B40 cuts out on gear change/higher gears

[highboy_coupe]

Hello, I have a bit of an issue as I keep using my B40 to get to work, natively believing that the issue will clear itself!

It hasn't and after a very sweaty and sweaty trip to the office I feel like the thing has given up.

It was running very well a couple of weeks ago, last week I noticed once I got it into 4th and was travelling at along at about 40 it stated to misfire. I didn't think too much about it as it seemed to clear itself up.

I checked the plug and it's a good colour.

Then last Friday I was on 3rd and it started to splitter and bog, switching down to 2nd and revving seemed to sort it temporarily ,then on going back up to 3rd or 4th was misfiring badly and not revving.

It then stalled and was a pig to kick over, bit once it did it idled.

Pulling away it spluttered bit keeping it in a low gear and revving it seemed to be the only way to keep going. As soon as I went up a gear it just splitters and dies.

Same story today but more of it, easy to start first thing, will run in 1st and second with the odd misfire, 3rd patchy and 4th unrideable. 

Carb is original amal, was ultrasonically cleaned and has new jets,
It has vape ignition and is 12v.

It idles nice and cleanly, it revs up on the stand in neutral ok.

It now pops and bangs on the overrun where I believe it wasn't so much a couple of weeks ago, sounds like an air leak.. bit why not all the time, and why does changing gear affect it so much?

I did notice an rear indicator bulb has blown .

Thanks for any help!!!

[highboy_coupe]

I wonder perhaps, as it feels like a misfire, if maybe over a few rides an electrical connection has become loose somewhere. This wouldn't count for such a drastic change between gears though, changing up is definitely the catalyst and it can be like night and day!

My other thought was my fuel tap looks a bit small, I wasn't that happy with the ID of the tap, but then in a high gear with low revs this wouldn't matter would it?

[Rog1]

It may be a charging issue. When you change up and the engine revs go down, the generator is then capable of producing less power. If you ride with the headlight on but do not turn it on until the bike is running, and if you rev it up on the stand with the headlight off, then less power is needed and you'll get the results you describe.

[highboy_coupe]

Ah, good point. I was trying to tie the gear change to the cutting out. It may be that the reg/rec I have is not great, (wassell), or the charging system is not giving the electronic ignition enough ? The battery is fully charged though as I had it on a trickle charger overnight.

[V500]

Well, you're right, Wassell stuff isn't great!
Having the battery on charge overnight doesn't necessarily mean that it's all good to go. What's it (battery) like under load? Turn on the headlamp and see it if holds up for a while.

[rhyatt]

The only way to check charging is with a meter.
Check across battery standing, nothing on, should be 12.5volts.
Run the engine and across the battery should be 13.9 or 14v at a fast tickover or close to these volts.

[idie]

Sounds like a coil or condenser braking down.

[DAVE BRADY]

Hi,

Very Basic but is the breather hole in the filler cap blocked, partially blocked.  When you take it off you may be able to blow through it but press together as if on the tank and see if it is restricted.

Dave.

[highboy_coupe]

Well, I am stupid. Thank you for your answers. I tracked it down to a petrol tap that has too high of a stand-off inside the tank. So even when there is plenty of fuel in the tank it starves the carb as it gets near the top of the stand off.

Why this reveals itself on gear change (slosh?) is beyond me.
I wish I had checked that before taking apart the ignition and charging system to check for bad wiring.

Thank god I didn't take the carb apart, I've got it just how I want it!!

[highboy_coupe]

... Or so I thought...
The same thing happened today. After riding him with the tank pretty much full it was fine.
Today as I rode of I felt it stumble in 4th again, then 3rd, then cut out, it had gone about a mile at this point.

It would t run reliability after that so I took it home.

I removed the online filter, just in case as it was horizontal and wanted to eliminate that. I noticed after a few seconds idling there is pressure in the fuel line, bubbles forming around the tap.

What would cause pressure here? And is that causing any issues. I am going to check the float bowl now to see if anything is wrong there

[KernowPete]

Do you mean bubbles on the outside of the fuel tap or bubbles inside a clear fuel line?

[rhyatt]

Have you carried out the charging checks described earlier using a voltmeter?
This can then exclude this part of the diagnostic process to help you.