Amal Mk2 Settings and Issue

Connie73 · Dec 27, 2020

Hi all,

Having an issue setting up some Amal Mk2's on an Atlas.

With the idle screws all the way out the engine response when bliping the throttle is great.
Once I set the idle if I blip the throttle again the revs stay high and won't come back down unless I either wind out the throttle stops or I choke the air supply. I have checked that the idle screws are not keeping the slides up,
I have new number 3 slides, 25 pilot jets, 106 needle, 220 main, needle in middle position. New cold start plungers have been fitted. There is a 35 jet in the cold start system.
Timing and AAU has been checked multiple times.
The float heights have been checked.
I have made custom inlet manifolds which I forgot to include a fitting for a balance tube.
Could the issue be that?
I have read elsewhere that if the carbs are inclined that it is better to have the pilot jet located in the mixing chamber body instead of in the float bowl.
Any suggestions as to what to change or adjust?
Thanks

Tornado · Dec 27, 2020

No idea on ho MK2's are setup, but one obvious thing to confirm...is the choke lever moved to correct position for choke off? It is a not so obvious position on the lever,m at least for MK1 amal...cable needs to pulled out the max at lever for chokes to be held high in the bores (off).

Connie73 · Dec 27, 2020

Tornado said:
No idea on ho MK2's are setup, but one obvious thing to confirm...is the choke lever moved to correct position for choke off? It is a not so obvious position on the lever,m at least for MK1 amal...cable needs to pulled out the max at lever for chokes to be held high in the bores (off).

Hey Tornado,
Thanks for the reply.
The cold start system plungers have been replaced and are seated correctly to eliminate and potential leak of fuel into The cylinder.

Matchless · Dec 28, 2020

Sounds like you have an air leak between the carbs & head. The pilots should be moved to the carb body if the fitted angle is over 15deg. or the bike is a two stroke. I've just built a pair of 30mm MK 2s for my Atlas but haven't tried them yet, but the spec. is the same as you have chosen execept for a 50 start jet.

Martyn.

cash · Dec 28, 2020

The choke/enrichment plungers work when lifted, the circuit bypasses the throttle slides and increases the tick over revs. The more you lift the throttle slides the less effect the enrichment circuit has.
It sounds like you've got the plungers lifted.

MichaelB · Dec 28, 2020

1. Possible crud holding choke plunger up.
2. Cracked/worn/missing O rings on pilot/idle screws.
3. Manifold leak.

Fast Eddie · Dec 28, 2020

What’s the history of the carbs, new or used? If used, what were they off of?

Schwany · Dec 28, 2020

The lack of a balance tube won't make the idle remain high. Make sure they are synced perfectly, or as close to perfect as possible. Air leak is certainly possible, but that usually results in a poor off idle response as well. Anyway, just wanted to say the balance tube has nothing to do with what you are talking about.

seattle##gs · Dec 28, 2020

I fought a pair of MK@ carbs on a bonneville, spending months pulling my hair out. I even went down the "European jetting" rabbit hole. I replaced the choke plungers. Every possible combination was tried.
WHAT I FOUND OUT: The enriching circuit is the culprit. If all attempts fail then block off any passageway where it can get air or fuel. I used a bit of tissue paper in two or three holes for a temporary blockage. Will require dropping the float bowl and stuffing the enriching hole. All this is a test and very temporary. The main airway is a 1/4" horizontal hole in the filter side of the carb. I did all this and suddenly gained control over the carb. If this is your problem I do not have a long term solution. I believe there was some real quality control problems with the MK2 carbs. Some work well and others are impossible. Now, if you are EXTREMELY CLEVER you could devise a tickler system to replace the choke plungers.

baz · Dec 28, 2020

seattle##gs said:
I fought a pair of MK@ carbs on a bonneville, spending months pulling my hair out. I even went down the "European jetting" rabbit hole. I replaced the choke plungers. Every possible combination was tried.
WHAT I FOUND OUT: The enriching circuit is the culprit. If all attempts fail then block off any passageway where it can get air or fuel. I used a bit of tissue paper in two or three holes for a temporary blockage. Will require dropping the float bowl and stuffing the enriching hole. All this is a test and very temporary. The main airway is a 1/4" horizontal hole in the filter side of the carb. I did all this and suddenly gained control over the carb. If this is your problem I do not have a long term solution. I believe there was some real quality control problems with the MK2 carbs. Some work well and others are impossible. Now, if you are EXTREMELY CLEVER you could devise a tickler system to replace the choke plungers.

I had a mikuni VM fitted with an Amal tickler not sure how well it worked as I never used it

Fast Eddie · Dec 28, 2020

Amal mk2 carbs have the option of having something called a ‘primary air jet’ installed. Amal and Triumph struggled hard to get the T140E through US emissions. They succeeded by fitting VERY strange jet combinations and a specific primary air valve.

Many don’t know about or ignore this primary air valve. If you wish to convert T140E spec mk2 carbs to ‘normal’ (ie none EPA) jetting, this primary air valve needs to be REMOVED.

Item # 41 in the diagram below:

Connie73 · Dec 28, 2020

Matchless said:
Sounds like you have an air leak between the carbs & head. The pilots should be moved to the carb body if the fitted angle is over 15deg. or the bike is a two stroke. I've just built a pair of 30mm MK 2s for my Atlas but haven't tried them yet, but the spec. is the same as you have chosen execept for a 50 start jet.

