'95 CB400FII :: Intermittent stalling issue

[zen61]

My problem is an intermittent stalling issue and I'm hoping someone here can shed some light on it. I have tried to read as many of the posts here and elsewhere before posting this. If you have already posted the answer to my issue and I've missed it then please accept my apologies.

As introduction, my new bike appears to be a Japanese Domestic Market (JDM) 1995 CB400 FII. My import VIN says it's a 1997 but the JDM VIN, Engine numbers and the conspicuous circled S on the sticker under the seat suggests otherwise. It reports around 24,000Km on the clock--if it's the original clock? I live in the dry tropics of Nth Queensland, Australia so it's quite warm here 28-30 Celsius. I've had the bike about a week. I've had no previous experience fixing bikes and barely a few weeks riding them.

The situation:

I can ride the bike for a random period; it might be 3kms, 1KM or only 500mtrs whereupon it will lose power and I'm forced to coast off the road. The electrics are still running such as indicators head lamp etc. Engine cranks just fine and tries to fire.

If I try to start the bike right after it's stalled, it may idle roughly but any attempt to wind-on some throttle just kills it again. Also, if I apply full choke just after it's stalled, the bike will start and run roughly but any attempt to wind-on throttle will kill it again. If I wait a minute or so and then start it again on full choke and just let it idle it will idle rough but after 10 seconds or so the idle will smooth out and the RPM will quickly wind-up to around 3000 RPM and that's when I know I can shut off the choke and head off again with the bike running perfectly until it happens again maybe 1-2KM down the road.

Things I have noticed:

Whenever I have left the bike parked on it's side-stand for some time and I pop the fuel cap it will make a sound as if the air is equalising in the tank. I'm not sure if the sound is due to a vacuum in the tank i.e. lack of pressurisation or too much pressure i.e. fume buildup. But it nearly always gives the cap a kick when I unlock it. I have got into the habit now of popping the fuel cap before I head off on a ride and as soon as it stalls upon pulling over. I don't notice the fuel cap kick when unlocking it directly after a stall however.

Things I have attempted to look at:

• 1/. I have unbolted the tank, raised it slightly and blown through the short rubber tube connected to the metal tank breather tube. With the fuel cap open I can push air through the tube into the tank and feel it coming out of the fuel cap opening with my hand. If the fuel cap is closed however, and I try to blow back through the breather pipe into the tank I can feel increasing resistance as the air I'm blowing pressurises the tank like blowing-up a balloon and when I take my mouth away that air rushes back out the breather tube into my face.

• 2/. I have undone the bottom of the fuel tap to see if there was any gunk in there and it's as clean as a whistle and the O-ring looks like new.

• 3/. I have tried running the bike with the fuel tap set to both ON and RESERVE and it appears to make no difference with my intermittent stalling.

In one of the posts here I read someone say that you should be able to blow through the breather tube back into the tank with the fuel cap closed and that the air should flow freely and not pressurise the tank. Is this correct? This would mean that there is another means for air to escape the tank apart from the breather tube itself-yes? Or is that post wrong? Or am I missing something? Or both

I guess at this stage I'm just trying to identify if the fuel tank breather system is the cause of my problem before I go on to to look elsewhere for the cause of my stalling issue.

Cheers...Zen
P.S. Before someone suggests it, I can't open the fuel tank while the bike is running and listen for a woosh as I only have one key

[RICKNC31]

Hi when you open the filler cap you will see two holes one is the larger overflow hole the other is the smaller tank breather hole i recomend you push a piece of flexable wire or plastic from the tank down the holes right through the hoses rather than blow up the hoses i find this clears the breather pipe and drain hole better also check the breather pipe is not being pinched somewhere as it deff sounds like your breather is obstructed somewhere hope this helps rick

[zen61]

Thanks for the reply Rick, I can push some heavy gauge plastic whipper snipper cord down through the top of the breather hose but it seems to hit an obstruction about 38cm (15") down that I can't get past. Conversely, I can push the same cord up from the bottom of the metal breather tube at the base of the tank about 5cm (2") and it hits an obstruction that I can get past.

