Amal Carb size

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Lesson learnt: never assume anything. I've had my bike for years, a 72 750, not combat. It has 32 mm amals. I ran calipers across the inlet ports today and discovered they are only 30mm. How significant an effect is this likely to have? Is it worth changing to the correct 30mm amals?
 

Fast Eddie

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No, you won’t gain anything, 32mm carbs feeding 30mm ports is good. I assume your manifold tapers from 32-30 ?
 
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Roland Pike accidentally found while undertaking tuning on Gold stars that a larger carb feeding a smaller inlet increased power. The MK2 850 with the RH10 head has 32mm carbs feeding 30mm inlet ports through tapered manifolds.
 

baz

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Roland Pike accidentally found while undertaking tuning on Gold stars that a larger carb feeding a smaller inlet increased power. The MK2 850 with the RH10 head has 32mm carbs feeding 30mm inlet ports through tapered manifolds.
Wasn't the BSA story about an apprentice sticking a 500 carb on to a 350? IE no tapered inlet?
 
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Wasn't the BSA story about an apprentice sticking a 500 carb on to a 350? IE no tapered inlet?
With the Goldstar it was a step between the carb and inlet port. I have Pikes memoirs somewhere but from memory no mention of apprentice or 350 in his telling of the test.
 
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Fast Eddie

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Mr Aardvark, you definitely have the best port size at 30mm. And 32mm carbs on tapered manifolds will work great, better than 30mm carbs.

If you wanted to improve further, as Comnoz posted in the thread below, you’d be better off using 32mm manifolds and simply putting a small radius at the step where the manifold meets the port.

 
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Mr Aardvark, you definitely have the best port size at 30mm. And 32mm carbs on tapered manifolds will work great, better than 30mm carbs.

If you wanted to improve further, as Comnoz posted in the thread below, you’d be better off using 32mm manifolds and simply putting a small radius at the step where the manifold meets the port.

Good to hear I should keep my relatively new premiers. The manifolds are 32mm end to end. I'm planning to pull the engine down before long as it's got a worn cam, so I can look at head work then. If anyone can recommend someone who does good Norton head work in this hemisphere please let me know.
 

Fast Eddie

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Good to hear I should keep my relatively new premiers. The manifolds are 32mm end to end. I'm planning to pull the engine down before long as it's got a worn cam, so I can look at head work then. If anyone can recommend someone who does good Norton head work in this hemisphere please let me know.
I’m sure if you post a thread about head work you’ll get good recommendations for your neck of the woods.

But whatever you do, do not let anyone open up those ports ! Lots, in fact LOTS, of info on the do’s and don’ts of porting in the link I posted above.
 
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If you have space for a phenolic spacer, you can file or dremel the 32mm to 30mm taper in a 30mm spacer between the head and intake manifold. A 30mm hole is only 1mm smaller than a 32mm hole at the radius and perimeter edge. The taper won't make a big difference, but it might make you feel better knowing you got rid of the 90 degree step.
 
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If you have space for a phenolic spacer, you can file or dremel the 32mm to 30mm taper in a 30mm spacer between the head and intake manifold. A 30mm hole is only 1mm smaller than a 32mm hole at the radius and perimeter edge. The taper won't make a big difference, but it might make you feel better knowing you got rid of the 90 degree
Thanks all. Happy that though not 'correct' it's not a real world problem, Schwany's solution seems eminently sensible, so that's what I'll do.

Cheers
 
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Lesson learnt: never assume anything. I've had my bike for years, a 72 750, not combat. It has 32 mm amals. I ran calipers across the inlet ports today and discovered they are only 30mm. How significant an effect is this likely to have? Is it worth changing to the correct 30mm amals?
If it like mine it will have a RH5 head, slightly lower compression than the normal head. Maybe something to do with the combat problems.
 
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Lesson learnt: never assume anything. I've had my bike for years, a 72 750, not combat. It has 32 mm amals. I ran calipers across the inlet ports today and discovered they are only 30mm. How significant an effect is this likely to have? Is it worth changing to the correct 30mm amals?
Been following this thread with some interest in 750 head evolution. The following post has more info on head versions and port sizes.
"RH1 28.5mm port and RH3 RH5 RH6 32mm port"
 

L.A.B.

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If it like mine it will have a RH5 head, slightly lower compression than the normal head.

RH5 was the 32mm inlet, standard compression post-Combat head often referred to as 'Low' compression as there was also the 32mm inlet 'High' compression (but not as high as the Combat) RH6 head.
 
