Carburation issue

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Tom P said:
Well, New enrichener fitted along with new chromed brass slide and for good measure I replaced the standard float with a new stay up one. Glad to say that after a bit of tinkering with the airscrew it runs better than ever. I will reserve judgement on the cold idling for the moment as it idled fine when I fired it up, but the airscrew was set on the rich side. In addition I had a lovely ride down to the coast for a bag of chips in the sun. Thanks for all your help and advice.
Click to expand...
Brilliant
 
Fast Eddie said:
The hole is flowing fuel, not air, keep the plunger close to the hole and it’ll reduce the flow.

I assume…
Click to expand...
That hole is flowing fuel AND air.
The cable rig allows a talented user to BARELY unseat the plunger, allowing a less than full-tilt-bozo rich mixture. We use a THREE position cable actuater on sleds/ATV's. Works great.
 
concours said:
That hole is flowing fuel AND air.
The cable rig allows a talented user to BARELY unseat the plunger, allowing a less than full-tilt-bozo rich mixture. We use a THREE position cable actuater on sleds/ATV's. Works great.
Click to expand...
Well, that first line got me thinking… and looking…

I only have a pair of mk1.5 Amals on the shelf at the moment, I believe the cold start mechanism is the same as the MK2.

Actually, the circuit does indeed supply both air and fuel to the main throttle body / choke. But the rubber seal only seals the hole that supplies fuel.

The air passage joins the plunger body ’chamber’ above the fuel seal point.

I had never noticed this before, but it prompts some thought…

1. The air passage is not actually ‘sealed’ off. It is only ‘blocked’ by the body of the plunger. Therefore, a worn plunger body or plunger chamber bore will presumably allow air to pass, even when the cold start circuit is ‘off’ thus effecting carburation.

2. A worn / damaged / hardened rubber plunger pad will allow extra (almost neat) fuel to be sucked into the throttle body / choke, even when the cold start circuit is ‘off’ thus effecting carburation.

3. As the lever is operated and the plunger passes the air passage, it will clearly have a huge impact on the air that can pass unimpeded through the passage. The passage will only be fully open when the plunger is lifted sufficiently high, and I’m guessing that will be close to the max lift of the lever. There is a LARGE range of movement below that point where the plunger will still allow full fuel flow, but limited air flow. So there is actually a large range of lever movement whereby the mixture is greatly effected.

4. Basically, the higher the plunger, the max air flow = the weaker the cold start circuit mixture will be. The lower the plunger, the more obstructed the air flow = the richer the mixture, up to the point where the air flow will be reduced to the point that the cold start circuit mixture will be very, very rich indeed (virtually only adding neat fuel).

So, that little cable operated lever has a huge impact! As does how the cables have been set up, what the free play is, if both plungers are synched (with twin carbs) etc.

Perhaps that’s why the original design was a fixed on / off… to remove such variability ?!

No freakin’ wonder there’s so much variation between different bikes !!
 
Fast Eddie said:
Well, that first line got me thinking… and looking…

I only have a pair of mk1.5 Amals on the shelf at the moment, I believe the cold start mechanism is the same as the MK2.

Actually, the circuit does indeed supply both air and fuel to the main throttle body / choke. But the rubber seal only seals the hole that supplies fuel.

The air passage joins the plunger body ’chamber’ above the fuel seal point.

I had never noticed this before, but it prompts some thought…

1. The air passage is not actually ‘sealed’ off. It is only ‘blocked’ by the body of the plunger. Therefore, a worn plunger body or plunger chamber bore will presumably allow air to pass, even when the cold start circuit is ‘off’ thus effecting carburation.

2. A worn / damaged / hardened rubber plunger pad will allow extra (almost neat) fuel to be sucked into the throttle body / choke, even when the cold start circuit is ‘off’ thus effecting carburation.

3. As the lever is operated and the plunger passes the air passage, it will clearly have a huge impact on the air that can pass unimpeded through the passage. The passage will only be fully open when the plunger is lifted sufficiently high, and I’m guessing that will be close to the max lift of the lever. There is a LARGE range of movement below that point where the plunger will still allow full fuel flow, but limited air flow. So there is actually a large range of lever movement whereby the mixture is greatly effected.

4. Basically, the higher the plunger, the max air flow = the weaker the cold start circuit mixture will be. The lower the plunger, the more obstructed the air flow = the richer the mixture, up to the point where the air flow will be reduced to the point that the cold start circuit mixture will be very, very rich indeed (virtually only adding neat fuel).

So, that little cable operated lever has a huge impact! As does how the cables have been set up, what the free play is, if both plungers are synched (with twin carbs) etc.

Perhaps that’s why the original design was a fixed on / off… to remove such variability ?!

No freakin’ wonder there’s so much variation between different bikes !!
Click to expand...
There is a Triumph or AMAL (don't remember which) bulletin telling you to elongate the hole in the link (71-7053) for the far carb to ensure that when the lever is up, the plunger on that carb is all the way down. It does fix a big problem - add that to your pipe and smoke it :) Since the carbs on a T140E are rubber mounted, the distance between them varies slightly and without the slot it's a crap shoot which enricher will close first and which can stay open. The side-effect is that the two enrichers are not synchronized and you must open all the way.
 
marshg246 said:
There is a Triumph or AMAL (don't remember which) bulletin telling you to elongate the hole in the link (71-7053) for the far carb to ensure that when the lever is up, the plunger on that carb is all the way down. It does fix a big problem - add that to your pipe and smoke it :) Since the carbs on a T140E are rubber mounted, the distance between them varies slightly and without the slot it's a crap shoot which enricher will close first and which can stay open. The side-effect is that the two enrichers are not synchronized and you must open all the way.
Click to expand...
Greg,
You are referring to Triumph Service Bulletin # RB793. I just looked at it for the first time in years. Interesting to note that Euro models have different jets, needles and slides, including a #35 choke [aka enrichener] jet.

