BSA 1955 B31

[Graham M Cherrett]

I have recently purchased a 1955 B31 ad I am having a problem with the carb in as much as it will not shut down although the slider is at its bottom most position, it has new gaskets all round  but I suspect air is getting in from somewhere, or could this be another problem.

[R]

Check that your ignition timing is not overly advanced, before chasing carb problems.
Did BSA have auto advance by 1955, could it be sticking at full advance ?

Try tickling the float chamber when this happens, just to rule out that the carb is starving for fuel.

Otherwise it could well be a very worn slide in the carb, allowing too much air in at idle.
This can be indicated by the idle mixture screw being nearly all the way in,
instead of at the suggested 1 & 1/2 turns out.
hopethishelps.

[Graham M Cherrett]

Thanks for the reply I have tried all of the advice given and even replaced the carb, makes very little difference now I am totally stumped.

[iansoady]

Are you absolutely sure the slide isn't hanging on the cable ie not fully bottoming? Unscrew the throttle stop completely then sharply close the twistgrip. You should hear a clunk as the slide hits the bottom of the mixing chamber.

[Graham M Cherrett]

Thanks I will give it go

[Rex]

I have recently purchased a 1955 B31 ad I am having a problem with the carb in as much as it will not shut down although the slider is at its bottom most position, it has new gaskets all round  but I suspect air is getting in from somewhere, or could this be another problem.

When you say "not shut down" do you mean the tick-over stays too high or the engine does not stop (some people like the engine to stop on a totally closed throttle...I don't know why but there you go)

[Graham M Cherrett]

From tick over the engine speed increases for no obvious reason to an extremely fast tick over, then when revved up it won't drop back to normal tickover,also when revved up it stays revved up with the slide right down. It runs best with the mixture screw right in. Could it be worn inlet valve guide?

[R]

If the inlet guide was worn that much, it would likely also start up with copious clouds of blue smoke.
Replaced a guide recently for this very reason.
It did affect the tuning, but not to that extent.

It does sound to be running very lean.
Have you investigated the float height, and for blocked fuel/air passages in the idle system.
Is the breather in the fuel cap clear, and good strong flow from the fuel taps and pipes.
What did tickling the float do when it shows this problem.

[Graham M Cherrett]

Got day to myself tomorrow, I am going check and try all advice given in one day, I thank every one for the advice given. The nextt problem is a non charging G3,  but it that can wait a bit.

[R]

If you haven't checked it out yet then, 1st thing to look at is that an automatic advance unit
(if fitted ?) is actually retarding at idle.

[Graham M Cherrett]

Spring weights are free and well oiled, carb changed back to original, have noticed when it is running extremely fast tick over if I depress the tickler the engine stops!, also noticed today that although full operational the choke has no effect at all.

[R]

Are you doubly sure the slide is not hanging on the throttle cable ?
It would be very strange indeed if the slide can be fully bottomed out and the engine will keep running.
If you wind the slide stop screw right out, can you hear the click as the slide hits rock bottom.

Does the idle mixture screw alter the idle when you adjust it in and out ?

Amals are rather prone to the little fuel and air passages becoming corroded or gunged up.
It can take a lot of cleaning to get them all clear.
In the meantime, if the carb has been adjusted to idle on the needlejet, it might do like you say.
But dropping the slide right down should kill it stone dead.

What happens if you turn off the fuel, and let it idle and run the carb dry.

[R]

P.S. Someone on another forum  has just described a carb that was badly worn, and required the slide to be resleeved to bring back a reliable and adjustable idle.
It also would keep running with the slide down quite low - but required much choke when cold.
Curious and curiouser ?

[dunboyne]

most likely well fixed but an air leak on the intake manifold will do this .

[Foghorn]

Re my carby post- I have been fiddling with the carby on my 1951 B31.  I did find an odd thing or two:
After removing the jet block (by tapping it down out the bottom of the slide tunnel, I found very slight deformations at the point where the vertical slots in which the throttle slide runs, and this was causing the slide to catch at that point.  I used a 3 square scraper to gently remove the high spots found there.  The throttle slide slipped freely through the entire length of the slide tunnel then, and also freely over the jet block.
When I re-inserted the jet block and then slid the throttle slide down into the slide tunnel, it slipped nicely over the jet block as it should.
However, when I tightened the large nut over the bottom of the jet block, a distortion occurred so as to cause the throttle slide to jam when it was well down over the jet block.  By loosening the nut slightly the jammed slide was freed.  I think the carby body was being distorted ever so slightly.  So-
How tight should the nut be?
My carby is well used/worn. The carb had been dropped on the top of the slide tunnel as some time and there was a tight spot just at the top thread area so that the slide was tight until it cleared the very top edge of the tunnel (it didn't affect the slide through its operating range though).  When I tried to fit a new throttle slide I found I had to hone the tunnel bore to get it to fit, and even then It was sticky.  Eventually I went back to the original slide and I think that is why in my own post I am after a new carb.
I hope my story helps.
Foghrn.