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i have a similar issue on my BSA resto project - 32mm carbs on 30mm ports. from the limited research i've done on the subject, no ill-effects in going up 2mm. however, IMO, putting 32's on 30 ports is like putting a 32 inch door on a 30 inch hallway. one thing to note, the smaller carb should give a bit better low end throttle responce, whereas, the larger carb will see the best gains at the higher RPM's i doubt the average person will feel any SOTP difference. BTW, install new 30mm amal premier 's on the spitfire.
 
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Off topic drivel

The intake ports measure 31mm in my Norton 750 head. I don't actually know what size the ports were before I got the head ported and surfaced. I never measured them. My guess is they were at most 30mm, since the bike was sold new with 930 Amal carburetion. The head has no identifying marks visible. No clue about the RH factor.

My current carburetors are 35mm. There are no sharp steps from 35mm to 31mm in the intake track. Lots of dremel shaping work in the intake manifolds. Do the larger carburetors work on the street? You bet they do. I can't ride at 25mph in 4th gear though unless I'm pointed downhill and coasting. Point is you can put bigger carburetors on a Norton or any engine, if you have enough patience to tune all aspects of the setup. Is it worth doing? Not for many Norton owners, and certainly not for a restoration. Yadda yadda... end BS

I wouldn't be concerned about putting the 32mm carburetors back on the 72 750. If you didn't mind spending the money, instead of the spacer idea, getting intake manifolds that were 30mm at both ends would be the a viable solution. It would only take a hour or so per manifold to port match and smooth the carburetor ends of the intake manifolds to 32mm. Use a 32mm intake gasket as a guide for the hole shape and blend the taper into the manifolds about 3/4 of an inch. You could do it with emery cloth (although it would take longer) if you don't have a dremel and cutters laying around. Easy for me to say. I like doing that kind of thing.
 
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Off topic drivel

The intake ports measure 31mm in my Norton 750 head. I don't actually know what size the ports were before I got the head ported and surfaced. I never measured them. My guess is they were at most 30mm, since the bike was sold new with 930 Amal carburetion. The head has no identifying marks visible. No clue about the RH factor.

My current carburetors are 35mm. There are no sharp steps from 35mm to 31mm in the intake track. Lots of dremel shaping work in the intake manifolds. Do the larger carburetors work on the street? You bet they do. I can't ride at 25mph in 4th gear though unless I'm pointed downhill and coasting. Point is you can put bigger carburetors on a Norton or any engine, if you have enough patience to tune all aspects of the setup. Is it worth doing? Not for many Norton owners, and certainly not for a restoration. Yadda yadda... end BS

I wouldn't be concerned about putting the 32mm carburetors back on the 72 750. If you didn't mind spending the money, instead of the spacer idea, getting intake manifolds that were 30mm at both ends would be the a viable solution. It would only take a hour or so per manifold to port match and smooth the carburetor ends of the intake manifolds to 32mm. Use a 32mm intake gasket as a guide for the hole shape and blend the taper into the manifolds about 3/4 of an inch. You could do it with emery cloth (although it would take longer) if you don't have a dremel and cutters laying around. Easy for me to say. I like doing that kind of thing.
Semi-retired now so plenty of time for projects, and I do have a dremel.
 
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On the BSA a special tapered manifold was made to try and improve on the accidental power gain found with the oversized carb.
With the tapered manifold in place the initial power gain disappeared.
So it was back to the stepped setup for best result.

Glen
 

Fast Eddie

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IMO, putting 32's on 30 ports is like putting a 32 inch door on a 30 inch hallway. one thing to note, the smaller carb should give a bit better low end throttle responce, whereas, the larger carb will see the best gains at the higher RPM's
It’s not quite as simple as that…

Partly because of the drag that is caused. Also the effective size on the carb is reduced by the needle and emulsion tube that protrudes. And flow is hampered by the machining for the slide etc.
A bigger carb will have less drag and will provide some compensation for the above.
Provided the carb is not stupidly big (big enough to not cause a proper Venturi effect and screw up the fuelling) then a slightly bigger carb is good as it ensures the port will get all it can use.
Velocity is created in the port mainly, so again, provided we’re not talking crazy carbs, a bigger carb should not negatively effect velocity.
Comnoz’ testing showed the best thing to bolt onto 30mm Norton ports is 32mm manifolds and 32mm carbs (at least).
 
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