Fitting an overflow to a Concentric.

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I think vibration at some rpm is making a little fuel get past the float valve on at least one of the twin Amal Mk1s.

The only symptom is a brief splutter coming off a longish, fastish period of over-run.

I’m somewhat minded to experiment with a spillway or overflow at a level that prevents the suspected flooding. I suppose it has to be lower than the horizontal pilot and compensation air drillings in the carburettor body.

I think people on Accessnorton have experience of just about every modification that the human mind can devise and someone has probably tried something like this and can advise, disparage, or whatever.

Pity it’s not a Commando- I’d have a bigger audience. 😃
 
What float needles do you have?
As I have heard the brass ones can be problematic
And some people go for the aluminium ones as opposed to the original plastic
I was wondering if that could cause your symptoms
 
I think vibration at some rpm is making a little fuel get past the float valve on at least one of the twin Amal Mk1s.

The only symptom is a brief splutter coming off a longish, fastish period of over-run.

I’m somewhat minded to experiment with a spillway or overflow at a level that prevents the suspected flooding. I suppose it has to be lower than the horizontal pilot and compensation air drillings in the carburettor body.

I think people on Accessnorton have experience of just about every modification that the human mind can devise and someone has probably tried something like this and can advise, disparage, or whatever.

Pity it’s not a Commando- I’d have a bigger audience. 😃
If you have StayUp floats you could try lowering the fuel level a little. At higher speeds, if a little gas is leaking by, it shouldn't hurt anything if it's really a little and if the fuel level is a little lower more could leak by without hurting.

You could try polishing the seat and replacing the needles since, AFAIK, what you think is happening is not a known issue. You could also swap the two bowls and see if it changes.
 
Check the floats' fuel levels. Book says what/where it should be
You can do that with clear tubing connected to the inside so that you can see the fuel's level.
Or, just try lowering the fuel level. Bend/adjust to lower it. Risk of too low is you could go too lean on a long wide open throttle run.
 
All sensible answers thanks. It’s hard to investigate a problem which only lasts a second. I guess it could be investigated on a dynamometer with air/fuel reading.

I think I can get my hands on at least one old float bowl, float and needle to try substitution.
 
As English is my second language, I'm not certain if I understand fully what the symptoms are. Could it be lack of fuel instead?
 
I found that the aluminum float needles didn't have enough weight to seal reliably.
I went back to brass rubber tipped needles and they work fine.
Jaydee
 
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