I tried to attach the SB with this post using PostImage, but it would not let me [JPG files]
 
chaztuna said:
Greg,
You are referring to Triumph Service Bulletin # RB793. I just looked at it for the first time in years. Interesting to note that Euro models have different jets, needles and slides, including a #35 choke [aka enrichener] jet.

I tried to attach the SB with this post using PostImage, but it would not let me [JPG files]
Click to expand...

Yes… different jets… but also…

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:

Carburation issue
 
Regarding the discussion about the Amal Mk2 carb, Les Emery was adamant in his conviction twin Mk2 carbs could never be made to work when fitted to the Commando.
Right or wrong? Maybe the knowledge base has expanded in a couple of decades. It seems like a topic not well understood in the past, which is strange.
So, is the Amal Mk2 too advanced or too poorly engineered to enable them to be properly tuned for a Norton 750/850?

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- Knut
 
As a user of a Commando in the UK or America, do you tune your motor to get the best performance, or to meet the emissions requirements ? Do you tune it to meet noise requirements or to go better ?
I am wondering about the comment about not being able to get Mk2 Amals to work properly on Commandos. The jetting for best performance is lean to a point just before causing internal damage to the motor, and still be safe. The slightest bit too rich will take the edge off the motor. Emissions are probably affected by both high combustion temperatures due to lean mixture, and poor combustion due to rich mixture.
I use Mk2 Amal carbs because most Mikuni carbs have complex needle jets which you cannot make on a lathe at home, out of a piece of brass hex. Mikunis are much more expensive to tune, but they are probably better.
If the enrichener leaks, it is game over. You will never get the jetting right.
 
Last edited:
acotrel said:
As a user of a Commando in the UK or America, do you tune your motor to get the best performance, or to meet the emissions requirements ? Do you tune it to meet noise requirements or to go better ?
I am wondering about the comment about not being able to get Mk2 Amals to work properly on Commandos. The jetting for best performance is lean to a point just before causing internal damage to the motor, and still be safe. The slightest bit too rich will take the edge off the motor. Emissions are probably affected by both high combustion temperatures due to lean mixture, and poor combustion due to rich mixture.
I use Mk2 Amal carbs because most Mikuni carbs have complex needle jets which you cannot make on a lathe at home, out of a piece of brass hex. Mikunis are much more expensive to tune, but they are probably better.
If the enrichener leaks, it is game over. You will never get the jetting right.
Click to expand...
I tune mine as lean as I dare go
But I cheat I use a Koso narrow band fuel air guage to set it up
I have my throttle drum marked and I make a note of the reading at each throttle position and adjust accordingly
I'm not sure what the law is on decibels for a pre bsau 193 marked exhaust which all commandos are ,this bsau 193 was introduced in the 1980s
 
Fast Eddie said:
Well, that first line got me thinking… and looking…

I only have a pair of mk1.5 Amals on the shelf at the moment, I believe the cold start mechanism is the same as the MK2.

Actually, the circuit does indeed supply both air and fuel to the main throttle body / choke. But the rubber seal only seals the hole that supplies fuel.

The air passage joins the plunger body ’chamber’ above the fuel seal point.

I had never noticed this before, but it prompts some thought…

1. The air passage is not actually ‘sealed’ off. It is only ‘blocked’ by the body of the plunger. Therefore, a worn plunger body or plunger chamber bore will presumably allow air to pass, even when the cold start circuit is ‘off’ thus effecting carburation.

2. A worn / damaged / hardened rubber plunger pad will allow extra (almost neat) fuel to be sucked into the throttle body / choke, even when the cold start circuit is ‘off’ thus effecting carburation.

3. As the lever is operated and the plunger passes the air passage, it will clearly have a huge impact on the air that can pass unimpeded through the passage. The passage will only be fully open when the plunger is lifted sufficiently high, and I’m guessing that will be close to the max lift of the lever. There is a LARGE range of movement below that point where the plunger will still allow full fuel flow, but limited air flow. So there is actually a large range of lever movement whereby the mixture is greatly effected.

4. Basically, the higher the plunger, the max air flow = the weaker the cold start circuit mixture will be. The lower the plunger, the more obstructed the air flow = the richer the mixture, up to the point where the air flow will be reduced to the point that the cold start circuit mixture will be very, very rich indeed (virtually only adding neat fuel).

So, that little cable operated lever has a huge impact! As does how the cables have been set up, what the free play is, if both plungers are synched (with twin carbs) etc.

Perhaps that’s why the original design was a fixed on / off… to remove such variability ?!

No freakin’ wonder there’s so much variation between different bikes !!
Click to expand...
Great research work Nigel! Again, in my haste ;)had defaulted to my primary "language", which is "fluent Mikuni". So, my comment was inaccurate as it did not pertain to Amal MK2.
 
baz said:
snipped
I'm not sure what the law is on decibels for a pre bsau 193 marked exhaust which all commandos are ,this bsau 193 was introduced in the 1980s
Click to expand...
Baz, please translate for us Colonials. I have no idea what bsau 193 means.
 
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