94 CB 400 STalling Issues

[danman]

Got this little intermittant issue that I hope I can get some help with. The bike will start and ride fine but every now and again will jut loose power. You pull up by the side of the road wait awhile and then it starts again. There seems to be no reason as to why it does it as it seems to happen in the evening when cold and during the day when warm.

I've changed the fuel filter and drained the carbies. I've been thinking it's fuel related. It still has a fair amount of fuel in the tank as well and changing from reserve to normal does not seem to help.

Any thoughts?

[Superjack44]

Got this little intermittant issue that I hope I can get some help with. The bike will start and ride fine but every now and again will jut loose power. You pull up by the side of the road wait awhile and then it starts again. There seems to be no reason as to why it does it as it seems to happen in the evening when cold and during the day when warm.

I've changed the fuel filter and drained the carbies. I've been thinking it's fuel related. It still has a fair amount of fuel in the tank as well and changing from reserve to normal does not seem to help.

Any thoughts?

Try a run on the bike when it starts to lose power open the fuel cap if you hear an inrush of air the breather is blocked, this must be clear in order for the fuel to enter the carbs.
Lift the tank there are two rubber pipes connected to the rear of it the large one is for draining the fuel cap well the second smaller one is a breather for the tank its self this must be clear.
Take it off and try and blow through it if you cant then either renew or clear it .
If the rubber pipe is clear check that the small bore pipe is clear by replacing the rubber pipe and blowing through it if you cant I cleared mine by running a broken throttle (Boden) cable through it and twisting it at the same time to clear the blockage well it worked for me.
Underneath the petrol tap there is there is a small water trap held in place by a 10mm nut remove and check that first for dirt that may prevent petrol getting through.
If the tank breather is clear then drain the petrol tank and remove the petrol tap strip and clean it

[danman]

Thanks for the info. There was no 'woosh' when I opened the tank. I'll have a look at the points you raised over the weekend.

I was beginning to think it could be electrical. Do the CBs have a tendancy for leads and condensors to fail after a while?

The electrical equipment looks original but the bike has only done 15,000 kms.

I want to get to the bottom of it as it's beginning to put doubts in my mond about this bike (and I do really like it)

[Superjack44]

Sorry I may not have been clear on this but.
First run the bike until it starts to lose power then as soon as you can open the filler cap to see if you get an inrush of air you should be able to hear this.
You could also try this when the bike starts to fail quickly loosen a drain tap on a carb to see if petrol drains out .
From the symptoms you describe I would put my bet on fuel starvation.
Where abouts are you a member may be not far away.

I have never had a problem with leads or coils, I replaced a reg/rec once to cure a charging problem but thats the only problem I have had

[danman]

I'm located in Western Australia.

I had a go at looking at the fuel tap today. It all looked pretty clean in there tto be honest. There does seem to be a 'crap' in teh tank but I do not think that would be a problem though. Anyone got any ideas on how to clean out a tank.

I assume ylou mean the 10mm at the bottom of the tap is actually part of the tap itself?

Will spend some time checking various connections,

[CMSMJ1]

can you try it without the inline filter? It might not flow enough fuel and after a run..the carbs get low and you have issues...?

[berno2020]

if theres crap in the tank it cud be in your fuel tap aswell worth a look

[Superjack44]

I'm located in Western Australia.

I had a go at looking at the fuel tap today. It all looked pretty clean in there tto be honest. There does seem to be a 'crap' in teh tank but I do not think that would be a problem though. Anyone got any ideas on how to clean out a tank.

I assume ylou mean the 10mm at the bottom of the tap is actually part of the tap itself?

Will spend some time checking various connections,

Yes the 10mm bolt head is part of the watertrap bowl just unscrew it to see if there is any sediment or crap in there it must be clean.
If the tank does contain any significant amounts of rubbish its best to clean it out, what I do to clean a tank is to remove the tap then place a number of smallish nuts and bolts (not brass) about 13mm into the tank and give it a good shake to see if it loosens any scale from the sides then remove the nuts and bolts with a magnet on a string after this remove any more debris using a vacuum cleaner you might have to cobble a flexi tube up to get in the bottom of the tank finally rinsing out with a drop of petrol.

[danman]

There's a little bit of 'scale' in the tank but not that much. And my thoughts were that the scale would sit to the bottom of the tank and not really be an issue with the tap on normal (It would be an issue on reserve though).

Water trap seemed pretty clean.

Hmm the point about the fuel filter seems an interesting one. It was doing the stalling thing before I replaced the filter and again after. The filter I replaced it with was a car one so I assumed that it would have the required flow!

The plan is to have a look at the fuel/vent connections next and see if there is anything obvious there.

With the tank off I'll also see if there are any obvious loose connection with leads etc.

[Superjack44]

You need to remove the tank to check breather pipe is clear.
Once all connections are disconnected just lift the rear of the tank and ease it back towards the rear of the bike the front sits on two rubber mounts if its been awile since it was off it may stick on these rubbers slightly.
Tank is simple to remove, remove seat , diconnect the fuel pipe from the tap and also the vacuum pipe that opens the tap, at the rear of the tank there are two 10mm bolts remove these and lift the rear of the tank underneath there are two rubber pipes one (large)to drain the petrol cap well and the smaller one to vent the tank this must be clear to allow the tank to breathe blow through the rubberto check and then reconnect a rubber pipe to the small metal breather pipe and blow back into the tank ,make sure the petrol tank cap is closed when you do this you should be able to continually blow through if there is any resistance find out what it is and clear it.
Since you now have the tank off remove and completely strip the petrol tap and give it a good clean once assembled connect a small pipe to the vacuum pipe that opens the tap if you suck on the pipe you should be able to feel the tap open nip the pipe to try and contain the vacuum then you should be able to blow through the main petrol feed (the long pipe that sticks above the tap and goes inside the tank you should be able to blow through quite easily if you cant there is a problem in there.