Uneven Revving B40 - Any ideas ?

[Crazy Mike]

I've had my restored 1961 B40 for just 3 weeks so I'm still getting used to it !
Here's the problem . After starting it ticks over ok but if I 'flip' the throttle, the revs won't settle back down to a normal tick over speed and it 'races'. This has happened during a short ride and I'm sat at traffic lights with a racing tick over. The next set of lights it could be back to normal.  Flipping the throttle has no effect.

It's a new (300 miles) Amal 376/253 Monobloc Carb with in-line filter fitted to fuel line. I noticed the throttle was rather 'sticky' so this was removed and mechanism and cable was lightly greased. Both throttle mechanism and throttle cable are new and now feel fine. I removed the throttle slide and checked for any wear marks and gave a light clean. Both needles have been removed and blown through with no dirt apparent. No apparent air leaks from manifold. The float bowl is clean with new gasket fitted.
When running, the engine behaves fine - no misfires etc. It's a b****d to start, especially from warm but i'm still learning the knack ! Not sure if the starting has anything to do with the revs issue.

The only thing that has effect when it's over-revving, is a quarter / half a turn on the throttle stop adjust screw, which then drops the revs back to normal.
Any advice would be very welcome.
Thanks

[A10 JWO]

Sounds like the slide is sticking, but it also reminds me of the symptoms of an air leak on the carb or manifold. I would get the carb on the workbench and have a close look at it. If you need any spares call Hitchcocks, overnight delivery. Not too much wrong there my friend.

[Crazy Mike]

Thanks for the reply. A Sticky slide was what prompted me to initially check the same, which looked ok to me.
However, I may have found the cause ! I say May, as until I can thoroughly road test it, I can't be 100% sure.

When I acquired the bike, there was no air filter - just a hole in the battery / tool panel with the moulded fitting for a circular type filter. In a box of supplied bits, I found the metal mesh and retaining clip but no actual filter, so I decided to order the rubber connecting tube between the air intake of the carb and the hole in the battery box purely to try and add more originality. There is not much room to fit the hose so I cut it down to size and managed to squeeze it, sort of in place. The hole and the air intake do not truly align (as someone else on this forum was earlier querying) so I suspect this may have been 'standard' at manufacture. Needless to say, once the shortened rubber hose was in place, it may have been exerting certain pressure against the carb body and it's possible that this pressure may have caused the slide to stick occasionally.

I have now removed the hose, back to how it was and the revs drop back down to normal whilst stationary. Maybe it's my imagination but the bike now seems easier to start (probably my imagination !). The proof will be in the road test later this week I hope. I'll report back !

[AdrianS]

Good luck with your findings.

I can't see how the pressure from a bit of rubber hose can distort the carb enough o allow the slide to stick.
My carb and tinware hole for the airfilter did not align well at all so I use a bit of fork gaitor of a size that fits the carb flange and compresses against the tinware. It may not give a good airtight seal but I feel it is better than nothing! A lot of carbs were run without an air filter many years ago - in fact my C15 in 1977 never had an air filter and it ran fine on standard jetting as far as I can remember (though I remember going to Amal at Witton in Birmingham where one of the technicians opened up the air slide a little!)

[Crazy Mike]

Road test now completed. After first starting and running for a few hundred yards, the revs reverted back to a normal tick-over level when stationary but as the ride progressed, the revs would not return to normal tick over when stopped at lights etc and remained at a uncomfortably high level. So, it's back to square one !  I removed the tank to get access to the carb to remove the slider mechanism and all looks ok with no serious scoring or marks, although in my opinion, it feels like a stronger spring may be required to return the slider back to the top in a more positive manner.
I have no previous  experience of the Amal monobloc carb and would be interested to hear from others whether the slider returns quickly or is what I am experiencing, in fact normal. The carb is new'ish with new cables and throttle mechanism. The cables are not held too tightly by nylon ties and are positioned correctly.
I'm happy to remove and disassemble the carb if necessary but would rather avoid it if I can ! Any advice / thoughts gratefully received.
Thanks.

[ducati2242]

could be that the float height is a little high that would cause the problems you describe but so does worn valve guides and air leaks .

[johnan]

I like to hear an nice “click” when the slide drops down when I release the twist grip when the engine is not running.
When your engine/carb has warmed up, if you switch the engine off does the throttle slide drop down with an audible click? If not it would suggest that it is sticking and possibly causing your erratic idle.