So, it sounds from what you're saying that when I push a wire down through the top of the breather tube at the fuel cap opening that I should be able to push it all the way through so that the tip of the wire should appear through the metal outlet tube at the bottom of the tank--yes?

Looks like I need to get out the coat-hanger wire and ram it through. I was worried about doing this before in case I perforated something I wasn't supposed to.

Cheers again.

[zen61]

Ok, now I can get the wire all the way through, not the coat hanger wire because it's too inflexible but it appears to have shifted the obstruction. Now, a few days of riding to see if the problem is fixed for good.

Cheers...Zen.

[Superjack44]

I think the only way to clear the breather hole is from underneath the tank ( well it is on mine a 92 superfour) if you lift the rear of the tank you should see two pipes a small bore pipe and a larger bore pipe both connected to rubber pipes that go down through the frame towards the ground.
The large pipe is the drain from the filler cap recess and is not the tank breather pipe, the small bore pipe is the breather pipe and must be clear of any obstruction disconnect this rubber pipe from the tank and blow through it if you cant then clear the obstruction and try it,if you can blow through it connect it back to the small bore metal pipe and try blowing back into the tank with the cap open if you cant then there is an obstruction in there and must be cleared, I cleared the obstruction in mine by pushing and twisting at the same time a length of flexi boden cable with end slighty just frayed but not enough to prevent entry into the small metal pipe (push bike brake cable) until It met the obstruction then I continued to push and twist until it eventually broke through the obstruction.
The only other problem I had was the vacumm operated petrol tap and combined fuel gauze water trap I dismantled mine and gave it a good clean (it had stood 12 years before I got it) tank and tap were full of debris but once cleaned no more problems.
Ps, the small rubber pipe that is connected to the lefthand cylinder operates the petrol tap this must be in good condition otherwise if it drags air in it may mean that it is not operating the vacumm in the petrol tap correctly.

[nictecninja]

Wellcome to the forum,
& my I say what a nice part of the world to live I have relatives in Denmark, W.A.
I have to agree with Superjack,
I have a 92 Superfour FII & I have had the same problems with the vacuum tap & the blocked breather Pipe (the small pipe on the right hand side of the tank veiwed from the rear).
I unblocked mine by twisting some 0.8 mm mig welding wire through mine from the bottom up then squirted some WD40 through from the top, (this will blow through any dislodged particles & give the inside of the tube some level of rust prevention).
I recommend doing this annually as a prevention rather than cure! & if this does not help then check the condition of the diaphragm as this can perrish & split with age & will cause your running issues.
Hope this helps Paul.

[zen61]

Hey, thanks a lot guys for the advice. Like the WD40 trick and I'll keep and eye on the fuel tap diaphragm too. Bike's been running fine without a hitch. It did stall the other day but that was only because someone forgot to turn the fuel tap on

Cheers again...Zen.

[hoppy41]

Why is there a rubber insert over the top of the breather tube. This appears to block the airflow when caps closed ??

You can see it with cap removed Its pushed on cap screw

[Superjack44]

Why is there a rubber insert over the top of the breather tube. This appears to block the airflow when caps closed ??

You can see it with cap removed Its pushed on cap screw

I am fairly certain that rubber is a seal between the breather pipe and the actual petrol cap there should be a small hole in the cap (somewhere) that allows the tank to breathe thus allowing fuel to flow freely into the carbs .
Without that seal dirt could possibly get into the petrol cap vent and really block it to the extent that a new cap is required.
I may well be wrong but thats the way I look at it.

[hoppy41]

Thanks Jack. Think I'm making progress. This is what my bike does.


1. Rubber seal in and cap closed no airflow when I blow through breather tube.
2. Rubber seal removed and cap closed HAS airflow when I blow through breather tube.


I presume this means my cap is the problem and blocked.



Running bike without that rubber insert seems OK so far




Dave