pushing the slide out of the way, that should let you see down to where the restrictor is.

[Blau Zedong]

The NC24 does 128mph, same as an unrestricted NC30. There is no speed limiter on the 21 or 24.
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18:54:48 Rob Fzs: jews don't give away stuff for free

18:54:59 Rob Fzs: unless its their clothes/ hair/ golden teeth

[Bishbash]

Definitely not the speedo, but thanks for the reply.

Robbie, you might have saved me quite a bit of work, fingers crossed I can see that far down.


____________________
My current bikes are - YZF-R1 & GS 500 E

Trackday videos - https://www.youtube.com/user/bishbash13

[Bishbash]

OK, I had a quick look as Robbie suggested, but I could not see anything, so before I rip the bike apart, here is a speed test of the bike, anything abnormal, let me know, it could help.

https://www.youtube.com/watch?v=Vn89-Fid3Kw

Oh let me know what you think of my new bullet camera/sound too.
____________________
My current bikes are - YZF-R1 & GS 500 E

Trackday videos - https://www.youtube.com/user/bishbash13

[P.]

well its orgasmic up until 90..
is it definitely not over fuelling? the revs bounced, not sure if it was your clutch doing so, but it went up then down then up again then down.
my gs pulsed a bit like that when it was over fuelling, only when going fast though, i say fast 80-100ish

[Bishbash]

[Bikeless]

Looks like it has a 33bhp restriction.

[P.]

if its the yellow bike you were doing up for your partner, you included pics of restriction washers.. dunno if you put them in though.

My GS was a bag of shit when it was running rich, like overly rich.

[Bishbash]

Yeah it is the same bike for the Mrs, but I didn't put the restrictions that I bought in the carbs.

I guess it means I am pulling the bike to bits again....Well the carbs anyway.
____________________
My current bikes are - YZF-R1 & GS 500 E

Trackday videos - https://www.youtube.com/user/bishbash13

[Billing]

Might be worth getting one of those colourtune things and seeing if your mixture is good on all the cylinders, i think the vfr takes the 10mm one (the spark plug number should begin with a C i.e CR8EA)


How about seeing if a motorbike shop will put the bike on the dyno and see if that reveals anything
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'84 TS50X, '91 TZR125, '89 CBR400RR, '91 VFR400R NC30, '98 R1
"Hey copernicus! Why don't you navigate yourself to the back of the line with your feet and stand there with your shit."
BCF Eastern members map here ---> g.co/maps/2tm8b PM me to be added to it!

[Bishbash]

That costs bucks though, not too keen on spending more dosh on it.

I will take it apart on Saturday morning and see what I can see.

[Billing]

Yup, I've been reading about them, they're basically a clear spark plug, you can see the colour of the flame inside the cylinder, just youtube 'colortune' and it should give you some idea of how it works.


if nothing else I'll see what they are on trade at halfords (they're ?35 on the shelf) and if its cheap enough I'll get one, may be able to lend it to you for a bit.
____________________
'84 TS50X, '91 TZR125, '89 CBR400RR, '91 VFR400R NC30, '98 R1
"Hey copernicus! Why don't you navigate yourself to the back of the line with your feet and stand there with your shit."
BCF Eastern members map here ---> g.co/maps/2tm8b PM me to be added to it!

[Bishbash]

[Billing]

Any updates Bishbash? If you're still having problems someone over on 400greybikes.info might be able to help you, they seem a fairly good bunch
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'84 TS50X, '91 TZR125, '89 CBR400RR, '91 VFR400R NC30, '98 R1
"Hey copernicus! Why don't you navigate yourself to the back of the line with your feet and stand there with your shit."
BCF Eastern members map here ---> g.co/maps/2tm8b PM me to be added to it!

[0l0dom0l0]

from the video it definatly looks like somethings up. it is fairly unlikely, but have you checked for a throttle body restrictor in the carbs? (ie the other form of restriction instead of the washers) as it should pull way harder than that up past 60. if the carb dismantle reveals nothing then im out of ideas . keep us posted!

[Bishbash]

I havn't had any time what so ever to look at the bike, I am planning to do it on saturday, so we'll see.

I will take a peek at the throttle restriction, another good idea.
____________________
My current bikes are - YZF-R1 & GS 500 E

Trackday videos - https://www.youtube.com/user/bishbash13

[Bishbash]

OK, finally had a chance to take a look at the carbs, there are no restriction kit in there

I have no clue what's wrong with it now, would it be simply due to it's age? It has done 32000 miles, it's not a great deal I know, but it is how it has been treated in those miles......
____________________
My current bikes are - YZF-R1 & GS 500 E

Trackday videos - https://www.youtube.com/user/bishbash13

[0l0dom0l0]

hmm. well i would be going down the compression test route now. check those rings are okay. as long as the bike had a full service history then it should be fine. im just trying to think of any other things that cause this problem

another idea, i dont know if anyone mentioned about the sprockets but check the bike hasnt been seriously up geared by accident and it just simply doesnt have the power to pull past 90.

strange question but if you lie on the tank will it go any faster? or is it like just complete stop at 90?

good luck!!