Undoing the twistgrip screws and repositioning it on the handlebars, rotationally, has usually cured any stickiness . My carb. is quite new too, so it is fine.

I expect you have checked the above as part of your trouble shooting, but thought I would mention it anyway!

[Crazy Mike]

Thanks for the replies here. I have removed the carb and stripped it down but it all looks very clean with no dirt visible. Re-assembled with new gaskets and O-ring rubber, re-fitted making sure the flange nuts were not overtightened. It re-started just fine and after setting the air and throttle settings, it ticks over very consistent but on twisting the throttle and releasing, it still doesn't feel that the throttle return is as 'sharp' as I think it should be. In saying that, the revs do revert to tick-over speed. All this is in the garage and a further road test is needed. The throttle slide does make a solid 'click' as it hits the bottom but it just seems a bit slow but perhaps I'm being unrealistic and comparing it to my other bike BMW r1150r. which snaps back quickly.

However, whilst playing with the carb it made me think about how this carb actually works, in particular the throttle slide / needle and the integral choke slide. The choke slide puzzled me somewhat as my understanding is that the choke restricts the air supply when cold, enriching the mixture thus improving starting. With the choke lever on the handlebars (RH side) in the 'off' (lever to left) position, the choke slide is actually 'down' in the carb body thus restricting air flow. Moving the choke lever on the bars to the 'On' (lever to the right) position, this then lifts the choke slide thus allowing full air flow.
Perhaps I've been sniffing too much petrol / exhaust fumes recently but I felt very confused !
I searched on line for the correct operation of the choke and ended up speaking to a very helpful person at Burlen (Amal) who informs me that what I thought was 'Off' is actually 'On' and vice-versa. It looks very odd to have the choke lever in the closed or 'Off' position and to have the choke slide restricting air flow and then having the choke control 'open', that is to the right so the inner cable is exposed to the elements etc and this being the 'normal' riding position ! Can anybody here with a B40, confirm what I have been told ? What's the operation on your choke lever ?

So, if what I have been told is correct, I have been running the bike with the choke 'on' all the time !! This might answer the other Rev problem in some way. Now waiting for dry weather until the next road test. More to follow ….

[berniej]

Hi Mike,

Yup! If the last time you saw a choke knob was (like me a few years ago) on the dashboard of some 70's motor from Leyland you would assume that you pull to choke and push when warm.

Just to confuse things more, that is the way that the Amal Mk2 works - but we don't need to bother with such new-fangled devices on our BSAs as I think they were only fitted to the later T140 Bonnevilles.

It does help to think of it as an air slide rather than a choke. Push/release to lower and restrict, pull to raise and open. Sounds like you've been running rich for a while so a good ride out with it open (if it ever stops raining) will hopefully sort things out for you.

Cheers,
Bernie

[Phil C]

I remember when I first got my (1965) B40, trying to work out why I had a black sooty plug. It turned out that what I thought was the open setting for the choke was actually closed, and of course vice-versa. I had been using the lever the wrong way round. Took ages, and much discussion on this forum, before I realised one day when I peeped into the carb.  Phil.

[chaz]

Hi Mike,

Yup! If the last time you saw a choke knob was (like me a few years ago) on the dashboard of some 70's motor from Leyland you would assume that you pull to choke and push when warm.

Bernie

not so far back, my A50 chop has a Mikuni fitted with a choke knob.

[Crazy Mike]

Thanks all. Yes Bernie, you are about right there ! My sanity is just about restored ! Now I'm really looking forward to a test ride with the choke Off ! (or is it On ??)
Will it sort out the high revving ?? We shall see !

[Willie Dee]

Ahhhhh, really? Choke lever to the right and slider up=choke off and lever to the left and slider down=choke on??? This could explain a lot.....

[johnan]

I am happy with how my air slide operates ie slider up means choke is off, but it seems my handlebar lever works differently to the general rule , and the RR book.

My slide is up, or off, when the lever is fully anti-clockwise and down when the lever is fully clockwise!
That is the opposite of what some people have said.

Or am I misinterpreting what has been written here?

I don’t use it anyway, a float tickle is enough for starting, it just seems I have a “backward” lever.

[berniej]

John, it may be a lever that was designed to go on the left of the bars - maybe an advance/retard lever that is now used as a choke lever?

As long as you remember that pull cable = choke off you should be fine!

Cheers,
Bernie