Cb400 Super four hyper vtec NC39 Gearbox problem

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Tarlowb
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Joined: Tue Oct 11, 2016 9:37 am
Bike owned: CB400SF Hyper VTEC Spec II
Location: West Midlands
Cb400 Super four hyper vtec NC39 Gearbox problem

Post by Tarlowb » Sun Jul 08, 2018 10:39 pm

Hello Greybikers

In April 2017 i was riding my bike, and hit a false neutral between 5th and 6th. i quickly pulled the clutch lever but it wouldn't find either 5th or 6th. At this point the road revs were much higher than the idle speed the engine was now running at and in the confusion, there was a series of clacks and crunches and it found 5th gear.

I pulled over to investigate and the engine would rev fine, so i tried riding it, no problem through 1st - 5th but 6th was skipping, my heart sank. :cry: :shock: :down:

It has been back on the road for two weeks, and i have documented as much of the process as i could.

This is my write up detailing how i fixed it for anyone who has got this issue,had this issue or may have this issue in the future. it will include how i removed the entire engine, sourced parts and managed to get her working again. i appreciate there are people who know what they are doing on this forum, but I've learnt a lot from this experience as well as using the pages for information to so if nothing else i'm happy to give some knowledge/experience back.


Quick History: Im not a mechanic, just learning as i go, I've been riding since 2012 on a 125, passed my full test in 2016, had a 650 bandit, sold it after a year and got the NC39 and love it :grin:

Tarlowb
Settled in member
Reactions:
Posts: 33
Joined: Tue Oct 11, 2016 9:37 am
Bike owned: CB400SF Hyper VTEC Spec II
Location: West Midlands
Re: Cb400 Super four hyper vtec NC39 Gearbox problem

Post by Tarlowb » Sun Jul 08, 2018 11:43 pm

To start with, here is a photo of my bike :grin:

Image

Tarlowb
Settled in member
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Posts: 33
Joined: Tue Oct 11, 2016 9:37 am
Bike owned: CB400SF Hyper VTEC Spec II
Location: West Midlands
Re: Cb400 Super four hyper vtec NC39 Gearbox problem

Post by Tarlowb » Sat Jul 14, 2018 4:11 pm

The bike was stuttering in 6th gear, not smooth like clutch slip, more like the symptom of worn dogs on the gears. I didn't think this was the case as it only happened in the v-tec threshold. but after a little more riding, it realized it WAS also present under the VTEC threshold too, anything more than 40% throttle and its skipping.

I had a chat with a few biker friends who had different opinions ranging from adjusting the clutch, changing fuel, checking for speed limiters etc (its a Japanese market bike, but because it doesn't have the traditional front wheel speedo drive like the NC31,that was unlikely to be a factor. the speedo is electronic and there is a sensor above the engine, but below the airbox, which must have been converted to mph. the dials still go up to 200 but there is a sticker that says MPH where it used to say KPH)

All were incorrect. I have a powertech end can on it and replaced with the stock can, but still the same, I even checked the spark plugs, HT leads and ignition but all were fine. I then contacted Grame France of GFracing in his workshop, he said it was definitely a gearbox failure, but he could fix it if I took the engine out and delivered it to his workshop near Scarborough.

This may be no big deal to a mechanic or a motorcycle restorers etc, but I was seriously hesitant and apprehensive about the task ahead of me. right, some more photos next (having difficulty uploading them doesn't matter what size for some reason? any tips other than the ones on the main board?)

Tarlowb
Settled in member
Reactions:
Posts: 33
Joined: Tue Oct 11, 2016 9:37 am
Bike owned: CB400SF Hyper VTEC Spec II
Location: West Midlands
Re: Cb400 Super four hyper vtec NC39 Gearbox problem

Post by Tarlowb » Sat Jul 14, 2018 4:33 pm

Image

alright, photobucket seems to be ok for hosting and resizing. :peace:

I waited a few months to start as I was about to move into my first house (garage to work in 8-) ) and did the things I already knew how to do which was remove the seat and tank. to remove the tank remove the two side panels and after that, undo the big bolt at the back of the tank. there are a five connections to the bike. fuel and vacuum lines on the left side of the bike, fuel sender electrical connector on the right hand side and two breathers/overflow? rubber tubes towards the rear of the bike. after this, the whole thing should slide backward a few inches and then be free to lift up.

At this stage I had brimmed the tank with petrol to prevent condensation and added some fuel stabilizer, drained the oil, and the coolant. the coolant hose can be seen looped back on itself into the reservoir, but was connected just under the brass coloured coolant top up tap, top right.


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