Hyper vtec spec II issue

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Tarlowb
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Bike owned: CB400SF NC39, '88 NC23 tri-arm
Location: West Midlands
Re: Hyper vtec spec II issue

Post by Tarlowb » Sun Jul 15, 2018 10:28 pm

I have ordered a new fuel tap as the issue is still present. i've noted that it only happens when im on 2 out of 6 bars of fuel on the gauge, mainly if i have been riding, stop somewhere and then set off again, but especially if i have started riding up/down a hill.

My thinking is that when i have a full tank, gravity plus the vacuum opening the diaphragm allow fuel to flow freely, but once the weight of the fuel pushing down has reduced, the diaphragm is maybe getting stuck? either way i had a look at a few "rebuild kits" on amazon etc and im a bit sceptical of them.

Today I rode from Redditch to Rhayader in wales and back with the slip on end can, 202 miles with two fuel stops to keep the tank above 2/3s full and didn't have any issue :grin:

fingers crossed this will sort it for good, the fuel tap is 17 years old, has anyone else had any issues like this?

Tarlowb
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Re: Hyper vtec spec II issue

Post by Tarlowb » Sun Apr 21, 2019 12:46 pm

Alright this is really getting me confused now.

Bike still bogs down after a run and stalls, any throttle position, either WOT or just cruising around, it also behaves fine when the tank is full so I'm confident its not carb related. sometimes its after a run, then a pause ( for a coffee etc) then it runs fine for a few mins and then bogs down a mile or so down the road. I have already replaced the fuel tap and filter and its making a good seal from the vacuum hose from the engine, breathers clear and hoses un-kinked, I'm running out of ideas GB's

any thoughts on why it would only happen at low fuel? less head pressure?

Next time it happens I will whip off the fuel line/try to drain the carb bowl to try to confirm the hypothesis that for some reason, fuel isn't flowing.

very frustrating... :cry:

Superjack44
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Re: Hyper vtec spec II issue

Post by Superjack44 » Sun Apr 21, 2019 8:42 pm

Next time it happens open the fuel cap as quick as you can and listen for any inrush of air (there should not be any) if you hear any that indicates the tank is not breathing properly if it's not breathing properly fuel is not flowing freely , if the small rubber hose is clear try blowing back into the tank it should be quite easy if it's not you need to remove the obstruction .
In my case something was blocking the small metal pipe that runs down the inside of the tank to where the small rubber hose connects under the tank the small rubber hose may be clear but double check the one inside the tank .
Keep a steady pressure on it, there should be no build up of pressure in the tank the vent in the cap should be capable of dealing with you blowing into it.
I dealt with the small pipe blockage by inserting a Bowden cable into the pipe once I removed the petrol filler cap and spun it using a drill and gradually pushing the cable until I met the obstruction then kept it spinning until the cable passed the obstruction.
I bunched the cable just a bit just enough to go down the hole but it worked for me

Tarlowb
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Bike owned: CB400SF NC39, '88 NC23 tri-arm
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Re: Hyper vtec spec II issue

Post by Tarlowb » Sun Apr 21, 2019 11:03 pm

Thanks for the reply superjack44,

I saw your post on the other post earlier and tried this. Took the tank off, removed the hoses and checked for blockages, both clear, both the larger and smaller diameter holes were clear too but to be sure, lubed up some guitar string and worked it through the holes.

Both clear and a miniscule amount of dirt came out but nothing serious, put the tank back on and went for a ride, didn't even put the overflow pipes on in case they got kinked.

This was a 40 minute ride, stopped for 10-15 mins to make a phone call and then started it up, it acted completely normal and a few minutes in... Bogged down and stalled. Checked the filler cap, no air rushing.

I did check the float bowls though, each of the four had petrol in so that rules out that line of thought. What could be affecting the bike this way?

Graham04
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Re: Hyper vtec spec II issue

Post by Graham04 » Fri Apr 26, 2019 3:58 pm

Is it possible to completely bypass the vacuum tap as a temporary measure , it would mean blocking the vacuum pipe and relying on the float valves to stop petrol from leaking when stopped. I fitted a manual tap to an old xs400 which cured an intermittent running problem. Just a thought as it seems to be straw clutching time.

Tarlowb
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Re: Hyper vtec spec II issue

Post by Tarlowb » Mon Nov 25, 2019 1:10 pm

Cheers Graham,

I had a good look at the vacuum line and played about with a few different types of system, tried blanking it off but it seemed to be only when the fuel was low, combined with a hill. i presume it may have had an issue if that tap didn't have the weight of fuel on the diaphragm or something, but i still couldn't work it out.

The NC39 as now been sold,i let the guy know what was happening and he didn't seem that bothered by it, its a shame as it was such a beauty, and i have now bought a cbr400RR tri-arm as a project :)


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