Amal MK1 Pilot jet and 4S Camshaft

Persohn

Persohn

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Apr 26, 2019
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Hi all ! Looking for infos about 4 S camshaft and MK1 pilot jet. Found nothing about this here...

The main question is: is it correct (or not) to think that with a bigger inlet aperture, you have to increase the pilot jet size ?

As you know, the idle circuit is important until 1/3 opening throttle valve.

Thanks !
 
The pilot circuit is separate from the main carb venturi so you would not expect a larger carb to need a pilot jet change. If you look at the old Amal specs per machine 99% of 4 strokes used a 25 pilot jet equivalent to premier pilot 17, if you go premier the more tortuous route the fuel takes makes a premier pilot 19 more likely to work than a 17.
 
kommando said:
The pilot circuit is separate from the main carb venturi so you would not expect a larger carb to need a pilot jet change. If you look at the old Amal specs per machine 99% of 4 strokes used a 25 pilot jet equivalent to premier pilot 17, if you go premier the more tortuous route the fuel takes makes a premier pilot 19 more likely to work than a 17.
Click to expand...
I don't have a vast experience with Premiers, but on a bike currently in my garage, the LH carb is a Premier with a #17 pilot, while on the RH side is a Wassel Evolution with #19. The former carb requires around 1 turn out on the air screw, and will still spit on occasion when opening the throttle. I find this an issue with the few Premiers I've worked with. The Wassel requires around 1,5 turns out as per the original Concentrics, and works really well.

SR
 
tri-man said:
I don't have a vast experience with Premiers, but on a bike currently in my garage, the LH carb is a Premier with a #17 pilot, while on the RH side is a Wassel Evolution with #19. The former carb requires around 1 turn out on the air screw, and will still spit on occasion when opening the throttle. I find this an issue with the few Premiers I've worked with. The Wassel requires around 1,5 turns out as per the original Concentrics, and works really well.

SR
Click to expand...
Why don’t you fit a #19 in the Premier ?
 
Hi Eddie, the owner was in a hurry, I didn't have time to do it when I fitted the carbs. May do it now that it's back, but it runs pretty nice anyway.
It's a 71 Bonnie with a TR6 head and a Webco twin carb manifold. Sounds backwards maybe, but it goes better than a "proper" Bonnie. The smaller ports suits a 650 better I think.
Sorry, not a Norton...

SR
 
tri-man said:
Hi Eddie, the owner was in a hurry, I didn't have time to do it when I fitted the carbs. May do it now that it's back, but it runs pretty nice anyway.
It's a 71 Bonnie with a TR6 head and a Webco twin carb manifold. Sounds backwards maybe, but it goes better than a "proper" Bonnie. The smaller ports suits a 650 better I think.
Sorry, not a Norton...

SR
Click to expand...
I would do so Tri-man. I run #19s in my 68 T120, I just could not get the #17s to work. The Trident boyz even fit #19s and they’re only feeding a 250cc cylinder.
 
Thanks guys. My question was about 4s vs STD camshafts for the idle circuit.
It's something hard to understand for me....:confused:
 
See post 2, it will make no difference, but your idle will be less reliable/lumpy purely from the extra overlap on the cam.
 
Thank you for your answers.
I had the opportunity to try a single carb 34 mm MK 2 fitted on a RGM ( Stewart) pipe.
What I saw is when I fit a bigger pilot jet ( 25 vs 30, even 32) it's better and the bike is more soft on the throttle and
maybe more torque. At this point , I want to try for more kms with this single carb as it is more pleasant to drive.
Probably in high rpm, the loss of power is more significant...
 
Persohn said:
Thank you for your answers.
I had the opportunity to try a single carb 34 mm MK 2 fitted on a RGM ( Stewart) pipe.
What I saw is when I fit a bigger pilot jet ( 25 vs 30, even 32) it's better and the bike is more soft on the throttle and
maybe more torque. At this point , I want to try for more kms with this single carb as it is more pleasant to drive.
Probably in high rpm, the loss of power is more significant...
Click to expand...
If I am reading you correctly, you have a 4S cam installed with a single 34mm Mk1 Amal carb.

