33mm mk1 amal
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The slide makes the venturi - a variable venturi too, no less.
Can't help thinking it wouldn't need the extra venturi built in.
Could be wrong.
But someone has probably tried this already - and discarded it as unjettable (?).
You'd need a big cam to want this, to see any difference ??
Can't help thinking it wouldn't need the extra venturi built in.
Could be wrong.
But someone has probably tried this already - and discarded it as unjettable (?).
You'd need a big cam to want this, to see any difference ??
lcrken
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Norton specified the Amal 32 bored out to 33 mm as standard on the short stroke 750. I bored out the 32 mm carbs that came on my PR to 33 mm, and used them for several seasons of racing. That was before I had access to a dyno, so I don't have comparative numbers, and I don't recall if I noticed the difference (that was 40 years ago). At least I can say they worked well enough that I kept using them.
Ken
Ken
Perhaps my comments/wording weren't entirely clear.
It was boring it as a venturi that I was referring to as possibly not a good idea.
Not that 1mm will make a huge difference either way.
The carbs had been bored to 33mm on the FACTORY hotrod 850 that Baker-Rawlins built,
and it did 142 mph at Elvington in 1973.
But it had had the full house hot-rod treatment = cams carbs compression blueprinted.
So this carb boring to 33mm has been around for a long time.
It was boring it as a venturi that I was referring to as possibly not a good idea.
Not that 1mm will make a huge difference either way.
The carbs had been bored to 33mm on the FACTORY hotrod 850 that Baker-Rawlins built,
and it did 142 mph at Elvington in 1973.
But it had had the full house hot-rod treatment = cams carbs compression blueprinted.
So this carb boring to 33mm has been around for a long time.
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This adem:
This explainsit :
BOTH , actually ! :wink: http://s181.photobucket.com/user/kcanag ... 5.jpg.html
This explainsit :
BOTH , actually ! :wink: http://s181.photobucket.com/user/kcanag ... 5.jpg.html
They don't look like concentric carbs, Matt. !!
But are Commandos taken to the (racing) max....
It was previously discussed here that 32mm concentrics were bored to 33mm ( as far as they could be taken),
because Amal didn't make 900 series concentrics any bigger than 32mm.
The 1000 series Amals are too wide to side side-by-side inside the Commando frame tubes,
unless you do extreme things with the manifolds.
But are Commandos taken to the (racing) max....
It was previously discussed here that 32mm concentrics were bored to 33mm ( as far as they could be taken),
because Amal didn't make 900 series concentrics any bigger than 32mm.
The 1000 series Amals are too wide to side side-by-side inside the Commando frame tubes,
unless you do extreme things with the manifolds.
lcrken
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Rohan said:
No, I think you were quite clear, Rohan. You were addressing the venturi part of his question,and I was addressing the other half of it.
And you are quite right about boring them to 33 being around a long time. I first heard about it in 1972 or 1973, but didn't try it myself until 1977.
Ken
Thanks for that. I actually have a spare pair of 30mm concentrics. There is 16mm in length behind the slide and that is where I thought about putting the venturi. I was thinking 20 degrees in and 5 degrees out for the venturi. So if I bore from the front of the carb to the rear of the slide at 33mm then bore the venturi 20 degree in and 5 degrees out starting at the rear the slide. The venturi will reduce the throat size down to 30.75mm. I guess there is no harm in trying as i can refit the 32mm concentrics.
Thanks again
ando
Thanks again
ando
lcrken
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Bernhard said:ando said:
You must take the diameter of the carb slide into account-if the hole is bigger there will be nowhere for the slide to be held into the carb body :!:
True, but in the case of the 900 series Amals (930 and 932) discussed here, we already know from experience that the slide is large enough that it still works with a 33 mm bore.
Ken
With all else staying the same, boring the carb - is very unlikely to improve performance overall on its own. The volume to be filed in the cylinder is the same, the only thing that changes is the speed at which you can fill it, engine speed, and thus the air speed through the carb. It is a trade off, smaller dia, more lift on the fuel, but not enough air to fill the cylinder or large dia, less lift on the fuel, but not enough fuel in the mixture. Best bet just change the jets to get similar effect which is a lot less hassle.
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The carbs on the John Player Norton are Amal GP's the advantage of these is that the needle is remote from the slide and doesn't compromise the airflow, also the float chamber can be remote and angled to suit a vertical orientation while the carb can be downward pointing.
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Theyre BOTH , ACTUALLY ,
The Dual Muffler ( Daytona ) has the bored concentrics. as is clear on the photobucket link .
Presumeably the concentrics @ 33 bore flow the same CFM as the G.P.s .
If you want your 750 to do 155 mph . . . you need 33 bore Concentrics !
Book I have refers to the carbs as 33 on that ( 72 ) J.P.N. , Phil Reads machine.
The Dual Muffler ( Daytona ) has the bored concentrics. as is clear on the photobucket link .
Presumeably the concentrics @ 33 bore flow the same CFM as the G.P.s .
If you want your 750 to do 155 mph . . . you need 33 bore Concentrics !
Book I have refers to the carbs as 33 on that ( 72 ) J.P.N. , Phil Reads machine.
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The Amal GPs flow better not only because of the remote needle but also because they are "smoothbores;" at wide open throttle they have an absolutely clean shot, no slots or slide bores or needles in the sides of the bore.
Most used 30 or 40 year old Concentrics are going to be worn in the first place. That is, the slide/body fit won't be great. The slide, of course, should be a tight fit to the body so that air is not pulled around the side of the slide instead of being forced under the cutaway so that its full Bernoulli effect is "seen" at the venturi.
If the carb's body is bored larger, there will be less bearing area and the slide will have a less tight fit to the body.
Most used 30 or 40 year old Concentrics are going to be worn in the first place. That is, the slide/body fit won't be great. The slide, of course, should be a tight fit to the body so that air is not pulled around the side of the slide instead of being forced under the cutaway so that its full Bernoulli effect is "seen" at the venturi.
If the carb's body is bored larger, there will be less bearing area and the slide will have a less tight fit to the body.
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