mk 1 amal pilot jet

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I want to fit a pilot jet into the original vacant hole after drilling and removing the fixed tube pilot. it is on. a mk1 Amal Atlas 750 engine in the P11. twin carbs are fitted. what size pilot jet should I use as there are various different sizes available. thanks in advance
 
25 was the default for 4 strokes, you may have some issues with the pilot jet being in the vacant hole as that was for 2 strokes. After the introduction of the Concentric the 4 stroke pilot jet was quickly moved from the vacant hole to the bush due to issues on the Triumph 500 twin.
 
Shouldn’t be a problem fitting 25 pilot jet in vacant hole. Yes it is used on 2 stroke carbs but unless you’re looking for last bhp it will be ok. It is quite common as the only way to clear muck or swarf out of the cavity behind the pilot bush is to drill it out and use the screw in jet.
 
25 was the default for 4 strokes, you may have some issues with the pilot jet being in the vacant hole as that was for 2 strokes. After the introduction of the Concentric the 4 stroke pilot jet was quickly moved from the vacant hole to the bush due to issues on the Triumph 500 twin.
That's interesting, seeing as the Premiers have re-introduced the detachable pilot jet, albeit slightly different in design. What was the original issue thrown up by the 500 Triumph?
 
Starting, idle and pickup from idle, jet was too far away to get the full signal from the engine. Premier keeps the position nearer the engine as the pilot bush but detachable for regular cleaning.
 
A
Do you mean Norton 850 Mk II or Amal Mk II?
Amal. But are they not the same? I thought the Norton 850 MKII came with Amal MKII carbs. Anyhow, I have several versions of Amal carbs on various bikes and wonder if I could update them to use Premier idle jets. They are in good enough condition that I'm not anxious to replace them.
 
Nortons from 1967-8 came with Amal Mk 1 carburetors. They were never sold with Amal Mk II carbs. (Although Triumphs were).
 
A

Amal. But are they not the same? I thought the Norton 850 MKII came with Amal MKII carbs. Anyhow, I have several versions of Amal carbs on various bikes and wonder if I could update them to use Premier idle jets. They are in good enough condition that I'm not anxious to replace them.
No commandos came with MK2 amal carbs
Are you thinking of a triumph t140e ?
 
My very truncated notes on converting an Amal Concentric MK1 to premier spec.

Converting an old but unworn Concentric to Premier condition.

Revised removable pilot jet

1. Press out old pilot bush from the air screw side using a 2mm punch, the Pilot jet has a 2mm recess on this side and is 2.4mm overall diameter. Retrieve pilot bush and set to side.

2. From current blocked side using a 4mm drill drill out the new opening, stop when the drill comes to free air, then change to a 4mm end mill and make the hole a total of 0.825" or 20.95mm deep, the end mill creates a seat for the premier jet tapered tip to seal against.

3. Tap first 4mm section to 2BA, do not tap the second 4mm section or you will lose the jet tip seal.

4. Clean out and install Premier pilot jet.

It all hinges on centralising the first 4mm hole and being able to stay in that fixed position so best done on a vertical mill with DRO.

The old 25 pilot jet is a 17 thou hole so Amal use a 17 Premier pilot jet but the new routing appears to lean out the jet so the fix is to use a 19 Premier pilot jet.

Slide

Fit either an Amal Anodised slide or a Surrey Cycles brass slide with chrome plate.

Float

Fit the 'Stay up Float'

Float Needle
Amal fit an aluminum rubber tipped float jet.


I do mine on a Bridgeport Vertical mill with DRO using a 4mm milling cutter, this allows an accurate centering on the cast ring on the body. If you use a pillar drill then there is a greater risk of not getting the hole in the right position and the new premier pilot jet not sealing on the internal hole where the bush originally sat.
 
My very truncated notes on converting an Amal Concentric MK1 to premier spec.

Converting an old but unworn Concentric to Premier condition.

Revised removable pilot jet

1. Press out old pilot bush from the air screw side using a 2mm punch, the Pilot jet has a 2mm recess on this side and is 2.4mm overall diameter. Retrieve pilot bush and set to side.

2. From current blocked side using a 4mm drill drill out the new opening, stop when the drill comes to free air, then change to a 4mm end mill and make the hole a total of 0.825" or 20.95mm deep, the end mill creates a seat for the premier jet tapered tip to seal against.

3. Tap first 4mm section to 2BA, do not tap the second 4mm section or you will lose the jet tip seal.

4. Clean out and install Premier pilot jet.

It all hinges on centralising the first 4mm hole and being able to stay in that fixed position so best done on a vertical mill with DRO.

The old 25 pilot jet is a 17 thou hole so Amal use a 17 Premier pilot jet but the new routing appears to lean out the jet so the fix is to use a 19 Premier pilot jet.

Slide

Fit either an Amal Anodised slide or a Surrey Cycles brass slide with chrome plate.

Float

Fit the 'Stay up Float'

Float Needle
Amal fit an aluminum rubber tipped float jet.


I do mine on a Bridgeport Vertical mill with DRO using a 4mm milling cutter, this allows an accurate centering on the cast ring on the body. If you use a pillar drill then there is a greater risk of not getting the hole in the right position and the new premier pilot jet not sealing on the internal hole where the bush originally sat.
Do you mean drill the 4mm hole in the blocked side first?
As I'm not understanding how you remove the pilot jet bush once you have drifted it out?
 
Do you mean drill the 4mm hole in the blocked side first?
As I'm not understanding how you remove the pilot jet bush once you have drifted it out?
Too long ago for me to remember if I drilled the hole first and then pressed the bush out or pressed it out first. All I can remember is the bush too some force to push it out and I got to it from the mixture screw side with the drift and hammer.
 
It has been 50 years since I ran a Bridgeport vertical mill. I wish I had one. Seems to me that several Amal premier carbs might be less expensive than a good used Bridgeport. it does not sound like something I could do with a drill press.
 
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