This doesn't sound like the prospect for a happy marriage. The 4S isn't really working well until 4000rpm, and the single carb will limit performance over 5000rpm.

If you were starting from an engine build, many of us would recommend that a 'softer cam', for example a standard cam, would be a better combination with your single carb. And in any case a better choice for road riding. You would have a much nicer power delivery up to 5000rpm.

Changing to a single carb would seem to be aimed at achieving the opposite of the high lift, wide overlap cam.

But I understand changing the cam is a lot of work and expense with the 4S already installed.

Whereabouts in France are you located?
 
Indeed, there may be contradictions in this marriage... But I bought the bike with this camshaft. It's a 750 S, and I'm not a fan of high revs, even though 5000 rpm is in the max torque range, and I never go there.
I'm actually not happy with how the Premier MK1's work, that's why I was also looking into idle jets size, and then I had the opportunity to try the MK 2, and the bike just works better in the revs I use it. I'm not looking for performance, but driving pleasure.
 
Persohn said:
Indeed, there may be contradictions in this marriage... But I bought the bike with this camshaft. It's a 750 S, and I'm not a fan of high revs, even though 5000 rpm is in the max torque range, and I never go there.
I'm actually not happy with how the Premier MK1's work, that's why I was also looking into idle jets size, and then I had the opportunity to try the MK 2, and the bike just works better in the revs I use it. I'm not looking for performance, but driving pleasure.
Click to expand...
I think I'd consider fitting a standard camshaft
 
The timings on a standard 850 cam are still quite hot. With separate exhausts, I would expect a surge of power at about 4,000 revs, even with a single carb - with more over lap, the surge might be a bit higher in the rev range. For low speed riding around towns a slightly restrictive 2 into 1 exhaust system might be better. But it is not always easy to fit one, unless it can go under the motor.
 
Does anyone know the correct tank level on an MK 2?

I know on the MK 1 it's between 4mm and 6mm, but on the MK2 Amal doesn't say anything about it.
I read somewhere that the top edge of the float may match the gasket face of the bowl, but I'd like to be sure.

Acotrel, I own a Gus Kuhn 2 in 1 complet exhaust, but you need to remove the centre stand, and the noise is a little bit "too much"...
 
Hi Persohn
I ran a 34mm Mk2 for a while worked great but I found it hard to start!
Later ran a 38mm Dellorto unfortunately I didnt persevere. Kept nicking bits for racing.
Ran my Mk3 Seeley 850 on a pair of Dellorto 34mm pumpers, fabulous but I never really fully sorted them.
Keep at it.
 
Chris said:
Hi Persohn
I ran a 34mm Mk2 for a while worked great but I found it hard to start!
Later ran a 38mm Dellorto unfortunately I didnt persevere. Kept nicking bits for racing.
Ran my Mk3 Seeley 850 on a pair of Dellorto 34mm pumpers, fabulous but I never really fully sorted them.
Keep at it.
Click to expand...
I ran 34mm smoth bore and standand twin carbs for many years with 4s and 7s cam and never had a problem with starting always second kick from cold after the choke was applied...The problem i had was with the slides sticking at near full throttle ...not nice....which is why I changed them out for Keihin 35mm pumpers which I have found to be nice instruments...a friend of mine (ex army engineer) had reasonable success with a single Delorto twin choke side draft on his 920... it ran like a clock...
 
Chris, my Seeley for the track is equipped with MK 2 34, they were fitted when I bought it, and they work very well, so I didn't touch them.

KiwiShane, I want to check the float level of this MK 2 of 34 for this Commando S with which I never drive very fast, so I will probably never be at full throttle.

But I haven't found anything yet on the tank level for MK 2...Even on Burlen website.
 

Concentric mk2 float level

I've set a fair few float levels in various carbs, but I can't get the Mk2 Concentrics on my T140E right. I can't get the floats level. I've adjusted (tapped down) the brass seats as far as they'll go, but the floats are still on an angle down, sloping away from the pivot pin perhaps 1.5mm...
www.britbike.com www.britbike.com

If it could help you ..............
 
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