B40 stops after 5 mins and won’t start again.

[Hancock]

Hello all, my 1962 b40 just stops running after about 5mins then won't start again.starts ok initially.

[DerekAnderson8]

fuel cap might not be breathing causing airlock.

[Mark B]

Does it just cut dead, or splutter, or backfire?  Apart from the fuel starvation possibility, how about the coil breaking down?

[AdrianS]

Have you checked for a spark when the bike stops?
Checked fuel supply - is the carb running dry; fuel tank not venting or float needle sticking or dirt in the fuel supply at the carb ( gauze filter at the carb banjo blocked)?

[ducati2242]

There are so many reasons why it could stop and not restart . My son had a chinese honda copy , cant remember what it was but it was a single cylinder 125, it would start great but after 5 minutes it would stop and not restart . Turned out to be that there was no valve clearance and when the push rods got warm they got longer and kept the valves open . 8 to 10 though clearance and it never stopped again .

[A10 JWO]

Sounds electrical, check spark as soon as it stops. Likewise check fuel as soon as it stops. I have never had a blocked fuel cap in 50 years, but the answer is frequently used on all forums.. Good luck and keep us all updated.

[Hancock]

Checking plug when it's stopped it's a bit sooty indicating rich mixture. Screwed out pilot air screw as manual says. That's makes the bike fire in the silencer. Looked at Amal tuning process but don't really understand what they are saying.

[Hancock]

The bike doesn't have an air valve ,should it?

[royblackburn1@btinternet.]

Hi I had a similar problem, took bike for a run came back and it would not start again till the next day took it for a run again same thing happened but started next day after weeks of asking questions Classic Bike mag suggested I had a dry connection in the ammeter I bypassed this and it started even after a run bought a new ammeter job sorted. hope this might help

[Hancock]

What's the thinking behind that.thanks

[royblackburn1@btinternet.]

When Classic Bike explained it, it sounded like when the power went through  the ammeter it warmed up and broke the circuit and the bike would not start but  would the next day (I don't know why they used the term dry) just a thought

[Bees]

Tuning the carb is quite straight forward. It sounds more complicated than it is.

I read a link posted on here that sums it up simply.

You adjust the air screw to make the engine run as fast it will go. Then adjust the throttle stop to make it to run as slow as possible.

Repeat the the process 3 or 4 times. Worked a treat for me.

Your problem is a matter of trial and error. It's either fuel starvation, the spark breaking down as things get hot or it could be the valve clearances closing up as per the above.

Whats the history of this, Has the bike been running fine and things have changed or is it a re-built engine with teething troubles?

[Hancock]

History,had the bike about a year been doing bits and pieces but not been out on it, I had it ticking over nicely in the shed .thought all was ok .my son took it out for the first time and it stopped after about 5mins. What are the symptoms of a over tightened carb. Not that's what I've done but seems quite common. Thanks for all your help plenty to look at.

[AdrianS]

Not sure what you mean by an overtightened carb? If the carb flange is overtightened there is a risk of distorting the mounting flange which could introduce an air leak. This will affect the mixture but I doubt it would stop a running engine completely dead!

Check the basic - fuel in the carb when it cuts out and also that there is a spark. Could be a number of causes but need to find if it is fuel or electrics causing the problem. Electrics are often harder to sort out; could be a coil breaking down, condenser playing up, wiring breaking down when hot etc. If you are concerned about the fuel side, take the carb off and clean it out. It is a simple device but has fine jets and airways in it which have to be clean. Also check the flow from the tank.

[Hancock]

I cleaned the carb and it has new jets. I've checked the resistance of the coil ,ok. I'll keep tweaking things I guess.