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B175 pilot bush
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Bee175
Felixstowe, Suffolk
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September 25, 2020 - 5:17 pm
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Purely out of curiosity, does anyone know the size of the fixed pilot bush in a 626/17 B175 carb?

Thanks,

Gary.

BSA Bantam B175 1971 and BSA C15 1966

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mikef
Chatham Kent
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September 25, 2020 - 5:50 pm
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Hi Gary.
         Quoted as 25 from the information that I have.
Regards.
        Mike.

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Bee175
Felixstowe, Suffolk
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September 25, 2020 - 6:00 pm
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Thanks Mike, to be honest, that's what I thought.

So I wonder why Amal supply the premier carb with all the relevant 4 stroke bits, but the removable pilot jet is a 17?

Mine came with a 17 a few years back (changed to 21 as it worked better), and they still sell them with a 17 to this day.

I've read that 4 strokes using the 626 carb often have a 17 pilot. I wonder if that's where Amal have got it from.

BSA Bantam B175 1971 and BSA C15 1966

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Alan.Moore
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September 25, 2020 - 8:31 pm
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Gents

From what I understand the fixed pilot jet bush (part number 622/107 as shown in the B175 manual specs) is a 'standard / fixed size' in the MK1 concentric having a ''hole' diameter of 0.016". No alternatives are available. This is confirmed in that to clean them out you need to use a #78 drill which is 0.016" dia

The screw in type jets for MK1's have a different 'numbering/sizing' system and they are defined by the cc of fuel they will pass. The screw in jet for the D14 is shown as being 25 (as Mike says) so that's 25cc. Apparently it flows the same amount of fuel as the fixed 622/107 bush type jet. 

I understand that the premier pilot jets 'size' are quoted in relation to the diameter of its 'hole' so #17 is 0.017" but its flow rate is equivalent to the 0.016 bush and the 25cc MK1 screw in jet.

So if all of this is correct a premier with a #17 pilot is correct. 

I have also read that on premier carbs some folk say a #19 pilot jet works better.

Cheers

Alan

1939 Ariel VH, 1942 Ariel WNG, 1951 Triumph 6T Thunderbird, 1970 BSA Bantam B175, 1980 Honda CB250N, 1986 Yamaha SRX600

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Bee175
Felixstowe, Suffolk
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September 25, 2020 - 9:15 pm
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That's a terrific answer thanks Alan. Very nicely explained.

I can confirm that a 17 in my premier light weight carb didn't work well for me, and that a 19 was better, but even better than that was a 21 that I've had in for several years now.

It's nice to know how all these different jets worked, so thanks again for making it clear.

Gary.

BSA Bantam B175 1971 and BSA C15 1966

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cocorico
Central France
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September 26, 2020 - 7:47 am
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It's no wonder that tuning carbs has such a 'black arts' reputation with the variations in numbering / size systems used in reference to jets. I've read the guides, etc, but whenever I try to put it to use I usually end up at the 'Darrel Doo' stage. wink

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