Martyn.

Martin, thanks for the reply. I can pull the carbs off and my hand is stuck to the rubber adaptor under vacuum. It's highly unlikely that there would be a leak between the carbs and the rubber seeming they are shaped to fit the groove on the carb inlet stub.

Connie73 · Dec 28, 2020

Fast Eddie said:
What’s the history of the carbs, new or used? If used, what were they off of?

They are used, not sure what they are initially off. Would any of the numbers give it away with the numbered components?

Connie73 · Dec 28, 2020

cash said:
The choke/enrichment plungers work when lifted, the circuit bypasses the throttle slides and increases the tick over revs. The more you lift the throttle slides the less effect the enrichment circuit has.
It sounds like you've got the plungers lifted.

I tested the enrichment circuit. The new plungers I received had a weaker spring pressure so I changed them over and tested the passage for leaks. I could push air past the plunger but not past the seal to the bowl. I got the bike running and there was vacuum at the enrichment intake. I then blocked them to no avail. I also moved the pilots to the car bodies. Are there any other passages the enrichment circuit draws from?

Connie73 · Dec 28, 2020

Fast Eddie said:
Amal mk2 carbs have the option of having something called a ‘primary air jet’ installed. Amal and Triumph struggled hard to get the T140E through US emissions. They succeeded by fitting VERY strange jet combinations and a specific primary air valve.

Many don’t know about or ignore this primary air valve. If you wish to convert T140E spec mk2 carbs to ‘normal’ (ie none EPA) jetting, this primary air valve needs to be REMOVED.

Item # 41 in the diagram below:

View attachment 20665

I initially did some reading about this. I have since removed it and it hasn't changed the running of the bike that I know of.

Connie73 · Dec 28, 2020

seattle##gs said:
I fought a pair of MK@ carbs on a bonneville, spending months pulling my hair out. I even went down the "European jetting" rabbit hole. I replaced the choke plungers. Every possible combination was tried.
WHAT I FOUND OUT: The enriching circuit is the culprit. If all attempts fail then block off any passageway where it can get air or fuel. I used a bit of tissue paper in two or three holes for a temporary blockage. Will require dropping the float bowl and stuffing the enriching hole. All this is a test and very temporary. The main airway is a 1/4" horizontal hole in the filter side of the carb. I did all this and suddenly gained control over the carb. If this is your problem I do not have a long term solution. I believe there was some real quality control problems with the MK2 carbs. Some work well and others are impossible. Now, if you are EXTREMELY CLEVER you could devise a tickler system to replace the choke plungers.

Thanks for the reply. Was the running on after the throttle shuts a symptom? The plunger and seals are sealing the port to the bowl but when I had it running I could feel vacuum in the enrichment inlet port. Do I need to block the main passage the the main jet, the one with the primary air jet?

Fast Eddie · Dec 28, 2020

Connie73 said:
I initially did some reading about this. I have since removed it and it hasn't changed the running of the bike that I know of.

Bugger.

The only other thing I can think of is that the foam floats with steel tangs can have sharp edges where they’ve been pressed out. This can catch on the float needle and cause running issues.

I’m clutching a straws here, but it’s a quick and simple thing to rule out.

Connie73 · Dec 29, 2020

Fast Eddie said:
Bugger.

The only other thing I can think of is that the foam floats with steel tangs can have sharp edges where they’ve been pressed out. This can catch on the float needle and cause running issues.

I’m clutching a straws here, but it’s a quick and simple thing to rule out.

Checked over them to and fitted new floats. Do you think the cutaway on the slides are an issue?

Fast Eddie · Dec 29, 2020

Connie73 said:
Checked over them to and fitted new floats. Do you think the cutaway on the slides are an issue?

Number 3 slides should be fine on a 750cc 4 stroke twin. 3.5 may suit it better, may not, but that’s into the realms of fine tuning and isn‘t your issue.

You haven’t mentioned the needle. What needle type do you have and which clip position are they in? Again, T140Es had odd ball needles that you don’t want.

And another possible needle issue is that the washer (#5 in the diagram above) must be correctly located under the spring and on top of the needle clip to hold it in place properly.

But the float tang thing... it is the new floats that have the issue. So fitting new floats isn’t the solution! A bit of dressing may be in order. Mine had a bur that had cut away a tiny slither of the float needle. A few minutes with a file and some emery is all we’re talking about here.

MichaelB · Dec 29, 2020

34mm Mk2's on a 750 Combat.
They are NOS, 15+ years old.
#3, 106, 25
Had wierd tuning issues, unstable idle.
Went down to a 20. Better, but.........
Completely depotted them, again.
Had a piece of grit on one choke holder, knackered O ring at pilot on other.
After clean up, new orings all around, back to 25, runs great

Edited.. No balance tube.

Amal Mk2 Settings and Issue

Connie73

Tornado

Connie73

Matchless

cash

MichaelB

"Sons of Arthritus"

Fast Eddie

Schwany

seattle##gs

baz

Fast Eddie

Connie73

Connie73

Connie73

Connie73

Connie73

Fast Eddie

Connie73

Fast Eddie

MichaelB

"Sons of Arthritus"

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